Plain bearings come in dozens of materials and variations. They sometimes get a bum rap because they are simpler and less expensive than rolling-element bearings. Plain bearings generally function better with lighter loads than rolling element designs, but the plain variety handles higher speeds and generally dirtier environments. Even these generalities are occasionally challenged. A brief examination of several designs shows where they work best.
The simplest bearings are a plastic sleeve that rides on a smooth, stiff round-steel rod. A few designs cover the plastic with an aluminum housing that allows mounting it dozens more ways. The engineering plastic often needs no lubrication, tolerates dirt and shaft damage, and damps vibration. What’s more, it absorbs shock loads without damaging components.
The bearings also allow simultaneous rotary and linear motion. Their coefficient of friction is predictable over the life of the bearing, and they do not fail catastrophically. Many plain bearing manufacturers make similar performance claims. Most of the engineering by each company goes into the material that rides on the steel rod.
A few sizing guidelines involve first calculating PV where P = the pressure load, psi; and V = velocity, ft/min. For example, carrying a 100-psi load at 200 ft/min requires a bearing with a PV of 20,000. Units are often omitted. The accompanying bearing articles detail the most recent developments.
