lubricant friction and wear testing

MNL37-EB/Jun. 2003

Lubricant Friction and Wear Testing Michael Anderson^ and Frederick E. Schmidt^

THIS CHAPTER PROVIDES VARIOUS METHODS TO EVALUATE THE FRIC

TION AND WEAR PROPERTIES of lubricants and materials. If friction and wear can be controlled then the engineer can select materials and lubricants with a high degree of confidence. Many laboratory tests are used to evaluate the interaction of materials under a broad range of test conditions and controlled environments.

In this chapter, the following topics will be discussed:

• History of tribology testing • Basic types of tribology test systems and reasons for their

use • Fundamentals in designing tribology tests • How to select a test device to simulate a field condition • Contact geometry used in bench tests • Standard and commonly used test devices • Designing special application bench tests • Common terminology relating to friction and wear testing

HISTORY OF TRIBOLOGY [1]

From the beginning of time, man has tried to overcome friction and wear. The earliest application of friction is its use for building fires. To early man, fire offered many advantages including safety, light, warmth, and cooked food. Man also needed weapons to kill animals for food. Primitive techniques were used to sharpen sticks and stones. As simple as these would appear, this use of friction greatly enhanced man's quality of life during this primitive period.

Later, as man began to cultivate the land to provide food to supplement his diet of animal meat and fish, agricultural tools became a necessity. Not only must they be durable, but they also had to be shaped. Simple manufacturing techniques were employed such as grinding. More durable materials were more difficult to make. As time went on, man used new techniques to help in this manufacturing stage. Simple engineering methods were employed such as pottery wheels.

At the time the great pyramids and monuments in ancient Egypt were being built, man was beginning to use not only engineering techniques, such as rolling elements (logs) to reduce friction, but he was also introducing liquid media between the surfaces. Sometimes, these liquids were simply hy-drated earth (clays, soaps, or other materials). Nevertheless, lubrication was becoming a part of life.

1020 Airpark Drive, Sugar ' Vice President, Falex Corporation, Grove, IL 60554. ^ Manager Services for Industry, Engineering Systems Inc., 3851 Exchange Avenue, Aurora, IL 60504.

Even though man was employing simple engineering principles and lubrication for manufacturing, it wasn't until the late 15th century, when Leonardo DiVinci first deduced laws governing the motion of a block over a flat surface, that the science of friction and lubrication was developed. During this time, primitive testing devices were developed to measure the force of one object moving against another. Scientist during this time also realized that measured forces were less when a material such as pig fat was introduced between sliding or moving surfaces; hence, the study of lubrication had begun.

During the years that followed, friction, wear, and lubrication studies increased. As the industrial revolution brought more advanced machines for transportation and power generation, engineering became part of the curriculum at universities. These studies included the fundamentals of friction, lubrication, and wear. With new extraction techniques for obtaining crude oil and the ability to refine this oil, lubricants became more commonplace. As lubricants became more widely used, technology was needed to eveduate the differences in properties and in various applications. In 1927, the first commercial tribomoter was introduced to blenders and manufacturers of finished lubricants. This tester "Pin and Vee Block test machine" provided suppliers with a method of measuring anti-wear and extreme pressure properties of the lubricants they were selling. Subsequently, tribometers such as the Timken®^ tester. Four Ball Wear and Four Ball EP, Block-on-Ring, and others were introduced to evaluate lubricants and materials under a variety of test conditions. These machines are described in this chapter.

Further developments in transportat ion, medicine, and space exploration have provided impetus for the development of new lubricants and materials. With these technologies has come the development of test machine designs and test methods to meet the challenges of these new applications. Today, over 225 commercial and independent testing devices [2] have been developed.

BASIC TYPES OF TRIBOLOGY TEST SYSTEMS

Laboratory testing of lubricants used in a tribology system involves different levels of sophistication. A tribological system consists of all relevant test parameters, materials in contact including the lubricant, if present, and any external, environmental conditions [3]. Each level of test sophistication

^ Timken Corporation, Canton, OH.

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1018 MANUAL 37: FUELS AND LUBRICANTS HANDBOOK

has benefits and drawbacks. The three levels of test sophistication Eire as follows [4]: • Laboratory bench test devices (simple geometric contacts) • Component bench test devices (use of actual parts and as

semblies in a laboratory test rig) • Field tests (materials and lubricants tested in actual

systems) The most representative test program is one that uses the

proposed material combination in the actual field situation [4,5]. Generally, this approach is not practical for several reasons. Costs can be prohibitive; the time to develop a meaningful program may be too long; and the environmental or ambient conditions are difficult if not impossible to control. These limitations leave many uncontrollable variables and possibly, a wide scatter of test data. Even component testing, which is a laboratory test rig that uses the particular section of the machine (or field application) that is of interest and in which the parts are made of the materials under evaluation, is more cost effective but rarely used as a first approach [3,6]. These instrumented laboratory test devices possess the same limitations as field tests, except that the ambient conditions are more controllable. However, laboratory bench tests are designed to move test pieces with simplified geometry under a variety of test loads, speeds, and environmental conditions. Although these simplified devices cannot produce exact operating conditions, they have the potential to produce results that provide meaningful data for a range of similar applications [5]. The wide use of these test devices, such as the Four Ball and Timken machines, for determination of extreme pressure properties reflects the low cost and ease of such measurements and the belief that test results correlate to some extent with performance [7]. The use of simple bench testing reduces the test evaluation to a single, specific tribo-logical condition simulating, as close as possible, the operating conditions for the material and lubricants. Data generated from these tests are compared and those materials and lubricants are selected that jaeld the best wear life or performance for further testing under more specific test designs. Repeatability of the obtained test results can be better when the test is kept as simple as possible. Figure 1 gives a relative economic comparison versus repeatability for bench tests, component tests, and field tests.

The ability to use a bench test offers many benefits including: • Simplicity of operation • Lowest testing cost • Accelerated test results • Real time presentation of data to facilitate recognition of

changing conditions.

BENCH TESTS Test tvDe

bench

component

Relative cost

$

$$$

Repeat-abiltv

*

* * *

field

FIG. 1—Economic comparison of test types.

• Accurate and precise indication of wear rates and performance properties given the test parameters under which the test is conducted

• Inexpensive and uniform consumable test pieces • Test pieces from a wide range of materials and conditions • Small volumes of test fluid • Controlled test environments and ambient conditions • Convenience of operation.

Commercial test devices offer significant benefits over test equipment made in-house. Because commercial test machines are made in quantity to the same manufacturing specifications, they can offer better test result comparisons between the laboratories using them. Commercial testers are often used when developing standardized test methods because of the availability of users willing to cooperate in the development of precision statements. In most cases, the test parameters are listed in standardized test methods. However, they may not provide the user all the necessciry information for evaluating his materials. Therefore, standardized test methods can be suitable starting points, but the user may need to modify the test parameters to achieve meaningful test results [3,8]. Usually more data can be obtained throughout the test rather than just the final specified endpoint or reported test result. Many data occur during the course of a test, including but not limited to, changes in lubricating mechanisms, changes in surface areas giving different contact pressures, development of lubricating films and surfaces, and so on. Therefore, the operator must identify these changes and develop test methods that facilitate obtaining as much pertinent information as is possible or required.

Commercial test devices provide the following benefits: • Established and known precision • Simplicity of test operation • Many meet ASTM, SAE, ISO and other standard test

methods • Flexible test procedures • Ability to compare results worldwide • Correlation with previously published field results • Support and assistance in operation and method develop

ment by the manufacturer

FUNDAMENTALS IN DESIGNING THE TRIBOLOGY TEST

One of the most important concepts in understanding tribo-logical testing is that it is a system. As a system, each test parameter affects the test result. Changing any test parameter can effect differences in wear rates and/or frictionaJ properties. One must identify the components and possible conditions that exist during the test as part of the test program and try to match these as closely as possible with those that are occurring in the field. The objective of testing is to produce, on the test rig, similar surface damage to that which occurs after failure in service [5,6].

The first step is to obtain a complete understanding of the field condition. The second step is the selection of the most representative bench test available and the development of the test procedure to be used. The third step is to conduct the test. And the fourth and final step is the review of the test data and development of conclusions [3]. Formulation


CHAPTER 37: LUBRICANT FRICTION AND WEAR TESTING 1019

of conclusions at the end of the test requires the user to analyze the data obtained with respect to actual field conditions. The user then draws the appropriate conclusions and then may develop models for predicting wear of future applications involving similar materials and operating conditions [9].

When designing a laboratory test program, the basic steps for successful testing are as follows:

F i e l d P r o b l e m

1. Identify the location of the wear problem in the test system 2. Determine the failure mechanism 3. Identify the tribological conditions

• Motion Sliding, unidirectional Sliding, bidirectional or reversing Rolling Fretting

Speed Linear velocity Rotational velocity

Contact Geometry Point Line Area

Pressure/Load Normal loads Surface area of contact

Temperature Bulk lubricant Contact temperature

Type of Lubricant Fluid Solid Semi-solid (grease) Dry film None

Lubricant Performance Properties Anti-wear Extreme pressure Chemistry

• Lubricating Mechanism Enclosed chamber (flooded sump) Constant circulation Spray Coated

Contacting Materials External Operating Conditions

Ambient temperature Atmosphere Humidity Vibration Contaminants

B e n c h Tes t S e l e c t i o n

1. Experience 2. Standardized test methods 3. Available test equipment 4. Analytical methods such as Tribological Aspect Number

(TAN) [3]

T e s t P a r a m e t e r S e l e c t i o n

1. Motion • Sliding • Rolling • Combination roll/slide • Unidirectional • Reciprocating

2. Speed • Constant • Ramping • Changing

3. Load • Constant • Ramping (to failure) • Cycling

4. Contact Pressure (Normal Load Over Area of Contact) • Hertzian (constant) • Hertzian (changing) • Area (constant)

5. Materials • Composition • Hardness • Surface finish • Micro-structure • Coatings • Surface treatments

6. Duration Fixed (compare amount of wear) • Time • Number of cycles • Total linear distance Time to failure (compare life of test) • Excessive friction/torque • Excessive temperature • Excessive wear rate • Excessive total wear

T e s t R e s u l t s

1. Wear • Volume of material lost • Weight loss • Dimension of wear scar • Dimension change of material

2. Friction Force or Torque 3. Coefficient of Friction

• Static Coefficient of Friction • Dynamic Coefficient of Friction • Maximum • Minimum • Average

4. Correlation with Field Results • Type of wear mechanism • Type of failure mechanism • Relative amount of wear • Comparative ranking of different material 's perfor

mance with field results • Meets specification

5. Lubricant Analysis • Additive depletion • Wear particle count • Viscosity change


1 0 2 0 MANUAL 37: FUELS AND LUBRICANTS HANDBOOK

All controlled laboratory testing yields test data that reflect what is happening in the test device under the selected test conditions. If the test results do not correlate with the field results, then either the test device is not representative of the field application or an incorrect test procedure was chosen. Therefore, it is of the utmost importance that the user develops a meaningful test procedure and utilizes the type of test equipment that best represents the field condition. There Eire many approaches that are used to select the best test machine and the test parameters. This chapter will reference and discuss a practical, analytical approach to selecting the best bench test for simulating a particular field condition.

SELECTING AND DESIGNING THE TEST

The first and most challenging step in conducting a successful laboratory test program is the selection of a test that will accurately simulate the field condition being investigated. As mentioned previously, there are three basic types of tests: bench, component, and field.

Field tests are time consuming, and generally very expensive. They tend to have poor repeatability as they are subject to many complications in the control of their test parameters and ambient conditions. For these reasons this chapter is limiting the discussion to laboratory tests. Materials selected using laboratory tests must be qualified in a field test before actually being put into service. This is ultimately where the tested materials must work, but considerable testing can be done in the laboratory far more economically.

Two types of tests that will be discussed in detail are bench tests and component tests. A good rule is to attempt the simplest test possible that accurately simulates what occurs in service. To that end, test devices have been developed that are used to simulate service conditions in the laboratory [2]. Selection of a laboratory test rig should be based on the ability to simulate as closely as possible the conditions of the field application. There are many ways to select a bench test, including experience, s tandardized test methods , and published analjrtical approaches. One such approach is a paper written by R. Voitik, "Realizing Bench Test Solutions to Field Tribology Problems by Utilizing Tribological Aspect Numbers," incorporates an analytical approach to selecting the best bench test [3].

Laboratory tests are used in two basic functions: specification testing (or quality assurance) and research testing. Specification testing uses standardized laboratory bench tests, such as those based on the Pin and Vee Block, Timken, Four-Ball Wear, and Four-Ball EP test machines. Considerable testing has been performed on these test machines in the evaluation of materials for specific applications. Industrial laboratories have a sufficiently high degree of confidence that test results obtained on these test machines under prescribed conditions will correlate with performance in the field. These tests work well for screening formulations during lubricant development, for monitoring the quality of lubricant production and for lubricant specification [7]. Other uses for specification testing include evsiluation of a new formulation to meet an existing application, competitive performance properties evaluation, and approving new batches of

materials. Laboratory tests for specifications are generally accepted for verifying material performance.

A widely used test procedure is generally submitted to a standardization organization to develop a test method for determining certain properties. A standardized test method is selected for use in specifications because the user determined that standardized test results correlate with field experience. Qualifying materials to a specification allows further development and improvement of materials for field use, and provides a means for assuring performance quedity. A critical part of the standardization process is the development of the precision of the test method. Precision is developed through the efforts of many testing laboratories participating in a cooperative testing program. In such a program, participants test selected materials according to a prescribed sequence. The laboratories obtain back-to-back duplicate test results. These test results form the basis for determining both repeatability and reproducibility of the test procedure. Precision gives the user of the test method the confidence to determine whether the test results obtained are significant, in other words, whether the results obtained are merely within the repeatability of the test method and are not actually different. Additionally, uncertainty could be reduced by a duplicate test or comparison with reference oil [7].

When there is no established or agreed upon test procedure, test method development can be more involved and create significant delays. The test method developer must select the test device, design the test method, run the tests and then determine the validity of the test results [3,6]. This process of developing a laboratory test method to predict field conditions can be difficult due to lack of information about the field conditions, availability of the certain test equipment and desired acceleration of the test. The volume of field performance data available for various test materials will determine the level of difficulty in developing base case information for test validation.

Sometimes the designer chooses a component test as being the preferred approach for materials evaluation. Frequently, the customer requires data to be produced on the actual components. Or quite possibly, preliminary test data was generated on a simple bench test and subsequent developmental testing must be performed at the component level. Nevertheless, the fundamental criteria for selecting test conditions and parameters Eire similar whether one selects a component test or a bench test.

B e n c h Tes t

Bench Test, as used in this chapter, is the term used to describe laboratory test devices that are simple in design, yet complex enough to rank a materiEil's performance for a specific property or to simulate an actual field condition. Unlike component test stands, bench tests are designed to isolate specific contacts, motions, loads, and geometric contacts. These physical characteristics combined with the selected test parameters give the researcher a means of easily evaluating materials for their effects in field applications or further component testing. In a properly designed test, operating under a lubrication regime similar to the field condition, the asperities at the point of contact will react the same as they would in the field condition that the bench test is represent-


CHAPTER 3 7: LUBRICANT FRICTION AND WEAR TESTING 1021

ing. Correlation has been shown by Faville [8] when using the Pin and Vee Block for evaluating transmission and other lubricants.

C o m p o n e n t T e s t D e s i g n

Component testing, which includes, but is not limited to, pump tests and engine test beds, uses actual parts or components from manufactured equipment. The test stand designer must carefully select these parts to be within well-defined manufacturing tolerances for dimensions, surface finish, and hardness. The component fixturing is typically designed to hold the components and test them in a manner that represents, as close as possible, the actual field conditions and environment while maintaining the desired test parameters . The test device design should be sufficiently flexible to permit a wide range of operating conditions.

To achieve reasonable repeatability and reproducibility with the test results obtained from the test stand, close control of the test parameters must be maintained. In the actual field condition it is virtually impossible to control all of the ambient conditions, due to the wide fluctuations in surrounding conditions. These test parameters for a component test stand should be selected to represent conditions that might occur in actual applications. The selected parameters must be closely controlled, and the test stand must be designed to provide provisions for monitoring the selected test parameters. Monitoring the test parameters will provide the operator with a recorded history indicating whether these parameters were maintained during the test. Often times controlled atmospheres, large sumps for test fluids, special air and fluid filtration, temperature control, load control, and other more specific systems must be designed into the test stand to maintain test parameters or for better simulation of field conditions. These test devices are designed to simulate a particular aspect of actual operation and are valuable for the development of additives and lubricant formulations [10]. In engine test stands, for example, proper simulation of actual driving conditions includes cycling of load, speed, and temperatures according to a designed test program. It is essential to integrate the cyclic characteristics to simulate the actual driving conditions on the laboratory test rig. The use of a computer to control test parameters provides more consistent test operation and facilitates data acquisition

Monitoring test variables during the test sequence is critical. The computer, with its capability of acquiring and storing the test data, has offered considerable benefit to the test operator by recording data throughout the test. This data will advise the operator that the test has run within the selected controlled parameters and that the test is operating as desired. It will also alert the operator to a change or failure in any of the measured properties. Because ambient conditions can also influence test results, it is important that these conditions such as tempera ture and relative humidity be recorded. The computer may provide more rapid data collection when one or more of the variables exceed an alarm condition. This rapid data collection will give more detailed information of the test results when unusual conditions are present. The computer can also activate the control function of the test stand, eliminating the need for the operator to make the control adjustments after the test has initiated. The

computer can also cause the test to terminate if a predetermined set-point is exceeded or if a dangerous condition exists. And finally the computer can organize, calculate, and present the data in tabulated or graphic format for ease of interpretation. This valuable tool is used in most component test formats and for an increasing number of standard bench tests stands.

With component tests, the parts chosen for the test fixturing must have some sensitivity to the materials under evaluation [11]. For example, pumps that are used to evaluate wear of hydraulic fluids should be sensitive to formulated fluids that have sufficient anti-wear properties and those that do not [12]. There should always be some check that the test device and selected test parameters and sequences are robust enough to discriminate between materials with known poor field performance and those with acceptable field performance. The user should prove that the component test has an acceptable degree of precision, both in repeatability and in reproducibility. Repeatability is the closeness of the agreement of test data on back-to-back testing on the same test stand, in the same laboratory, with the same operator, on the same test materials or fluids, within a short time span. Reproducibility is the agreement of test results using the same test materials or fluids but on different test stands, with different operators, in different laboratories, and run at different times.

When designing any laboratory test rig, the design should provide for wide latitude of test parameters. This will assist in discriminating between materials or components tested with materials of varying performance properties. When a series of test parameters has been chosen that demonstrates differences in materials of known field performances, the test stand should successfully rank materials with unknown field performance. Extreme care should be exercised when selecting existing manufactured parts for use as testing components, as these parts must have some guarantee of consistency of manufacture and known tolerances for dimensions and materials. Nevertheless, successful testing has been developed and has given the test engineer considerable information on the performance of materials predicted from the results of component testing under actual or near field conditions. Properly designed test stands should mimic the field conditions as closely as possible so that the contacting materials will exhibit properties exactly as if they were in the actual field condition.

TEST PARAMETER SELECTION

The basic test parameters to be considered when developing any type of test method are discussed in this section.

T e m p e r a t u r e

Temperature control can be effected by the use of heating systems if elevated temperatures are required and/or cooling systems if lower than ambient test temperature is required. The test fixturing can be designed to contain heaters or cooling tubes to maintain the bulk temperature of the lubricant or the test pieces. It is virtually impossible to measure the temperature at the point of asperity contact, but by



locating the temperature sensor in close proximity to the contact surface, one can get an idea of the contact temperature versus the bulk lubricant temperature. In some test designs, the atmosphere surrounding the test area must have a controlled temperature. This is usually achieved through some type of air circulation if lower than ambient temperature is required, or through the use of heaters external to the test component but within the enclosed testing chamber. Large fluid reservoirs can help in maintaining the stability of test temperature by providing a large volume for heat absorption. A large reservoir also permits the use of the fluid in extended duration testing with a minimal amount of additive depletion.

L o a d

Load control must be considered when selecting a test. Test load can be applied by a static or dead weight system, a pneumatic system, hydraulic system, or by fluid pressure. Consideration should be given to including an initial "break-in" or "wear-in" at a lighter load representing partial EHD [13]. Inclusion of a break-in can form a more uniform surface rather than testing under machined conditions. This more uniform surface can add to the consistency of test results.

It can be difficult to select test loads that will be representative of the loads encountered in the field, yet of a magnitude large enough to challenge the test system without introducing any complex or other type wear mechanisms. Selecting loads that are excessive could produce results that are not representative of what is occurring in the actual field condition. If the operator chooses a test load that is low, he may encounter very long test duration before failures occur. Another way to determine an appropriate test load is to conduct a step load test and look for an erratic change in the torque or friction force as load is increased [8]. This variability of the torque indicates a breakdown of the lubricant film, allowing for metal to metal contact. The load immediately prior to the load that corresponds to the erratic torque or friction force can be looked at in terms of the threshold limit of the test system load. The operator can choose a test load at or below the threshold load that should give a controlled amount of wear in a reasonable time. Note that too light a test load may not give enough weeir to discriminate between samples and too heavy a load may yield too much wear to discriminate. At this point, it is basically an educated guess as to the best test load. Several loads in this area may have to be tried in order to select the test load that gives the best discrimination with regards to the other test variable. Ideally, if materials with known field data are being used to set up the test procedure, discrimination should be the focus. If discrimination is obtained, then this test load should be used as the starting point for the tests, that is to say that at some point in the testing with other materials, these test parameters may have to be reevaluated.

Often times the load must be controlled to allow for cycling of test loads to simulate in-field conditions. Cycling versus static load can better simulate the stresses encountered in the test system and is used to better maintain the temperature of the test system in long-term endurance or life tests.

S p e e d

If possible, select the linear test speed to be the same as the field condition. This is done in rotational tests by taking the speed of the field condition and dividing by diameter of the point of contact or wear track, and by pi, msiking the appropriate conversions for distance units results in the corresponding rpm. Certain applications require very slow or very fast test speeds. Test speeds have a profound effect on lubrication regime in the test system. The operator should consult the Stribeck curve for general effects of the change in load or speed on the coefficient of firiction and lubricating regime.

The Stribeck curve (Fig. 2) shows the relation of coefficient of friction to the ratio of viscosity, speed, and the inverse of load, known as the Sommerfeld Number [14] with respect to coefficient of friction.

Sommerfeld Number = speed X viscosity/load

This fundamental curve gives an illustration of the effect of changing viscosity, speed or load on the coefficient of friction in the various lubrication regimes of boundary, elastohydro-dynamic, mixed and hydrodynamic [15].

1. Hydrodynamic lubrication: the surfaces are separated by the lubricant film resulting in low friction.

2. Mixed lubrication: the load is carried by the lubricant and the interacting asperities

3. Boundary lubrication: the load is solely carried by the interacting asperities, resulting in high friction. Although it may be impractical to construct the entire clas

sic Stribeck curve, specific portions of the curve for vEirious test systems can be determined as illustrated in the curves listed. It should also be noted that different materials, geometries, and test systems can yield curves that are different in shape than the classic Stribeck Curve. For example, certain systems may not have a boundary lubrication region and may rise directly to a very high friction value indicating severe metal-to-metal contact.

The curve in Fig. 3 depicts Stribeck curves in the mixed lubrication region as a function of lubricant thickness [15]. By changing speed and/or load in the Sommerfeld number, the resulting change in coefficient of friction is determined and plotted for each lubricant thickness. In the self-lubricated

speed X viscosity/ioad

FIG. 2—Theoretical Stribeck Curve. Reprinted with permission of STLE.



0.14

0.12

*- 0.08

0.04

0.02 -

1.E

^ ^ v •"~~-—— \ ^ ^S«-«.-.--''>^

%\. —

-06 1.E-05 1.E-04 1.E-03 1.E -02

L

. . — ... 1.00E-04

1.00E-05

1.00E-06

5.00E-07

3.00E-07

2.00E-07

— 1.00E-07

8.00E-08

4.50E-08

4.20E-08

»-4.00E-08

——3.90E-08

— - 3 . 8 0 E - 0 8

370E-08

3.65E-08

3.64E-08

3.63E-08

3.60E-08

FIG. 3—Stribeck curves for starved lubricated line contacts as a function of the applied lubricant layer thickness in )j.m [15]. Reprinted with permission of Prof. D. J. Schipper, University of Twente, The Netherlands.

0.20

•U 0.15

U-

•p: 0 . 1 0 -c <p "o o O

0.05 N=1.0N/cm

N=0.2 N/cm2

N=0.4 N/cm

N=0.8 N/cm

I

10" IQ- 10 -6 10" 10" 10" Sommerfeld Number

FIG. 4—Coefficient of friction as a function of Sommerfeld number for a plastic on a PMMA disk lubricated by a saline solution [14]. Reprinted with permission from the Society of Plastics Engineers.

condition, as illustrated in Fig. 4, changes in speed with constant loads are plotted against coefficient of friction determined by testing. Similarly, tests could be conducted by varying the load maintaining constant viscosity and speed. Although it is possible to vary more than one variable in determining Sommerfeld numbers and their relation to coefficient of friction, it is more typically studied by varying just one of the variable's affect on the coefficient of friction, as illustrated in the above examples.

Duration

Test duration is also important. Often tests are conducted on many different samples at the same predetermined test dura

tion and the wear of each material combination is compared. Test duration in this case is either by time or by number of cycles. If the materials are being severely challenged, tests can be run to a predetermined failure point such as a friction, wear, or temperature limit. In this type of test, the result is the time to failure or in the case of increasing load or speed, the load or speed at which failure occurred.

When selecting test parameters to represent a field condition, it may not be practical in the laboratory to have failure occur in the same time frame or duration as it would in the field, or it may be desired to accumulate data in a relatively short period of time. This is referred to as an "accelerated test." Therefore, the test parameters must be selected to enhance the desired wear condition or to produce a desired wear condition in a shorter period of time. Careful consideration of accelerating test results must be given since it is important not to introduce any wear mechcinisms into the test system that are not occurring in the field. Verification must be made at the end of test to insure that the wecir mechcuiism or failure is the same as that which occurs in the field. Otherwise, it is possible to obtain test results that do not correlate with field results.

Materials

Standardized test methods generally specify the materials cuid conditions of the test pieces consumed in the test. Simulation tests should utilize consumable test pieces from the same materials and conditions that are present in the system being modeled. The use of different materials than present on the system being modeled, can result in poor correlation of the test with the system [11].

Special Atmospheres

If special atmospheres are present in the field condition, the test designer should consider these when designing the laboratory test procedure. Special atmospheres would include humidity, pressurization, and inert or reactive gases. Gases can be introduced into any closed test chamber. Inert gases can be introduced in an attempt to exclude the formation of oxide layers at the surface. Specialty gases, for example Freon, can be introduced into the lubricant, often under pressure, to understcuid the effect of these gases when solublized in the test lubricants. This approach is used for eveduating lubricants for use in compressors or refrigeration systems [16].

Test Fluids

Test design of lubricated systems will always include the presence of test fluids and their influence on the prevention of wear or on load carr5dng ability. The method of lubricant introduction into the contact zone will affect test results. The most commonly used system is the test fluid bath. This lubricated system uses a contained quantity of test fluid, into which the moving test pieces are placed and remain as the test is performed. The contact may be submerged in the test fluid or a portion of the test piece enters the test fluid and carries the lubricant into the contact zone. Another lubricated system involves the use of an external reservoir, which intro-


1024 MANUAL 3 7: FUELS AND LUBRICANTS HANDBOOK

duces the test fluid into the contact zone either by continuously changing the fluid in a flooded test chamber or by injecting or depositing a controlled volume of test fluid directly into the contact zone.

Special Testing

Special testing is the addition of a test variable that is unique to a situation to better understand certain phenomena. These would include the introduction of abrasives or solid particles, the testing of used oils or test fluids with known history, or the addition of contaminants such as water or some other solid or liquid. Special testing is generally used in the laboratory test program when the field condition typically contains these special materials. Anything that can be done to better simulate field results will improve the simulated laboratory test results.

TEST SELECTION AND USE OF THE TRIBOLOGICAL ASPECT NUMBERS

Selecting the best bench test is the first part of a test development program. Prudently selecting the bench test will provide for a more efficient development of test data. There are many ways used to select the best bench test to evaluate ma-terieJs for a specific application. A common and important method is to draw on experience. Although experience is critical in developing any test method, it is also a limitation when evaluating materials and processes for new applications. Trial and error is another method that can be successful, but can also be very tedious and frustrating. Others have expanded geometry, speed, load, etc. and studied their effect on friction and wear [8,11,12]. In 1993, a paper was presented at the ASTM symposium on Wear Test Selection for Design and Application, entitled, "Realizing Bench Test Solutions to Field Tribology Problems by Utilizing Tribological Aspect Numbers" [3]. This paper and a subsequent review paper by Anderson [17] describe an analytical approach to selecting the best bench test for simulating a particular field application. This procedure identifies the tribological condition and characterizes it in a four-digit number based on motion, geometry, load (pressure), and entry angle of the test fluid. This approach can be used to select laboratory bench tests for simulating field conditions.

Field applications can involve many different tribological conditions in the same system. It is the premise of using the Tribological Aspect Number (TAN) approach to isolate the tribological condition under consideration and then select the bench test that has the same TAN.

The TAN is a four digit number that characterizes a tribological condition. The first digit identifies the velocity or motion characteristic. The second digit identifies the contact area. The third digit identifies the load or pressure. And finally, the fourth digit identifies the entry angle of the lubricant.

The first step in the process of using the TAN system is to identify the TAN of the field condition. Second, the user selects the bench test that has the same TAN number. Note that a testing device may have been designed to provide for more than one TAN and there may be more than one tester that can

yield the same TAN. After identifying the bench tester to be used, the researcher must select the test parameters under which the test will be performed. These conditions should be selected to mimic or reflect the actual field conditions. These parameters are discussed earlier in this chapter. With the exact match of the TAN for the field and the tester, and selecting the same test parameters, the proposed test program is identified as a simulation. A simulation is a set of conditions that exactly represents the field condition. It is under this condition that the contact asperities should behave in an identical manner as those contact asperities in the field condition.

First Digit—Speed

The contact velocity or motion characteristic is subdivided into four types. Unidirectional, identified as 1, is a motion that does not change directions. It is unidirectional and is the sliding motion encountered in most typical wear testers. Cyclic motion, identified as 2, is motion that changes direction. It is also known as reciprocating motion and is more specific to select applications involving this type of motion. Roll/Slip, identified as 3, is motion that includes partial or complete rolling. Complete rolling occurs in ball or roller bearings; partial rolling, or those conditions that only have a percentage of rolling, is typical of gear simulations. Finally, fretting, identified as 4, is given its own, unique condition.

Second Digit—Contact Area

The contact area describes the geometry of the areas in which the moving pieces contact each other. The descriptions simply and accurately describe the type of geometric contacts that are present in the system. The open nature of the TAN 7 and 8 is unique in that one of the contact surfaces is being contacted in a previously untouched area. The fixed refers to non-changing contact geometry; the variable refers to changing contact geometry.

Third Digit—Contact Pressure or Loading Characteristic

Unidirectional, third digit of 1, identifies a condition of constant load or increasing load. High frequency, third digit of 2, is a quickly changing test load. Cyclic Loading, third digit of 3, refers to a condition of cycling the test load.

Fourth Digit—Entry Angle

The entry angle identifies the geometric angle of the leading edge of the tribological contact. It indicates the facility at which the lubricant can enter the contact zone. An angle of 0°-0°, fourth digit of 1, describes a contiguous contact. That is to say, there is no provision for the lubricant to enter the contact zone. This TAN is typical of thrust washer or seal applications. Because no lubricant can enter into the contact zone, these applications would do well using solid bonded lubricants, self-lubricating plastics, or high wear resistant ceramics. An angle of 90° to 75°, TAN of 2, is a very steep angle and would typically allow very little lubricant into the contact zone acting like a plow. As one moves from a fourth digit of



2 through Tan of 7, the angle of entry gets increasingly smaller. This smaller entry angle permits entry of the lubricant with ever increasing ease. An entry angle of < 10° - >2°, TAN of 8, has a very small angle of entry. This small angle of entry allows for the lubricant to come into the contact zone and reveals the effectiveness of the ability of a journal bearing, with its small entry angle, to easily carry the lubricant into the contact zone. And finally, >2°->0°, fourth digit of 9, illustrates the very, very small entry angle as is t5T3ical of rolling elements.

Once the TAN for a field condition has been identified, matching the bench test's TAN to the field condition's TAN can easily affect selecting the bench test. However, this is not always possible. Many times there is no exact match available to the researcher. This occurs because of the complexity on one or more of the TAN digits or in the fact that the best test device just is not available to the researcher. In such cases, the researcher should select a bench test with a TAN as close as possible to the field condition. He may then have to make some test procedural compromises to permit as close a representation of the selected laboratory tester with the field application. This condition represents a ranking rather than a simulation. A ranking test basically will provide performance test data for a particular property based on the value obtained from running the test.

Additional considerations when testing in the laboratory are to select the material for the consumable test pieces to be representative of the field condition material. This includes selecting not only the type of material, but the condition of the material such as hardness, case depth, surface finish, coatings, surface treatments, and microstructure. In the laboratory environment, if one were to simulate the test parameter conditions exactly as in the field, the test would take about the same time to fail in the laboratory as in the field. This just is not feasible; therefore, it is always desirable to acquire the data under accelerated conditions. Care should be taken not to introduce any complex or catastrophic wear mechanisms that are not present in the field and may adversely affect representative test results.

This chapter has discussed some of the techniques available for selecting bench tests, choosing test parameters, acquiring test data and evaluating the results with respect to the test procedure. The following sections will contain some of the more common test devices available and their respective standardized test methods.

The basic design of the Pin and Vee Block consists of two opposing Vee blocks loaded against a rotating journal pin (Fig. 6). It conducts tests in four-line contact, unless optional C-Blocks are used. C-Blocks give a conformal area contact. The test is run with the pins and blocks submerged in the test lubricant (D 2670, D 3233) (Fig. 7), with the test pieces coated with a bonded film lubricant (D 2625), or with the lubricant coating the test pieces (D 5620). Load is applied via a ratchet wheel and eccentric pawl. Each turn of the motor will advance the ratchet wheel one tooth when the pawl is engaged. Tests can be run at constant load (D 2670, D 2625 procedure A) or at increasing load until failure (D 3233, D 2625 procedure B). In the increasing load test, failure is indicated by a break in either the shear pin or test pin or in the inability of

FIG. 5—Pin and Vee Block Tester (Falex Lubricant Tester).

FIG. 6—Pin and Vee Block test pieces.

COMMON T Y P E S OF B E N C H T E S T DEVICES

Pin and Vee Block

This Pin and Vee Block (Fig. 5) is the most widely used commercialized wear tester for evaluating lubricants [18] Also known as the Falex®'* Lubricant Tester, the Pin and Vee Block test machine has been successfully used for evaluating lubricating and wear preventing properties of lubricants, both fluid and solid, for over 75 yccirs. This tester is used for evaluating metalworking fluids, automotive and industrial lubricants, and bonded solid film lubricants [8].

'' Falex Corporation, Sugar Grove, IL FIG. 7—Pin and Vee Block test piece configu

ration.


1 0 2 6 MANUAL 3 7: FUELS AND LUBRICANTS HANDBOOK

FIG. 8—Falex Test Pieces [8]. A: Unused (new); B: After ASTM D 2670 wear test; C: After ASTM D 3233 EP test, torque failure; D: After ASTM D 3233 EP test, weld failure.

the test system to maintain test load. Careful monitoring of the torque with respect to the test load can yield valuable information as to the lubricating properties of the test fluid as it interacts with the selected test piece materials [8,19] Changes in the slope of the torque curve can reveal changes in the lubricating regime of the test system. Careful examination of the torque, load, and wear values gives information on the anti-wear and extreme pressure properties of the tri-bology system.

Tests can be run at constant load for evaluating anti-wear properties and also under increasing load conditions to evaluate lubricating effects at different load conditions. Although the ASTM test methods for evaluating extreme pressure properties of lubricants directs the user to increase load up to the point where either the test or shear pin breaks, the information obtained during the entire test can provide important data as to the performance of the lubricating properties. An important phenomenon described by Faville [8] and later elaborated by Helmetag [19] is the occurrence of a sudden increase in the torque, also referred to as the torque "pop-up." Anti-weld evaluations can be made only in the load range immediately following this initial seizure. Products lacking anti-weld properties tear out metal, resulting in weld type seizure (Fig. 8D), while some products develop high torque, which result in twisting off the shear pin without any occurrence of scoring (Fig. 8C). The latter failures cire referred to as torque seizures [20].

The ASTM test methods that relate to the Pin and Vee Block test machine and their typical test results (Fig. 8) Eire:

• ASTM D 2625, Endurance (Wear) Life and Load-Carrying Capacity of Solid Film Lubricants (Falex® Pin and Vee Method)

• ASTM D 2670, Measuring Wear Properties of Fluid Lubricants (Falex® Pin and Vee Block Method)

• ASTM D 3233, Measurement of Extreme Pressure Properties of Fluid Lubricants (Falex® Pin and Vee Block Methods)

• ASTM D 5620, Evaluating Thin Film Fluid Lubricants in a Drain and Dry Mode Using a Pin and V-Block Test Machine

P i n o n D i s k

Pin-on-Disk test is the simplest and most common wear test device [21]. It consists of a rotating disk upon which is loaded a pin or ball (Figs. 9 and 10) In the simplest versions, the ball rotates on the Scune wear scar. Other mechanisms can be incorporated such that the pin or ball comes in contact with an untouched portion of the rotating disk. This results in a spiral type of wear track. Pin-on-disk testing is used most widely

FIG. 9—Pin-on-Disk.

FIG. 10—Pin on disk test configuration.

for determining wear rates and endurance life of coatings and bonded film lubricants, but can be used with liquid lubricants for determining wear rates and coefficients of friction (G 99). As wear progresses, material can be removed from the pin or ball or from the disk or transferred from one piece to the other. Therefore, as with any wear test, both pieces should be examined for wear or material transfer. The pin or ball can be eveJuated by measuring the diameter of the wear scar and observing any material transfer. The wear scar on the disk can best be characterized by a profilometric trace across the surface to determine depth juid width of the wear scar or by another surface characterization method (Chapter 35). Pin on Disk tests will give relative wear rates only, as few actual field components match a pin-on-disk configuration.

The test methods that relate to the Pin on Disk test machine are:

• ASTM G 99, Wear Testing with a Pin-on-Disk Apparatus • ASTM G 132, Pin Abrasion Testing

F o u r B a l l

Four Ball tests include the Four Ball Wear Test, the Four Ball Extreme Pressure (EP), Rolling Four Ball, and Ball on Three Disks. Both the Four Ball Wear and the Four Ball EP test machine have three lower balls that are either held stationary (Fig. 11) or allowed to roll in a race. The upper ball is held in



a chuck and rotated. The tests in which the three balls are held stationary are the most common of the Four Ball tests. It provides for pure sliding wear in initial three-point contact. After initial motion begins, the point contact develops into a load bearing, area contact. At the end of the test run, the width of the wear scar on each ball is measured using a microscope designed for this purpose (Fig. 12). Measurements are taken, once with the striations of wear and again at 90° (Figs. 13 and 14). The six scar diameter measurements are averaged for the reported wear scar diameter. The main advantage of testing with balls is that they are very consistent in shape and properties and that if they Eire available, they are very cost effective. The main disadvantage is that if they are not available, they can be very expensive to manufacture and perhaps impossible to manufacture depending on the shape of the bulk material. For instance, if the materiEd is available only in sheet stock, a ball cannot be made. In this case an adapter called the Ball-Three-Disks can be used to effect sim-

FIG. 11—Four Ball.

FIG. 12—Typical scar measurement system (microscope).

FIG. 14—Ball scar measurement.

FIG. 13—Typical wear scars.

FIG. 15—Four Ball Wear test machine.

ilar geometrical contact. It allows geometry similar to the Standard Four-Ball configuration while permitting the testing of additional materials. This test has found application for testing for Einti-wear properties of diesel fuels containing lubricity additives [22,23,24].

Four Ball Wear

In the Four Ball Wear tests (D 2266, D 4172), the upper ball is allowed to rotate under load for an extended, predetermined period of time, tj^pically 60 min. The wesir scar average is reported Eind compared against a specification or other wear scars obtained by testing comparative fluids under the same conditions. The Four Ball Wecir test machine (Fig. 15) has a very precise loading system Eind because of this, has a limited range of test loads. Coefficient of friction is often a desired result of the test. ASTM D 5183, using the Four Ball



Wear test machine, was standardized because the friction trace in standard wear tests (D 2266, D 4172) can be erratic under normal operating conditions, making it difficult to determine a reportable value. This alternate test provides for a break-in run using a mineral oil to intentionally create a consistent load bearing area contact prior to the introduction of the test lubricant. The minereJ oil is drained from the ball cup; the balls aire cleaned without their removal from the ball cup; and the test is run in a series of 10 kg increment loads for 10 min each to determine the coefficient of friction at each load tested. Incipient seizure, which is the localized fusion of metal between the rubbing surfaces of the test pieces, can also be observed. Incipient seizure indicates that the film is breaking down and allowing metal-to-metal contact. One can report the coefficient of friction at any desired test load. This test will generally give a steady, coefficient of friction value that will be easy to determine.

The standardized test methods that use the Four-Ball "Wear test machine are as follows: • ASTM D 2266: Wear Preventive Characteristics of Lubri

cating Greases (Four Ball Method) • ASTM D 4172: Wear Preventive Characteristics of Lubri

cating Fluids (Four Ball Method) • ASTM D 5183: Determination of Coefficient of Friction of

Lubricants Using the Four-Ball Wear Test Machine

Four Ball EP

In the Four Ball EP test (D 2596, D 2783), the upper ball is allowed to rotate under load for 10 s, after which the resultant wear scars are measured and averaged. In this test, a series of test runs is performed at logarithmically increasing loads up to the weld point. The weld point is the load at which the lubricant film breaks down and the temperature at the point of contact is so high that it melts the metal, causing the test balls to weld together, indicating that the extreme pressure level of the lubricant has been exceeded. When the lubricant is performing as designed, the wear scars will be very smzdl, only slightly larger than the corresponding theoretical Hertzian scar diameters for the given materials, load, and radii of the test balls. The Hertzian scar diameter is the average diameter of an indentation caused by the deformation of the balls under static load (prior to test). The line that parallels the Hertzian line is referred to as the compensation line. When the lubricating film breaks down, metal-to-metal contact occurs and mild to severe incipient seizure occurs. This seizure is evidenced by the disproportionate increase in the average scar diameter. When incipient seizure is present, the test ball scar diameter is no longer on the compensation line. The highest test load that yields a scar diameter within 5% of the compensation line value for the corresponding load is the last non-seizure load. The ASTM D 2596 and D 2783 test methods provide an index of the relative wear performance with respect to load for the lubricant under evaluation., which is shown in a graph of the wear scar versus test load (Fig. 16). This term is called the Load Wear Index (LWI).

Because of the wide range of test loads (8-1000 Kg) 2ind the severe conditions that occur when a weld point is reached, the Four Ball EP test machine (Fig. 17) is designed to be very robust in construction. ASTM D 2266, D 4172, and D 5183 warn against using the Four Ball EP test machine for running

APPLIED LOAD, kgf ABE—compensation line. B—Point or last nonseizure load. EC—Region of incipient seizure. CD—Region or immediate neizure. D—Weld point.

FIG. 16—Schematic plot of scar diameter versus applied load.

FIG. 17—Four Ball EP test machine.

wear tests, as it lacks the necessary precision. When running wear tests, the Four Ball Wear Test Machine should be used.

The Four Ball EP test machine is also used for testing rolling elements (Fig. 18). This is because of the high test loads that are available. These tests are referred to as contact fatigue tests and are used to predict the life of a lubricant when used in ball or roller bearings. A vibration detection device is required to identify the onset of surface fatigue. Another test of interest is a lubricant shear test. Also known as the KRL test (CEC L-45-T-93), this test uses a tapered roller bearing under high test load to shear polymer containing lubricants. The viscosity is measured before and after shearing to determine viscosity loss. Results of this test have correlated well with the shear losses experienced with multi-viscosity gear oils used in manual transmissions.


CHAPTER 37: LUBRICANT FRICTION AND WEAR TESTING 1 0 2 9

The standardized test methods that use the Four Ball EP test machine are as follows:

• ASTM D 2596, Measurement of Extreme Pressure Properties of Lubricating Greases (Four Ball Method)

• ASTM D 2783, Measurement of Extreme Pressure Properties of Lubricating Fluids (Four Ball Method)

Block on Ring

The Block-on-Ring test machine (Fig. 19) is more of a research tool. It is primarily used to determine wear rates of materials and to rank materials in pure sliding motion. The tester is designed to accommodate different test fixtures to effect point, line, ellipsoid, and area contact. The standard block-on-ring test uses a rectangular block on a rotating ring and starts as Hertzian line contact. As motion begins, a load carrying bearing surface forms, allowing the formation of anti-wear and/or EP films to form on the surface. The wear scar width is measured and reported at the end of the test (Fig. 20). The preferred method of reporting is volume loss; however, if the same metals are being used, simply reporting the wear scar diameter for comparative wear is acceptable. A table in ASTM G 77 gives block scar volumes for measured wccir scar widths. Oscillating drive mechanisms can be installed to effect reciprocating (back and forth) motion. This motion is used in test methods for evaluating greases and bonded film lubricants. Testing with a ball on ring combination results in initial high Hertzian point contact. After motion begins, the Hertzian point contact area develops into a load carrying bearing surface. With increasing loads, moni-

FIG. 20—Block on ring test piece configuration. TImken size (left); Falex size (right).

RING a. BLDCK

BOTATIDN ROTATION

FIG. 21—High pressure (rectangular) block (left); Low pressure (conformal) block (right).

FIG. 18—Rolling four ball.

FIG. 19—Block-on-ring test chamber.

toring the friction force of lubricants containing EP additives has shown good correlation with predicting load limits obtained from more sophisticated component tests [11]. Area tests that simulate journal bearing applications are effected on this test machine using the conforming or curved test block (Fig. 21). This configuration is most effective when testing polymeric or plastic materials. When testing under area contact, it is advisable to perform an initial break-in to achieve complete contact between the mating surfaces. If complete contact is not achieved, limited contact will give higher than desired pressures on the contacting aireas, resulting in premature failure. Recently, a new adapter has shown promise in predicting anti-wear properties and coefficient of friction values of thin film lubricants such as automatic t ransmission fluids. The canted cylinder adapter holds a cylinder against the test ring with initial Hertzicin point contact. An additional benefit is the ease of alignment during setup. As wear develops, an elliptical wear scar develops. The resultant wear scar provides for easy measurement eind determination of wear and friction properties.

The standardized test methods that use the Block-on-Ring test machine sire as follows:

• ASTM D 2714, Calibration and Operat ion of the Falex Block-on-Ring Friction and Wear Testing Machine

• ASTM D 2781, Wear Life of Solid Film Lubricants in Oscillating Motion



• ASTM D 3704, Wear Preventive Properties of Lubrication Greases Using the (Falex) Block on Ring Test Machine in Oscillating Motion

• ASTM G 77, Ranking Resistance of Materials to Sliding Wear Using the Block-on-Ring Wear Test

• ASTM G 137, Ranking Resistance of Plastic Materials to Sliding Wear Using a Block-on-Ring Configuration

Timken The Timken Extreme Pressure Test Machine (Fig. 22) was developed in 1932 to measure load carrying capacity of EP lubricants for use in steel production. It too, is a block-on-ring type test (Fig. 20, Fig. 23) and is CcJled out in many specifications for oils and greases requiring extreme pressure

properties or high levels of load carrying ability. This tester is designed to evaluate lubricants for low, medium, and high levels of extreme pressure for lubricating greases (D 2509) and fluids (D 2782). The test is carried out by running a series of 10 min duration test runs at increasing test loads to the point where scoring or seizure is indicated on the wear scar. This scoring is evident as lines that extend past the edge of the wear scar or as a scar in which scuffing is evidenced by jagged irregularities (Fig. 24) The tester is also designed to provide friction data, but is seldom used for this function [25].

The standardized test methods that use the Timken Extreme Pressure test machine are as follows: • ASTM D 2509, Measurement of Load Carrying Capacity of

Lubricating Grease • ASTM D 2682, Measurement of Extreme Pressure Proper

ties of Lubricating Fluids

FIG. 22—Timken test machine.

Tapping Torque

The Tapping Torque test machine (Fig. 25) is designed to perform actual metalworking applications in a laboratory environment. Originally designed to perform thread cutting and thread forming, the Tapping Torque test machine can perform additional metalworking functions. There are two basic metalworking applications: metal removal and metal deformation. Metal removal techniques remove material to achieve the desired final shape, while metal deformation techniques reshape or form the existing material into the desired shape. Under metal removal, there are thread cutting (tapping), drilling, and reaming. Under metal deformation, there are thread forming (tapping), roll forming, drawing, and rolling mill simulation.

The tool rotates and descends at a rate determined by the rotational speed and pitch of the lead screw of the tapping head. The test machine measures the torque as the tool descends and enters the material to be machined (Fig. 26). The piece to be machined will have various forms depending on the test selected. It is most important to observe the tight tolerances required for the consumable test pieces. Even slight variances can have an affect on the precision of the test results.

The ASTM standardized test method that uses the Tapping Torque test machine is as follows: • ASTM D 5619, Comparing Metal Removal Fluids Using the

Tapping Torque Test Machine

FIG. 23—Timl<en ring and block configuration.

Multi-Specimen/Multi-Purpose (Thrust Washer Tester)

The Multi-Specimen test machine (Fig. 27) is designed to be a versatile tribology test apparatus. It consists of two opposed, vertical test shafts. One rotates; the other is stationary. It is called Multi-Specimen because of the use of different adapters that can be placed between the opposing vertical shafts. These adapters affect many different tribological configurations. The Multi-Specimen can measure friction and wear under point, line, and area contacts, in pure sliding, pure rolling or combination roll/slide motion in unidirec-tioneJ or oscillation. Because a lubricant is another parameter for the tribological system, virtually any of the adapters



T>pic4l OK No Storing Improper Sclup

Scoring (failure)

FIG. 24—Typical Timken test wear scars.

PILOT PLUG SEALS WITI

O'RINC

FIG. 25—Tapping Torque - exploded view of test area.

can accept a lubricant to determine its effect on friction and wear of the selected tribological system.

Some of the adapters that are most specific to the evaluation of lubricants are as follows: • Vane on Disk test for evaluating friction and wear of hy

draulic fluids under cycling stressed pressures (Fig. 28) • Thrust Washer test for area contact wear of plastic and ce

ramic materials (D 3702)

• Pin-on-Disk (G 99), (Figs. 9,10) • Oscillating Roll Slide for evaluating greases used in con

stant velocity joints of front wheel drive automobiles (Fig. 29) [26]

• Stick/Slip for determining static coefficients of friction of way lubricants

• Gear Cam Contact and Hypoid Gear with combination rolling and sliding motion for evaluating wear and friction of lubricants used in gear applications (Fig. 30) [27]

• Ball Bearing tests for evaluating a lubricant's effect on wear in a ball bearing assembly

• Sliding Bottle test for evaluating lubricants used on conveyors in beverage bottle filling machines

• Sheet Metal Forming for evaluating the friction obtained during the forming of a flat piece of sheet stock into a grooved surface

Linear Reciprocation

Linear reciprocation test machines are popular because they add the element of reciprocation or back and forth motion. This type of motion has proven effective in studying the friction, wear, and lubricating films occurring in applications such as a piston ring on a cylinder wall [28-30]. High-speed linear reciprocation has also shown good correlation in testing greases used in constant velocity joints of front wheel drive automobiles.

Linear reciprocation can be achieved in point, line, or area contacts. Load is applied vertically. The speed is controlled by the rate of reciprocation, stroke length and test machine design and is not constant, but rather a sinusoidal wave shape, due to the start/stop reversal of the reciprocation function. The rate of reciprocation is measured in Hertz, or cycles per second. Therefore, the user must specify the rate of re-



T I M E (RCJTATION) •

FIG. 2B—Tapping Torque trace of tap entering test piece (insets show position of tap in specimen blank).

FIG. 27—Multi-Specimen type tester.

FIG. 29—Oscillating roll/slide.

GEAR/CAM CONTACT

R0TAT1C3N

TORQUE

FIG. 28—Vane-on-disk. t LOAD

FIG. 30—Gear cam contact.



ciprocation (Hertz), stroke length, load, and temperature. The wear scar on the ball and disk can be evaluated as a measurement for wear. In EP tests, the load at which incipient seizure occurs is typically identified as the load carrying capacity of the lubricant with this test apparatus. The ball on flat reciprocating test is used for evaluating the lubricity of diesel fuels (D 6079) [31-33].

Depending on the design of the test machine, longer stroke lengths may compromise the rate of reciprocation. The user should verify desired requirements with the capabilities of the test equipment. The commercially available linear reciprocating test machines are the Flint and Partners TE77, High Frequency Friction Machine [34] (Figs. 31 and 32) and the SRV®5 [35] (Figs. 33 and 34). The TE77 test machine also provides for contact resistance measurements for evaluating lubricating surface films in elastohydrodynamic and boundary lubrication conditions.

The ASTM standardized test methods that use linear reciprocating test machines are as follows: • ASTM D 5706, Determining Extreme Pressure Properties

of Lubricating Greases Using a High-Frequency, Linear-Oscillation (SRV®) Test Machine

• ASTM D 5707, Measuring Friction and Wear Properties of Lubricating Grease Using a High-Frequency, Linear-Oscillation (SRV®) Test Machine

• ASTM D 6425, Measuring Friction and Wear Properties of Extreme Pressure (EP) Lubricating Oils Using SRV® Test Machine

• ASTM G 133, Linearly Reciprocating Ball-on-Flat Sliding Wear

COMMON TYPES OF WIDELY U S E D C O M P O N E N T T E S T DEVICES

FZG Gear

The FZG Gear Test Rig [36], (Fig. 35) is a standard laboratory test machine, designed to test wear and load carrying capacity of fluid lubricants. The tester uses a matched set of spur gear (Fig. 36), referred to as Type A test gears, as the consumable test pieces. These are used for evaluating fluids for wear (scuffing) [37] and load carrying properties. Alternatively, a gear set of slightly different geometry having greater surface area is available, referred to as Type C gears.

' Optimol Insturments, Munich, Germany. FIG. 33—SRV® test machine. Reprinted with permission of Optimol Instruments Priiftechnik GmbH.

FIG. 31—High speed linear reciprocating test rig (TE77). Reprinted with permission from Phoenix Tribology.

m<\ •tanOtrii tpacRwm

Rntrr h«Mr*i(5 gurtx w rvp n « anl fvt f «|Wf n nr i n .

I •

t-^^i^ -^Bjfijl^j

point twm am»

FIG. 34—Schematic of SRV® test pieces. Reprinted with permission of Optimol Instruments Pruftechnilc GmbH.

FIG. 32—Schematic of the test area geometries for TE77. Reprinted with permission from Phoenix Tribology.



Type C gears are used for evaluating pitting and micro-pitting tendencies of industrial gear oils [38]. As with most component tests, this gear tester uses actual parts, in this case, gear sets.

A constant load test is used for evaluating anti-wear properties of tractor hydraulic oils (D 4998). The test gears are weighed before and after the test. A load stage test of increasing test loads is used for evaluating industrial gear oils for their ability to carry a load. After each test load, the gear teeth are evaluated for signs of scoring (D 5182) (Figs. 37 and

38). Lubrication can be either dip lubrication in which the test gears are submerged in a known quantity of test oil, or in jet spray lubrication. Jet spray lubrication uses a nozzle to deliver the test lubricant directly onto the test gears.

The ASTM standardized test methods using the FZG Gear Test Rig are as follows: • ASTM D 4998, Evaluating Wear Characteristics of Tractor

Hydraulic Fluids • ASTM D 5182, Evaluating the Scuffing Load Capacity of

Oils (FZG Visual Method)

® PlBion

® GMrWiietl

® Oriva QitMm

® UMd Clutch

® LodUng Pin

® Uver Ann with WelgM Kk

® TOKpi* Mtaswing Ciuteh

® TtmptnMm Stnser

FIG. 36—FZG test gears on test rig. Reprinted with permission from Strama-IV1PS IVIaschinenbau GmblH & Co. KG.

FIG. 35—Diagram of FZG test rig.

Polishing Scoring

Scoring and Scuffing

FIG. 37—Examples of FZG gear distress.

Scuffing


CHAPTER 37: LUBRICANT FRICTION AND WEAR TESTING 1 0 3 5

Distress; • Scratches & Scoring

Pass

3 |

Rating :

•1 Scorinq (Smm)

See 10.4.2

5 m

Distress:

7 M

Oistmss:

Rating :

Scoring (20mm)

Fail

Scoring & Scuffing (Smm)

See 10.4.2

Scoring (15mm)

Rating See 10.4.2

eBj

R8tlns

8 | M

[)istress:

Rating :

Scratches & Scuffing (2mm)

See 10.4.2

Scuffing (20mm)

Fail

FIG. 38—Examples of FZG gear distress.

V i c k e r s P u m p S t a n d

The Vickers Pump Stand is a controversial yet widely used component bench test. Although under current scrutiny for improvement of its precision, this test s tand evaluates hydraulic fluids for wear using an actual pump. The pump parts are inspected metrologically and corrected, cleaned, weighed, and assembled prior to beginning each test. The test load is the fluid pressure, which can be either 1000 psi or 2000 psi depending on the test method. In some tests the loaded pressure exceeds that pressure recommended for normal operation. This pressure is chosen to challenge the test system in order to screen lubricants. At the end of the test, the p u m p cartridge pieces are again inspected for damage, cleaned, and weighed.

This test is undergoing considerable modifications. It is recommended to review the latest draft prior to beginning ciny test program using this tester.

The ASTM standardized test methods using the Vickers Pump Stand are as follows:

• ASTM D 2271, Preliminary Examination of Hydraulic Fluids (Wear Test)

• ASTM D 2882, Indicating the Wear Characteristics of Petroleum and Non-Petroleum Hydraulic Fluids in a Constant Volume Vane Pump

C O N C L U S I O N

In this chapter, a systematic method to evaluate how to select appropriate bench or laboratory tests has been described using the TAN approach. Test equipment, methods, and their advantages have been detailed. Using the tools available in the market today, researchers can develop meaningful tests, acquire test data easily and rapidly, and then draw appropriate conclusions for improving the life and performance of industrial equipment.

In summary, extensive progress has been made to formalize the procedures, practices, and test methods required to obtain predictive information on the performance of materials and lubricants. Additional information on the use of test equipment and the significance of test results can be obtained from the meuiufacturer and can be found in the Significance of Test section in the respective ASTM test methods.

A S T M S T A N D A R D S

F r i c t i o n a n d W e a r P r o p e r t i e s

No. D1367

D2266

D2271

D2625

D2670

D2714

D2882

D2981

D3336

D3704

Title Standard Test Method for Lubricating Qualities of Graphites (general laboratory test) Standard Test Method for Wear Preventive Characteristics of Lubricating Grease (Four Ball Method) (general laboratory test for wear of greases in sliding contact) Standard Test Method for Preliminziry Examination of Hydraulic Fluids (Wear Test) (general laboratory test for wear of hydraulic fluids under low pressures in pump test) Standard Test Method for Endurance (Wear) Life and Load Carrying Capacity of Solid Film Lubricants (Falex Pin and Vee Method) (genercil laboratory tests for load carrying and wear properties of solid lubricants) Standard Test Method for Measuring Wear Properties of Fluid Lubricants (Fcdex Pin and Vee Block Method) (general laboratory test for sliding wear) Standard Test Method for Calibration and Operation of the Falex Block- on-Biing Friction and Wear Testing Machine (generzJ laboratory test in sliding motion)

Standard Test Method for Indicating the Wear ChEiracteristics of Petroleum and Non-Petroleum Hydraulic Fluids in a Constant Volume Vane Pump (general laboratory test, known as the Vickers pump stand test) Standard Test Method for Wear Life of Solid Film Lubricants in Oscillating Motion (general laboratory test) Standard Test Method for Life of Lubricating Grease in Ball Bearings at Elevated Temperatures (general laboratory test) Standard Test Method for Wear Preventive Properties of Lubricating Greases Using the (Falex) Block-on-Ring Test Machine in Oscillating Motion (general laboratory test)



D 4172 Standard Test Method for Wear Preventive Characteristics of Lubricating Fluid (Four Ball Method) (general laboratory test)

D 4173 Standard Practice for Sheet Metal Forming Lubricant Evaluation (general methodology for testing)

D 4998 Standard Test Method for Evaluating Wear Characteristics of Tractor Hydraulic Fluids (general laboratory test)

D 5001 Standard Test Method for Measurement of Lubricity of Aviation Turbine Fuels by the Ball-on-Cylinder Lubricity Evaluator (BOCLE) (general laboratory test)

D 5183 Standard Test Method for Determination of the Coefficient of Friction of Lubricants Using the Four-Ball Wear Test Machine (general laboratory test)

D 5619 Standard Test Method for Comparing Metal Removal Fluids Using the Tapping Torque Test Machine (general laboratory test)

D 5620 Standard Test Method for Evaluating Thin Film Fluid Lubricants in a Drain and Dry Mode Using a Pin and V-Block Test Machine (general laboratory test)

D 5707 Standard Test Method for Measuring Fiction and Wear Properties of Lubricating Grease Using a High-Frequency, Linear-Oscillation (SRV) Test Machine (general laboratory test)

D 6078 Standard Test Method for Evaluating Lubricity of Diesel Fuels by the Scuffing Load Ball-on-Cylinder Lubricity Evaluator (SLBOCLE) (general laboratory test)

D 6079 Standard Test Method for Evaluating Lubricity of Diesel Fuels by the High-Frequency Reciprocating Rig (HFRR) (general laboratory test)

D 6425 Standard Test Method for Measuring Friction and Wear Properties of EP Lubricating Oils Using the SRV® Test Machine (general laboratory test)

G 77 Standard Test Method for Ranking Resistance of Materials to Sliding Wear Using Block-on-Ring Wear Test (general laboratory test)

G 83 Standard Test Method for Wear Testing with a Crossed Cylinder Apparatus (general laboratory test)

G 99 Standard Test Method for Wear Testing With a Pin-on-Disk Apparatus (general laboratory test)

G 115 Standard Guide for Measuring and Reporting Friction Coefficients (guide for methodology)

G 118 Standard Guide for Recommended Data Format for Sliding Wear Test (guide for methodology)

G 133 Standard Test Method for Linearly Reciprocating Ball-on-Flat Sliding Wear (general laboratory test)

Extreme Pressure Properties

D 1947 Standard Test Method for Load-Carrying Capacity of Petroleum Oil and Synthetic Fluid Gear Lubricants (general laboratory test for testing load carrying capacity of oils in sliding and rolling contact)

D 2509 Standard Test Method for Measurement of Load Carrying Capacity of Lubricating Grease (Timken Method) (general laboratory test for extreme pres

sure using block-on-ring type tester for seizure, galling and scuffing)

D 2596 Standard Test Method for Measurement of Extreme-Pressure Properties of Lubricating Grease (Four Ball Method) (general laboratory test for extreme pressure properties in sliding motion)

D 2782 Standard Test Method for Measurement of Extreme Pressure Properties of Lubricating Fluids (Timken Method) (general laboratory test)

D 2783 Standard Test Method for Measurement of Extreme Pressure Properties of Lubricating Fluids (Four-Ball Method) (general laboratory test)

D 3233 Standard Test Method for Measurement of Extreme Pressure Properties of Fluid Lubricants (Falex Pin and Vee Block Method) (general laboratory test)

D 5182 Standard Test Method for Evaluating the Scuffing Load Capacity of Oils (FZG Visual Method) (general laboratory test)

D 5706 Standard Test Method for Determining Extreme Pressure Properties of Lubricating Greases Using a High-Frequency, Linear Oscillation (SRV) Test Machine (general laboratory test)

O T H E R S T A N D A R D S

Friction and Wear Properties

DIN-Deutsches Institut fur Normung

No. Title 50280 Running Test on Radial Plain Bearings; General

Plain Bearings; Testing of the Tribological Behavior of Plain Bearings with Hydrostatic and Mixed Lubrication in Bearing Testing (General Laboratory Test for Sliding Wear of Bearings)

50281 Friction in Bearings; Definitions; Tjrpes; Conditions; Physical Quantities (definitions)

50320 Wear; Terms; System Analysis of Wear Processes; Classification of Wear Phenomena (definitions of wear terms and classifications)

5032l.G Wear Quantities (definitions of various wear types)

50322 Wear; Wear Testing Categories (definitions of scales of testing of all tjrpes of wear)

50323 Tribology; Terms (4 parts) (terms and definitions of wear t5fpes)

50324 Tribology; Testing of Friction and Wear Model Test for Sliding of Solids (Ball on Disc System)

51350 Testing in the Shell Four-Ball Tester (lubricant characterization) • Determination of the Wearing Characteristics of

Liquids (Part 3); • Determination of the Wearing Characteristics

for Consistent Lubricants (Part 5); • Determination of Shear Stability of Lubricating

Oils Containing Polymers (Part 6) 51354 Mechanical Testing of Lubricants in the FZG Gear

Rig Test (gear lubricant classification in sliding/rolling contact)



• General Working Principles, (Part 1); • Method 1/8.3/90 for Lubricating Oils, (Part 2) • Shear Stability of Polymer Containing Oils

(Parts)

51389 Mechanical Testing of Hydraulic Fluids in the Vane-Cell Pump (lubricant characterization of hydraulic fluids)

• General Working Principles (Part 1) • Method A for Anhydrous Hydraulic Fluids

(Part 2) • Method B for Aqueous Not Easily Inflammable

Fluids (Part 3)

51509 Selection of Lubricants for Gears (recommended use of hydraulic fluids in sliding/rolling contact)

• Gear Lubricating Oils (Part 1) • Semi-Fluid Lubricants (Part 2)

51834 Testing of Lubricants—Tribological Test in Trans-latory Oscillation Apparatus

• General Working Principles (Part 1) • Determination of Friction and Wear Data for

Lubricating Oils (Part 2) • Determination of Tribological Behavior of Ma

terials (Part 3)

ISO-International Standards Organization

7148/1 Testing of the Friction and Wear Behavior of Bearing Material / Mating Material / Oil Combinations under Conditions of Boundary Lubrication (bearings, friction and wear tests)

TR6281 Testing Under Conditions of Hydrodynamic and Mixed Lubrication in Test Rigs-Guidelines (guidelines for bearings in sliding wear)

JIS - Japanese Agency of International Science and Technology Standards Department

K2519 Testing Methods for Load-Carrying Capacity of Lubricating Oil (note: Japanese Timken Test) (lubricant testing in sliding or rolling contact)

IP - Institute of Petroleum-UK

281 Determination of the Anti-Wear Properties of Hydraulic Fluids—Vane Pump Method (lubricant characterization of hydraulic fluids in a vane pump)

334 Determination of Load Carrying Capacity of Lu-bricants-FZG Gear Machine Method (lubricant characterization of gear oils in sliding and rolling contact)

166 Load Carrying Capacity for Oils-IE A Gear Machine (testing of geeir oils in gear test rig)

239 Extreme Pressure Properties: Friction and Wear Tests for Lubricants: Four Ball Machine (seizure and welding characteristics using Four Ball EP)

240 Standard Method for Measurement of Extreme Pressure Properties of Lubricating Fluids (Timken Method) (general laboratory test for extreme pressure properties of oils)

326 Standard Method for Measurement of Extreme Pressure Properties of Lubricating Grease (Timken Method) (general laboratory test for extreme pressure properties of greases)

ASTM RELATED STANDARDS

D4175

G40

Standard Terminology Relating to Petroleum, Petroleum Products, and Lubricants (terminology compilation) Standard Terminology Relating to Wear and Erosion (terminology compilation)

REFERENCES

[1] Dowson, D., History ofTribology, Professional Engineering Publishing Limited, London, UK, 1998.

[2] Benzing, R., Goldblatt, I., Hopkins, V., Jamison, W., Mecklenburg, K., and Peterson, M., Friction and Wear Devices, STLE, Park Ridge, IL, 1976.

[3] Voitik, R. M., "Realizing Bench Test Solutions to Field Tribology Problems," Tribology: Wear Test Selection for Design and Application, ASTM STP 1199, A. W. Ruff, and Raymond G. Bayer, Eds., ASTM International, West Conshohocken, PA, 1993.

[4] Sture, H. and Staffan, J., "Hints and Guidelines for Tribotesting and Evaluation," Lubrication Engineering, Vol. 48, No. 5, 1991, pp. 401-409.

[5] Neale, M. J. and Gee, M., Guide to Wear Problems and Testing for Industry, y^ ed., Williams Andrew Publishing, Norwich, NY, 2001.

[6] Calabrese, S. J. and Muray, S. F., "Methods of Evaluating Materials for Icebreaker Hull Coatings," Selection and Use of Wear Tests for Coatings, ASTM STP 769, R. G. Bayer, Ed., ASTM International, West Conshohocken, PA, 1982, pp. 157-173.

[7] Banniak E. A. and Fein R. S., "Precision of Four Ball and Timken Tests and Their Relation to Service Performance," NLGI Spokesman, January 1973.

[8] Faville, F. and Faville, W., "Falex Procedures for Evaluating Lubricants," Journal of the American Society of Lubrication Engineers, STLE, Park Ridge, IL, August 1968.

[9] Bayer R. G., Shalkey A. T., and Wayson A. R., "Designing for Zero," Machine Design, IBM Corporation, Endicott, NY, 1969.

[10] Wei D.-P., "Future Directions of Fundamental Research in Additive Tribochemistry," Lubrication Sciences, Vol. 7, April 1995.

[11] Mizuhara K. and Tsuya Y., "Investigation of a Method for Evaluating Fire-Resistant Hydraulic Fluids by Means of an Oil-Testing Machine," JSLE International Tribology Conference, Tokyo, Japan, 8-10 July 1985.

[12] Feldman, D. G. and Kessler, M., "Development of a New Appli-cation-RelatedTest Procedure for Mechanical Testing of Hydraulic Fluids," Hydraulic Failure Analysis: Fluids, Components and System Effects, STP 1339, G. E. Totten, D. K. Wills, and D. G. Feldman, Eds., ASTM International, West Conshohocken, PA, 2001, pp. 75-89.

[13] De Gee, A. W. J., "Characterization of Five High-Performance Lubricants in Terms of IRG Transition Diagram Data," Proceedings of the IMechE, International Conference on Tribology-Fric-tion. Lubrication, and Wear, Fifty Years On, London, Mechsinical Engineering Publications Lmtd., Bury St. Edmunds, 1987, Vol. 1, pp. 427-436.

[14] Vaim, J. A. and Jising, T.-B, "Measurement of the Friction and Lubricity Properties of Contact Lenses," Proceedings of the AN-TEC1995, Boston, MA, May 7-11, 1995.

[15] Schipper, D. J. and Faraon, I. C, "Stribeck Curves for Starved Line Contacts," University of Twente, The Netherlands, Report number TROl-2227, 2001.

[16] Sanvordenker, K. S., "Lubrication by Oil-RefrigerEint Mixtures: Behavior in the Falex Tester," ASHRAE Transactions, KC-84-14, No. 3, pp. 799-805.

[17] Anderson, M., "The Use of Tribological Aspect Numbers in Bench Test Selection-A Review Update," Bench Testing of In-



dustrial Fluid Lubrication and Wear Properties Used in Machinery Applications, ASTM STP 1404, G. E. Totten, L. D. Wedeven, J. R. Dickey, and M. Anderson, Eds., ASTM International, West Conshohocken, PA, 2001.

[18] Condensed Catalog ofFalex Test Equipment and Custom Services, QG-18, Falex Corporation, Sugar Grove, IL, 1999.

[19] Helmetag, K., "A New Look at an Old Idea: The Torque Curve Revisited," Bench Testing of Industrial Fluid Lubrication and Wear Properties Used in Machinery Applications, ASTM STP 1404, G. E. Totten, L. D. Wedeven, J. R. Dickey, and M. Anderson, Eds., ASTM International, West Conshohocken, PA, 2001.

[20] "Extreme Pressure Lubricant Test Machines," Mobil Technical Bulletin, Mobil Oil, Roswell, GA, 1970.

[21] "A Compilation of International Standards for Friction and Wear Testing of Materials," VAMAS Technical Working Area 1, Peter J. Blau, Ed., Report No. 14, NIST, Gaithersburg, MD, 1993.

[22] Voitik, R., and Ren, N., "Diesel Fuel Lubricity by Standard Four Ball Apparatus Utilizing Ball on Three Disks, BOTD," SAE Paper #950247, Society of Automotive Engineers, Warrendale, PA, 1995.

[23] Voitik, R., "Diesel Fuel Lubricity BOTD Status-1995," Presented at the Fuels & Lubricants Meeting & Exposition, Toronto, Ontario, 16-19 Oct. 1995, SAE Paper #952371, Society of Automotive Engineers, Warrendale, PA, 1995.

[24] Nikeinjam, M., " Diesel Fuel Lubricity: On the Path to Specifications," presented at the International Spring Fuels & Lubricants Meeting & Exposition, Dearborn, MI, 3-6 May 1999, SAE Paper #1999-01-1479, Society of Automotive Engineers, Warrendale, PA, 1999.

[25] The Timken Company Model 1750 Lubricant and Wear Tester, The Timken Company, Canton, OH, 1977.

[26] Anderson, M., "Oscillating Roll/Slide Test Machine for Screening Properties of Greases Used in Constant Velocity Joints," Journal of the National Lubricating Grease Institute: NLGI Spokesman, Vol. 54, No. 3, June 1990.

[27] Voitik, R. M., and Heerdt, L. R., "Wear and Friction Evaluation of Gear Lubricants by Bench Test," Journal of the American Society of Lubrication Engineers: Lubrication Engineering, Vol. 40, No. 12, pp. 719-724.

[28] Bell, J. C. and Delargy, K. M., "Lubrication Influences on the Wear of Piston-Ring Coatings," Proceedings of the 16''' Lees-Lyon Symposium, Mechanics of Coatings, 1989, pp. 371-377.

[29] Patterson, D. J., Hill, S. H., and Tung, S. C, "Bench Wear Testing of Engine Power Cylinder Components," Lubrication Engineering, Vol. 49, No. 2, 1993, pp. 89-95

[30] Hartfield-Wunsch, S. E., Tung, S. C, and Rivard, C. J., "Development of a Bench Wear Test for the Evaluation of Engine Cylinder Components and the Correlation with Engine Test Results," Tribological Insights and Performance Characteristics of Modem Engine Lubricants, SAE SP-996, SAE Paper #932693, 1993.

[31] Lacey, P. I. and Lestz, S. J., "Fuel Lubricity Requirements for Diesel Injection Systems," SWRI Report No. BFLRF 270, Southwest Research Institute, San Antonio, TX, 1991.

[32] Spikes, H. A., Meyer, K., Bovington, C, Caprotti, R., and Krieger, K., "Development of a Laboratory Test to Predict Lubricity Properties of Diesel Fuels and its Application to the Development of Highly Refined Diesel Fuels," presented at the 9*̂ International Colloquium, Ecological and Economical Aspects of Tribology, Paper 3.22, Technische Akademie Esslingen, Germany, 1994, pp. 1-16.

[33] Hadley, J. W., Owen, G. C, and Mills, B., "Evaluation of a High Frequency Reciprocating Wear Test for Measuring Diesel Fuel Lubricity," SAE Paper #932692, Society of Automotive Engineers, Warrendale, PA, 1993.

[34] Plint and Partners Catalog of Tribology Test Equipment, Flint and Psutners, Newbury, Bershire, England, 2001.

[35] SRV® Test System for Evaluating the Tribological Properties of Lubricants and Materials, Optimol Instruments Priiftechnik GmbH, Miinchen, Germany, 1996.

[36] D-94303: FZG Zahnrad-Verspannungs-Prufstand FZG Gear Test Rig, Strama GmbH & Co. KG, Straubing, Germany, 1995.

[37] "Method to Assess the Scuffing Load Capacity of Lubricants with High EP Performance Using an FZG Gear Test Rig," FVA Information Sheet, Research Project No. 243, Forschungsvere-inigung Antriebstechnick E.V., 60528 Frankfurt/Main, 1995.

[38] "Test Procedure for the Investigation of the Micro-Pitting Capacity of Gear Lubricants," FVA Information Sheet, Research Project No. 54/1 - IV, Forschungsvereinigung Antriebstechnick E.V., 60528 Frankfurt/Main, 1993.

A P P E N D I X

T e r m i n o l o g y R e l a t e d t o F u e l s a n d L u b r i c a n t s T e s t i n g

abrasive wear, n—wear due to hard peirticles or hard protuberances forced against and moving cdong a solid surface. additive, n—a material added to another, usueJly in small amounts, to impart or enhance desirable properties or to suppress undesirable properties. adhesive wear, n—wear due to localized bonding between contacting solid surfaces leading to material transfer between the two surfaces or loss from either surface. apparent area of contact, n—in tribology, the area of contact between two solid surfaces defined by the boundaries of their macroscopic interface. (Contrast with real a r ea of contact.) asperity, n—in tribology, a protuberance in the small-scale topographical irregularities of a solid surface. break-in, n—in tribology, an initicJ transition process occurring in newly established weEuing contacts, often accompanied by transients in coefficient of friction or wear rate, or both, which are uncharacteristic of the given tribological system's long-term behavior.

catastrophic vwear, n—rapidly occurring or accelerating surface damage, deterioration, or change of shape caused by wear to such a degree that the service life of a part is appreciably shortened or its function is destroyed. coefficient of friction, fi or f, n—in tribology, the dimen-sionless ratio of the friction force (F) between two bodies to the normal force {N) pressing these two bodies together.

/x or f = (F/N)

Discussion—a distinction is often made between static coefficient of friction and kinetic coefficient of friction. corrosive wear , n—^wear in which chemical or electrochemical reaction with the environment is significEuit. debris, n—in tribology, pjirticles that have become detached in a wear or erosion process. dry solid film lubricants, n—dry coatings consisting of lubricating powders in a solid matrix bonded to one or both surfaces to be lubricated. extreme pressure (EP) additive, n—in a lubricant, a sub-stcince that minimizes damage to metcil surfaces in contact under high stress rubbing conditions (D 4175). fatigue wear, n—^wear of a solid surface caused by fracture arising from material fatigue. fretting wear, n—a form of attiitive wear caused by vibratory or oscillatory motion of limited amplitude characterized



by the removal of finely-divided particles from the rubbing surfaces. Discussion—^Air can cause immediate local oxidation of the wear particles produced by fretting wear. In addition, environmental moisture or humidity can hydrate the oxidation product. In the case of ferrous metals, the oxidized wear debris is abrasive iron oxide (FeaOs) having the appearance of rust, which gives rise to the nearly synonymous terms fretting corrosion and friction oxidation. A related, but somewhat different, phenomenon often accompanies fretting wccir. Fedse brinelling is localized fretting weeir that occurs when the rolling elements of a bearing vibrate or oscillate with small amplitude while pressed against the bearing race. The mechanism proceeds in stages: (1) asperities weld, are torn apart, and form wear debris that is subsequently oxidized; (2) due to the small-amplitude motion, the oxidized detritus cannot readily escape, and being abrasive, the oxidized wear debris accelerates the wear. As a result, wear depressions are formed in the bearing race. These depressions appear similar to the Brinell depressions obtained with static overloading. Although false brinelling can occur in this test, it is not chcu-acterized as such, and instead, it is included in the determination of fretting wear.

fretting corrosion, n—a form of fretting wear in which corrosion plays a significEuit role. fretting wear, n—wear arising as a result of fretting (see fretting). friction, n—the resistance to sliding exhibited by two surfaces in contact with each other. Basically, there are two fric-tional properties exhibited by any surface: static friction EUid kinetic friction. friction force, n—^the resisting force tangential to the interface between two bodies when, under the action of an external force, one body moves or tends to move relative to the other. (See also coefficient of friction.) Hertzian contact area, n—the apparent area of contact between two nonconforming solid bodies pressed against each other, as calculated from Hertz's equations of elastic deformation. Hertzian contact pressure, n—the magni tude of the pressure at any specified location in a Hertzian contact Eirea, as calculated from Hertz's equations of elastic deformation. kinetic coefficient of friction, n—the coefficient of friction under conditions of macroscopic relative motion between two bodies. kinetic friction, n—the force that resists motion when a surface is moving with a uniform velocity; it is, therefore, equal and opposite to the force required to maintain sliding of the surface with uniform velocity. lubricant, n—any substance interposed between two surfaces for the purpose of reducing the friction or wear between them. lubricating grease, n—a semi-fluid to solid product of a dispersion of a thickener in a liquid lubricant.

Discussion—the qualifying term, lubricating, should always be used. The term, grease, used without the qualifier refers to a different product, namely certain natural or processed animal fats, such as tallow, lard, cind so forth (D 128).

Discussion—the dispersion of the thickener forms a two-phase system and immobilizes the liquid lubricant by surface tension eind other physical forces. Other ingredients cire commonly included to impart specied properties (D 217).

lubricating oil, n—a liquid lubricant, usually comprising several ingredients, including a major portion of base oil and minor portions of various additives (D 5966). lubricity, n—a qualitative term describing the ability of a lubricant to minimize friction between and damage to surfaces in relative motion under load (D 4857, D 4863). precision, n—the degree of agreement between two or more results on the same property of identical test material. In this practice, precision statements are framed in terms of the repeatability and reproducibility of the test method (D 3244). pitting, n—in tribology, a form of wear characterized by the presence of surface cavities, the formation of which is attributed to processes such as fatigue, local adhesion, or cavitation. repeatability, n—the quantitative expression of the random error associated with a single operator in a given laboratory obtaining repetitive results with the same apparatus under constant operating conditions on identiced test material. It is defined as the difference between two such results at the 95% confidence level.

Discussion—interpret as the value equal to or below which the absolute difference between two single test results obtained in the above conditions may expect to lie with a prob-abihty of 95%.

Discussion—the difference is related to repeatability standard deviation but is not the standard deviation or its estimate. reproducibility R, n—quEintitative expression of the random error associated with operators working in different laboratories, each obtaining single results on identical test material when applying the same method. result, n—the value obtained by following the complete set of instructions of a test method. rolling contact fatigue, n—a damage process in a triboele-ment subjected to repeated rolling contact loads, involving the initiation and propagation of fatigue cracks in or under the contact surface, eventually culminating in surface pits or spalls. rolling wear, n—wear due to the relative motion between two nonconforming solid bodies whose surface velocities in the nominal contact location are identical in magnitude, direction, and sense. Discussion—Rolling wear is not a synonym for rolling contact fatigue, although the latter can be considered one form of rolling wear. run-in, n—in tribology, an initial transition process occurring in newly established wearing contacts, often accompanied by transients in coefficient of friction, or wear rate, or both, which ju-e uncharacteristic of the given tribological system's long term behavior. {Synonym: break-in, wear-in.) run-in, v—in tribology, to apply a specified set of initial operating conditions to a tribological system to improve its long term frictional or wecir behavior, or both. {Synonym: break in, V. and wear in, v. See also run-in, n.) scoring, n—in tribology, a severe form of wear characterized by the formation of extensive grooves and scratches in the direction of sliding. scratching, n—^the formation of fine lines in the direction of sliding that may be due to asperities on the harder slider or to hard particles between the surfaces or embedded in one of them. Discussion—Scratching is considered less damaging than scoring or scuffing.



scuff, scuffing, n—in lubrication, damage caused by instantaneous localized welding between surfaces in relative motion, which does not result in immobilization of the parts. scuffing—n, a form of wear occurring in inadequately lubricated tribosystems that is characterized by macroscopiccJly-observable changes in surface texture, with features related to the direction of relative motion.

Discussion—features characteristic of scuffing include scratches, plastic deformation, and transferred material. (Related terms: galling, scoring.) seizure, n—in lubrication, welding between surfaces in relative motion that results in immobilization of the parts. Localized fusion of metal between the rubbing surfaces of the test pieces (D 5707). Discussion—Seizure is usually indicated by an increase in coefficient of friction, wear, or unusual noise and vibration. In this test method, increase in coefficient of friction is displayed on the chart recorder as rise in the coefficient of friction from a steady state value. sliding wear, n—wear due to the relative motion in the tangential plane of contact between two solid bodies. speilling, n—in tribology, the separation of macroscopic particles from a surface in the form of flakes or chips, usually associated with rolling element bearings and gear teeth, but also resulting from impact events. standard test, n—a test on a calibrated test stand, using the prescribed equipment according to the requirements in the test method, and conducted according to the specified operating conditions. Discussion—the specified operating conditions in some test methods include requirements for determining a test's operational validity. These requirements are applied after a test is completed, and can include (1) mid-limit ranges for the average values of primary and secondary parameters that are narrower than the specified control ranges for the individual values, (2) allowable deviations for individual primary and secondary parameters from the specified control ranges, (3) downtime limitations, and (4) special parameter limitations. static coefficient of friction, n—the coefficient of friction corresponding to the maximum friction force that must be overcome to initiate macroscopic motion between two bodies. stick-slip, n—in tribology, a cyclic fluctuation in the magnitudes of friction force and relative velocity between two elements in sliding contact, usually associated with a relaxation oscillation dependent on elasticity in the tribosystem and on a decrease of the coefficient of friction with onset of sliding or with increase of sliding velocity.

Discussion—Classical or true stick-slip, in which each cycle consists of a stage of actual stick followed by a stage of overshoot "slip," requires that the kinetic coefficient of friction is lower than the static coefficient. A modified form of relaxation oscillation, with near-harmonic fluctuation in motion, can occur when the kinetic coefficient of friction decreases gradually with increasing velocity within a certain velocity range. A third t5^e of stick-slip can be due to spatial periodicity of the friction coefficient along the path of contact. Random variations in friction force measurement do not constitute stick-slip. test oil, n—any oil subjected to evaluation in an established procedure.

test sample, n—a portion of the product taken at the place where the product is exchanged, that is, where the responsibility for the product quality passes from the supplier to the receiver. Actually, this is rarely possible and a suitable sampling location should be mutually agreed on. triboelement, n—one of two or more solid bodies that comprise a sliding, rolling, or abrasive contact, or a body subjected to impingement or cavitation. (Each triboelement contains one or more tribosurfaces.)

Discussion—Contacting triboelements may be in direct contact or may be separated by an intervening lubricant, oxide, or other film that affects tribological interactions between them. tribology, n—the science and technology concerned with interacting surfaces in relative motion, including friction, lubrication, wear, and erosion. tribosurface, n—any surface (of a solid body) that is in moving contact with another surface or is subjected to impingement or cavitation. tribosystem, n—any system that contains one or more triboelements, including all mechanical, chemical, and environmental factors relevant to tribological behavior, (See also triboelement.) thin film Quid lubricant, n—fluid lubricants consisting of a primary liquid with or without additives or lubricating powders and without binders or adhesives, which form a film on one or both surfaces to be lubricated and perform their function after application and after excess material has drained from the application area, and without additional material being supplied by either a continuous or intermittent method. wear, n—damage to a solid surface, generally involving progressive loss of material, due to relative motion between that surface and a contacting substance or substances. wear, n—the removal of metal from the test pieces by a mechanical or chemical action, or by a combination of mechanical and chemical actions. wear rate, n—the rate of material removal or dimensional change due to wear per unit of exposure parameter; for example, quantity of material removed (mass, volume, thickness) in unit distance of sliding or unit time. wear coefficient, n—in tribology, a wear parameter that relates sliding wear measurements to tribosystem parameters. Most commonly, but not invariably, it is defined as the di-mensionless coefficient k in the equation

wear volume = k (load X sliding distance/hardness of the softer material)

(1) The equation given above is frequently referred to in the literature as "Archard's equation" or "Archard's law." (2) Sometimes the term wear coefficient has been used as a sjrnonym for wear factor. While this usage is discouraged, the term should always be fully defined in context to prevent confusion. wear rate, n—the rate of material removal or dimensional change due to wear per unit of exposure parameter, for example, quantity of material removed (mass, volume, thickness) in unit distance of sliding or unit time. Discussion—Because of the possibility of confusion, the manner of computing wear rate should always be carefully specified.



welding, n—in tribology, the bonding between metallic surfaces in direct contact, at any temperature.

Terminology Specific to Standards

load-canying capacity, n—as determined by D 2782, the maximum load or pressure that can be sustained by the lubricant (when used in the given system under specific conditions) without failure of the sliding contact surfaces as evidenced by scoring or seizure or asperity welding (D 2509, D 2782). OK value, n—as determined by D 2782, the maximum mass (weight) added to the load lever weight pan at which no scoring or seizure occurs (D 2509, D 2782). score value, n—as determined by D 2782, the minimum mass (weight) added to the load lever weight pan, at which scoring or seizure occurs (D 2509, D 2782).

Discussion—When the lubricant film is substantially maintained, a smooth scar is obtained on the test block, but when there is a breakdown of the lubricant film, scoring or surface failure of the test block takes place, see Fig. 24. In its simplest and most recognized form, scoring is characterized by the furrowed appearance of a wide scar on the test block and excessive pick-up of metal on the surface of the test cup. The form of surface failure more usually encountered, however, consists of a comparatively smooth scar, which shows local damage that usually extends beyond the width of the scar. Scratches or striations that occur in an otherwise smooth scar and that do not extend beyond the width of the scar are not considered as evidence of scoring. seizure or asperity welding, n—localized fusion of metal between the rubbing surfaces of the test pieces. Seizure is usually indicated by streaks appeciring on the surface of the test cup, an increase in friction and wear, or unusual noise and vibration. Throughout D 2782, the term seizure is understood to mean seizure or asperity welding (D 2509, D 2782). load-wear index, n—(or the load-carrying property of a lubricant)—an index of the ability of a lubricant to minimize wear at applied loads. Under the conditions of this test, specific loadings in kilograms-force (or Newtons) having intervals of approximately 0.1 logcirithmic units, are applied to the three stationary balls for ten runs prior to welding. The load-wear index is the average of the sum of the corrected loads determined for the ten applied loads immediately preceding the weld pair (D 2596, D2783).

weld point, n—under the conditions of this test, the lowest applied load in kilograms at which the rotating ball welds to the three stationary balls, indicating the extreme-pressure level of the lubricants-force (or Newtons) has been exceeded.

Discussion—Some lubricants do not allow true welding, and extreme scoring of the three stationary balls results. In such cases, the applied load that produces a maximum scar diameter of 4 mm is reported as the weld point (D 2596, D 2783).

corrected load, n—the load in kilograms-force (or Newtons) for each run obtained by multiplying the applied load by the ratio of the Hertz scar diameter to the measured scar diameter at that load (D 2596, D 2783). Hertz scar diameter, n—the average diameter, in millimeters, of an indentation caused by the deformation of the balls under static load (prior to test). It may be calculated from the equation

Dh = 8.73 X \Q-^ (P)''3

Where: Dh = Hertz diameter of the contact area, and

P = the static applied load. (D 2596, D 2783)

compensation scar diameter, n—the average diameter, in millimeters, of the wear scar on the stationary balls caused by the rotating ball under an applied load in the presence of a lubricant, but without causing either seizure or welding (D 2596, D 2783).

Discussion—The wear scar obtained shall be within 5% of the values noted in Table 1, Column 3 of ASTM D 2596/D 2783. Hertz line, n—a line of plot on logarithmic paper, as shown in Fig. 16, where the coordinates are scar diameter in millimeters and applied load in kilograms-force (or Newtons), obtained under static conditions. compensation line, n—a line of plot on logarithmic paper, as shown in Fig. 1, where the coordinates are scar diameter in millimeters and applied load in kilograms-force (or Newtons), obtained under dynamic conditions. (D 2596, D 2783)

Discussion—Coordinates for the compensation line are found in Table 1, Columns 1 and 3 of ASTM D 2596/D 2783.

Discussion—Some lubricants give coordinates which are above the compensation line. Known examples of such fluids are methyl phenyl silicone, chlorinated methyl phenyl silicone, silphenylene, phenyl ether, and some mixtures of petroleum oil and chlorinated paraffins. last nonseizure load, n—the last load at which the measured scar diameter is not more than 5% above the compensation line at the load (D 2596, D 2783). incipient seizure or initial seizure region, n—that region at which, with an applied load, there is a momentary breakdown of the lubricating film. This breakdown is noted by a sudden increase in the measured scar diameter and a momentary deflection of the indicating pen of the optional friction-measuring device (D2596, D2783). immediate seizure region, n—that region of the scar-load curve characterized by seizure or welding at the startup or by large wear scars. Initial deflection of indicating pen on the optional friction-measuring device is larger than with non-seizure loads (D 2596, D 2783).


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