In a piston engine, the main bearings are the bearings on which the crankshaft rotates, usually plain or journal bearings. The bearings hold the crankshaft in place and prevent the forces created by the piston and transmitted to the crankshaft by the connecting rods from dislodging the crankshaft, instead forcing the crank to convert the reciprocating movement into rotation.
While some small single-cylinder engines have only one main bearing, designed to deal with the bending moment exerted by the crank as a result of the force from the connecting rod, most engines have at least two main bearings, one at each end of the crank shaft and may have as many as one more than the number of crank pins. Increasing the number of bearings in an engine will generally increase the size and cost of the engine, but also provides stability to the crankshaft, which would otherwise have to endure greater bending moments from having the crank pin further from a bearing. The combination of bearing number and crankshaft design can also have an effect on engine balance; a more stable crankshaft will better avoid unbalance from flexing, but increasing the crankshaft length or girth to accommodate an additional bearing increases the rotational inertia of the crankshaft, which can have the opposite effect.
Most engine bearing failures are caused by one of two factors:
- Mixed lubrication with direct metal-to-metal contact between the bearing and crankshaft surfaces.
- Fatigue of the bearing material.
Bearing failures due to metal-to-metal contact:
Mixed lubrication is one of the main causes of engine bearing failures. Metal-to-metal contact may appear in the following forms of bearing wear:
- Accelerated wear: is when the bearing is not overheated and only a shiny appearance of the bearing surface is observed.
- Wiping or heavy wear: this appears in the form of overheating and partial melting of the overlay.
- Severe wear or Hot Short: it results in torn surfaces, severe overheating, melted overlay and lining material
Causes of mixed lubrication and the methods of solving the problem are presented in the table below:
Engine bearing failures due to fatigue:
Bearing material fatigue is the second cause of bearing failure.
- Fatigue of an aluminum lining: The fatigue cracks form on the surface and propagate 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 oil contamination and eventual bearing failure. Fatigue of aluminum alloys may also cause extrusion of the lining material past the length of the bearing edges.
- Fatigue of a tri-metal overlay: Spider web-like cracks are seen on the surface. 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 bearing to fail. However, running the bearing with fatigued overlay may cause partial flaking of the overlay, lowering the oil film thickness and eventual seizure of the exposed intermediate layer.
- Fatigue of a copper based intermediate layer: Fatigue of a copper based lining starts from a fatigue of the overlay. The overlay flakes out from the copper lining resulting in an interruption of the oil film, breaking the hydrodynamic lubrication regime. The load localizes at the contact area causing formation of small cracks on the lining surface. The cracks then propagate throughout the lining thickness, meeting the steel back surface and continuing to advance along the steel-copper boundary. As a result, parts of the intermediate layer detach from the steel surface.
The table below shows factors that cause fatigue and methods of preventing bearing failures due to fatigue. Engine bearing failures due to fatigue:
Two examples of the effect of geometric irregularities:
- Local wear with shiny appearance.
- The overlay fatigue in form of spider web like cracks on the areas of metal-to-metal contact.
- Distorted (bent or twisted connecting rod): 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 non-parallelism causes localized excessive wear of the bearing surface due to metal-to-metal contact (boundary or mixed lubrication) occurring near the bearing edge. Localized metal-to-metal contact may also cause fatigue cracking of the bearing material in the locations of the contact.
- Imperfect journal geometry: This is another 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 bearing materials may occur in the contact areas.
- Cavitation erosion of the overlay: Cavitation erosion is another type of engine bearing failure differing from both fatigue and metal-to-metal contact. Cavitation is a phenomenon related to hydrodynamics. Cavitation occurs when the load applied to the bearing fluctuates at high frequency (high RPM). The oil pressure instantly falls causing formation of bubbles (cavities) due to fast evaporation. When the pressure rises the cavitation bubbles collapse at high velocity. Such collapse results in impact pressure, which can erode the bearing material. Soft lead based overlays of tri-metal bearings are prone to the cavitation erosion. Therefore replacement of tri-metal bearings with babbitt overlay with bi-metal material or with high strength tri-metal bearings (e.g. GP) will preventthe failures due to cavitation.