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Back to Basics Bearings
By Larry Carley
Bearings are one of the most important parts inside an engine, so it’s important to understand their role in the overall operation of the engine, their design features, and how to install them properly. Engine bearings are a relatively inexpensive component compared to the cost of labor and many of the other parts that go into rebuilding an engine, but if one fails or causes a problem that results in a warranty claim, it can cost you plenty! So with that in mind, let’s review some of the basics about bearings.
Bearings actually have a variety of roles inside an engine, including:Supporting the crankshaft and camshaft; Limiting the fore and aft movement of the crankshaft (this job belongs to the thrust bearing); Reducing friction; Lubricating the rotating shafts and connecting rods; Providing splash lubrication for the pistons, rings and cylinder walls (which also helps cool the pistons); Conducting heat away from the rotating parts; and Affecting how much oil pressure the engine develops at idle and higher rpms.
In overhead cam (OHC) engines, the cam bearings may be either type, or there may be no bearing inserts at all, i.e., the machined surface of the cam bores serve as the bearing surface for the cam(s).
Sleeve style wet bearings are used instead of ball bearings or roller bearings in most automotive applications because they are cheaper, lighter and are capable of supporting high loads. Even so, ball or roller bearings are sometimes used to support balance shafts in some engines, as well as the crankshaft in some motorcycle, marine and racing engines.
Though ball bearings and roller bearings are called "anti-friction" bearings because they spin easily and produce little drag, a rotating shaft supported on a film of oil inside a pressurized sleeve style bearing also spins with minimal resistance. The only drag on the shaft is that created by the shearing characteristics of the oil film. Also, the oil film helps spread the bearing load over a broader surface unlike a ball bearing or roller bearing, which concentrates the load at a single point or line.
At a microscopic level, oil molecules are like tiny ball bearings and glide easily past one another. That’s why oil feels slippery and makes such a good lubricant. And, the thinner the viscosity of the oil, the more easily it shears and the less friction and drag it creates. But, the oil must have a certain amount of viscosity so it can maintain film strength and not gush out of the bearing too quickly.
Temperature is also a factor to consider, too, because the hotter the oil gets the thinner it gets. If low viscosity oil is subjected to too much heat, it may not be able to maintain ad...