Plain bearings are cylindrical sleeves that bear light to moderate radial loads. They slide radially or axially over shafts to allow rotary motion or linear motion (or sometimes both) of these loads. Plain bearings of all types are compact and lightweight with high strength-to-weight ratio.

Plain bearings have none of the moving parts that rolling-element bearings have, so minimize fail points; they’re also cost effective for even fairly rugged applications. Common variations are metallic sleeve bearings (which often ride loads on a hydrodynamic or full film of lubrication) and self-lubricating plastic bearings in an array of geometries for bushing, thrust bearing, and integral-slide applications.

Plain-bearing ratings are based in part on test results and its material modulus of elasticity, flexural strength, shore-D hardness, maximum surface pressure and running speed, rotating, and maximum load capacity — with the latter related to the plain bearing’s material compressive limit. (Here, recall that the compressive limit is the point at which 0.2% permanent deformation occurs.)

In addition, a pressure-speed (PV) value expresses plain-bearing load capacity — usually in in psi times the shaft rpm. However, note that PV values are only one to help determine a plain bearing’s overall load capacity — especially where a PV expressions might mislead engineers into thinking that a plain bearing can bear excessively high loads if the speed is very low. In other words, use of PV values requires concurrent consideration of real-world speed and load limits.

For more information, read Plain bearings (sleeve bearings) and the shafts they ride.