How To Prevent Bearing Fail ure and Determine Their Causes Engine Bearings depend on a film of oil to keep shaft and bearing surfaces separated ( figure A). Bearings fail when the oil film breaks down or when the bearing is overloaded. The oil film is generated by shaft rotation (figure B). At rest, the shaft and bearing are in contact. n start up the shaft rubs the bearing briefly. !unning, the shaft pulls oil from the clearance space into the wedge shape area between the shaft and bearing. The oil wedge lifts the shaft off its bearing and supports it during engine operation. "ith normal operating conditions and a continuous supply of clean oil the shaft and bearing surfaces will remain separated. "hen bearing damage occurs the cause must be determined and corrected before installin g new parts
How To Prevent Bear ing Fa i l ure and Dete rmine The i r Causes
Engine Bearings depend on a film of oil to keep shaft and bearing surfaces separated ( figure A).Bearings fail when the oil film breaks down or when the bearing is overloaded. The oil film is generated byshaft rotation (figure B). At rest, the shaft and bearing are in contact. n start up the shaft rubs the bearingbriefly. !unning, the shaft pulls oil from the clearance space into the wedge shape area between the shaftand bearing. The oil wedge lifts the shaft off its bearing and supports it during engine operation. "ith normal
operating conditions and a continuous supply of clean oil the shaft and bearing surfaces will remainseparated. "hen bearing damage occurs the cause must be determined and corrected before installing new parts
Appearance: Uniform wear pattern over approximately 2/3 of the bearing’s surface.Wear should diminish near the parting line ends of the bearing, and the wear pattern
should extend uniformly across the bearing in the axial direction.
Appearance: etwor! of fine crac!s in surface layer of a "rimetal bearing.
Causes: #verloading $lugging engine at low speed under high load, overfueling,detonation%. &ocali'ed concentration of load due to misalignment $edge loading, bentrod, tapered, hourglass or barrel shaped housing or (ournal%.
Note: )oderate overlay fatigue especially in locali'ed areas may be considered part of the brea!*in process.+earing may be re*used. ever overlay fatigue, especially in a high*performance engine, may be due to thewrong bearing selection. Use of special competition parts with thinner overlay is recommended.
Appearance: +earing surface deeply scratched and torn.
Causes- xcessive foreign particle contamination. oor cran!shaft surface finish.0nsufficient lubrication.
Appearance: +earing surface dar!ened, spongy, etched by chemical attac!.
Causes: 1cids in oil. xcessive operating temperature. xcessive blow*by. oolantcontamination of oil. Use of high sulfur fuel. xcessive oil change interval.
Appearance: +earing surface spec!led, dar!ened and lightly or heavily scratched.
Causes: oreign particle contamination. ngine components not thoroughly cleanedprior to assembly. Wear particles from another engine component. aulty air
Appearance: Wear or fatigue near bearing parting lines on opposite sides in upperand lower bearing halves.
Causes: )ixed bearing caps. 5eversed bearing cap. oor doweling of cap tohousing. Use of oversi'ed soc!et. 6ousing not machined and assembled at same bolttor7ue. )ating faces of housing not flat and parallel.
Appearance: With main bearings arranged as installed in the engine,
bearings show a progression of damage from one to another.
Causes: Main bearing bores out of alignment. Engine overheating. Impropertightening of engine components bearing caps, heads, manifolds, etc.!Engine not properl" or uniforml" supported large stationar" engines!.
Appearance: +earing surface strea!ed and smeared with worst damage at center.6at discoloration. )ay show pic!*up of bearing material on shaft depending onseverity.
Appearance: Wall thic!ness reduced from original dimension. +earing surface wornand polished but not smeared, torn, or scored. o evidence of heat, no embeddedforeign particles.
Causes: oor (ournal surface finish. Wear in the presence of ade7uate lubrication toprevent heat build*up and wiping is caused by pea!s in the (ournal surface finish
profile which penetrate the oil film and abrade the bearing. 1lways grind opposite to
rotation and polish in the direction of rotation.
Appearance: +earing surface wiped and torn, blac!ened from heat, with patches oflining material torn cleanly from steel bac!ing.
Causes: +rea!down of lubrication and resulting high friction elevates operatingtemperature. &ead in bearing material melts and allows shaft to tear away patchesof bearing lining. &ac! of lubrication. Wiping. 4irt contamination. oncentratedloading $misalignment, etc.%
Appearance: oncentrated area of distress on bearing 0.4. with correspondingmar! or discontinuity on #.4.
Causes: oreign particle trapped between bearing bac! and housing. 4amage tobearing bac! or housing bore $nic!, burr, etc.%. 6igh spot on bearing bac! or housingbore due to fretting.
Appearance: +earing surface smeared or scratched and torn. +earing metal melted
Appearance: +earing surface crac!ed, areas of lining bro!en out leaving craterswith ragged edges.
Causes: #verloading $lugging engine at low speed under high load, overfueling,
detonation%. +earing material of inade7uate fatigue strength for application.&ocali'ed concentration of load due to misalignment $edge loading, bent rod,
tapered, hourglass, or barrel shaped housing or (ournal%. +earing lining wea!ened bycorrosion.
Aluminum Bearings
Appearance: +earing surface crac!ed,
areas of lining bro!en out, leaving craterswith smooth bottoms and rough sharp
edges.
Causes: #verloading $lugging engine at lowspeed under high load, overfueling,detonation%. +earing material of inade7uatefatigue strength for application. &ocali'edconcentration of load due to misalignment$edge loading, bent rod, tapered, hourglass,
Appearance: Wear patter coveringapproximately 2/3 of the bearing surface.Wear should diminish near the part line endsof the bearing surface. attern may beintermittent in both axial and circumferential
directions on bimetal aluminum dependingon geometry of mating surfaces.
Appearance: +earing surface deeplyscratched and torn.
Causes: xcessive foreign particlecontamination. oor cran!shaft surfacefinish. 0nsufficient lubrication.
