TRIBOLOGY

DHIRENDRA BIHARI

TRIBOLOGY IN MARINE APPLICATIONS

EXPLAIN TRIBOLOGY- DEALING WITH FRICTION, WEAR,

AND LUBRICATION OF INTERACTING SURFACES IN

RELATIVE MOTION (AS IN BEARINGS OR GEARS) ENGINE

OIL DEGRADATION AND CONTAMINATION

Tribology is the science and engineering of interacting surfaces in relative motion.

It includes the study and application of the principles of friction, lubrication

and wear.

The tribological interactions of a solid surface's exposed face with interfacing materials and environment may result in loss of material from the surface. The process leading to loss of material is known as "wear". Major types of wear include abrasion, friction (adhesion and cohesion), erosion, and corrosion. Wear can be minimized by modifying the surface properties of solids by one or more of "surface engineering" processes (also called surface finishing) or by use of lubricants (for frictional or adhesive wear).The dynamic Coefficient of friction = Tangential force/ normal force.

Oiliness:-

A property of lubricating oil called oiliness assumes a

significant proportion while considering lubrication of

bearing surfaces under thin film condition. The property of

oiliness may be defined as the ability of oil to adhere to or

wet the surfaces with which they are in contact. It is as if a

molecular layer of lubricant is absorbed at the surfaces and

the bond prevents their squeezing out of the space.

Diesel Engine Lubrication

If one examines the requirements of lubrication amongst the large number of

moving parts in diesel engine, it will be noticed that the conditions are varied

and the requirements are divergent. Considering the nature of motion that exists

between the surfaces the diesel engine bearings are divided under the following

groups:

•Journal pin, crankshaft, camshaft and other bearings where the motion is purely

rotational.

•Cross head pin, rocker arm etc. where the motion is oscillating.

•The meshing teeth in the gear train, chain rollers and sprocket wheel, ball

bearings etc. where nominal line or point of contact exists in rolling motion.

•Cross head guide, piston rings, valve stem etc. Where high sliding velocity

exists.

Though the purpose of lubrication is primarily to reduce friction between

working surfaces, there is, in addition, another important function in a diesel

engine of maintaining an effective piston ring seal and transfer of heat thus

cooling. It will be appreciated from the fore going that the lubricating oil will

have to satisfy a variety of requirements in order to be really effective in service.

The many available lubricant choices are associated with a few specific lubricationregimes. These are in turn differentiated by the film associated film thickness as follows:• Hydrodynamic Fluid Film. The layer completely separates and prevents direct contactof the solid surfaces involved. The film thickness is several times larger than themagnitude of the composite standard deviation of surface heights of the contactingsurfaces, usually of the order of 100 micrometers.• Elastohydrodynamic. The layer is severely compressed and thinned by the applied load.Direct contact is still prevented but contacting solids deform elastically because of thehigh film pressure. The film thickness is only slightly larger than the magnitude of thecomposite standard deviation of surface heights of the contacting surfaces, usually ofthe order of 1 micrometer.• Transition or Mixed Lubrication. The film thickness becomes of the order of the surfaceasperities (i.e. of the order of 0.1 micrometer) and intermittent contact is obtained• Boundary Lubrication. The film can be as small as a single adsorbed layer. Intermit-tent solid contact may take place.

Elasto-Hydrodynamic LubricationAt large loads lubricating films in converging gaps are capable of supporting much greaterpressures than those estimated using standard lubrication theory. Two key effects that must be considered are• the pressure sensitivity of the viscosity, and• the elastic deformation of the solid surfaces.Elastohydrodynamic (EHD) lubrication analysis accounts for the above two effects and allows determination of fluid film thickness and pressure distribution which are in good agreement with experimental measurements on heavily loaded lubricated contacts.In heavily loaded bearings, high pressures develop inside the entrapped fluid film. Sincelubricant viscosity is pressure dependent the lubricating film exhibits solid-like behaviorunder these conditions.

Boundary LubricationIt is now known that lubricating layers as small as a single molecule are capable of producing significant improvements in tribological performance (i.e. reduced friction and wear). This is the subject of boundary lubrication.Even single molecular layers of particular substances attached to solid surfaces can haveimportant effects on tribological behavior. This is the basis of boundary lubrication technol-ogy. With the availability of the surface force apparatus (SFA) it is now possible to measure not only the thickness of the lubricating film down to atomic dimensions but also the friction forces involved.The key to boundary lubrication is the formation and maintenance of a single or multi-molecular layer of lubricating material so as to prevent as much as possible the direct drycontact of the solid surfaces in the tribological couple. Intervening films such as oxide and sulfide layers have demonstrated to be effective in reducing friction and wear. This is a good example of a chemical film. The oxide material in the film is in intimate contact with the metal surface underneath.Many other such chemical films are possible and particularly important are those formedwhen certain organic compounds react with the metal surfaces. Specifically, pure paraffin oil and combinations with small amounts of a fatty acid such as lauric acid, can be very effective in reducing friction. The resulting metallic soap molecules formed at the surface perform well until the temperature becomes high enough that soap melting and film breakdown takes place.

As a general rule, polar molecules exhibit strong affinity for bare metal surfaces and arethus ideal candidates as boundary lubricants. Such molecules may attach to the substrateby physical adsorption, chemical adsorption or by chemical reaction. Most appropriate arestraight chain organic molecules with one polar end such as alcohols and soaps of fatty acids.Often, the presence of more than one molecular layer of lubricating material leads toimproved tribological performance. Experimental data shows that some 50 layers of stearic acid deposited onto a stainless steel surface produce a low friction surface over a large number of repeated sliding contacts.Another way of increasing friction performance with boundary lubrication is to use stillsingle layers but of longer chainmolecules. The performance is also improved by the increased stability of the longer chain molecules on the metal surfaces.A number of intervening solid layers are capable of reducing friction. Surface coatingsof materials with layered crystallographic structures, specifically graphite and molybdenum disulphide have been found useful in reducing friction and wear.

SummaryIn sum, the engineer and the designer have available the following spectrum of unlubricanted and lubricated joint types:• Solid-solid contact• Few-atom thick molecular layer lubricants• Fluid lubricants: Animal fat, vegetable and petroleum-based oils, mineral oils, syn-thetic oils, and additives• Greases• Solid lubricants: Layer- and nonlayer-lattice solids, fullerenes and polymer plastics1

TYPES OF OILS-MINERAL AND SYNTHETIC

Lubricants can be classified by their origin—animal (e.g., sperm oil, goose

grease), vegetable (e.g., soybean oil, linseed oil), or mineral (e.g., petroleum,

molybdenum sulfide). From ancient times until the late 19th cent. lubricants were

obtained from vegetable oils or animal fats and oils. Today most are derived from

mineral oils, such as petroleum and shale oil, which can be distilled and

condensed without decomposition. Synthetic lubricants, such as silicones, are of

great value in applications involving extreme temperatures. In certain types of

high-speed machinery films of gas under pressure have been successfully used

as lubricants.Differing widely in viscosity, specific gravity, vapour pressure, boiling point, and other properties, lubricants also offer a wide range of selection for the increasingly varied needs of modern industry. But whatever their derivation or properties, the purpose of lubricants is to replace dry friction with either thin-film or fluid-film friction, depending on the load, speed, or intermittent action of the moving parts. Thin-film lubrication, in which there is some contact between the moving parts, usually is specified where heavy loads are a factor. In fluid, or thick-film, lubrication a pressure film is formed between moving surfaces and keeps them completely apart. This type of lubrication cannot easily be maintained in high-speed machinery and therefore is used where reciprocating or oscillating conditions are moderate.

Lubricant Types and CharacteristicsAccording to their physical characteristics lubricants are classified as follows:• Mineral Oils. A complex mix byproduct of fractional distillation of crude oil.• Synthetic Oils. Produced by polymerization of low molecular weight hydrocarbons.• Greases. Mixtures of lubricating oils and thickeners obtained by adding alkali and fattyacid to oil.• Boundary Lubricants. Molecules with strong affinity towards the surface being lubri-cated.• Solid Lubricants. Layered and non-layered lattice solids; fullerenes.In fluid film hydrodynamic lubrication both mineral and synthetic oils are commonlyused. Key properties of these lubricants which must be considered in engineering designinclude• Viscosity: dynamic and kinematic.• Physical properties: density, conductivity, specific heat, surface tension, refractive in-dex, additive compatibility and solubility, impurity content.• Stability: pour, cloud, flash and fire points; volatility, oxidation rate.

TIMED CYLINDER LUBRICATION – CYLINDER OIL

PROPERTIES

This cylinder lubrication, shown in the figure is based on a lubricator which injects

a specific volume of oil into each cylinder for each (or for every second, third, etc.)

revolution. The oil fed to the injectors is pressurised by means of Alpha lubricator

on each cylinder, equipped with small multi piston pumps. The amount of oil fed

to the injectors can be finely tuned with an adjusting screw, which limits the length

of the piston stroke.

The dosage of oil can be adjusted means of an adjustment screw which limits the

stroke of the main lubricator piston. After a predetermined time interval,

the computer transmits an OFF signal to the solenoid valve, which shuts off the

system pressure and opens the return oil system.

The amount of oil injected varies as required, e.g. at load changes, start/stop,

or increased engine load. Alternatively, the dosage of oil fed to the individual

cylinders can be adjusted by injecting a calibrated amount of oil, a number of

times, at a given number of revolutions. A combination of the two systems can

also be used.

A pump station delivers lube oil to the lubricators at 45 bar pressure. The

lubricators have a small piston for each lube oil quill in the cylinder liner, and the

power for injecting the oil comes from the 45 bar system pressure, acting on a

larger common driving piston. Thus, the driving side is a conventional common

rail system, whereas the injection side is a high-pressure positive displacement

system, thus giving equal amounts of lube oil to each quill and the best possible

safety margin against clogging of single lube oil quills.

For the larger bore engines, each cylinder has two lubricators (each serving half of

the lube oil quills) and an accumulator, while the small bore engines (with fewer

lube oil quills per cylinder) are served by one lubricator per cylinder. The pump

station includes two pumps (one operating, the other on stand-by with automatic

start up), a filter and coolers.

The lubricator can be delivered for our conventional engines in which case it

is controlled by a separate computer unit comprising a main computer, controlling

the normal operation, a switchover unit and a (simple) back-up unit. A shaft encoder

supplies the necessary timing signal in that case. When used on ‘Intelligent

Engines’, these functions are integrated in the engine control computers and their

shaft encoders.

The lubrication concept is intermittent lubrication – a relatively large amount of lube

oil is injected for every four (or five or six, etc.) revolutions, the actual sequence

being determined by the desired dosage in g/bhph. The injection timing is

controlled precisely and – by virtue of the high delivery pressure – the lube oil is

injected exactly when the piston ring pack is passing the lube oil quills, thus

ensuring the best possible utilisation of the costly lube oil.

The safety features of this system are as follows:

In the event of malfunctioning solenoid valve or transducer, the oil dosage will

automatically be increased to the maximum volume. If the oil pressure falls, the

computer will start stand-by pump, close down the faulty pump and sets on the

alarm.

In this system if one lubricator malfunctions (980-700 mm bore engines), the

oil dosage from the other lubricator will be automatically doubled, and an alarm

will be given whereas for 600-260 mm bore engines, alarm and slow down ensue.

An inductive sensor in each lubricator monitors the movement of the lubricator

piston a signal is sent to the control computer system which has a backup for

safety.In the shipping industry, two giants – MAN Diesel and Wartsila have introduced a remarkable technology for modern electronically controlled marine engines. Known as Alpha and Pulse lubrication systems, this new technology is one-of-its-kind.In this article we will understand what does pulse lubrication means and how it helps to reduce the cylinder oil feed rate and eventually the operating costs of the ship.Wartsila- A major player in the marine engine manufacturing industry has introduced an intelligent cylinder lubrication system in its electronically controlled engine. This system is popularly known as the pulse lubrication system.What is Pulse Lubrication System?A pulse lubrication system is an electronically controlled cylinder oil lubrication system for Wartsila engines, wherein metered quantity of cylinder oil is injected in to the liner, depending on the engine load. This ensures that accurate amount of cylinder oil is delivered inside the liner at the correct set-time for that particular engine load.

Construction and Working of Pulse Lubrication SystemThere are normally eight quills attached to the cylinder liner in a single row, which gets the oil supply from the electronically controlled dosage pumpThe oil is supplied to the dosage pump from daily tank via fine filter of 40 micronsThe quills consist of a duct passage to store metered quantity of oil. The area of this duct passage and the quantity of oil can be altered by changing the position of the central pistonThere are crank angle sensors attached to the engine which give signals to the control unit in order to inject oil at the correct position of piston movement200 bar high pressure servo oil reduced to 50 bars are supplied to the lubricator unit, which pressurises the centre piston in the quills. This injects oil inside the liner at adequate pressure for even distributionWECS (Wartsila Engine Control System) which is the master controller of the Pulse lubrication system controls the solenoid valve opening and the oil injectionEach unit is provided with 8 lubricating quills, 2 piping systems of Cylinder oil and servo oil, and A 4/2 solenoid valve to servo oil flow.After receiving signal from the crank angle sensor, at the correct position i.e. between the pack of piston rings, WECS allows the solenoid valve to open and pass the servo oil. This in turn presses the central piston and delivers the oil stored in the duct passage of the quills.

CONSEQUENCES OF OVER AND UNDER LUBRICATING

Over lubrication will lead to excessive deposit build up generally in the form of carbon deposits. This can lead to sticking of rings causing blowpast and loss of performance, build up in the underpiston spaces leading to scavenge fires, blockage and loss of performance of Turboblowersas well as other plant further up the flue such as waste heat recovery unit and power turbines.Under lubrication can lead to metal to metal contact between liners causing microseizure or scuffing. Excessive liner and piston wear as well as a form of wear not only associated with under lubrication but also with inadequate lubrication called cloverleafing CausesInsufficient cyl l.oIncorrect cyl l.o.Blocked quillIncorrect cyl at each stroke.

The fine adjustment operates in such away that by screwing it in the stroke of each pump may be accurately metered. Additionally it may be pushed into give a stroke enabling each p/p to be tested. The eccentric stroke adjuster acts as a coarse adjustment for all the pumps in the block. Additionally it may be rotated to operate all the pumps, as is the case when the engine is pre-lubricated before starting. Correct operation of the injection pumps whilst the engine is running can be carried out by observing the movement of the ball

For many years there has been a prevailing perception that the more oil you use for cylinder lubrication, the better. Nothing could be further from the truth!Our experience has shown that the main reasons for increased cylinder wear, broken piston rings and overall poor cylinder condition is over lubrication. It is therefore a problem which should be taken seriously.

Over lubrication of a two stroke engine can be very harmful for the cylinder condition and can lead to high cylinder liner and piston ring wear, and also breakage of the piston rings or they get stuck.Many crew members are under the impression that the larger amount of oil they use for cylinder lubrication, the better. They often try to solve a problem by increasing feed rate. However, this is far from true.

Excess of cylinder oil and thereby also excess of additives will, as only part of the additives are used for neutralization of the sulfuric acid formed during the combustion, burn off and form a layer of residues on the piston top land to an extent that the residues will touch the cylinder liner and consequently wipe off the cylinder oil from the cylinder liner running surface.Correct dosage of cylinder oil is a very important part of optimizing consumption and cylinder condition.

Always be aware of the sulphur content in the fuel oil in use, and adjust the cylinder oil feed rate accordingly.Especially when low sulphur fuels are taken into use, frequent port inspections are recommended.For recommendations about feed rate, for instance according to sulphur contents in fuel oil, we refer to our service letter about this isssue for relevant recommendations.

The building up of a dangerous layer of residues can under special circumstances happen within 24 to 48 running hours, after which wear is accelerated.The Hans Jensen SIP lubrication systems are, due to the injection timing, more sensitive to over lubrication than other systems, wherefore it is of very high importance to adjust the feed rates to the recommended limits given in the graphs for breaking in, running in and during normal service.

At many industrial facilities, the task of equipment lubrication is often assigned to a newly hired maintenance technician or mechanic with little or no lubrication training who is just learning the ins and outs of the plant. Often times these mechanics are handed a grease gun and told to lubricate the points on a particular line or maybe the entire plant. To the maintenance supervisor, this seems like a good way to familiarize the new mechanic with the plant’s equipment. To the new mechanic, he is performing an important task that is helping to increase bearing life. Both the maintenance supervisor and mechanic are right but they are also wrong.

Certainly, assigning a new mechanic the task of equipment lubrication will help familiarize him with the plant’s equipment, but at what cost? The new mechanic is correct in believing that he is performing an important task, but is the way he performs the task actually increasing bearing life? The answer depends upon how well the new mechanic has been trained. More than 35% of bearing failures can be attributed to improper lubrication. An enthusiastic but untrained lube tech with a grease gun is more than likely to cause premature bearing failures due to over greasing than he is due to under greasing.

Over greasing a bearing will cause the rollers or balls to slide along the race instead of turning, and the grease will actually churn. This churning action will eventually bleed the base oil from the grease and all that will be left to lubricate the bearing is a thickener system with little or no lubricating properties. The heat generated from the churning and insufficient lubricating oil will begin to harden the grease (see Fig. 1). This will prevent any new grease added to the bearing from reaching the rolling elements. The end result is bearing failure and equipment downtime. Ironically, an attempt to sufficiently lubricate a bearing by giving it several extra pumps from a grease gun will eventually result in its failure due to under lubrication.

Over lubricating the bearings in an electric motor causes an additional problem that will negatively effect the efficiency of the motor resulting in higher operating costs. The excess grease pumped into the bearing will eventually work its way into the stator body and the rotor assembly will distribute the grease throughout the windings (see Fig. 2). This will not only cause the motor to operate inefficiently because the grease will be insulating the windings, but it could also effect the operation of the fan and cause excessive heat within the motor.

The key to preventing the over lubrication of bearings is to ensure that all maintenance personnel are trained on proper lubrication techniques including how to determine the correct amount of grease to pump into a bearing. Establishing a sound overall maintenance program that includes lubrication intervals for each asset in your facility or even condition monitoring using ultrasonic technology will not only decrease maintenance costs; it will decrease downtime as well.

ELECTRONIC ALPHA CYLINDER LUBRICATION SYSTEM –

PRINCIPLE OF OPERATION

Alpha ACC (Adaptive Cylinder oil Control)

The principle of the Alpha ACC

The basic feed rate control should be adjusted in relation to the actual fuel

quality and amount being burnt at any given time. The sulphur percentage is a

good indicator in relation to wear, and an oil dosage proportional to the sulphur

level will give the best overall cylinder condition.

The following two criteria determine the control:

The cylinder oil dosage shall be proportional to the sulphur percentage in the

fuel

The cylinder oil dosage shall be proportional to the engine load (i.e. the

amount of fuel entering the cylinders).

The implementation of the above two criteria will lead to an optimal cylinder

oil dosage, proportional to the amount of sulphur entering the cylinders.

With the introduction of the electronically controlled Alpha Lubricator system,

featuring the easy-to-operate “HMI” panel, such adaptive lubrication has

become feasible.

The Alpha Lubricator system offers the possibility of saving a considerable

amount of cylinder oil per year and, at the same time, to obtain a safer and

more predictable cylinder condition.

The basic feed rate control should be adjusted in relation to the actual fuel quality

being burnt at a given time.

This new cylinder oil control principle is called the “Alpha Adaptive Cylinder oil

Control”, or abbreviated “Alpha ACC”.

the ACC factor can only be assessed when the fuel sulphur level has been high

enough to ensure that the lubrication has been in the ACC active area (the blue

area marked in Fig. 1), at lower fuel sulphur levels the engine is excessively

protected against corrosion because of the active minimum feed rate.One of the key parameters in Alpha ACC lubrication is part-load control proportional to engine load.This is important in order to prevent over-lubrication at low loads, and it is one of the main parameters to save oil, compared with conventional lubrication.When starting to burn new bunker oil, the HMI setting of the Alpha ACC should be adjusted according to the bunker analysis results.

INJECTOR UNIT FITTED TO MODERN CAMSHAFTLESS

SLOW SPEED ENGINES

Exact injection timing of cylinder lube oil is essential for efficiency. A move to electronics for the control of this has been made by some large slow speed engine manufacturers.

The system is based on an injector which injects a specific volume of oil into each cylinder on each ( though more normally alternate) revolution of the engine. Oil is supplied to the injector via a pump or pumps. A computer, which is synchronised to the engine at TDC each revolution, finitely controls the timing . Generally most efficient period for lubrication is taken at the point when the top rings are adjacent to the injection points.

The injection period is governed by the opening of a return or 'dump' solenoid which relieves system pressure.

Quantity can be adjusted by manually limiting the stroke of the main lubricator piston, by altering the injection period or by the use of multiple mini-injections per revolution.

The high degree of accuracy with this system allows for lower oil consumption rates.

Shown is the injector unit fitted to modern camshaftless slow speed engines. The motive force is via a dedicated or common hydraulic system. The hydraulic piston acts on multiple plungers one for each quill. At the dedicated time the electric solenoid valve energises an allows hydraulic oil to act on the piston commencing oil injection. One or two pumps per unit may be fitted dependent on cylinder diameter and oil flow requirements.

Precise control of the timing of injection allows oil to be delivered into the ring pack, something which has proved impossible with mechanical means. This has reduced oil consumption by as much as 50%.

Pre- lubrication for starting may be built into the bridge remote control system or carried out manually

STERNTUBE LUBRICATION- DEVELOPMENTS,

HYDROX 21 LUBRICANTS

HYDROX 21 – AT A GLANCE

DescriptionMineral oil based sterntube lubricant for use in the event of outboard

leakage and with some stabiliser fin shafts.

Typical Viscosity (cst @40°C) 275

Viscosity Index 100

Density (kg/ltr@15°C) 0.9

Pour Point IP15 (°C) < –5

Anti corrosion SKF Water Emcor Test Passes

Emulsifiable

These mineral based lubricants provide an excellent level of lubrication in both

neat and emulsion forms. They are developed primarily for use in sterntubes. The

products provide:

•Superior level of lubrication even when water ingress occurs

•Excellent wear protection

•Excellent corrosion protection

•Compatibility with metals commonly used

HYDROX 21 is compatible with the elastomers used for lip seal systems and are

approved by the major lip and face stern seal manufacturers.

HYDROX 21 will absorb any sea or fresh water entering the sterntube to form a

fluid emulsion. This reduces the risk of free water being present and continues to

provide the required lubrication and corrosion protection. (Conventional oils do

not emulsify in the same way and tend to separate, exposing components to free

water and potential wear damage).

The emulsions, once formed, have excellent stability, therefore free water is not

released. This ensures corrosion protection is maintained even during prolonged

standing and during the critical time of start-up.

HYDROX 21 (both neat oil and emulsion including 20% water contamination) has

been successfully tested by Class and their stability, lubricating and corrosion

protection performance have been verified by Lloyds Register.

HYDROX 21 is recommended for use particularly where problems of oil leakage

past the aft seals is experienced.

HYDROX 21 is compatible with most engine oils and system oils commonly used

in the sterntube and can therefore be introduced by top-up procedure to the

existing sterntube oil. A minimum of 50% is recommended as the initial charge,

however, the lubricant offers the greatest benefit when used to completely fill the

sterntube system.

Stern Tube Lubricant Absorbs Costs

In many industries and businesses, equipment failure or unexpected maintenance does not present a major problem. Standby systems can be brought online quickly, service engineers can repair machinery within a few hours, with spare parts that are readily available, or temporary equipment can be hired at a moment's notice.For some industries, however, failure or serious malfunction of machinery can be very damaging, whether financially or commercially. Such industries include steel making, oil and gas processing, underground mining, and shipping.Some systems aboard ships do not have standby backups, spare parts may be difficult or impossible to replace while at sea, even if they are carried on the vessel, and delays to voyages (whether through stoppage or slowdown) are likely to be costly. One item of equipment on ships that is not possible to replace or repair while at sea is a stern-tube bearing.

Benjn. R. Vickers & Sons Ltd., based in Leeds, England, offers the Hydrox range of specialized stern-tube oils. Hydrox stern-tube oils absorb water, which may enter the stern-tube bearing, and the resulting emulsions continue to provide a high standard of lubrication and corrosion protection to the shaft and bearings. All of the oils in the Hydroxrange are approved under the Lloyds Product Verification scheme. They are the first lubricants to have been approved in this way, with benefits of the Hydrox oils acknowledged by other leading Classification Societies.Oil lubricated bearings are more than likely white metal, but can be manufactured from specialized resin material. In either case the stern-tube is filled with oil, which is retained by means of a seal system designed not only to keep the oil in. but also to prevent seawater from entering. A header tank for the oil maintains a static head of pressure, which supports the sealing system in discouraging water entry.Stern-tube bearings can operate satisfactorily — with few or no problems. Sometimes, however, the stern tube seals can become worn or damaged. Equally, conditions can arise whereby seawater bypasses the seal.

For example, the oil/sea-water pressure relationship can be radically affected if extreme pitching is experienced, or stern tube vibration can result in seawater being drawn in and/or oil being forced out. It is not at all uncommon for oil to leak out of the stern-tube, creating a pollution problem, or more often for seawater to leak in or indeed both —water can do enormous damage to plain bearings. Where a conventional oil is being used, the presence of water can cause rusting and corrosion, and it can seriously compromise the quality of lubrication offered to the bearings. Any of these factors is likely to lead eventually to bearing damage and failure. Obviously, a stern-tube should be fitted with high performance seals, but a specialist stern-tube lubricant, which is able to lubricate bearings even in the presence of significant quantities of seawater, can provide very real cost savings. The Hydrox range of stern-tube oils manufactured by Vickers are specifical- lyformulated to do just that.Conventional engine oils that are often used to lubricate stern-tubes do not form stable emulsions with water, particularly seawater. Some "emulsifiable oils" may form emulsions, which are unstable, thereby allowing free water, leading to a breakdown in lubrication.Water ingress into a stern-tube lubricated with a conventional oil or unsuitable "emulsifiable oil" may therefore lead to serious bearing failure and damage to the propeller shaft should there be any significant degree of contamination with seawater. Hydrox 550 has been developed especially for use in sterntubes.

It forms stable emulsions with water and these continue to provide a high standard of lubrication and corrosion protection. Consequently, unscheduled repairs are unnecessary.Vickers calculates, for example, that an offshore supply vessel could achieve net savings of more than $21,538 where Hydrox 550 is in use and there is significant water entry into the system. These savings arise largely from the avoidance of unscheduled docking. Similarly, a large crude oil tanker could save more than $215,385 on a similar basis. The significance of the unit price premium is further diminished when the modest volumes of oil used in the stern-tube are taken into account.For example, Hydrox 550 is currently being used on the Queen Elizabeth 2 (QE2), the Oriana and the Ocean Princess. It is also being used on Stolt- Nielsen ships and on Andros Maritime ships. Chris Zukowski, Atlantic Fleet Manager at Stolt-Nielsen Transportation Group, says, "Stolt-Nielsen vessels use Hydrox 550 as a stern-tube lubricant because experience has shown that seawater entry into the stern-tube can occur on occasions. Using Hydrox 550 greatly reduces the likelihood of interrupted sailing schedules due to emergency repairs."G. Foustanos, superintendent engineer at Andros Maritime adds, "Andros Maritime, the tanker fleet agent, is typical of the growing number of Hydrox 550 users who value the longterm benefits provided by the product." Hydrox 550 has been formulated so that it will form stable emulsions with up to 20 percent of seawater, which may enter the stern-tube.

The oil is suitable for use in stern-tube systems fitted with circulatory oil-feed systems and is approved by leading seal manufacturers having been tested for compatibility with their seal materials. It may well be the case that a conventional oil is not compatible with the seal in which case excessive swelling or embrittlement of the seal can occur with consequent seal failure. Hydrox 550 is also used in cruise liner stabilisers, again to combat water ingress into the lubrication system.The oil is approved by a number of stabiliser manufacturers, including Sperry Marine and Brown Brothers.

BEARINGS

DESIGN CHANGES OF MAIN, BOTTOM END AND CROSSHEAD BEARINGS – BEARING METAL - TIN ALUMINIUM THIN SHELL

BEARINGS, JOURNALS/PINS -SURFACE ROUGHNESS,

BEARING DEFECTS-CAVITATION EROSION,

ELECTRICAL EROSION DAMAGE, ELECTROSTATIC

EROSION DAMAGE, FRETTING DAMAGE, WHITE

METAL BEARING CORROSION.

The textbook cases of distress modes are especially useful in diagnosing problems prior to the damage that occurs when a bearing can no longer support an oil film. Through the prudent use of temperature and vibration monitoring equipment, routine oil analysis, lubrication system evaluations and machine operational performance reviews, bearing distress may be identified and evaluated before catastrophic failure occurs.Bearing health is commonly monitored through the use of temperature measurements. Temperature sensors may be mounted in a wide variety of locations, with a corresponding variation in temperature. The specific location and type of sensor must be known for the measured temperature data to have any real value.Being able to properly identify the damage resulting from pitting (from arcing), fatigue, erosion and corrosion is key to diagnosing bearing problems.

VoidsElectrical PittingElectrical pitting appears as rounded pits in the bearing lining. The pits may appear frosted (Figure 1), or they may be blackened due to oil deposits. It is not unusual for them to be small and difficult to observe with the unaided eye. A clearly defined boundary exists between the pitted and unpitted regions, with the pitting usually occurring where the oil film is thinnest.As pitting progresses, the individual pits lose their characteristic appearance as they begin to overlap. Pits located near the boundary should still be intact. The debris that enters the oil begins abrasion damage. Once the bearing surface becomes incapable of supporting an oil film, the bearing will wipe. The bearing may recover an oil film and continue to operate, and pitting will begin again. This process may occur several times before the inevitable catastrophic bearing failure.Electrical pitting damage is caused by intermittent arcing between the stationary and rotating machine components. Because of the small film thicknesses relative to other machine clearances, the arcing commonly occurs through the bearings. Although the rotating and other stationary members can also be affected, the most severe pitting occurs in the soft babbitt.Electrical pitting can be electrostatic or electromagnetic in origin. Although both sources result in pitting damage, they differ in origin and destructive capabilities.Electrostatic shaft current (direct current) is the milder of the two. Damage progresses slowly, and it always occurs at the location with the lowest resistance to ground. It can be attributed to charged lubricant, charged drive belts or impinging particles.

Figure 1. Shaft Currents / Electrical Pitting (Frosting)

This type of shaft current can be eliminated with grounding brushes or straps. Bearing isolation is also recommended.Electromagnetic shaft current (alternating current) is stronger and more severe than electrostatic current. It is produced by the magnetization of rotating and/or stationary components.This type of current will not always occur at the location of lowest resistance. Because the current is stronger, bearing damage is often accompanied by journal, collar or runner damage.Electromagnetic currents are best eliminated by demagnetizing the affected component. Grounding brushes or straps may or may not be helpful. The bearings should also be isolated.The lubricating oil must be filtered or replaced. Pitting damage often blackens the oil and fills it with debris. In addition to filtering or replacing the oil, the entire bearing assembly, oil reservoir and piping should be flushed and cleaned. The original bearing finish should also be restored. Journal shoes typically must be replaced, but if the correction leaves the bearing within design tolerance, the bearing may be reused. The condition of the rotating journal, collar or runner surfaces must also be evaluated. It must be restored to original condition, either by lapping, hand stoning or replacement.

FatigueFatigue damage may represent itself as intergranular or hairline cracks in the babbitt. The cracks may appear to open in the direction of rotation. Pieces of babbitt may spall out or appear to be pulled away in the direction of rotation. The cracks extend toward the babbittbond line, and may reveal the shoe backing (Figures 2, 3, 4).

Figure 2. Edge Load Pivoted Shoe Showing Babbitt Mechanical Fatigue

Figure 3. Edge Load Journal Shell withBabbitt Mechanical Fatigue

Figure 4. Babbitt Fatigue in a Thin Thrust Plate

A combination of causes contributes to fatigue damage, but concentrated cyclic loading is usually involved. The fatigue mechanism involves repeated bending or flexing of the bearing, and damage occurs more rapidly with poor bonding.It is important to note that fatigue damage will occur without poor bonding. Fatigue can occur when conditions produce concentrated cyclic loads, such as:MisalignmentJournal eccentricityImbalanceBent shaftThermal cyclingVibrationPerformance data should be reviewed to determine if a vibration increase occurred. The leveling plate wings should be examined for signs of excessive wear, indicating the rotating collar or runner is not perpendicular to the shaft axis.High bearing temperature may also be considered as a contributing factor to fatigue damage. As temperatures increase, the fatigue strength of bearing materials decreases.The lubricating oil must be filtered or replaced. In addition to filtering or replacing the oil, the entire bearing assembly, oil reservoir and piping should be flushed and cleaned. Depending on the damage, voids in the babbitt can be puddle-repaired. The original bearing finish must be restored. Journal shoes may also be puddle-repaired and refinished. If this cannot be done, the shoes must be replaced.Although the babbitted surface is usually damaged more severely, the rotating collar or journal surface must also be evaluated. This surface must be restored to original condition, either by tapping or hand stoning.

CavitationCavitation damage appears as discreet irregularly shaped babbitt voids which may or may not extend to the bond line. It may also appear as localized babbitt erosion. The location of the damage is important in determining the trouble source (Figures 5, 6, 7).Often called cavitation erosion, cavitation damage is caused by the formation and implosion of vapor bubbles in areas of rapid pressure change. Damage often occurs at the outside diameter of thrust bearings due to the existence of higher velocities. This type of damage can also affect stationary machine components in close proximity to the rotor. Based on its source, cavitation can be eliminated in a number of ways. These include the following:Radius/chamfer sharp stepsModify bearing groovesReduce bearing clearanceReduce bearing arcEliminate flow restrictions (downstream)Increase lubricant flowIncrease oil viscosityLower the bearing temperatureChange oil feed pressureUse harder bearing materials

The lubricating oil must be filtered or replaced. In addition to filtering or replacing the oil, the entire bearing assembly, oil reservoir and piping should be flushed and cleaned.Depending on the extent of damage, voids in the babbitt can be puddle-repaired. The original bearing finish must be restored. Journal shoes may also be puddle-repaired and refinished. If this cannot be done, the shoes must be replaced.Although the babbitted surface is usually damaged more severely, the rotating collar, runner or journal surface must also be evaluated. This surface must also be restored to original condition, either by lapping or hand stoning.

Figure 5. Cavitation Damage on Outside Diameter of Collar

Figure 6. Thrust Shoe CavitationToward Outside Diameter

Figure 7. Thrust Shoe CavitationDamage in Babbitt Face

ErosionErosion damage may appear as localized babbitt voids with smooth edges, particularly in the direction of rotation. Damage is more likely to occur in stationary members.As a rule of thumb, if the babbitt has been affected, the cause was cavitation damage, not erosion. Because erosion is caused by sudden obstructions in oil flow, it is more likely to occur in other areas, because the babbitt is under high pressure. Once damaged, however, babbitt erosion may occur.Corrective action is similar to that employed in eliminating cavitation damage, with the emphasis on streamlining oil flow through the bearing.

CorrosionCorrosion damage is characterized by the widespread removal of the bearing lining by chemical attack. This attack produces a latticework appearance. The damage may be uniform with the affected elements being washed away, leaving the corrosion-resistant elements behind. Corrosion may also affect the rotating collar, runner or journal, appearing as random, widespread rust or pitting. The pits are easily distinguished from electrical pitting, because they are not as uniform or smooth-bottomed.Corrosive materials may appear in the lubricating oil through:Decomposition of oil additivesAcidic oxidation products formed in serviceWater or coolant in lube oilDirect corrosive contaminationBearing housing seals, oil additive packages, and oil reservoir operating temperatures should be evaluated as an initial step in eliminating corrosion. The integrity of cooling coils should also be examined.The cause of corrosion is best detected by knowledge of the babbitt composition and an oil analysis. Corrosion can be eliminated by replacing the lubricating oil. In addition, the entire bearing assembly, oil reservoir and piping should be flushed and cleaned. If the original bearing finish cannot be restored, the bearing must be replaced.The rotating collar, runner or journal surface must also be evaluated and restored to original condition, either by lapping or hand stoning.

Collar/ Runner/ Journal SurfaceThe most commonly overlooked bearing component is the collar. It is the single most important part of the bearing. Collar rotation draws oil into the region between the collar and shoe surfaces. Oil adheres to the collar and is pulled into pressurized oil wedges. This occurs due to the collar surface finish. If the collar finish is too smooth (better than 12 root mean square (RMS)), it will not move an adequate supply of oil; too rough, and the bearing shoes will be damaged. Ideally the finish should be between 12 to 16 RMS.Each time a bearing is inspected, the collar should be inspected and worked as necessary. Glossy areas on the collar can easily be removed by hand-scrubbing with a soft 600-grit oilstone. Collars with significant operating time may have lost their original surface flatness. This flatness, as well as the surface finish, should be restored.If a split runner is used, it should be separated into halves and evaluated. Relative motion between the halves will result in fretting damage to the runner, as well as potential cavitation-like damage to the bearing surfaces.It is important that the collar faces be parallel, and perpendicular to the centerline of the shaft. If the collar is not within tolerance, the resultant wobble will force the shoes and leveling plates to constantly equalize, causing rapid leveling plate wear (Figure 8).

Figure 8. Leveling Plate Wear due to Collar Wobble

Oil AnalysisA quick visual examination of the oil or oil filter may be all that is required to determine that a problem exists and that further investigation is necessary. Cloudy or discolored oil indicates a problem.A thorough oil analysis can provide useful data to assist in diagnosing bearing or machine distress. The usefulness of the analysis is directly related to the information requested. As a minimum, the following should be supplied:Particulate densityParticulate breakdownViscosityWater contaminationChemical breakdownThe amount of particulate, as well as its content, can identify potential trouble spots. Oil viscosity will decrease in time, and whether or not distress is suspected, it should be periodically evaluated. Water contamination is extremely unwanted, because it can cause rust and oil foaming, and if it is drawn into the oil film, bearing failure. A chemical breakdown of the oil will help to determine the integrity of the additive packages and the presence of unwanted contaminants.

Operational DataAnother important source of diagnostic information is unit operational data. Identifying periods of load or speed changes, recent maintenance, or the performance of related machinery may help determine the root cause of distress.Vibration data or an analysis may help uncover existing problems, as well as examine the remaining bearings in a troubled unit.In a perfect world, hydrodynamic bearings theoretically have an infinite life. Equipment operators know that their world is far from perfect. By taking a forensic approach to plain bearing failures, the operator can uncover and correct system-related problems and ultimately increase machine availability and output.

ALTERNATE DESIGN OF STAY BOLT

FOR MAIN BEARING OF MAN B & W.

To understand the importance of the role played by the tie bolts or tie rods, it is necessary to appreciate what is happening inside the cylinder of the engine.When the piston is just after top dead centre the pressure inside the cylinder can rise as high as 140 bar (14000kN/m2). This acts downwards through the piston rod and con-rod, pushing the crankshaft down into the bearing pockets. At the same time, the pressure acts upwards, trying to lift the cylinder cover. The cylinder head studs screwed into the entablature prevent this happening and so this upward acting force tries to lift the entablature from the frames and the frames from the bedplate, putting the fitted location bolts into tension.As the piston moves down the cylinder the pressure in the cylinder falls, and then rises again as the piston changes direction and moves upwards on the compression stroke. This means that the fitted bolts are under are cyclic stress. Because they are not designed to withstand such stresses they would soon fail with disastrous consequences.To hold the bedplate , frames and entablature firmly together in compression, and to transmit the firing forces back to the bedplate, long tie bolts are fitted through these three components and then tightened hydraulically. To prevent excessive bending moments in the transverse girders, the tie bolts are positioned as close to the centre of the crankshaft as possible. Because the tie bolts are so close to the crankshaft, some engines employ jack bolts to hold the crankshaft main bearing cap in position instead of conventional studs and nuts.

Operating the engine with loose tiebolts will cause the fitted bolts holding the bedplate, frame and entablature in alignment to stretch and break. The machined mating surfaces will rub together, corrode and wear away (this is known as fretting). Once this has happened the alignment of the engine running gear will be destroyed. Loose tie bolts will also cause the transverse girders to bend which could lead to cracking, and main bearing misalignment.

Once fretting between the mating surfaces has occurred, then tightening of the tie bolts will pull the engine out of alignment. The crosshead guides, the cylinder liner, and the stuffing box will no longer be in line and excessive wear will occur. Because the tie bolts will no longer be pulled down squarely they will be subject to forces which may lead to them breaking. If fretting has occurred, then the only solution is to remove the entablature or/and frame and machine the fretted mating surfaces (a very costly exercise).

Tie bolts can break in service. To reduce the risk of this happening they must be checked for tightness; not overtightened; and the engine not overloaded. If a breakage does occur, this is not disastrous, as the engine can be operated with care for a limited period (the load on the engine may have to be reduced). The position of the fracture will dictate how the broken pieces are removed. However in the worst possible scenario where the bolt is broken at mid length, then one solution is to lift out the top half, remove the bottom nut, and then feed a loop of braided wire cable (about 7mm diameter) down the tie bolt tube, down the side of the broken tie bolt and once it emerges at the bottom a supporting piece can be fitted to the wire enabling the broken tie bolt to be withdrawn. Another method is shown here

On the MAN B&W MC-C engine the tie bolts do not pass through the bedplate transverse girder in the traditional way. Instead there are two pairs of tie bolts fitted either side of the single plate A frame and screwed into the bedplate transverse girder. This, it is claimed, reduces the distortion of the bedplate during engine operationWhen checking the tightness of tie bolts, refer to manufacturers instructions for tightening pressures for the jacks and the order in which to carry out the check. The normal order is to start at the centre and work outwards checking the bolts in pairs. The MC -C engine with its twin tie bolts is an exception, starting at the fwd end and working aft. If the engine is fitted with bearing jacking bolts, then these must be slackened before tightening the tie bolts. Any pinch bolts fitted must also be slackened off