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The surface finish is very important to provide excellent sealing performance, a low friction and a good seal life. The wear rate is a major factor in the life of a dynamic cylinder, and depends on the quality of the rod and the cylinder bore surface finishes. It is impossible to define numerically surface finish characteristics. There are however two systems for defining roughness, which estimate the most important characteristics.
Roughness Rt = maximum distance between the highest peak and the lowest groove over a distance “L” (see Figure SF-1).
Roughness Ra = Average (1/L ? L/0 | y | dx) of profile variation on “Y” axis from the center line (see Figure SF-2).
Figure SF-1 (Measure of roughness Rt.)
Figure SF-2 (Measure of roughness Ra.)
The relationship between roughness Rt and Ra is as follows:
Rt = (7 Ra + 0.2) ± 35%
This formula is only valid for roughness over 0.1 µm.
Rt values and especially Ra values are expressed by µm. Even though they do not completely define the surface finish, it is possible to indicate an acceptable level of roughness, for different parts of the cylinder. The most common metal surface finish is a roughness of between 0.02 µm to 3.5 µm Ra.