As mentioned earlier — in the FAQ Plain bearings: What are they? — plain bearings are made of many graphite, bronze, and plastics that include PTFE, nylon, and polyacetal. Material improvements have made plastic plain bearings (or sleeve bearings) increasingly common, even in demanding motion applications.
Shaft material and wear of plain bearings
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 sooth 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.