General guidance for people who work on engines. For exact figures always follow the vehicle or engine maker's workshop manual.
A film of oil, not metal on metal
An engine bearing is a curved shell that sits between a rotating shaft and the part that carries it. In a running engine the shaft does not touch the bearing: it rides on a thin, pressurised film of oil that the bearing surface helps to hold in place.
Metal only touches metal for a moment when the engine starts and stops. That is why the surface of a bearing must be gentle on a shaft that is, for that moment, rubbing against it.
The kinds of bearing in an engine
- Main bearings (fixed bearings) hold the crankshaft in the engine block.
- Rod bearings (moving bearings) sit between the crankshaft and the big end of each connecting rod.
- Thrust washers control how far the crankshaft can slide along its own axis.
- Bushes such as camshaft bushes and connecting-rod bushes, are used in heavy-vehicle engines.
How a bearing is built
Most shells have a steel back, which gives strength and a firm fit in the housing. On it sits a lining of bearing alloy that carries the load, often with a very thin, soft surface layer.
Polymer coatings are a modern way to cut friction and wear under high pressure and temperature. The polyamide layer on Golden Bearing's polymer bearings combines solid lubricants, conductive metal particles and heat-resistant polymers; in the company's internal tests it resisted wear more than 15 times better than an uncoated bearing.
What makes a good bearing
Making an ideal bearing is not simple, because improving one property usually costs another. In practice a good bearing balances these:
- Ability to hold oil on its running surface
- Accurate dimensions
- Resistance to load pressure and to combustion shocks
- Softness, so that it can embed small foreign particles
- Fatigue resistance
- Corrosion resistance
- Wear resistance
- Good heat conduction