Surface roughness (roughness depth) refers to the unevenness of actual surfaces based on the material involved and how is has been processed. Surface roughness is largely responsible for the wear experienced by seals. In order to provide some protection for elastomers, which are liable to become scratched, very strict regulations are laid down with regard to surface roughness:
For the purpose of sealing non-moving surfaces, an R a of 0.5 to 1.5 μm is often used. However, in order to minimise the debris from seals on moving surfaces, the R a for these must not exceed 0.25 μm.
However, since the R a value — just like R t or R max — does not say anything about whether the profile is "open" — which is not good for seals (Figure O 1 a) — or "closed" — and therefore good for seals (b) — the R t value for the profile must always be used. This should not exceed 0.5.
Figure O 1: Rp value for various roughness profiles