rear axle lubrication

3Rear Axle Lubrication

Arup GangopadhyayFord Motor Company

Farrukh QureshiThe Lubrizol Corporation

3.1 Rear Axle Lubrication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-23.2 Viscosity Classifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-23.3 Gear Oil Classification. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-3

Service Designations in Current Use • Service DesignationsNot in Current Use

3.4 Performance Requirements for Gear Oils . . . . . . . . . . . . 3-43.5 Gear Oil Composition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-5

Base Oil • Viscosity Modifiers and Pour Point Depressants •Performance Package

3.6 Issues and Challenges for Rear Axle Fluids. . . . . . . . . . . 3-83.7 Fuel-Efficient Gear Lubricants . . . . . . . . . . . . . . . . . . . . . . . . 3-8

Axle Efficiency Tests • Vehicle Tests • Spin Loss Tests •Effect of Gear Surface Finish on Efficiency • Effect of ColdStart on Axle Efficiency • Limited Slip Differentials

3.8 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-19References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-20

This chapter will briefly discuss axle hardware, viscosity classifications, fluid requirements, and lubricantcomposition. The primary focus will be on development of fuel-efficient lubricants while maintainingaxle durability.

An automotive engine develops maximum power at a relatively high speed. The torque of the engine ismodified in various stages until it becomes a propulsive force at an appropriate speed at the interface oftires and the road. The rear axle is the final stage in the drive train of a vehicle, which is responsible forappropriately transforming the engine power into useful propulsive force. The power from the engineand transmission is transferred to the rear axle through the driveshaft. The driveshaft is connected tothe pinion gear inside the axle housing, which is partly filled with lubricant. Rotation of the driveshaftturns the pinion gear, which in turn rotates a contacting ring gear, as shown in Figure 3.1. The ring gear isconnected to two shafts on both sides which extend outside the axle housing and connect to the wheels. Thepinion gear is supported on two roller bearings and the two shafts connecting to the wheels are supportedon two axle roller bearings mounted on the housing. Therefore, energy losses in the rear axle are due to(1) losses in the four bearings, (2) shearing of rear axle lubricant in the axle housing, and (3) frictionalloss in pinion and ring gear contacts. The amount of preload on the pinion bearing also contributes tofrictional loss. An important component of the rear axle system is the gear oil which is required to play acritical role in the efficient and durable operation of the rear axle. Axle durability is related to the pinionbearings and also depends on the wear of ring and pinion gears. The bearing durability is in turn relatedto lubricant temperature. Excessive gear wear results in gear whine, which causes customer dissatisfaction.

3-1

© 2006 by Taylor & Francis Group, LLC

3-2 Handbook of Lubrication and Tribology

Piniongear

Ring gear

FIGURE 3.1 A view of ring and pinion gears inside the axle housing.

3.1 Rear Axle Lubrication

Usually spiral-bevel or hypoid gears are used in rear axles. Fluids used in rear axles are required to reducewear, pitting, spalling, scoring, other types of gear tooth distress to increase the life and reduce thedowntime of the equipment. Additional requirements may also include protection against oxidation, rust,copper corrosion, and foaming. Since vehicles have to operate in diverse climates, viscometrics at both highand low temperatures must also be tailored to provide adequate fluid film for protecting the surfaces. Mostof these requirements, developed over the years, can be satisfied by passing industry specified standard tests.These tests have been shown to define lubricants with adequate properties. Additional requirements suchas improving fuel economy require close cooperation between equipment manufacturers and lubricantformulators so that desired fluid properties are tailored to axle design. Several of the standard requirementsand specifications are discussed in the following sections, followed by a detailed discussion on improvingthe efficiency and durability of rear axle fluids.

3.2 Viscosity Classifications

Gear oil viscosity is the most important parameter that governs the fluid film thickness between operatingsurfaces and along with chemical additives, technology determines the degree of protection available forgears and bearings in the rear axle system. The viscosity of a fluid is its resistance to shear deformation andis usually measured by shearing the fluid under controlled temperature and shear rates. Another importantparameter is viscosity dependence on temperature, as represented by the viscosity index of fluid. A detaileddescription of standard test methods will not be included in this work, but references will be provided asappropriate. Fluid viscosity also depends on shear rate and ambient pressure. Automotive gear lubricantviscosities are defined in SAE J306. The SAE gear oil viscosity classifications are shown in Table 3.1. SAEJ306 has been updated in October 2005 and new viscosity grades have been added. These designations areused to specify the viscosity requirements for manual transmission lubricants. Multigrade gear oils thatcan maintain their film-forming characteristics over a large temperature range are being increasingly used.In order to reduce the temperature dependence of viscosity these multigrade gear oils contain significantamount of polymers. However, polymers are also susceptible to shear degradation under service condi-tions. If these polymers shear in service, it will lead to a drop in viscosity, resulting in reduced film thickness,and may eventually lead to equipment failure. It is required that the lubricant remain in its viscosity grade


Rear Axle Lubrication 3-3

TABLE 3.1 Automotive Gear Lubricant Viscosity Classification

Max temperature for Kinematic viscosity Kinematic viscositySAE viscosity viscosity of at 100◦C, cStc at 100◦C, cStc

grade 150,000 cP (◦C)a,b Minimumd Maximum

70W −55e 4.1 —75W −40 4.1 —80W −26 7.0 —85W −12 11.0 —80 — 7.0 <11.085 — 11.0 <13.590 — 13.5 <18.5110 — 18.5 <24.0140 — 24.0 <32.5190 — 32.5 <41.0250 — 41.0 —

Note: 1 cP = 1 mPa · sec; 1 cSt = 1 mm2/sec.

a Using ASTM D2983.b Additional low-temperature viscosity requirements may be appropriate for fluids inten-ded for use in light duty synchronized manual transmissions.c Using ASTM D445.d Limit must also be met after testing in CEC L-45-A-99, Method C (20 h).

The precision of ASTM Method D2983 has not been established for determinationsmade at temperatures below −40◦C. This fact should be taken into consideration in anyproducer–consumer relationship.

following a 20-h shear stability test. The shear stability requirement provides assurance that the lubricantwill maintain sufficient viscosity and oil film thickness to prevent premature failure of equipment.

A set of standard viscosity identification and labeling guidelines was also provided in SAE J360. Theguidelines provide a standard worldwide vocabulary to identify automotive gear lubricants. This shouldmake it easier for consumers to select proper lubricant viscosity grades.

3.3 Gear Oil Classification

Since a number of different gear designs and geometries are used in automotive applications, gear contactsare subjected to varying loads, speeds, and kinematics. Depending on gear design and operating environ-ment, they require different gear lubricants for proper protection. In order to facilitate proper selection ofgear oils for different applications, the American Petroleum Institute (API) has published gear oil designa-tions. These designations are intended to assist manufacturers and users of automotive equipment in theselection of transmission, transaxle, and axle lubricants based on gear design and operating conditions.Some of these designations have been rendered obsolete but will be included in the discussion for the sakeof completeness. A brief description of API service designations of gears oils follows.

3.3.1 Service Designations in Current Use

API GL-1 denotes lubricants intended for manual transmissions operating under such mild conditionsthat straight petroleum oils may be used satisfactorily. Oxidation and rust inhibitors, defoamers, andpour depressants may be added to improve the characteristics of these lubricants. Friction modifiersand extreme pressure agents should not be used. Lubricants meeting MT-1 standards are an upgradein performance over the lubricants meeting API GL-1 and are preferred by major commercial manualtransmission vehicle manufacturers.

API GL-4 denotes lubricants intended for axles with spiral bevel gears operating under moderate tosevere conditions of speed and load or axles with hypoid gears operating under moderate speeds and



loads. These oils may be used in selected manual transmission and transaxle applications where MT-1lubricants are unsuitable.

Although this service designation is still used commercially to describe lubricants, some test equipmentused for performance verification is no longer available.

API GL-5 denotes lubricants intended for gears, particularly hypoid gears, in axles operating undervarious combinations of high-speed shock loads and low-speed, high-torque conditions. Lubricantsqualified under MIL-L-2105D satisfy the requirements of the API GL-5 specification, although the APIdesignation does not require military approval.

API MT-1 denotes lubricants intended for nonsynchronized manual transmission used in buses andheavy-duty trucks. Lubricants meeting the requirements of API MT-1 service provide protection againstthe combination of thermal degradation, component wear, and oil seal deterioration. API MT-1 does notaddress the performance requirements of synchronized transmissions and transaxles in passenger carsand heavy-duty applications.

3.3.2 Service Designations Not in Current Use

API GL-2 denotes lubricants intended for automotive worm-gear axles operating under such conditionsof load, temperature, and sliding velocities that lubricants satisfactory for API GL-1 service will not suffice.Products suited for this type of service contain antiwear additives for film-strength improvers specificallydesigned to protect worm gears.

API GL-3 denotes lubricants intended for manual transmissions operating under moderate to severeconditions and spiral-bevel axles operating under mild to moderate conditions of speed and load. Theseservice conditions require a lubricant with load-carrying capacities exceeding those satisfying API GL-1service but below the requirements of lubricants satisfying API GL-4 service. Gear lubricants designatedfor API GL-3 service are not intended for axles with hypoid gears.

API GL-6 denotes lubricants intended for gears designed with very high pinion offset. Such designstypically require protection from gear scoring in excess of that provided by API GL-5 gear oils. A shiftto more modest pinion offsets and the obsolescence of original API GL-6 test equipment and procedureshave greatly reduced commercial use of API GL-6 gear lubricants.

Another relatively new performance classification is MIL-PRF-2105E. This combines the performancerequirements of its predecessor (MIL-L-2105D) and API MT-1. This specification had not been widelyadapted globally. The primary reason was that it had not been possible for oil blenders and marketersin non-NATO countries to obtain a formal approval under this specification. This has changed with therelease of SAE J2360.

In a follow-up of a government directive in 1991, The Society of Automotive Engineers (SAE) releasedthe J2360 classification in 1998. Effective January 2004, this classification has replaced MIL-PRF-2105E.Qualification to the SAE J2360 standard will continue to require a review of test data and parts by thePerformance Review Institute (PRI). This is a process unique to the gear lubricant industry where apanel of industry experts meets to review performance data and parts to qualify a standard. The PRIwill administer the approval process and maintain a qualified products list on its web site. Under thisclassification, an oil marketer or blender anywhere in the world can now obtain a formal approval andhave its name and the name of its approved lubricant published on the qualified products list. This enablesit to demonstrate a measurable and recognized quality of performance for its lubricants and should leadto a greater return on investment in the development of these lubricants.

3.4 Performance Requirements for Gear Oils

In order to meet performance requirements, gear oils are subjected to different tests and they must meetcertain criteria to pass the tests. A list of performance requirements for popular designations of gear oils



TABLE 3.2 Performance Test Requirements

MIL-PRF-2105E andTest Procedure API GL-5 MIL-L-2105D API MT-1 SAE J2360

ASTM L-33 X X — XASTM D 6121 (L-37) X X — XASTM L-42 X X — XASTM L-60 X X — —ASTM D5704 (L-60-1) — — X XASTM D 5662 — — X XASTM D 5579 — — X XASTM D 5182 — — X —ASTM D 130 — X X XASTM D 892 X X X XStorage stability and compatibility — X — XControlled field test — X — X

TABLE 3.3 Automotive Gear Oil Tests

Test Description Characteristics measured

ASTM L-33 Gear test using differentialassembly

Resistance to corrosion in presence of moisture

ASTM D 6121 Gear test using complete axleassembly

Resistance to gear distress under low-speed, high-torqueconditions

ASTM L-42 Gear test using complete axleassembly

Resistance to gear distress (scoring) under high-speed,shock-load conditions

ASTM L-60 Bench test using spur gears Oxidative stabilityASTM D 5704 Bench test using spur gears Thermal and oxidative stability and depositsASTM D 5662 Bench test Seal compatibilityASTM D 5579 Gear test Transmission cyclic durability (waived for approved

lubricants)ASTM D 5182 Gear test Spur gear wearASTM D 130 Bench test Stability in the presence of copper and copper alloysASTM D 892 Bench test Foaming tendencies

is given in Table 3.2. Brief descriptions of these tests as well as the characteristics that are measured duringeach test are listed in Table 3.3.

3.5 Gear Oil Composition

Automotive gear oil is composed of base oil, viscosity modifier, pour point depressant, and performancepackage. A brief description of individual gear oil components will be provided in the following sections.

3.5.1 Base Oil

Base oil is a major constituent of any gear lubricant. Base oil acts as a lubricant, heat transfer medium,debris carrier, and carrier for the performance package. API has classified base oils into different groupsbased on the amount of saturates and sulfur level. API base oil groups are listed in Table 3.4.

The base oils can be either processed from mineral oils (API Group I–III) or chemically synthesized.A careful cost/benefit analysis must be carried out before base oil selection. Solvent-refined mineral oils arecheapest and cost increases as more steps in refining are involved. Synthetic fluids such as polyalphaolefins(PAOs), polyolesters (POEs), or polyalkyleneglycols (PAGs) provide unique properties but are usuallymore expensive than mineral-based fluids.



TABLE 3.4 API Classification of Base Stocks

API Group Sulfur Saturates Viscosity index

I >0.03% <90% 80–120II <0.03% >90% 80–120III <0.03% >90% >120IV PolyalphaolefinsV Those base stocks not

included above

Another class of base oils processed from natural gas known as Fischer–Tropsch fluids or gas to liquid(GTL) fluids is also emerging. GTLs are expected to be commercially available by 2006–2007. It is anti-cipated that these fluids will provide performance equivalent to or better than API Group III fluids at acompetitive price.

3.5.2 Viscosity Modifiers and Pour Point Depressants

Another major component of multigrade gear oils is viscosity modifier. Viscosity modifiers are simplypolymeric molecules whose molecular conformation in oil solution is sensitive to temperature. At lowtemperatures, the polymer chains remain curled up and do not appreciably impact the fluid viscosity.At higher temperature, the polymeric molecular chains relax and open up, imparting an increase inviscosity to offset some loss of base oil viscosity with increase in temperature. Other performance char-acteristics of viscosity modifiers are their solubility and their ability to resist chain scission due to shearunder high shear rates that may approach 10−7 sec. In order to ensure that the viscosity modifier remainsintact under severe shear rates, SAE J306 specifies that the fluid should remain within 10% of its original100◦C viscosity after shear in a tapered roller bearing device (CEC- L-45-T-93). The choice of viscositymodifiers will impact the pressure viscosity behavior of formulated lubricants, which in turn impacts thefilm-forming characteristics of gear oil under elastohydrodynamic (EHD) lubrication conditions as well asits fuel economy characteristics.

Pour point depressants are usually needed for mineral oils that have a tendency to form wax crystalsat low temperature. Pour point depressants are high molecular weight polymers that tend to prevent thegrowth and aggregation of wax crystals.

3.5.3 Performance Package

Since a gear oil is required to provide performance that cannot be delivered by a simple mix of base oil andviscosity modifier, a performance package is usually necessary in the gear oil. In general, gear oil consistsof 5 to 15% of performance package, which may contain:

• Antiwear/extreme pressure additives• Oxidation inhibitors• Corrosion inhibitors• Foam inhibitors• Friction modifiers

3.5.3.1 Antiwear/Extreme Pressure Agents

Antiwear/extreme pressure agents are organic compounds that may contain sulfur, phosphorus, boron,and zinc. These compounds react with metal surfaces to form protective films under boundary lubricationconditions.

Antiwear additives, such as organophosphorus additives and their degradation products, are believedto form polar species that react with ferrous-based surfaces to form iron organo/inorganophosphorusfilms. These films are effective at low to moderate temperatures and loads.



API GL-580W-90

Pass duration — 24 h

API GL-1SAE 90

Failed at: 3 min

API GL-1SAE 250

Failed at: 10 min

FIGURE 3.2 Gear test results from the L-37 test.

Extreme pressure agents are commonly used in gear oils; these are considered to be more effective underheavy load and high temperature conditions. The extreme pressure agents become active at the highercontact temperature that results from metal-to-metal sliding at the contact under heavy load. Extremepressure additives react with gear surfaces to form protective films which prevent metal-to-metal contact.

The presence of antiwear/extreme pressure additives is essential in gear oils, particularly with modernsystems which operate under high contact pressure, higher temperature, smaller sump size, and longerdrain intervals. The effectiveness of antiwear/extreme pressure additives is shown in Figure 3.2, whichcompares the results from an L-37 test for GL-1 oils without antiwear/extreme pressure additives andGL-5 gear oil (SAE 80W-90) that contains antiwear/extreme pressure additives.

A proper balance of antiwear/extreme pressure additives is essential to increase the gear life undervarious operating conditions. Addition of extreme pressure additives alone does not guarantee surfaceprotection. The fatigue life of gear components is slightly affected in the absence of extreme pressureadditives; however, the absence of antiwear additives significantly reduces gear fatigue life.

The effectiveness of antiwear/extreme pressure agents is determined using the L-37, L-42, and FZGscuffing and wear tests listed in Tables 3.2 and 3.3.

3.5.3.2 Oxidation Inhibitors

Most lubricants are susceptible to oxidation since they are made of hydrocarbon base oils. Oxygen presentin the atmosphere reacts with the hydrocarbon molecules at higher temperature resulting in peroxy orother radicals. Increase in temperature and presence of metallic wear particles accelerates the oxidationreactions. The oxidation of lubricants occurs via a free radical reaction, leading to base oil polymerization.The polymerization of base oil increases viscosity and impairs the flow characteristics of a lubricant,which is essential for adequate film formation. Due to the abrasive nature of some oxidation byproducts,elastomeric seals may also abrade and fail.

Oxidation inhibitors can be classified as hydroperoxide decomposers and radical scavengers, dependingupon the mode of action. Sulfur-containing compounds act as decomposers. These compounds react withhydroperoxide radicals and become oxidized to higher oxidation states. Nitrogen- and oxygen-containingcompounds such as arylamines and phenols act as radical scavengers and render radicals innocuous,usually by an oxidation–reduction reaction.

3.5.3.3 Corrosion Inhibitors

As a result of oxidative degradation of gear oils, acids may be introduced that chemically attack surfaces.If not properly formulated some extreme pressure agents may also attack surfaces. In order to preventcorrosion, certain inhibitors such as basic sulfonates and fatty amines may be used. The effectiveness ofcorrosion inhibitors is measured by the L-33 and ASTM D-130 tests.



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1992 1994 1996 1998 2000 2002 2004

Model Year ('93–'03)

Hor

sepo

wer Cars

SUVs

Pickups

All light duty

Light-duty vehicle horsepower trends

FIGURE 3.3 Vehicle horsepower trend over a 10-yr period.

3.5.3.4 Foam Inhibitors

The churning action of gears in transmission and axles causes air entrainment or foaming, particularlyat lower temperature. In order to avoid foaming, certain inhibitors such as polydimethylsiloxanes(“silicones”) and polyacrylates are used.

3.5.3.5 Friction Modifiers

Friction modifiers are adsorbed on metal and other surfaces and result in reduced friction. Frictionmodifiers are particularly needed for modern axle designs equipped with limited slip capability and mayalso reduce the operating temperature due to reduced frictional heating at the contact.

3.6 Issues and Challenges for Rear Axle Fluids

Over the years, light trucks, minivans, and sports utility vehicles (SUVs) have gained in popularity. In theyears 2002–2003, vehicle production and sales in North America for the light truck segment exceededthose for the car segment for the first time in history [1]. The increase in the number of vehicles in thelight truck segment has technical implications for delivering performance economically and efficiently.

There have been significant improvements in drive train technology, which have resulted in improve-ment in performance, efficiency, and durability. These gains have been offset by consumer craving forhigher performance. Technology improvements are being used to deliver greater power and perform-ance, while the components that deliver performance have either remained virtually the same size orhave become smaller. Increasing use of independent rear suspension has changed vehicle aerodynamics.The reduction in sump size, longer drain intervals, and restricted airflow for convective heat transfer haveresulted in a challenging atmosphere of higher axle operating temperature. Figure 3.3 indicates that therehas been an increase in horsepower of about 34% over a 10 yr period. There has been similar increase inpower density (measured as a ratio of horsepower to vehicle weight) of about 20% during the same timeperiod.

3.7 Fuel-Efficient Gear Lubricants

Over the past few years, customer preference for light-duty trucks (including minivans and SUVs) hasincreased significantly. However, a major point of customer dissatisfaction for light duty truck owners hasbeen their low fuel economy. Additionally, government regulatory agencies have continued to push forfurther increases in corporate average fuel economy (CAFE) requirements for cars and light-duty trucks.The CAFE standards will rise in the year 2005 and beyond. Therefore, automakers are considering severalnew engine and transmission concepts to improve fuel economy. The automotive rear axle also contributesto frictional loss, but less than major powertrain systems. The automakers are closely scrutinizing all



systems and exploring where further frictional losses could be reduced. As a result, even inherently efficientsystems such as gearboxes and axles have not escaped close scrutiny. It is anticipated that frictional lossesin rear axles can be reduced through the use of lower viscosity rear axle lubricant, novel gear surfacefinishes, improved gear design, lower friction bearings and so on.

During the last few decades, a number of well-documented attempts have been made to developfuel-efficient gear lubricants primarily through use of lower viscosity and multigrade viscosity grades.Naman [2] examined the effect of 1% addition of MoS2 in a 75W viscosity grade lubricant and observedno fuel economy benefit in a fully warmed-up vehicle. O’Connor et al. [3] studied the effect of syntheticbase content in axle lubricant on efficiency. They showed that the type of synthetic base oil had a relativelyinsignificant impact on axle efficiency, but a bigger effect was observed when the lubricant composition waschanged from part synthetic to full synthetic. Law [4] showed that use of synthetic base stocks resulted inimproved efficiency in automotive hypoid gears. Watts and Willette [5] measured axle efficiency and truckfuel economy with a few different viscosity grade lubricants. SAE 75W-90 and SAE 75W-140 viscosity gradelubricants improved axle efficiency over a SAE 90 grade lubricant by about 1.3 and 0.55%, respectively.This translated into about 1.2 to 0.7% improvement in fuel economy at 65 mph speed. Willermet andDixon [6] evaluated the effect of lubricant viscosity on axle efficiency and based on these results predictedabout 1% improvement in fuel economy by adjusting low- and high-temperature viscosities.

The additive package also plays an important role in improving axle efficiency. Adams et al. [7] evalu-ated the effect of two additive packages on truck fuel economy, one containing a conventional S-P additivepackage and the other containing a potassium triborate and zinc dialkyldithiophosphate-based package.The triborate additive package showed about 1.1% fuel economy improvement over the conventionalaxle lubricant. Also, the lubricant temperature was 30◦C lower than that observed with the conventionallubricant. Greene and Risdon [8] looked into the effect of molybdenum-based friction modifiers on axleefficiency. Molybdenum-based friction modifiers improved axle efficiency and the degree of improve-ment depended on the amount of Mo present in the lubricant. Increasing the Mo content from 0.1 to0.3% increased axle efficiency from 1–1.8% to 2.6–2.9% under the low-speed/high-torque condition.Further improvement in axle efficiency was observed under a high-speed/low-torque condition. Recently,Bala et al. [9] evaluated the effect of lubricant viscosity, additive package, and lubricant viscosity index(VI) on axle efficiency under simulated city and highway driving conditions using an axle efficiency rig.They observed that lubricant viscosity and VI affected axle efficiency under different load speed condi-tions and sometimes in opposite directions, but generally lubricants with high VI appeared to show higheraxle efficiency. Vinci et al. [10] formulated axle lubricants which provided a balance between temperaturecontrol and efficiency through proper selection of base oils, thickening agents, and performanceadditives.

In order to improve axle efficiency, lubrication regimes must be considered, factors leading to energydissipation in each regime be identified, and such energy dissipation be minimized using appropriatemeasures. Gear contacts operate in multiple lubrication regimes, resulting in energy dissipation bothinside and outside the contact region. Energy dissipation outside the contact leads to load-independentlosses and use of lower-viscosity fluids is of help. However, at higher operating temperature or underrealistic operating conditions, lower-viscosity fluids may not provide fluid films that are thick enoughto protect surfaces. This may lead to reduced bearing and gear life and premature failure. Gear contactsare heavily loaded and operate in EHD lubrication, as well as in mixed/boundary lubrication regimes. InEHD lubrication regime energy dissipation as well as the film formation characteristics of fluids dependon their pressure-viscosity behavior, in addition to nominal fluid viscosity. It has been shown that thepressure-viscosity coefficient of a fluid is directly proportional to average friction in a rolling/sliding EHDcontact. The average EHD friction in a rolling/sliding contact can be directly measured and is designatedas the traction coefficient. The traction coefficient of gear oils depends directly on the type of base oil andviscosity modifier. The traction coefficients of different base oils with similar nominal viscosity at 100◦Care compared in Figure 3.4.

Clearly, synthetic fluids such as PAOs exhibit lower traction coefficients than mineral oils. Viscositymodifiers are used in modern multigrade gear oils to reduce their viscosity–temperature sensitivity. They



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0 10 20 30

Slide to roll ratio

Trac

tion

coef

ficie

nt

Group I Group II Group III PAO-4

Traction coefficient 100°C at 1.25 GPa, 2.5 m/sec

FIGURE 3.4 Traction coefficient of base oils at 100◦C.

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0 500 1000 1500 2000

Output torque, N m

Effi

cien

cy, %

EPA FE range Performance range

FIGURE 3.5 A general axle efficiency curve. (Adapted from Gangopadhyay et al. SAE 2002-01-2821, withpermission.)

also profoundly impact the pressure–viscosity behavior and traction coefficient of gear oils. It is desirableto reduce EHD friction to improve the efficiency and reduce the operating temperature of axle fluids.As the EHD film starts to become thin due to operating conditions, contact transitions to boundarylubrication regime and surface friction start to contribute to energy dissipation.

3.7.1 Axle Efficiency Tests

Generally, improvement in axle efficiency is measured as a function of lubricant chemistry using a pro-duction rear axle which is coupled with a transmission and fired engine, as shown in Figure 3.5. Thetorque input to the axle is measured by an in-line torque meter and the output torques are measured bythe load cells connected to a dynamometer. Axle efficiency is calculated from these measurements using aknown gear ratio. It should be noted that a test axle is used for multiple runs. One should be cognizant ofthe fact that the axle performance continues to change as it accumulates test runs. It is essential that theaxle is periodically referenced for its performance. A frame work for effectively considering the hardwarechanges during the life of an axle, as outlined by Akucewich et al. [11] provides valuable guidance.

Figure 3.5 shows a typical axle efficiency curve, which consists of two parts: the low-torque range,which is related to Environmental Protection Agency (EPA) fuel economy cycle, and the high-torquerange, which represents axle performance and durability (particularly pinion bearing) under trailer/tow



FIGURE 3.6 A picture of experimental setup to measure axle efficiency. (Adapted from Gangopadhyay et al. SAE2002-01-2821, with permission.)

condition. The test is conducted under a variety of load and speed conditions reflecting idling, city driving,highway driving, and trailer/tow conditions. In this test large-capacity torque meters are used to cover theentire range, and therefore, there are some uncertainties in the accuracy of efficiency measurement in thelow-torque region. Although efficiency is higher than 95% in the high-torque region, in the low-torqueregion efficiency is much lower and, therefore, provides an opportunity for improvement. In the high-torque region, efficiency does not vary a lot, but lower lubricant temperature is desired for maintainingbearing durability.

For axle efficiency improvement studies, it is convenient to run tests under low-torque conditions toscreen potential candidates and then run tests under high-torque conditions for durability. Figure 3.6shows another setup to evaluate axle efficiency under low torque conditions. This setup utilized a 223 mm(8.8 in.) diameter ring gear set and 3.55 axle ratio from a full size pickup truck. The pinion was drivenby one motor while the energy output on the other side was absorbed by another motor. The electricdrive controlled the speed on the input motor and the torque on the output motor. The output shafton one side of the axle was shortened and prevented from rotation by welding a pair of side gear teeth.Two kinds of experiments were conducted: (1) where the lubricant was allowed to heat naturally due tofriction and (2) where the temperature of the lubricant was held constant. For the latter, the axle housingwas connected to an external oil sump where the lubricant was heated to the desired temperature by aheater and pumped in and out of the axle housing through a heat exchanger. The lubricant level wasalways maintained as specified in the owner’s manual. The input and output torques were measuredby the electric drive system within ±5% error. The lubricant temperature was recorded by inserting athermocouple in the axle housing.

3.7.1.1 Effect of Axle Speed and Lubricant Temperature on Axle Efficiency

The effect of axle speed on efficiency is shown in Figure 3.7 during natural heating of axle. Generally,efficiency increases with applied torque because the lubricant temperature also increases with torque,as shown in Figure 3.8. The lubricant temperature generally keeps rising with continued operation due toconstant frictional heat input. Therefore, axle efficiency was calculated when the temperature stabilizedand did not vary more than 1◦C for about 15 min. The increased temperature decreases lubricant viscosity,and therefore, decreases hydrodynamic drag, resulting in increased efficiency. Also, the higher the speed,the higher the efficiency because the film thickness is higher between gear surfaces, resulting in less severeasperity contact, and asperity friction is higher than fluid friction. The lubricant temperature is also higherat higher speed, leading to reduced lubricant viscosity, which will increase asperity friction. But the film



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FIGURE 3.7 The effect of speed on axle efficiency.

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FIGURE 3.8 The effect of torque on lubricant temperature.

thickness increase due to increased speed must be greater than the film thickness decrease due to increasedtemperature.

Figure 3.9 shows axle efficiency as a function of speed when tests were conducted at constant lubricanttemperature of 113◦C. The efficiency decreased with increasing speed at lower loads, which is oppositeto what was observed when the lubricant was heated naturally. At a constant temperature, the lubricantviscosity is constant and, therefore, at higher speed the film thickness is higher than at lower speed.Therefore, at low torque, efficiency decreased due to increase shear loss, but at high torque, efficiencyimproved due to increase in load-dependent losses as compared with load-independent losses. At a givenspeed, the increase in efficiency due to increased temperature is shown in Figure 3.10, where the efficiencyincreased with increase in temperature at low torque due to decreased shear loss.

3.7.1.2 Effect of Lubricant Formulations on Axle Efficiency and Temperature

Lubricant formulation plays a significant role in controlling axle efficiency and durability. In particular,the lubricant viscosity plays a more significant role in improving axle efficiency under low torque while theadditive package plays a significant role in durability, in particular clutch pack for limited slip differential.



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FIGURE 3.9 Axle efficiency as a function of torque at different speeds at a constant lubricant temperature of 113◦C.

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FIGURE 3.10 Axle efficiency as a function of torque at a constant speed.

Figure 3.11 shows axle efficiency under various torque conditions at 1000 rpm for various lubricantformulations, mostly on 75W-90 and a few 75W-140 viscosity grades.

The viscosities of the lubricants used in this test program are shown in Table 3.5. Lube A has thehighest viscosity at 40◦C and showed lowest efficiency, whereas Lube D has the lowest viscosity at 40◦Cand exhibited the highest efficiency at the lowest torque of 112 N m. The lubricant temperature at cor-responding torques is shown in Figure 3.12. Although Lube B and Lube E have similar viscosities, andalthough the lubricant temperatures at lower torques are similar, they showed a difference in efficiency,probably highlighting the impact of chemistry. A similar effect could be observed for Lube A and Lube C′,both of which are in the 75W-140 viscosity grade.

The axle efficiency curves can be better understood when the results are plotted in the form of Stribeckcurves, as shown in Figure 3.13, which shows that the test axle operates in both mixed and hydrodynamiclubrication regimes. The higher-viscosity Lube A operates primarily in the hydrodynamic regime at1000 rpm, and the frictional loss is higher than any of the other lubricants. The torque loss of 75W-90viscosity grade lubricants is comparable in the hydrodynamic regime, but they distinguish themselves inthe mixed lubrication regime. In the mixed lubrication regime, Lube A and Lube B shows the highest



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FIGURE 3.11 Axle efficiencies of various lubricants.

TABLE 3.5 Viscositites of Lubricants Investigated

Lubricants Viscosity, cSt

40◦C 100◦C Viscosity index

Lube A 75W-140 192.2 24.3 172Lube B 75W-90 110.4 17.7 177Lube C 75W-90 136.7 22.8Lube C′ 75W-140 170.6 25.7 190Lube D 75W-90 64.2 14.5 227Lube E 75W-90 116.1 18.1 168

Source: Adapted from Gangopadhyay et al. SAE 2002-01-2821, withpermission.

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FIGURE 3.12 The axle sump temperature for various lubricants.



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FIGURE 3.13 Stribeck curves for lubricants tested at 1000 rpm. (Adapted from Gangopadhyay et al. SAE 2002-01-2821, with permission.)

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FIGURE 3.14 Vehicle fuel economy obtained with various lubricants. (Adapted from Gangopadhyay et al. SAE2002-01-2821, with permission.)

torque loss, whereas Lube E shows the lowest. This provides an opportunity for Lube A and Lube B toimprove efficiency further by reducing losses in the mixed lubrication regime.

3.7.2 Vehicle Tests

Figure 3.14 shows the improvement in fuel economy over Lube A with different 75W-90 lubricants, withina 90% confidence interval. These results were obtained in chassis roll dynamometer tests under FederalTest Protocol (FTP) metro/highway cycles. The results show significant fuel economy improvement andthe level of improvement also depended on the vehicle being tested. Figure 3.15 shows axle temperatureprofile during chassis roll dynamometer tests in a Lincoln Navigator model year 2001, equipped with5.4-L four-valve engine. Again, the dependence of lubricant viscosity on lubricant temperature could beobserved. The highest-viscosity lubricant, Lube A, showed the highest lubricant temperature, whereas thelowest-viscosity lubricant, Lube C, showed the lowest.

3.7.3 Spin Loss Tests

Spin loss tests are another way of evaluating the fuel economy improvement potential of rear axle lub-ricants. Such tests measure the power required to drive the axle in coast down mode. Specially equipped



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FIGURE 3.15 Axle lubricant temperature during chassis roll tests.

dynamometers with high-precision torque transducers are used to measure torque loss when the axle isrotated either through the wheel or through the pinion from 70 to 0 mph. The measured torque loss isconverted into horsepower (hp) loss. Reduced hp loss translates into improved fuel economy. Bjornenet al. [12] demonstrated that a lower-viscosity grade 75W-90 lubricant exhibits lower spin loss, whichcorrelates with higher estimated fuel economy.

3.7.4 Effect of Gear Surface Finish on Efficiency

It is observed that the ring and pinion gear contact operates in mixed and hydrodynamic lubricationregimes, the relative portion of which depends on operating conditions including surface roughness andlubricant viscosity. Some of the 75W-90 lubricants pushed the contact well into mixed lubrication regimes.Therefore, it is expected that further reduction in friction in the mixed lubrication regime could be possibleby reducing the surface roughness of gears.

The ring and pinion gear surfaces were superfinished using a chemo-mechanical technique. The iso-tropic finishing process is a two-step chemo-mechanical process. The first step is called the refining step,where the parts are put into a slowly rotating standard vibratory device containing a mild acidic solutionand ceramic media. The acidic solution reacts with the metal surface and leaves a very thin soft film,which is then removed by abrasion with the ceramic media. The surface film is continuously reformedand removed, resulting in material removal from the surface. The second stage of the process is called theburnishing stage, where a basic solution is added in the vibratory bowl to neutralize the refining solutionand also to remove any soft film remaining on the surface. This process results in a very smooth surface,on the order of 0.07 µm Ra . The typical production surface finish is 1 to 1.5 µm Ra . Figure 3.16 comparesthe axle efficiency measured at 1000 rpm between isotropic finish gears and production gears. The resultsdemonstrated significant improvement in axle efficiency as well as lower axle operating temperatureswith isotropic finish gears compared to production gears as shown in Figure 3.17. The axle efficiencyimprovement resulted in about 0.5% improvement in fuel economy in chassis roll dynamometer testsunder metro/highway cycles.

Ring and pinion gear finish also plays an important role in durability. Generally, the gears are phosphatedto enhance wear resistance and assist in running-in of surfaces. These coatings have excellent lubricity andscuffing resistance under lubricated conditions; however, the coating itself is not very lubricous. Due to itsporous nature, the coating provides lubricity by absorbing a large amount of lubricant in its porousstructure. It has been claimed that the supply of lubricant from these pores helps to prevent severe damageto gear surfaces [13,14]. The coating morphology plays a critical role in its ability to absorb lubricant.The coating morphology depends primarily on coating process as well as on underlying roughness ofgear surfaces. The surface roughness of the gear surface also plays an important role in gear wear. A high



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FIGURE 3.16 Axle efficiency of isotropic finish ring and pinion gears in contact with Lube B.

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FIGURE 3.17 Axle lubricant temperature with isotropic finish ring and pinion gear set at 1000 rpm.

surface roughness will probably push the components to more boundary and mixed lubrication regimes,leading to wear. On the other hand, a smoother surface finish could potentially increase cost. Therefore,an optimum surface roughness needs to defined based on cost and performance.

3.7.5 Effect of Cold Start on Axle Efficiency

Fuel economy tests according to FTP cycles are conducted under a controlled room temperature. However,customers drive under a variety of temperatures depending on their geographic location. Generally,the axle warms up quite fast and, therefore, actual axle lubricant temperature during driving may notvary much whereas vehicle-starting temperature varies a lot depending on locations. A lower startingtemperature would result in higher lubricant viscosity, which may result in lower initial fuel economyuntil the axle is fully warmed up. Therefore, it is of interest to know the effect of cold start temperatureon axle efficiency.

In order to assess the effect of cold start temperature on axle efficiency, the axle housing was wrappedaround with nylon tubes through which liquid nitrogen was passed, as shown in Figure 3.18. The ring and



FIGURE 3.18 A close-up view of the arrangement to cool axle lubricant.

pinion gears were rotated slowly (around 10 to 15 rpm) to equilibrate the temperature of the componentsinside the axle housing without generating any significant frictional heat. When the desired temperaturewas reached, the liquid nitrogen flow was stopped and the test started at the required torque and speedcondition. Figure 3.19 shows the axle efficiency and lubricant temperature due to cold start and comparesthem with an ambient start under various torque and speed conditions. At any given condition, theefficiency was calculated when the lubricant temperature stabilized. Under ambient conditions, the initialefficiency is about 71% under 68 N m torque and 532 rpm speed and it increased gradually to about 73%when the lubricant temperature stabilized. For the cold start, it took about 40 min to cool the lubricant toabout −14◦C. Upon starting, the efficiency was only about 41%, significantly lower than for the ambientstart. The efficiency increased rapidly with the increase of lubricant temperature and finally leveled atabout 67%, less than that observed with an ambient start although the stabilized lubricant temperaturewas similar, 42◦C compared with 41◦C. As the torque was increased to 611 N m, the axle efficiency andthe lubricant temperature increased but the initial efficiency difference between the ambient start andthe cold start diminished to 4.6% (Figure 3.19[b]). The stabilized lubricant temperatures for ambientand cold starts are about the same but the difference in respective efficiencies was only less than 0.5%.At 68 N m torque, as the speed is increased from 532 to 1775 rpm, both the axle efficiency and the lubricanttemperature increased. The initial difference in axle efficiency between ambient and cold starts narrowed to13.5% and, after stabilization of lubricant temperature, the difference was less than 1%. At the highesttorque and speed condition investigated (Figure 3.19[d]), the initial efficiency difference widened to 11.5%and, after the lubricant temperature stabilized, the difference reduced to 6.6%. The larger difference inefficiency under these conditions could be due to the lower initial temperature of −22◦C compared withabout −13◦C for the other starts. The results demonstrated significant efficiency loss at the beginning ofa cold start, but the loss diminished as the lubricant warmed up. However, a significant efficiency loss wasobserved even after the lubricant temperature stabilized under low-torque, low-speed conditions.

3.7.6 Limited Slip Differentials

More vehicles are being equipped with limited slip differentials for improved handling. When one wheelbegins to spin, the limited slip differential provides more torque to the other wheel to move the vehicle.In other words, it limits the amount of differential action between the wheels. This differential action isobtained through the use of clutch packs, each consisting of a combination of steel plates and frictionplates. One of the issues with limited slip differential is shudder, which translates into chatter to the driver.The shudder is related to stick slip motion between the clutch plate friction material and the steel plate.Generally, a friction modifier is added with the axle lubricant to reduce stick slip. The effectiveness of thefriction modifier in reducing stick slip can be evaluated in a specially equipped rig where a friction material



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FIGURE 3.19 Comparison of axle efficiency between ambient start and cold start under (a) 532 rpm and 68 N mtorque; (b) 532 rpm and 611 N m torque; (c) 1775 rpm and 68 N m torque; (d) 1775 rpm and 611 N m torque.

is loaded against a steel plate under a given load and lubricant temperature. The friction coefficient ifmeasured as a function of speed. A positive slope of friction coefficient vs. speed curve is indicative ofresistance to shudder. Another method to evaluate the effectiveness of a friction modifier on shudderis to load the rotating friction material against the steel plate under a given load and then measure thetorque required to stop the rotation. The load, temperature, and speed of rotation are selected basedon field correlation. In this test configuration, the stick slip phenomenon is reflected as oscillations in thefriction coefficient vs. time curve. Another issue is the limited life of friction modifiers. The effectiveness offriction modifier decreases with aging. Therefore, it is very important to evaluate friction characteristics oflubricant both when fresh and used. Phosphorous-type friction modifiers are commonly used in gear oils,which can be acid phosphates, amine phosphates, or hydrogen phosphates. Okazaki et al. [15] investigatedthe structural change in friction modifiers in laboratory aging tests and blended a series of gear oils basedon the compounds formed during aging. The evaluation of friction characteristics of these blends led tothe development of a long-life friction modifier. Generally, the friction modifier acts on the clutch surfaceand does not necessarily impact friction at the ring and pinion gear surface [16].

3.8 Summary

A critical component in rear axle systems is gear lubricant, and it plays a very important role in theefficient and durable operation of the system. Traditionally, the lubricant is required to provide thermal



and oxidative stability, seal compatibility, adequate antifoaming tendencies, resistance to corrosion, andso on. However, modern vehicle designs and regulatory pressures are demanding additional performancefrom lubricants. The proliferation of higher horsepower, smaller sump size, longer drain intervals, andindependent rear suspensions is pushing the lubricant temperature higher, leading to decreased filmthickness while demand for increased fuel economy requires lower-viscosity lubricants. In contrast, highertowing capacity requires adequate durability, which requires higher viscosity lubricant to provide increasedfilm thickness to prevent metal-to-metal contact. These conflicting requirements are making lubricantformulation very challenging. Increased use of limited slip differentials makes lubricant formulation tomake a vehicle chatter-free during turns even more complex. These issues will continue to be significantas the CAFE requirements rise over coming years. The challenge to develop lubricants which improve theefficiency of rear axle systems while maintaining durability will continue to be faced by both the equipmentmanufacturers and fluid developers. A successful resolution to these issues will require close cooperationbetween equipment manufacturers and fluid formulators to use concurrent engineering techniques tooptimize fluids for hardware with improved surface finish, optimum coating on gears, improved frictionmaterials, and improved design of axle housing.

References

[1] 2004 Market Data Book, Automotive News, May 2, 2004.[2] T. Naman, “Automotive fuel economy — potential improvement through selected engine and

differential gear lubricants,” Society of Automotive Engineers Paper No. 800438, 1980, SAEInternational.

[3] B.M. O’Connor, L.F. Schiemann, and R.L. Johnson, “Axle efficiency — response to syn-thetic lubricant components,” Society of Automotive Engineers Paper No. 821181, 1982, SAEInternational.

[4] D.A. Law and C.N. Rowe, “The design of fuel efficient automotive hypoid gear lubricants,” Journalof Synthetic Lubrication, 11(1), 3–15, 1993.

[5] R.F. Watts and G.L. Willette, “Newtonian multigrade gear lubricants: formulation and performancetesting,” Society of Automotive Engineers Paper No. 821180, 1982, SAE International.

[6] P.A. Willermet and L.T. Dixon, “Fuel economy — contribution of the rear axle lubricant,” Societyof Automotive Engineers Paper No. 770835, 1977, SAE International.

[7] J.H. Adams, K.A. Frost, L.M. Hartman, and L.J. Painter,“The effect of gear lubricant on fuel economyas measured in a line haul truck fleet,” Society of Automotive Engineers Paper No. 810179, 1981,SAE International.

[8] A.B. Greene and T.J. Risdon, “The effect of molybdenum containing, oil-soluble friction modifierson engine fuel economy and gear oil efficiency,” Society of Automotive Engineers Paper No. 811187,1981, SAE International.

[9] V. Bala, G. Brandt, and D.K. Walters, “Fuel economy of multigrade gear lubricants,” Technis-che Akademic Esslingen, Tribology 2000-Plus, 12th International Colloquim, January 11–13, 2000,Ed. W.J. Bartz.

[10] J.N. Vinci, E.S. Akucewich, R.S. Cain, and F.S. Qureshi, “Developing next generation axle fluids:part II — systematic formulating approach,” Society of Automotive Engineers Paper No. 2002-01-1692, 2002, SAE International.

[11] E.S. Akucewich, J.N. Vinci, F.S. Qureshi, and R.W. Cain, “Developing next generation axle fluids:part I — test methodology to measure durability and temperature reduction properties of axle gearoils,” Society of Automotive Engineers Paper No. 2002-01-1691, 2002, SAE International.

[12] K.K. Bjornen, H. Chambers, and D. Degonia,“Development of a fuel efficient multipurpose 75W-90gear lubricant,” Society of Automotive Engineers Paper No. 2003-0260, 2003, SAE International.

[13] P. Hivart, B. Hauw, J.P. Bricout, and J. Oudin, “Seizure behavior of manganese phosphate coatingsaccording to the process conditions,” Tribology International, 30(8), 561–570, 1997.



[14] T. Oyamada and Y. Inoue, “Evaluation of the wear process of cast iron coated with manganesephosphate,” Tribology Transactions, 46(1), 95–100, 2003.

[15] K. Okazaki, K. Noguchi, K. Motoyama, and T. Wakizono, “A study of friction characteristics anddurability of LSD oils,” Society of Automotive Engineers Paper No. 932786, 1993, SAE International.

[16] A.K. Gangopadhyay, S. Asaro, M. Schroder, J. Sorab, and R. Jensen, “Fuel economy improvementthrough frictional loss reduction in light duty truck rear axle,” Society of Automotive EngineersPaper No. 2002-01-2821, 2002, SAE International.
