8 lubrication

8/9/2019 8 Lubrication

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Lubrication

Lubrication Fundamentals

Friction

Definition of friction

Friction is a force that resists relative motion between two surfaces incontact. Depending on the application, friction may be desirable orundesirable. Certain applications, such as tire traction on pavement and

braking, or when feet are firmly planted to move a heavy object, rely onthe beneficial effects of friction for their effectiveness. In other

applications, such as operation of engines or equipment with bearings andgears, friction is undesirable because it causes wear and generates heat,which frequently leads to premature failure.Wear

Wear is defined as the progressive damage resulting in material lossdue to relative contact between adjacent working parts. Althoughsome wear is to be expected during normal operation of equipment,excessive friction causes premature wear, and this creates significant

economic costs due to equipment failure, cost for replacement parts,and downtime. Friction and wear also generate heat, whichrepresents wasted energy that is not recoverable. n other words,wear is also responsible for overall loss in system efficiency.

!he primary objective of lubrication is to reduce friction and wear ofsliding surfaces. !his objective is achieved by introducing a materialwith a low shear strength or coefficient of friction between the wearingsurfaces. Although nature provides such materials in the form of oxidesand other contaminants, the reduction in friction due to their presence is

insufficient for machinery operation.Hydrodynamic or Fluid Film Lubrication

n heavily loaded bearings such as thrust bearings and hori"ontal journal bearings, the fluid#s viscosity alone is not sufficient tomaintain a film between the moving surfaces. n these bearingshigher fluid pressures are required to support the load until thefluid film is established. f this pressure is supplied by an outsidesource, it is called hydrostatic lubrication. f the pressure isgenerated internally, that is, within the bearing by dynamic action, itis referred to as hydrodynamic lubrication. n hydrodynamic


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lubrication, a fluid wedge is formed by the relative surface motionof the journals or the thrust runners over their respective bearingsurfaces. !he guide bearings of a vertical hydroelectric generator,if properly aligned, have little or no loading and will tend to operate

in the center of the bearing because of the viscosity of the oil.

n hydrodynamic lubrication, sometimes referred to as fluidfilm lubrication, the wearing surfaces are completelyseparated by a film of oil. !his type of lubricating action issimilar to a speedboat operating on water. When the boat isnot moving, it rests on the supporting water surface. As theboat begins to move, it meets a certain amount of resistanceor opposing force due to viscosity of the water. !his causes

the leading edge of the boat to lift slightly and allows a smallamount of water to come between it and supporting watersurface. As the boat$s velocity increases, the wedge%shapedwater film increases in thickness until a constant velocity isattained. When the velocity is constant, water entering underthe leading edge equals the amount passing outward fromthe trailing edge. For the boat to remain above thesupporting surface there must be an upward pressure thatequals the load.

!he same principle can be applied to a sliding surface. Fluidfilm lubrication reduces friction between moving surfaces bysubstituting fluid friction for mechanical friction. !o visuali"ethe shearing effect taking place in the fluid film, imagine thefilm is composed of many layers similar to a deck of cards.

!he fluid layer in contact with the moving surface clings to thatsurface and both move at the same velocity. &imilarly, the fluid layerin contact with the other surface is stationary. !he layers in between

move at velocities directly proportional to their distance from themoving surface. For example, at a distance of ' h from &urface (,the velocity would be ' ). !he force F required to move &urface (across

&urface * is simply the force required to overcome the frictionbetween the layers of fluid. !his internal friction, or resistance to flow,is defined as the viscosity of the fluid. )iscosity will be discussed inmore detail later.

!he principle of hydrodynamic lubrication can also be applied to amore practical example related to thrust bearings used in the

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hydropower industry. !hrust bearing assembly is also known as tiltingpad bearings. !hese bearings are designed to allow the pads to liftand tilt properly and provide sufficient area to lift the load of thegenerator. As the thrust runner moves over the thrust shoe, fluid

adhering to the runner is drawn between the runner and the shoecausing the shoe to pivot, and forming a wedge of oil. As the speed of the runner increases, the pressure of the oil wedge increases and therunner is lifted as full fluid film lubrication takes place. n applicationswhere the loads are very high, some thrust bearings have highpressure%pumps to provide the initial oil film. +nce the unit reaches( percent speed, the pump is switched off.

Although not as obvious as the plate or thrust bearing examplesabove, the operation of journal or sleeve bearings is also anexample of hydrodynamic lubrication. When the journal is at rest,the weight of the journal squee"es out the oil film so that the

journal rests on the bearing surface. As rotation starts, the journal has a tendency to roll up the side of thebearing. At the same time fluid adhering to the journal is drawn intothe contact area. As the journal speed increases an oil wedge isformed. !he pressure of the oil wedge increases until the journal islifted off the bearing. !he journal is not only lifted vertically, but is alsopushed to the side by the pressure of the oil wedge. !he minimumfluid film thickness at full speed will occur at a point just to the left ofcenter and not at the bottom of the bearing. n both the pivoting shoethrust bearing and the hori"ontal journal bearing, the minimumthickness of the fluid film increases with an increase in fluid viscosityand surface speed and decreases with an increase in load.

Film thickness

!he preceding discussion is a very simplified attempt to provide abasic description of the principles involved in hydrodynamiclubrication. For a more precise, rigorous interpretation refer to

American &ociety for -etals&implified equations have been developed to provide approximationsof film thickness with a considerable degree of precision. egardlessof how film thickness is calculated, it is a function of viscosity, velocity,and load. As viscosity or velocity increases, the film thicknessincreases. When these two variables decrease, the film thicknessalso decreases. Film thickness varies inversely with the load/ as theload increases, film thickness decreases. )iscosity, velocity, andoperating temperature are also interrelated. f the oil viscosity is

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increased the operating temperature will increase, and this in turn hasa tendency to reduce viscosity. !hus, an increase in viscosity tends toneutrali"e itself somewhat. )elocity increases also cause temperatureincreases that subsequently result in viscosity reduction.

Factors influencing film formation

!he following factors are essential to achieve and maintain thefluid film required for hydrodynamic lubrication0

!he contact surfaces must meet at a slight angle to allowformation of the lubricant wedge.

!he fluid viscosity must be high enough to support the load andmaintain adequate film thickness to separate the contacting

surfaces at operating speeds. !he fluid must adhere to the contact surfaces for conveyance

into the pressure area to support the load. !he fluid must distribute itself completely within the bearing

clearance area. !he operating speed must be sufficient to allow formation

and maintenance of the fluid film. !he contact surfaces of bearings and journals must be smooth

and free of sharp surfaces that will disrupt the fluid film.!heoretically, hydrodynamic lubrication reduces wear to "ero. nreality, the journal tends to move vertically and hori"ontally due toload changes or other disturbances and some wear does occur.1owever, hydrodynamic lubrication reduces sliding friction andwear to acceptable levels.

Boundary Lubrication

Definition of boundary lubrication

When a complete fluid film does not develop between potentiallyrubbing surfaces, the film thickness may be reduced to permitmomentary dry contact between wear surface high points orasperities. !his condition is characteristic of boundary lubrication.2oundary lubrication occurs whenever any of the essential factorsthat influence formation of a full fluid film are missing. !he mostcommon example of boundary lubrication includes bearings, whichnormally operate with fluid film lubrication but experience boundary

lubricating conditions during routine starting and stopping of

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equipment. +ther examples include gear tooth contacts andreciprocating equipment.

Oiliness

3(4 5ubricants required to operate under boundary lubricationconditions must possess an added quality referred to as 6oiliness7or 6lubricity7 to lower the coefficient of friction of the oil between therubbing surfaces. +iliness is an oil enhancement property providedthrough the use of chemical additives known as antiwear 3AW4agents. AW agents have a polari"ing property that enables them tobehave in a manner similar to a magnet. 5ike a magnet, theopposite sides of the oil film have different polarities.When an AW oil adheres to the metal wear surfaces, the sides of theoil film not in contact with the metal surface have identical polaritiesand tend to repel each other and form a plane of slippage. -ost oilsintended for use in heavier machine applications contain AW agents.

3*4 8xamples of equipment that rely exclusively onboundary lubrication include reciprocating equipment suchas engine and compressor pistons, and slow%movingequipment such as turbine wicket gates. 9ear teeth also relyon boundary lubrication to a great extent.

Extreme Pressure (EP Lubrication

Definition

AW agents are effective only up to a maximum temperature ofabout *: 8; 3nusually heavy loading will cause the oil temperature to increasebeyond the effective range of the antiwear protection. When the loadlimit is exceeded, the pressure becomes too great and asperities

make contact with greater force. nstead of sliding, asperities alongthe wear surfaces experience shearing, removing the lubricant andthe oxide coating. >nder these conditions the coefficient of friction isgreatly increased and the temperature rises to a damaging level.

Extreme !ressure additi"es

Applications under extreme pressure conditions rely on additives.5ubricants containing additives that protect against extreme pressureare called 8? lubricants, and oils containing additives to protectagainst extreme pressure are classified as 8? oils. 8? lubrication is

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provided by a number of chemical compounds. !he most commonare compounds of boron, phosphorus, sulfur, chlorine, orcombinations of these. !he compounds are activated by the highertemperature resulting from extreme pressure, not by the pressure

itself. As the temperature rises, 8? molecules become reactive andrelease derivatives of phosphorus, chlorine, or sulfur 3depending onwhich compound is used4 to react with only the exposed metalsurfaces to form a new compound such as iron chloride or ironsulfide.

!he new compound forms a solid protective coating that fills theasperities on the exposed metal. !hus, the protection is deposited atexactly the sites where it is needed. AW agents in the 8? oil continueto provide antiwear protection at sites where wear and temperatureare not high enough to activate the 8? agents.

Elasto#hydrodynamic (EHD Lubrication

Definition of EHD lubrication

!he lubrication principles applied to rolling bodies, such as ball orroller bearings, is known as elasto%hydrodynamic 381@4lubrication.

$olling body lubrication

Although lubrication of rolling objects operates on a considerablydifferent principle than sliding objects, the principles ofhydrodynamic lubrication can be applied, within limits, to explainlubrication of rolling elements. An oil wedge, similar to that whichoccurs in hydrodynamic lubrication, exists at the lower leadingedge of the bearing. Adhesion of oil to the sliding element and thesupporting surface increases pressure and creates a film betweenthe two bodies. 2ecause the area of contact is extremely small in aroller and ball bearing, the force per unit area, or load pressure, isextremely high. oller bearing load pressures may reach


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3(4 !he roughness of the wearing surfaces is an importantconsideration in 81@ lubrication. oughness is defined as thearithmetic average of the distance between the high and low pointsof a surface, and is sometimes called the centerline average

3;5A4.3*4 As film thickness increases in relation to roughnessfewer asperities make contact. 8ngineers use the ratio of filmthickness to surface roughness to estimate the lifeexpectancy of a bearing system. !he relation of bearing lifeto this ratio is very complex and not always predictable. ngeneral, life expectancy is extended as the ratio increases.Full film thickness is considered to exist when the value ofthis ratio is between * and


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applied to many other fluids that do not specifically perform thisfunction. 8xamples include power and heat transmission fluids,hydraulic fluids, dielectric fluids, process oils, and the others.

ncidentally, in this book the term 6lubricant7 pertains to a finished

lubricant, that is, it comprises base fluid and additives.

A lubricant performs many diverse functions, which help protectand prolong the life of the equipment. !hese include the following0

&' Lubrication (reduce friction and ear

5ubricant helps reduce friction and wear by introducing a

lubricating film between mechanical moving parts, such as gearsand bearings. 8ssentially the presence of a lubricating filmminimi"es the metal%to%metal contact and reduces the forcenecessary to move one surface against the other, thereby reducingwear and saving energy.

)' *ooling (heat transfer

5ubricant acts as a heat sink and dissipates the heat away fromthe critical moving parts of the equipment, thereby decreasing thepossibility of the machine component deformation and wear. !heheat is either frictional heat that results from the metal surfacesrubbing against one another, such as in gears, or is conducted andradiated heat, which is due to the close proximity of the parts to acombustion source, such as the combustion chamber in anautomobile engine.

+' *leaning and ,us!ending

5ubricant facilitates smooth operation of the equipment byremoving and suspending potentially harmful products, such ascarbon, sludge, and varnish, and the other materials, such as dirtand wear debris. !his lubricant function is important in operationsthat involve high operating temperatures, as in the case of aninternal combustion engine or a transmission. !his is because inthese applications the lubricant gets oxidi"ed to form depositprecursors that can separate on hot surfaces and get convertedinto deposits.

-' Protection

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5ubricant prevents metal damage due to oxidation products,corrosion, and wear. t achieves this by forming a physical film onmetal surfaces that is impervious to oxygen, water, and acids, orby forming physical and chemical films by additives, such as rust

and corrosion inhibitors, extreme%pressure 38?4 additives, andanti%wear agents, that are present in the lubricant.

.' %ransfer Po er

5ubricant is used as a power transfer medium in someapplications, for example, in hydraulic systems. !he lubricantperforms this function in addition to its normal function oflubrication. 8xamples of equipment that use hydraulics technology

include transmissions, circulating systems, lifts used in automotiveservice stations, log splitters, fork lifts, dump trucks, andunderground continuous mining equipment such as drills, loaders,and miners.

%he main !ro!erties of the lubricants/iscosity

!echnically, the viscosity of an oil is a measure of the oil$sresistance to shear.)iscosity is more commonly known as resistance to flow. flubricating oil is considered as a series of fluid layerssuperimposed on each other, the viscosity of the oil is a measureof the resistance to flow between the individual layers. A highviscosity implies a high resistance to flow while a low viscosityindicates a low resistance to flow. )iscosity varies inversely withtemperature. )iscosity is also affected by pressure/ higherpressure causes the viscosity to increase, and subsequently theload%carrying capacity of the oil also increases. !his propertyenables use of thin oils to lubricate heavy machinery. !he loadcarrying capacity also increases as operating speed of thelubricated machinery is increased. !wo methods for measuringviscosity are commonly employed0 shear and time.

(& ,hear'

When viscosity is determined by directly measuring shear stressand shear rate, it is expressed in centipoise 3c?4 and is referred toas the absolute or dynamic viscosity. n the oil industry, it is morecommon to use kinematic viscosity, which is the absolute viscosity

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divided by the density of the oil being tested. Einematic viscosity isexpressed in centistokes 3c&t4. )iscosity in centistokes isconventionally given at two standard temperatures0 < ; and (; 3( < F and *(* F 4.

() %ime

Another method used to determine oil viscosity measures the timerequired for an oil sample to flow through a standard orifice at astandard temperature. )iscosity is then expressed in &>& 3&aybolt>niversal &econds4. &>& viscosities are also conventionally givenat two standard temperatures0 8; and D= 8; 3( 8F and *(8F4. As previously noted, the units of viscosity can be expressed

as centipoises 3c?4, centistokes 3c&!4, or &aybolt >niversal&econds 3&>&4, depending on the actual test method used tomeasure the viscosity.

/iscosity index

!he viscosity index, commonly designated ) , is an arbitrarynumbering scale that indicates the changes in oil viscosity withchanges in temperature. )iscosity index can be classified asfollows0 low ) % below :/ medium ) % : to = / high ) % = to(( / very high ) % above (( . A high viscosity index indicatessmall oil viscosity changes with temperature. A low viscosity indexindicates high viscosity changes with temperature. !herefore, afluid that has a high viscosity index can be expected to undergovery little change in viscosity with temperature extremes and isconsidered to have a stable viscosity. A fluid with a low viscosityindex can be expected to undergo a significant change in viscosityas the temperature fluctuates. For a given temperature range, say%(= to 8; 3 % ( 8F4, the viscosity of one oil may changeconsiderably more than another. An oil with a ) of D: to ( wouldchange less than one with a ) of = . Enowing the viscosity indexof an oil is crucial when selecting a lubricant for an application, andis especially critical in extremely hot or cold climates. Failure touse an oil with the proper viscosity index when temperatureextremes are expected may result in poor lubrication andequipment failure. !ypically, paraffinic oils are rated at = ; 3 (F4 and naphthenic oils are rated at %(= 8; 3 F4. ?roper selectionof petroleum stocks and additives can produce oils with a verygood ).

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Pour !oint

!he pour point is the lowest temperature at which an oil will flow.!his property is crucial for oils that must flow at low temperatures.

A commonly used rule of thumb when selecting oils is to ensurethat the pour point is at least ( 8; 3* 8F4 lower than the lowestanticipated ambient temperature.

*loud !oint

!he cloud point is the temperature at which dissolved solids in theoil, such as paraffin wax, begin to form and separate from the oil.

As the temperature drops, wax crystalli"es and becomes visible.

;ertain oils must be maintained at temperatures above the cloudpoint to prevent clogging of filters.

Flash !oint and fire !oint

!he flash point is the lowest temperature to which a lubricant mustbe heated before its vapor, when mixed with air, will ignite but notcontinue to burn. !he fire point is the temperature at whichlubricant combustion will be sustained. !he flash and fire points

are useful in determining a lubricant$s volatility and fire resistance.!he flash point can be used to determine the transportation andstorage temperature requirements for lubricants. 5ubricantproducers can also use the flash point to detect potential productcontamination. A lubricant exhibiting a flash point significantlylower than normal will be suspected of contamination with avolatile product. ?roducts with a flash point less than = 8; 3(8F4 will usually require special precautions for safe handling. !hefire point for a lubricant is usually = to ( percent above the flashpoint. !he flash point and fire point should not be confused with theauto%ignition temperature of a lubricant, which is the temperatureat which a lubricant will ignite spontaneously without an externalignition source.


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%able & ,ynthetic Oils

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%able & (continued ,ynthetic Oils

0cid number or neutrali1ation number

!he acid or neutrali"ation number is a measure of the amount ofpotassium hydroxide required to neutrali"e the acid contained in alubricant. Acids are formed as oils oxidi"e with age and service.

!he acid number for an oil sample is indicative of the age of the oiland can be used to determine when the oil must be changed.

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Oxidation stability

+ne of the most important properties of new turbine oil is itsoxidation stability. Gew turbine oils are highly stable in the

presence of air or oxygen. n service, oxidation is graduallyaccelerated by the presence of a metal catalyst in the system3such as iron and copper4 and by the depletion of antioxidantadditives. Additives control oxidation by attacking thehydroperoxides 3the first product of the oxidation step4 andbreaking the chain reaction that follows. When oxidation stabilitydecreases, the oil will undergo a complex reaction that willeventually produce insoluble sludge. !his sludge may settle incritical areas of the equipment and interfere with lubrication and

cooling functions of oil. -ost rust inhibitors used in turbine oils areacidic and contribute to the acid number of the new oil. An increase in acid number above the value for new oil indicatesthe presence of acidic oxidation products or, less likely,contamination with acidic substances. An accurate determinationof the total acid number 3!AG4 is very important. 1owever, this testdoes not strictly measure oxidation stability reserve, which is better determined by the otating 2omb +xidation !est 3 2+!4, A&!-!est -ethod @ ** *.

Freedom from sludge

&ludge is the byproduct of oil oxidation. @ue to the nature of thehighly refined lubricant base stocks used in the manufacture ofturbine oils, these oils are very poor solvents for sludge. !his is themain reason why the oxidation stability reserve of the oil must becarefully monitored.

+nly a relatively small degree of oxidation can be permitted/otherwise, there is considerable risk of sludge deposition inbearing housings, seals, and pistons. Filtration and centrifugationcan remove sludge from oil as it is formed, but if oil deterioration isallowed to proceed too far, sludge will deposit in parts of theequipment, and system flushing and an oil change may berequired.Freedom from abrasi"e contaminants

!he most deleterious solid contaminants found in turbine oilsystems are those left behind when the system is constructed and

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installed or when it is opened for maintenance and repair. &olidcontaminants may also enter the system when units are outdoors,through improperly installed vents, and when units are opened formaintenance. +ther means of contamination are from the wearing

of metals originating within the system, rust and corrosionproducts, and dirty make%up oil. !he presence of abrasive solids inthe oil cannot be tolerated since they will cause serious damage tothe system. !hese particles must be prevented from entering thesystem by flushing the system properly and using clean oil andtight seals. +nce abrasive solids have been detected, they mustbe removed by filtration or centrifugation, or both.

*orrosion !rotection

!he corrosion protection provided by the lubricant is of significantimportance for turbine systems. Gew turbine oil contains a rust%inhibitor additive and must meet A&!-!est -ethod @ CC:. !he additive may be depleted by normalusage, removal with water in the oil, absorption on wear particlesand debris, or chemical reaction with contaminants.

2$E0,E, 03D 2$E0,E L4B$5*0%5O3

9rease is a semi%fluid to solid mixture of a fluid lubricant, a thickener,and additives. !he fluid lubricant that performs the actual lubricationcan be petroleum 3mineral4 oil, synthetic oil, or vegetable oil. !hethickener gives grease its characteristic consistency and issometimes thought of as a 6three%dimensional fibrous network7 or6sponge7 that holds the oil in place. ;ommon thickeners are soapsand organic or inorganic non%soap thickeners. !he majority ofgreases on the market are composed of mineral oil blended with asoap thickener. Additives enhance performance and protect thegrease and lubricated surfaces.

9rease has been described as a temperature%regulated feedingdevice0 when the lubricant film between wearing surfaces thins, theresulting heat softens the adjacent grease, which expands andreleases oil to restore film thickness.

Function6!he function of grease is to remain in contact with and lubricate

moving surfaces without leaking out under gravity or centrifugalaction, or be squee"ed out under pressure. ts major practical

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requirement is that it retain its properties under shear at alltemperatures that it is subjected to during use. At the same time,grease must be able to flow into the bearing through grease guns andfrom spot to spot in the lubricated machinery as needed, but must not

add significantly to the power required to operate the machine,particularly at startup.7

0!!lications suitable for grease

9rease and oil are not interchangeable. 9rease is used when it isnot practical or convenient to use oil. !he lubricant choice for aspecific application is determined by matching the machinery

design and operating conditions with desired lubricantcharacteristics. 9rease is generally used for0

3(4 -achinery that runs intermittently or is in storage for anextended period of time. 2ecause grease remains in place, alubricating film can instantly form.

3*4 -achinery that is not easily accessible for frequentlubrication. 1igh%quality greases can lubricate isolated or relativelyinaccessible components for extended periods of time without

frequent replenishing.!hese greases are also used in sealed%for%life applications such assome electrical motors and gearboxes.

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3 4 -achinery operating under extreme conditions such as hightemperatures and pressures, shock loads, or slow speed underheavy load. >nder these circumstances, grease provides thickerfilm cushions that are required to protect and adequately lubricate,

whereas oil films can be too thin and can rupture.3


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At start%up, grease has a resistance to motion, implying a high viscosity.1owever, as grease is sheared between wearing surfaces and movesfaster, its resistance to flow reduces.

ts viscosity decreases as the rate of shear increases. 2y contrast, anoil at constant temperature would have the same viscosity at start%upas it has when it is moving. !o distinguish between the viscosity of oiland grease, the viscosity of grease is referred to as 6apparentviscosity.7 Apparent viscosity is the viscosity of a grease that holdsonly for the shear rate and temperature at which the viscosity isdetermined.

Bleeding6 migration6 syneresis

2leeding is a condition when the liquid lubricant separates from thethickener. t is induced by high temperatures and also occurs duringlong storage periods. -igration is a form of bleeding that occurs whenoil in a grease migrates out of the thickener network under certaincircumstances. For example, when grease is pumped though a pipe in acentrali"ed lubrication system, it may encounter a resistance to the flowand form a plug. !he oil continues to flow, migrating out of the thickenernetwork. As the oil separates from the grease, thickener concentrationincreases, and plugging gets worse. f two different greases are incontact, the oils may migrate from one grease to the other and change

the structure of the grease. !herefore, it is unwise to mix two greases.&yneresis is a special form of bleeding caused by shrinking orrearrangement of the structure due to physical or chemical changes inthe thickener.

*onsistency6 !enetration6 and 3ational Lubricating 2rease5nstitute (3L25 numbers

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!he most important feature of a grease is its rigidity or consistency. Agrease that is too stiff may not feed into areas requiring lubrication,while a grease that is too fluid may leak out. 9rease consistencydepends on the type and amount of thickener used and the viscosity ofits base oil. A grease$s consistency is its resistance to deformation by anapplied force. !he measure of consistency is called penetration.?enetration depends on whether the consistency has been altered byhandling or working. A&!- @ *( and @ (< methods measurepenetration of unworked and worked greases. !o measure penetration,a cone of given weight is allowed to sink into a grease for : seconds ata standard temperature of *: ; 3 F4. !he depth, in tenths of amillimeter, to which the cone sinks into the grease is the penetration. Apenetration of ( would represent a solid grease while one of


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*ontaminants

9reases tend to hold solid contaminants on their outer surfacesand protect lubricated surfaces from wear. f the contaminationbecomes excessive or eventually works its way down to thelubricated surfaces the reverse occursJthe grease retainsabrasive materials at the lubricated surface and wear occurs.

*orrosion# and rust#resistance

!his denotes the ability of grease to protect metal parts fromchemical attack. !he natural resistance of a grease depends uponthe thickener type. ;orrosion%resistance can be enhanced bycorrosion and rust inhibitors.

Dro!!ing !oint

@ropping point is an indicator of the heat resistance of grease. Asgrease temperature rises, penetration increases until the greaseliquefies and the desired consistency is lost .

@ropping point is the temperature at which a grease becomes fluidenough to drip. !he dropping point indicates the upper temperature

limit at which a grease retains its structure, not the maximumtemperature at which a grease may be used. A few greases have theability to regain their original structure after cooling down from thedropping point.

E"a!oration

!he mineral oil in a grease evaporates at temperatures above ( ;3 : F4. 8xcessive oil evaporation causes grease to harden due toincreased thickener concentration. !herefore, higher evaporation rates

require more frequent relubrication.Fretting ear and false brinelling

Fretting is friction wear of components at contact points caused byminute oscillation. !he oscillation is so minute that grease is displacedfrom between parts but is not allowed to flow back in. 5ocali"edoxidation of wear particles results and wear accelerates. n bearings,this locali"ed wear appears as a depression in the race caused byoscillation of the ball or roller.

!he depression resembles that which occurs during 2rinell hardnessdetermination, hence the term 6false brinelling.7 An example would be

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fretting wear of automotive wheel bearings when a car is transportedby train. !he car is secured, but the vibration of the train over thetracks causes minute oscillation resulting in false brinelling of thebearing race.

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Oxidation stability

!his is the ability of a grease to resist a chemical union with oxygen.!he reaction of grease with oxygen produces insoluble gum, sludges,and lacquer%like deposits that cause sluggish operation, increased wear,and reduction of clearances. ?rolonged high%temperature exposureaccelerates oxidation in greases.

Pum!ability and slum!ability

?umpability is the ability of a grease to be pumped or pushed through asystem. -ore practically, pumpability is the ease with which apressuri"ed grease can flow through lines, no""les, and fittings ofgrease%dispensing systems. &lumpability, or feedability, is its ability tobe drawn into 3sucked into4 a pump. Fibrous greases tend to have good

feedability but poor pumpability. 2uttery%textured greases tend to havegood pumpability but poor feedability.

,hear stability

9rease consistency may change as it is mechanically worked orsheared between wearing surfaces. A grease$s ability to maintain itsconsistency when worked is its shear stability or mechanical stability. Agrease that softens as it is worked is called thixotropic. 9reases thatharden when worked are called rheopectic.

High#tem!erature effects

1igh temperatures harm greases more than they harm oils. 9rease, byits nature, cannot dissipate heat by convection like a circulating oil.;onsequently, without the ability to transfer away heat, excessivetemperatures result in accelerated oxidation or even carboni"ationwhere grease hardens or forms a crust. 8ffective grease lubricationdepends on the grease#s consistency. 1igh temperatures inducesoftening and bleeding, causing grease to flow away from neededareas. !he mineral oil in grease can flash, burn, or evaporate attemperatures above ( ; 3 : F4. 1igh temperatures, above % D ;3(C:%( : F4, can dehydrate certain greases such as calcium soapgrease and cause structural breakdown. !he higher evaporation anddehydration rates at elevated temperatures require more frequentgrease replacement.

Lo #tem!erature effects

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f the temperature of a grease is lowered enough, it will become soviscous that it can be classified as a hard grease. ?umpability suffersand machinery operation may become impossible due to torquelimitations and power requirements. !he temperature at which thisoccurs depends on the shape of the lubricated part and the power beingsupplied to it. As a guideline, the base oil$s pour point is considered thelow%temperature limit of a grease.

%exture

!exture is observed when a small sample of grease is pressed betweenthumb and index finger and slowly drawn apart. !exture can bedescribed as0

2rittle0 the grease ruptures or crumbles when compressed.

2uttery0 the grease separates in short peaks with no visiblefibers. 5ong fiber0 the grease stretches or strings out into a single

bundle of fibers. esilient0 the grease can withstand moderate compression

without permanent deformation or rupture. &hort fiber0 the grease shows short break%off with evidence of

fibers.

&tringy0 the grease stretches or strings out into long, finethreads, but with no visible evidence of fiber structure.

Water resistance

!his is the ability of grease to withstand the effects of water with nochange in its ability to lubricate. &oapBwater lather may suspend the oilin the grease, forming an emulsion that can wash away or, to a lesserextent, reduce lubricity by diluting and changing grease consistency andtexture.

usting becomes a concern if water is allowed to contact iron or steelcomponents.

%y!es of lubricants and their a!!lications

%y!es of Oil

Oils are generally classified as refined and synthetic'

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?araffinic and naphthenic oils are refined from crude oil whilesynthetic oils are manufactured. 5iterature on lubrication frequentlymakes references to long chain molecules and ring structures inconnection with paraffinic and naphthenic oils, respectively. !hese

terms refer to the arrangement of hydrogen and carbon atoms thatmake up the molecular structure of the oils. @iscussion of thechemical structure of oils is beyond the scope of this manual, butthe distinguishing characteristics between these oils are notedbelow.

Paraffinic oils

?araffinic oils are distinguished by a molecular structure composed

of long chains of hydrocarbons, i.e., the hydrogen and carbonatoms are linked in a long linear series similar to a chain. ?araffinicoils contain paraffin wax and are the most widely used base stockfor lubricating oils. n comparison with naphthenic oils, paraffinicoils have0

8xcellent stability 3higher resistance to oxidation4. 1igher pour point. 1igher viscosity index.

5ow volatility and, consequently, high flash points. 5ow specific gravities.

3a!hthenic oils

n contrast to paraffinic oils, naphthenic oils are distinguished by amolecular structure composed of 6rings7 of hydrocarbons, i.e., thehydrogen and carbon atoms are linked in a circular pattern. !heseoils do not contain wax and behave differently than paraffinic oils.

Gaphthenic oils have0 9ood stability. 5ower pour point due to absence of wax. 5ower viscosity indexes. 1igher volatility 3lower flash point4. 1igher specific gravities.

Gaphthenic oils are generally reserved for applications with narrowtemperature ranges and where a low pour point is required.

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,ynthetic oils3(4 &ynthetic lubricants are produced from chemical synthesisrather than from the refinement of existing petroleum or vegetableoils. !hese oils are generally superior to petroleum 3mineral4

lubricants in most circumstances. &ynthetic oils perform betterthan mineral oils in the following respects0

2etter oxidation stability or resistance. 2etter viscosity index. -uch lower pour point, as low as %


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3a4 &ynthesi"ed hydrocarbons

?olyalphaolefins and dialkylated ben"enes are the most commontypes.

!hese lubricants provide performance characteristics closest tomineral oils and are compatible with them. Applications include engine and turbine oils, hydraulic fluids, gearand bearing oils, and compressor oils.

3b4 +rganic esters

@iabasic acid and polyol esters are the most common types. !heproperties of these oils are easily enhanced through additives.

Applications include crankcase oils and compressor lubricants.3c4 ?hosphate esters

!hese oils are suited for fire%resistance applications.

3d4 ?olyglycols

Applications include gears, bearings, and compressors forhydrocarbon gases.

3e4 &ilicones

!hese oils are chemically inert, nontoxic, fire%resistant, and waterrepellant. !hey also have low pour points and volatility, good low%temperature fluidity, and good oxidation and thermal stability athigh temperatures. !able %( identifies several synthetic oils andtheir properties.

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Lubrication grease classification

9rease is classified by penetration number and by type of soap orother thickener. ?enetration classifications have been establishedby G59

A penetration number indicates how easily a grease can be fed tolubricated surfaces 3i.e., pumpability4 or how well it remains inplace. Although no method exists to classify soap thickeners, the

producer indicates which soap is in the product. !he type of soapthickener indicates probable water resistance and maximum

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operating temperature and gives some indication of pumpability. Although these are important factors, they are not the only ones ofinterest.

!hese simple classifications should be regarded as startingrequirements to identify a group of appropriate grease types. !hefinal selection must be made on the basis of other informationprovided in the producer#s specifications. )iscosity of the oilincluded in a grease must also be considered.

?roducers also provide information and specifications for grease inbrochures, pamphlets, handbooks, or on the product container orpackaging. 9rease specifications normally include soap thickener,

penetration, included oil viscosity, and dropping point. !heproducer may also include A&!- test information on wear, loading,lubrication life, water washout, corrosion, oil separation, andleakage. 9rease additives are not usually stated except for solidadditives such as molybdenum disulfide or graphite, or that an 8?additive is included. f 8? or solid additives are used, the producerwill often state this emphatically and the product name mayindicate the additive.

!he grades most common in use in engine rooms are ball androller bearing grease and extreme pressure grease.

Ball and $oller Bearing 2rease

2all and roller bearing grease is for general use in equipmentdesignated to operate at temperatures up to KF. Fortemperature applications above KF, high%temperature, electric%motor, ball and roller bearing grease must be used.

Extreme Pressure 2rease

8xtreme pressure grease has antirust properties and is suitable for lubrication of semi%enclosed gears, or any sliding or rolling metalsurfaces where the load may be high and where the equipmentmay be exposed to salt spray or moisture. t is intended for use intemperature ranges within K to (< KF.

2ra!hite 2rease

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9raphite grease may be applied with compression grease cups tobearings operating at temperatures that do not exceed (: KF. !hethree grades of graphite grease are as follows0

9rade ( &oft For light pressures and high speeds9rade * -edium For medium pressures and medium speeds

9rade -edium 1ard For high pressures and slow speeds

*hoice of grease

1ow do you know if you$re using the right greaseL Mou might beusing a high quality grease. Mou may have put a lot of effort and

money into selecting the best quality grease in the pursuit oflubrication excellence. 2ut don$t confuse the quality of the lubricantwith the quality of the specification. ;onsidering this lubricating oilanalogy, the best quality turbine oil would most likely not make agood engine oil.

-ost users are aware of the importance of selecting the rightlubricant for a given application. When it comes to selectinglubricating oils for manufactured equipment, it$s easy to determine

which products meet the original equipment manufacturer 3+8-4requirements. +8- specifications for a lubricating oil normallyinclude viscosity at operating or ambient temperature, additiverequirements, base oil type and even special considerations fordifferent environmental conditions. 9rease specifications, on theother hand, often lack the detail necessary to make a properselection, leaving it up to the lubrication engineer to create thespecification.

A common +8- grease specification might be to use an G593Gational 5ubrication 9rease nstitute4 Go. * lithium grease of goodquality. >sing this information alone, one could select the rightconsistency and thickener type. A similar specification for an oil%lubricated application would be to use a 6good quality lubricatingoil.7 WhatLN

@ue to the lack of specificity in most grease recommendations, it isimportant to learn how to properly select greases for eachapplication in the plant. ?roper grease specification requires all of

the components of oil selection and more. +ther specialconsiderations for grease selection include thickener type and

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concentration, consistency, dropping point and operatingtemperature range, worked stability, oxidation stability, wearresistance, etc. >nderstanding the need and the methods forappropriate grease selection will go a long way toward improving

lubrication programs and the reliability of lubricated machinery.5et$s walk through the grease selection process step by step,starting with the most important property.

*alcium grease

;alcium or lime grease, the first of the modern production greases,is prepared by reacting mineral oil with fats, fatty acids, a small

amount of water, and calcium hydroxide 3also known as hydratedlime4.!he water modifies the soap structure to absorb mineral oil. 2ecauseof water evaporation, calcium grease is sensitive to elevatedtemperatures. t dehydrates at temperatures around D ; 3( : F4 atwhich its structure collapses, resulting in softening and, eventually,phase separation. 9reases with soft consistencies can dehydrate atlower temperatures while greases with firm consistencies canlubricate satisfactorily to temperatures around D ; 3* F4. n spite

of the temperature limitations, lime grease does not emulsify in waterand is excellent at resisting 6wash out.7 Also, its manufacturing cost isrelatively low. f a calcium grease is prepared from (*%hydroxystearicacid, the result is an anhydrous 3waterless4 grease. &incedehydration is not a concern, anhydrous calcium grease can be usedcontinuously to a maximum temperature of around (( ; 3* F4.

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;alcium complex grease is prepared by adding the salt calciumacetate. !he salt provides the grease with extreme pressurecharacteristics without using an additive. @ropping points greaterthan *C ; 3: F4 can be obtained and the maximum usable

temperature increases to approximately ( ; 3 : F4. With theexception of poor pumpability in high%pressure centrali"edsystems, where caking and hardening sometimes occur calciumcomplex greases have good all%around characteristics that makethem desirable multipurpose greases.

,odium grease

&odium grease was developed for use at higher operating temperaturesthan the early hydrated calcium greases. &odium grease can be used attemperatures up to (*( ; 3*: F4, but it is soluble in water and readilywashes out. &odium is sometimes mixed with other metal soaps,especially calcium, to improve water resistance. Although it has betteradhesive properties than calcium grease, the use of sodium grease isdeclining due to its lack of versatility. t cannot compete with water%resistant, more heat%resistant multipurpose greases. t is, however, stillrecommended for certain heavy%duty applications and well%sealedelectric motors.

0luminum grease

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Aluminum grease is normally clear and has a somewhat stringytexture, more so when produced from high%viscosity oils. Whenheated above D ; 3( : F4, this stringiness increases andproduces a rubber like substance that pulls away from metal

surfaces, reducing lubrication and increasing power consumption. Aluminum grease has good water resistance, good adhesiveproperties, and inhibits rust without additives, but it tends to beshort%lived. t has excellent inherent oxidation stability but relativelypoor shear stability and pumpability.

Aluminum complex grease has a maximum usable temperature ofalmost ( ; 3*(* F4 higher than aluminum%soap greases. t hasgood water%and%chemical resistance but tends to have shorter lifein high%temperature, high%speed applications.

Lithium grease

&mooth, buttery%textured lithium grease is by far the most popularwhen compared to all others.

!he normal grease contains lithium (*%hydroxystearate soap. t has adropping point around * < ; 3< F4 and can be used attemperatures up to about ( : ; 3* : F4. t can also be used attemperatures as low as % : ; 3% ( F4 . t has good shear stability anda relatively low coefficient of friction, which permits higher machineoperating speeds. t has good water%resistance, but not as good asthat of calcium or aluminum. ?umpability and resistance to oilseparation are good to excellent. t does not naturally inhibit rust, butadditives can provide rust resistance. Anti%oxidants and extreme

pressure additives are also responsive in lithium greases.5ithium complex grease and lithium soap grease have similarproperties except the complex grease has superior thermalstability as indicated by a dropping point of *C ; 3: F4. t isgenerally considered to be the nearest thing to a true multipurposegrease.

Other greases

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!hickeners other than soaps are available to make greases. Althoughmost of these are restricted to very special applications, two nonsoapgreases are worthy of mention. +ne is organic, the other inorganic.

Polyurea grease

?olyurea is the most important organic nonsoap thickener. t is alow%molecular%weight organic polymer produced by reactingamines 3an ammonia derivative4 with isocyanates, which results inan oilsoluble chemical thickener. ?olyurea grease has outstandingresistance to oxidation because it contains no metal soaps 3whichtend to invite oxidation4. t effectively lubricates over a widetemperature range of * to ( ; 3%< to : F4 and has long life.

Water%resistance is good to excellent, depending on the grade. tworks well with many elastomer seal materials. t is used with alltypes of bearings but has been particularly effective in ballbearings. ts durability makes it well suited for sealed%for%lifebearing applications.

?olyurea complex grease is produced when a complexing agent,most commonly calcium acetate or calcium phosphate, isincorporated into the polymer chain. n addition to the excellentproperties of normal polyurea grease, these agents add inherentextreme pressure and wear protection properties that increase themultipurpose capabilities of polyurea greases.

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Organo#clay

+rgano%clay is the most commonly used inorganic thickener. tsthickener is a modified clay, insoluble in oil in its normal form, butthrough complex chemical processes, converts to platelets thatattract and hold oil. +rgano%clay thickener structures areamorphous and gel%like rather than the fibrous, crystallinestructures of soap thickeners. !his grease has excellent heat%resistance since clay does not melt. -aximum operatingtemperature is limited by the evaporation temperature of itsmineral oil, which is around ( ; 3 : F4. 1owever, with frequentgrease changes, this multipurpose grease can operate for shortperiods at temperatures up to its dropping point, which is about*C ; 3: F4. A disadvantage is that greases made with higher%viscosity oils for high thermal stability will have poor lowtemperature performance. +rgano%clay grease has excellentwater%resistance but requires additives for oxidation and rustresistance. Work stability is fair to good. ?umpability andresistance to oil separation are good for this buttery texturedgrease.

*om!atibility

9reases are considered incompatible when the physical orperformance characteristics of the mixed grease falls beloworiginal specifications. n general, greases with different chemicalcompositions should not be mixed. -ixing greases of differentthickeners can form a mix that is too firm to provide sufficientlubrication or more commonly, a mix that is too soft to stay inplace.

;ombining greases of different base oils can produce a fluidcomponent that will not provide a continuous lubrication film.

Additives can be diluted when greases with different additives aremixed.-ixed greases may become less resistant to heat or have lowershear stability. f a new brand of grease must be introduced, thecomponent part should be disassembled and thoroughly cleaned toremove all of the old grease. f this is not practical, the new greaseshould be injected until all traces of the prior product are flushed out.

Also, the first grease changes should be more frequent than normallyscheduled.

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2rease a!!lication and trouble#shootingWhen selecting a grease, it is important to determine the propertiesrequired for the particular application and match them to a specificgrease. A grease application guide is shown in !able :%*. t shows the

most common greases, their usual properties, and important uses.&ome of the ratings given are subjective and can vary significantlyfrom supplier to supplier.

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Ho to monitor the condition of lubrication system

Oil analysis 3+A4 is the sampling and laboratory analysis of alubricant#s properties, suspended contaminants, and wear debris.OA is performed during routine preventive maintenance to provide

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meaningful and accurate information on lubricant and machinecondition. 2y tracking oil analysis sample results over the life of aparticular machine, trends can be established which can helpeliminate costly repairs. !he study of wear in an machinery is

called tribology . !ribologists often perform or interpret oil analysisdata.

+A can be divided into three categories0

(. analysis of oil properties including those of the base oil andits additives,

*. analysis of contaminants,. analysis of wear debris from machinery

When field testing is inadequate or indicates that additional testingis required, oil samples should be submitted for laboratoryanalysis. 5aboratory analysis should include viscosity,neutrali"ation number, water contamination, and the identificationof wear metal and other contaminants. ?roperties to be tested,along with the A&!- test method to be used, are listed in !able (.

f possible, the oil#s manufacturer should perform tests periodically.&ince the composition and additive content of oils is usuallyconsidered proprietary information, only the manufacturer can

accurately determine the extent of additive depletion. Whenanalysis is conducted by independent laboratories, the oilmanufacturer should be contacted anytime the test results suggestquestionable serviceability of an oil.

When problems arise or abnormal situations develop, otherproperties may be tested or the testing frequency of therecommended properties should be increased. For example, if oilcolor suddenly becomes ha"y or dark, the oil should be tested

immediately for water or other contamination. !he tests included in!able ( are used to determine contamination and degradation ofthe oil. )iscosity, appearance, water content, and cleanliness arerelated to contamination. !otal acid number 3!AG4, color, and

otating 2omb

+xidation !est 3 2+!4 are related to degradation. !he 2+! and!AG tests are excellent for following the degradation of turbine oil.

f 2+! results for the new oil are known, these can be comparedwith the values for the used oil to determine the oxidation stabilityreserve of the used oil. ;hanges in the 2+! and !AG of the oils

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http://en.wikipedia.org/wiki/Tribologyhttp://en.wikipedia.org/wiki/Tribology


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are the best indication of the remaining useful life of the lubricatingoil.

Determining What %ests to $un

+nce a representative oil sample has been taken, the next step isto determine what analytical tests will provide meaningfulinformation about oil and equipment condition. !here are hundredsof different tests to select from, and often two or three thatmeasure the same properties. !here are variations in cost,accuracy and time required to complete each test.

%able & 7ey %ests for Oil 8uality *ontrol 9onitoring

!he key element in any analysis program is to balance accuracywith cost and the time required to complete each test. n mostsituations the real value of the data is in determining trends ratherthan in the accuracy of any one individual test. !here is little valuein measuring viscosity to within OB% .( &aybolt >niversal &econds3&>&4 if the normal sample variation is OB% &>&. n fact, pre%

occupation with excessive test accuracy can undermine the abilityto distinguish longer term trends in oil properties.

Another important aspect is the importance of the equipment beingsampled. A large bearing on a ( %cylinder diesel is much moreexpensive to replace than the main bearings on a gasoline engine.

An oil analysis program should take into consideration the cost ofequipment repairs and downtime, and the importance of thatparticular equipment in the overall plant production cycle. criticalequipment may warrant speciali"ed oil analysis testing that wouldnot be cost effective on less important equipment.

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Oil

+il is stored in active oil reservoirs, where it is drawn as needed,and in oil drums for replenishing used stock. 8ach mode has itsown storage requirements.

Filtered and unfiltered oil tanks. -ost hydroelectric power plantsuse bulk oil storage systems consisting of filtered 3clean4 andunfiltered 3dirty4 oil tanks to store the oil for the thrust bearings,guide bearings, and governors. +ccasionally the filtered oil tankcan become contaminated by water condensation, dust, or dirt. !oprevent contamination of the bearing or governor oil reservoirs, thefiltered oil should be filtered again during transfer to the bearing orgovernor reservoir. f this is not possible, the oil from the filteredtank should be transferred to the unfiltered oil tank to remove anysettled contaminants. !he filtered oil storage tank should beperiodically drained and thoroughly cleaned. f the area where thestorage tanks are located is dusty, a filter should be installed in thevent line. f water contamination is persistent or excessive, a waterabsorbent filter, such as silica gel, may be required.

Oil drums

f possible, oil drums should be stored indoors. &tore away fromsparks, flames, and extreme heat. !he storage location mustensure that the proper temperature, ventilation, and fire protectionrequirements are maintained. !ight oil drums breathe in responseto temperature fluctuations, so standing water on the lid may bedrawn into the drum as it 6inhales.7 ?roper storage is especiallyimportant when storing hydraulic fluids due to their hygroscopicnature. !o prevent water contamination, place a convex lid overdrums stored outdoors. Alternatively, the drums should be set ontheir side with the bungs parallel to the ground. !he bungs on thedrums should be tightly closed except when oil is being drawn out.

f a tap or pump is installed on the drum, the outlet should bewiped clean after drawing oil to prevent dust from collecting.

2rease9rease should be stored in a tightly sealed container to preventdust, moisture, or other contamination. 8xcessive heat may causethe grease to bleed and oxidi"e. &tore grease in clean areaswhere it will not be exposed to potential contaminants, and away

from excessive heat sources such as furnaces or heaters. !hecharacteristics of some greases may change with time. A grease

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may bleed, change consistency, or pick up contaminants duringstorage. !o reduce the risk of contamination, the amount of greasein storage should not exceed a one%year supply. 2efore purchasinggrease supplies, the manufacturer or distributor should be

consulted for information about the maximum shelf life and otherstorage requirements for the specific grease5ubrication system problems

$ecommended storage conditions and !ractices &tore lubricating oils and greases in a cool, dry indoor area

where airborne particles are at a minimum.ndoor storage also prevents label deterioration and the container

from weathering. !he ideal storage temperature range is from K; to*:K;.

f drums must be stored outside, apply one of the followingoptions0

&tore drums on their side or 6blocked7 into a tilted position, withdrum bungs at the three and nine o$clock positions, to allowwater to run off.

?lace a plastic cover on top of the drum to keep the topprotected from dust and water.

>se other equivalent methods to prevent the ingress of water or dust.

-ost lubricating oils and greases deteriorate with time. 1owever,good storage practices promote sufficient stock turnover so thatlubricants are used before performance loss occurs.

efrigeration oils and brake fluids are highly sensitive to water contamination and must not be stored outside. Always store greaseupright to prevent oil separation.

When necessary, bring grease to satisfactory dispensingtemperature just before it is used.

otate the inventory. ;heck the container fill date and use theoldest container first.

Eeep containers tightly covered or closed to avoidcontamination.

Wipe off the tops and edges of containers before opening themto avoid contamination.

>se clean tools and equipment when pumping or handlinglubricants and grease.

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Protection of the lubrication system

!he minimum alarms and trips recommended for each major driver and driven machine should be a low oil pressure alarm, a low oillevel alarm, a high oil filter differential pressure alarm, high bearing

metal temperature alarm, and a metal chip detector. &ee table (

%able &

8 lubrication

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