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.
As mentioned earlier, plain bearings are made of many graphite, bronze, and plastics that include PTFE, nylon, and polyacetal. Material improvements have made plastic plain bearings increasingly common, even in demanding motion applications.
Shaft material and plain-bearing wear
The shafts on which plain bearings ride have significant impact on plain-bearing performance and life. One common option is cold-rolled carbon steel. This shaft material makes for a suitable mating surface for plain bearings made of polymers. Ceramic shaft surfaces induce more wear, though are sometimes chosen for their ability to withstand harsh environmental conditions. Though aluminum shafts are lightweight and easy to machine, they also induce accelerated plain-bearing wear. Aluminum shaft made of anodized slightly improves the assembly performance.
In fact, shaft surfaces for mating with plain bearings shouldn’t be too smooth or rough. Overly smooth surfaces will cause stick-slip adhesion variations — in turn causing higher friction resistance to bearing movement. More of a disparity between dynamic and static friction will make for faster bearing wear and jerkier motion. In contrast, overly rough shaft surfaces quickly abrade plain bearings. In fact, the rates of wear induced by shaft-bearing interfaces can vary a hundredfold. Some manufacturers recommend shaft-surface finishes to 64 root mean square (rms) for precision applications needing low friction; a smoother shaft with roughness of 20 rms or so is more suitable where long plain-bearing life is a design objective.
Recall that the rms expression of surface roughness is derived from measurements of a surface’s microscopic peaks and valleys. Ra is an alternative measure some in industry use to quantify roughness — in this case, as an average roughness of a surface’s peaks and valleys. So the two measures express the same quality, only in different formats. Note that large and outlying peaks or flaws on a shaft surface will affect the RMS value more than its equivalent Ra value.