Dr. Dmitri Kopeliovich
Engine bearings are small and relatively inexpensive components of internal combustion engines however failure of an engine bearing commonly leads to serious reconditioning works of the engine including its disassembling, regrinding the crankshaft and replacing the bearings.
Factors affecting normal functioning of engine bearings:
Types of engine bearing failure:
Fatigue of an overlay appears in form of a network of thin cracks.
Thinner overlays form thinner fatigue cracks.
Fatigue limit of an overlay is determined by the strength of the material and the thickness of the overlay. The thinner the overlay the higher its fatigue strength.
Overlay fatigue itself does not cause the engine failure.
However running the bearing with fatigued overlay for a long time may cause partial flaking of the overlay and lowering the oil film thickness.
The conditions for boundary lubrication occurred at low oil film thickness lead to excessive wear and localized loading, which may result in the failure of the lining (intermediate layer).
Fatigue of a copper based lining starts from fatigue of the overlay.
The overlay flakes out from the copper lining resulting in breaking the oil film and changing the lubrication regime from hydrodynamic to boundary.
The load localizes at the contact area causing formation of small cracks on the lining surface.
The cracks then spread throughout the lining thickness, meet the steel back surface and continue to advance along the steel-copper boundary.
As a result parts of the intermediate layer detach from the steel surface.
Fatigue of copper lining causes immediate engine failure.
Appearance of fatigue of aluminum based lining is similar to that of copper base lining.
It is also result of overloading caused by running the engine under high loads (torque) at low rotation speeds for long period, localized loading of the bearing due to a misalignment, fuel detonation or other factors.
The fatigue cracks form on the surface and spread inside the lining reaching the steel back.
The cracks then progress along the bond line between the lining and the steel.
Pieces of the lining flake out from the steel back resulting in the engine failure.
Fatigue of aluminum alloys may also cause extrusion of the lining material out of the bearing edges.
Metal-to-metal contact (boundary lubrication) may be a result of different factors:
Metal-to-metal contact may appear in the following forms:
Distorted (bent or twisted connecting rod) is one of the causes of localized loading of engine bearings.
Overloading of an internal combustion engine due to detonation or running under high torque at low rotation speed may cause distortion of the connecting rods.
The distortion results in non-parallel orientation of the bearing and journal surfaces.
The misalignment causes localized excessive wear of the bearing surface due to metal-to-metal contact (boundary lubrication) occurring near the bearing edge.
Localized metal-to-metal contact may also cause Fatigue cracking of the Engine bearing materials in the locations of the contact.
Imperfect journal geometry is also a cause of localized loading of engine bearings.
Use of a worn stone in grinding a crankshaft results in obtaining an imperfect (out-of-shape) journal surface: taper shape, hour glass shape or barrel shape.
The parts of the journal surface having higher diameter (central part of the barrel shape journal, edge parts of hour glass shape journal) come to metal-to metal contact (boundary lubrication) with the bearing surface.
The metal-to-metal contact causes excessive wear.
Fatigue cracking of the Engine bearing materials may occur in the contact areas.
Cavitation occurs when the load applied to the bearing fluctuates at high frequency (high RPM).
The oil pressure instantly falls causing formation of cavities due to fast evaporation (boiling).
When the pressure rises the vapor cavities (cavitation bubbles) contract at high velocity. Such collapse results in impact pressure, which may erode the bearing material.
The oil pressure creates a supporting force separating the journal from the bearing surface.
Soft Lead based engine bearing overlays of tri-metal bearings are prone to the cavitation erosion.