Engine Bearing Failure Analysis

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. On start up the shaft rubs the bearing briefly. Running, 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 operations. With normal operating conditions and a continuous supply of clean oil the shaft and bearing surfaces will remain separated.

When bearing damage occurs the cause must be determined and corrected before installing new parts.

Hot Short

Appearance: Bearing surface wiped and torn, blackened from heat with patches of lining material torn cleanly from steel backing.

Causes: Breakdown of lubrication and resulting high friction elevates operating temperature. Lead in bearing material melts and allows shaft to tear away patches of bearing lining. Lack of lubrication. Wiping. Dirt contamination. Concentrated loading (misalignment, etc.)

Fretting

Appearance: Bearing back polished from movement in housing. Area of pock marks or build-up due to metal transfer between bearing and housing.

Causes: Insufficient crush. Oversize housing. Bearing cap not torqued properly. Foreign object between cap and housing faces. Over-stressed cap bolts.

Corrosion

Appearance: Bearing surface darkened, spongy, etched by chemical attack.

Causes: Acids in oil. Excessive operating temperature. Excessive blow-by. Coolant contamination of oil. Use of high sulfur fuel. Excessive oil change intervals.

Overlay Fatigue

Appearance: Network of fine cracks in surface layer of a trimetal bearing.

Causes: Overloading (lugging engine at low speed under high load, overfueling, detonation). Localized concentration of load due to misalignment (edge loading, bent rod, tapered, hourglass or barrel shapped housing bore).

Note: Moderate overlay fatigue especially in localized areas may be considered part of the break-in process. Bearings may be re-used. Severe overlay fatigue, especially in a high-performance engine. may be due to the wrong bearing selection. Use of special competition parts with thinner overlay is recommended.

Oil Starvation

Appearance: Bearing surface streaked and smeared with worst damage at center. Heat discoloration. May show pick-up of bearing material on shaft depending on severity. Note: This condition will progress into "Wiping and "Hot Short".

Causes: Low oil level, blocked oil pickup, oil pump failure, blocked oil hole or oil passage, excessive dilution of oil by fuel or coolant, lubrication system not primed before start-up, overspeed.

Wiped

Appearance: Bearing surface smeared or scratched and torn. Bearing metal melted and re-solidified along edges.

Causes: Lubrication system not primed before start up. Clogged oil passage. Oil pump failure. Improper installation bearing. Misalignment of shaft and bearing surfaces. Insufficient clearance.

Fatigue

Appearance: Bearing surface cracked, areas of lining broken out leaving craters with ragged edges.

Causes: Overloading (lugging engine at low speed under high load, overfueling, detonation). Bearing material of inadequate fatigue strength for application. Localized concentration of load due to misalignment (edge loading, bent rod, tapered, hourglass, or barrel shaped housing journal). Bearing lining weakened by corrosion.

Normal Wear

Appearance: Uniform wear pattern over approximately 2/3 of the bearing 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

Scoring

Appearance: Bearing surface deeply scratched and torn.

Causes: Excessive foreign particle contamination. Poor crankshaft surface. Insufficient lubricant.

Distorted Crank Case

Appearance: With main bearings arranged as installed in the engine, bearing show a progression of damage from one to another.

Causes: Main bearing bores out of alignment. Engine overheating. Improper tightening of engine components (bearing caps, heads, manifolds, etc.) Engine not properly or uniformly supported (large stationary engines).

Dirt Embedment

Appearance: Bearing surface speckled, darkened and lightly or heavily scratched.

Causes: Foreign partical contamination. Engine componentes not thoroughly clened prior to assembly. Wear particles from another engine components. Faulty air filtration. Neglected oil filter replacement. Dirt entering engine during oil addition.

Accelerated Wear

Appearance: Wall thickness reduced from original dimension. Bearing surface worn and polished but not smeared, torn or scored. No evidence of heat, no embedded foreign particles.

Causes: Poor journal surface finish. Wear in the presence of adequate lubrication to precent heat build-up and wiping is caused by peaks in the journal finish profile which penetrate the oil film and abrade the bearing. Always grind opposite to rotation and polish in the direction of rotation

Dirt on Back

Appearance: Concentrated area of distress of bearing I.D. with corresponding mark of discontinuity on O.D.

Causes: Foreign particle trapped between bearing back and housing. Damage to bearing on back or housing bore (nick, burr, etc). High spot on bearing back or housing bore due to fretting.

Cap Shift

Appearance: Wear or fatigue near bearing parting lines on opposite sides and in upper and lower bearing halves.

Causes: Mixed bearing caps. Reversed bearing cap. Poor doweling of cap to housing. Use of oversize socket housing. Housing not machined and assembled at same bolt torque. Mating faces of housing not flat and parallel.

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