Surface finish

    Abiding by the correct methods and specifications for surface finish can result in a prolonged lifetime for dynamic sealing systems. Upon a seal’s surface, microscopic deviations enable recesses to form, which allow the seal to retain an oil film between its surface and that of the exterior.

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    Measure and Maintain

    With the advance of technology, surface finish is gaining more relevance to system life and warranty savings. The equipment and ability to measure and maintain surface finishes has become more widely used. Traditional measuring gauges and visual inspection have become outdated. Profilometers are now used, which measure surface roughness of a profile far more easily by achieving precise and repeatable results.

    The most common parameter for gauging surface finish has been Root Mean Square (RMS, also referred to as Rq). However recently, Arithmetic Average Roughness (Ra) has become more commonly specified. Using either of these parameters alone is insufficient to define a reciprocating sealing surface. The image on the right shows why this is not enough to define a surface finish.

    The three surface finishes in the image all yield the same Ra value while all having very unique characteristics. The first surface finish profile (1) will have high wear characteristics due to the sharp peaks. The second profile (2) is a surface finish for dynamic seals as sharp peaks have been either minimised or removed. The final profile (3) will also tend to wear out quickly due to the peaks being too far apart.

    Ra defines the magnitude of the surface roughness, however it does not take into account actual surface shape and finish. Ra describes the average deviation from the mean line, not the nature of the valleys and peaks along the profile. To obtain accurate surface description, additional parameters are required. Parameters Rp, Rz and Rmr are used in defining the magnitude and spacing between peaks. The combination of these parameters can be used to identify if a surface is too rough or smooth as defined in the table below.

    Other parameters useful in surface evaluation are Rt, which considers only one measurement, and RZ, Rp and Rmr, which account for the full profile.

    Surface finish

    Roughness parameters as defined per ISO 4287:1997 and ISO 4288:1996.

    Ra* Arithmetic average deviation from the centreline of the sampling length.
    Rq* Square root mean deviation from the centreline of the sampling length.
    Rp* The maximum profile peak height within a sampling length (also referred to as Rpm).
    Ry* The maximum profile valley depth within a sampling length (also referred to as Rvm).
    Rz* The maximum height profile within a length. Rz = Rp + Rv.

    *NOTE: These parameters are first defined over a sampling length. When multiple sampling lengths are measured an average value is then calculated resulting in the final value. Standard number of sample lengths to find an average is five (except with Rz).

    Rt The maximum height profile within the evaluation length (evaluation length is typically 5 sample lengths).
    Rmr The relative material ratio at a height relative to a reference line. Estimates the amount of surface contact area at this height.

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