Ideal lu­bri­ca­tion regime

DEMM Engineering & Manufacturing - - CONTENTS - By Amin Al­masi

Rolling-el­e­ment bear­ings are among the most im­por­tant ma­chin­ery el­e­ments.

They may be de­signed as ball or roller bear­ings, ra­dial or thrust bear­ings; what they all have in com­mon is the trans­mis­sion of load and power via rolling el­e­ments lo­cated be­tween bear­ing rings. This has been a sim­ple and suc­cess­ful prin­ci­ple. The de­sign is ro­bust and re­li­able as long as the con­tact sur­faces re­main sep­a­rated, and wear and fail­ures could be pre­vented.

How­ever, if the sur­faces con­tact one an­other, there might be trou­ble ahead.

A vi­tal re­quire­ment for low-wear, or even wear-free op­er­a­tion of rolling-el­e­ment bear­ings, is the sus­tained sep­a­ra­tion of the sur­faces of rolling-el­e­ments and race­ways (the fric­tion bod­ies), by means of a suit­able lu­bri­ca­tion oil.

Un­der pure slid­ing con­tact con­di­tions, ex­ist­ing for ex­am­ple be­tween rolling el­e­ments and cage or be­tween rolling el­e­ment faces and lip sur­faces, the con­tact pres­sure, as a rule, is far lower than un­der rolling con­tact con­di­tions.

Rolling-el­e­ment bear­ings are usu­ally op­er­ated un­der elasto-hy­dro­dy­namic lu­bri­ca­tion regimes. Even un­der some lu­bri­ca­tion con­di­tions, with min­i­mum amount of oil and very thin lu­bri­ca­tion films, en­ergy losses due to fric­tion and wear are low.

There­fore, it is pos­si­ble to lu­bri­cate rolling-el­e­ment bear­ings with greases of dif­fer­ent con­sis­tency and oils of dif­fer­ent vis­cos­ity. This means that wide speed and load ranges might not create prob­lems if a proper lu­bri­ca­tion oil regime ex­ists be­tween slid­ing sur­faces.

Grease is a kind of lu­bri­ca­tion that re­sults from adding some thick­en­ing agents (usu­ally metal­lic soap) into oil to form a semi-solid jelly-like sub­stance. As the grease is of a three-di­men­sional frame struc­ture, its lu­bri­ca­tion regime is com­pli­cated and its lu­bri­ca­tion flow could not be a lam­i­nar flow; it usu­ally shows com­pli­cated, time-de­pen­dent vis­coplas­tic be­hav­iour.

There are many rolling-el­e­ment bear­ing greases that have been “tai­lored” to in­di­vid­ual ap­pli­ca­tions. An im­por­tant topic is de­vel­op­ing or select­ing right grease for each ap­pli­ca­tion re­quire­ment from a wide range of base oils and spe­cial thick­en­ers.

High-tem­per­a­ture greases con­sist of ther­mally sta­ble, prefer­ably syn­thetic base oils in­cor­po­rat­ing or­ganic or in­or­ganic thick­en­ers. The max­i­mum up­per op­er­at­ing tem­per­a­ture limit for some high-tem­per­a­ture lu­bri­cat­ing greases could be above 300°C. For life­time lu­bri­ca­tion, how­ever, many ex­perts rec­om­mend op­er­at­ing tem­per­a­tures that are con­sid­er­ably lower that the rated ones in or­der to achieve long run­ning times.

Lu­bri­cat­ing greases ex­hibit­ing min­i­mal con­sis­tency in­crease at low tem­per­a­tures pro­vid­ing ex­cel­lent lowtem­per­a­ture sta­bil­ity. Suit­able base oils for low-tem­per­a­ture duty are syn­thetic es­ters, per­flu­o­ri­nated polyether (PFPE) oils or polyal­phaolefins. Grease that shows good low-tem­per­a­ture sta­bil­ity will of­ten per­form poorly in high­tem­per­a­ture ap­pli­ca­tions.

How­ever, there could be some ex­cep­tions depend­ing on op­er­at­ing de­tails and the grease char­ac­ter­is­tics. Some ma­chin­ery for op­er­a­tion in the cold (such as start-up at a cold day) of­ten re­quires low tem­per­a­tures of -25°C whereas the ac­tual day-to-day op­er­at­ing tem­per­a­ture of the unit is for ex­am­ple more than 100°C. There are some grease types whose lower op­er­at­ing tem­per­a­ture range is clearly be­low -25°C whereas the up­per limit is more than 100°C.


The in­ter­ac­tion be­tween two sur­faces can be di­vided into two types, me­chan­i­cal and molec­u­lar. Me­chan­i­cal ac­tions in­clude many ef­fects such as elas­tic de­for­ma­tions, plas­tic de­for­ma­tions, etc. Ac­tions of sur­face mol­e­cules in­clude many ef­fects such as at­trac­tion, ad­he­sion, and oth­ers. Many com­plex lu­bri­ca­tion and wear regimes such as thin film lu­bri­ca­tion, boundary lu­bri­ca­tion and other could be re­lated to molec­u­lar in­ter­ac­tions.

The fail­ure of me­chan­i­cal parts or sur­faces mainly oc­curs be­cause of wear, fa­tigue and cor­ro­sion. Wear is usu­ally the largest fac­tor in all ma­chin­ery fail­ures con­tribut­ing about 50–65 per­cent of all fail­ures and un­sched­uled plant shut­downs.

As an in­di­ca­tion, the clas­si­fi­ca­tion of wear mech­a­nisms is usu­ally of four ba­sic types: abra­sive wear, ad­he­sive wear, sur­face fa­tigue wear, and

cor­ro­sion wear.

Too of­ten, af­ter the oc­cur­rence of one kind of wear, an­other or oth­ers may also ap­pear. On slid­ing sur­faces, the wear pro­duced by fric­tion and me­chan­i­cal ac­tions could in­clude abra­sive wear, sur­face plas­tic de­for­ma­tion, and brit­tle spalling.

Abra­sive wear is the phe­nom­ena that ex­ter­nal hard par­ti­cle, hard bumps or rough peaks cause sur­face ma­te­rial to break or peel off. The abra­sive wear is one of the com­mon forms of wear mech­a­nisms. Ad­he­sive wear is when sur­faces slide rel­a­tively then the fric­tion pairs are sheared and the ma­te­ri­als are cut off to form wear par­ti­cles.

When loads in­crease, the metal and ma­te­ri­als will pass elec­tric lim­its and plas­tic de­for­ma­tions would oc­cur. Plas­tic de­for­ma­tions make the metal sur­faces harden and be­come brit­tle. If the sur­faces with­stand re­peated elas­tic de­for­ma­tions, fa­tigue dam­age would oc­cur.

Fric­tion can cause high tem­per­a­ture on the con­tact sur­faces. Rapid cool­ing fol­low­ing a high tem­per­a­ture in­ci­dent can re­sult in re­crys­talli­sa­tion and de­com­po­si­tion of the solid. Ox­i­da­tion and chem­i­cal cor­ro­sions could also hap­pen.

There are usu­ally four ma­jor sur­face dam­ages that should be prop­erly un­der­stood for the study of wear and lu­bri­ca­tion, and con­se­quently proper op­er­a­tion and re­li­a­bil­ity of ma­chiner­ies:

Abra­sion: the plough­ing effect on the fic­tional sur­face pro­duces abra­sive par­ti­cles and grooves.

Pit­ting: the metal fa­tigue dam­age on the sur­face forms pits due to the re­peated ac­tions of the con­tact stresses.

Peel­ing: be­cause of the de­for­ma­tion strength­en­ing un­der the load, the metal sur­face be­comes brit­tle, gen­er­at­ing mi­cro-cracks and caus­ing some ma­te­ri­als to peel off.

Scuff­ing: be­cause of the ad­he­sive effect, the sur­face forms ad­he­sive points with high con­nec­tion in­ten­sity such that the shear breaks the points, caus­ing se­ri­ous wear.

Dam­age could also oc­cur in the mi­croscale, and are known gen­er­ally as mi­crowear mech­a­nisms.

Lu­bri­ca­tion con­di­tions play a sig­nif­i­cant role in the wear of sur­faces. In ad­di­tion to the lu­bri­ca­tion and fric­tion, the load and sur­face tem­per­a­ture are also im­por­tant for wear.

As an in­di­ca­tion, when the load reaches a cer­tain value, the wear scar area would sud­denly in­crease. Crit­i­cal load de­creases with in­crease in slid­ing ve­loc­ity. The sur­face pres­sure and slid­ing ve­loc­ity are two main fac­tors af­fect­ing tem­per­a­ture char­ac­ter­is­tics.


With re­gard to lu­bri­ca­tion oil fail­ure, there are five ma­jor causes, but an ef­fec­tive lu­bri­ca­tion pro­gramme should con­trol the im­pact of each:

• Oil con­tam­i­na­tion. • Oil leak­age. • Chem­i­cal in­sta­bil­ity. • Tem­per­a­ture in­sta­bil­ity. • Wear, ma­te­rial dis­tor­tion or mis­align­ment.

The con­tam­i­na­tion of lu­bri­ca­tion oil can re­duce com­po­nent and ma­chin­ery life. Con­tam­i­na­tion con­trol, for both wa­ter and solid par­ti­cle con­tam­i­na­tion, is a proven method to ex­tend ma­chin­ery life, and re­duce both start-up and ran­dom fail­ure oc­cur­rences.

For ex­am­ple, wa­ter con­tam­i­na­tion in lu­bri­ca­tion oil can re­duce bear­ing life. As a very rough in­di­ca­tion, in­creas­ing wa­ter con­tam­i­na­tion from 0.0025 per­cent (25 ppm) to 0.01 per­cent or (100 ppm) can re­duce bear­ing life by a fac­tor of 2.5 times.

In gen­eral, lu­bri­ca­tion regimes with lo­calised high-pres­sure zones (such as elasto-hy­dro­dy­namic lu­bri­ca­tions in rolling-el­e­ment bear­ings) have much greater sen­si­tiv­ity to small amounts of wa­ter and wear de­bris than low-pres­sure sys­tems.

Many bear­ing fail­ures re­lated to lu­bri­ca­tion (more than 70 per­cent) can be avoided by sim­ple checks if lu­bri­ca­tion oil is sup­ply­ing to bear­ings or not. Quan­tifi­able and highly sen­si­tive mea­sure­ments of lu­bri­ca­tion oil prop­er­ties, con­tam­i­na­tion lev­els, and wear con­di­tions also play sig­nif­i­cant roles for bear­ing health and op­er­a­tion.

Anal­y­sis of lu­bri­ca­tion oil for mon­i­tor­ing and main­te­nance pur­poses can be com­pared to anal­y­sis of blood for med­i­cal pur­poses. In both cases the f luid con­tains valu­able in­for­ma­tion that can be re­vealed through testing.


To im­prove the per­for­mance of lu­bri­ca­tion oil, small amounts of ad­di­tives are added to the base of the oil.

Un­der medium tem­per­a­ture and medium load, an oily ad­di­tive can form a thick high vis­cos­ity film. A good ad­di­tive should pos­sess the po­lar groups that have strong ab­sorp­tion en­ergy on the metal sur­face.

Ad­di­tives us­ing high molec­u­lar poly­mers have been de­vel­oped in last decade. Liq­uid crys­tals have also been de­vel­oped as anti-fric­tion ad­di­tives. All these mod­ern ad­di­tives should only be used at rea­son­able quan­ti­ties (rel­a­tively low per­cent­age) and with great care. As a very rough in­di­ca­tion, oily ad­di­tives should usu­ally be added less than eight per cent.

Anti-wear ad­di­tives can be em­ployed to form ad­sorp­tion films and pre­vent metal sur­faces from be­ing worn. The per­for­mances of anti-wear ad­di­tives are closely re­lated to ma­te­rial fric­tion sur­faces. In other words, proper anti-wear ad­di­tives should be cho­sen for each ap­pli­ca­tion.

The ad­sorp­tion film on com­mon an­ti­wear forms can­not usu­ally with­stand high tem­per­a­tures un­der heavy load con­di­tions. Of­ten, such harsh boundary lu­bri­ca­tion is called ex­treme pres­sure lu­bri­ca­tion. Ex­treme pres­sure ad­di­tives should be used to with­stand high pres­sures and pre­vent metal sur­faces from scratch and sin­ter.

Ex­treme pres­sure ad­di­tives should be ap­plied with great care. If they used in ex­ces­sive con­cen­tra­tions, the re­sult in un­pre­dictable be­hav­iours or cor­ro­sive wear on metal sur­faces.

If lu­bri­ca­tion oil con­stantly in­ter­acts with air, such as lu­bri­ca­tion oils in gas tur­bines, in­ter­nal com­bus­tion en­gine cylin­ders or air-com­pres­sors, ox­i­da­tion re­ac­tion may oc­cur.

An anti-ox­i­dant is used to de­lay the ox­i­da­tion process to pro­long the life of the lu­bri­ca­tion oil. Spe­cial care is needed when dif­fer­ent ad­di­tives are used in lu­bri­ca­tion oil. Ad­di­tives may in­ter­act with each other. When sev­eral ad­di­tives are used to­gether, their in­te­gral effect should be eval­u­ated.

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