Surface properties

A surface is usually defined by the surface roughness value µm Ra, i.e. the arithmetic mean deviation of the surface profile. This value does not, however, provide a complete scenario of how the surface can be expected to affect the seal. The reason for this is that two surfaces with the same value stated in µm Ra but with different surface profile characteristics can result in a varying degree of wear.
A surface with smooth profile characteristics provides a longer service life for the seal than a rough surface characterized by sharp profile details, but still with the same surface roughness value. A more important aspect is therefore the surface profile characteristics, or bearing capacity. The higher bearing capacity, the longer service life of the seal. Table shows the relation between different surface roughness values and the bearing capacity.
We recommend a surface roughness value for rod and cylinder tube surfaces working against seals of elastomeric materials of maximum 0,3 µm Ra and maximum 0,2 µm Ra surfaces working against PTFE seals. In this recommendation the bearing capacity is taken into consideration and is described as a relation between Rp and Rz according to the below formula:

For surfaces against seals of elastomeric materials:
Max Ra 0,3 µm for relation Rz 1,2/Rp max 45% of Rz

For surfaces against PTFE seals:
Max Ra 0,2 µm for relation Rz 1,2/Rp max 45% of Rz

We recommend a surface roughness value for the seal housing groove of maximum 0,8 µm Ra and maximum 1,6 µm Ra for other machined surfaces.