Shaft surfaces for mating with plain bearings shouldn’t be too smooth or rough. 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.