September 01, 2016
Ra is sufficient to define the magnitude of surface roughness, but is insufficient to define a surface entirely in that it only describes the average deviation from the mean line, not the nature of the peaks and valleys in a profile. To obtain an accurate surface description, parameters such as Rp, Rz and rmr (tp) can be used to define the relative magnitude of the peaks and the spacing between them.
Understanding and applying the benefits of appropriate hardware surface finish specifications can dramatically affect the longevity of a sealing system. In a dynamic surface, microscopic variations form recesses which hold an oil film between the lip seal and the moving surface. If the surface is too smooth, friction and seal wear will be high because this oil film will not be present. If the surface is too rough, the variations will create leak paths and accelerate lip wear.
These three very different surface finishes all yield the same Ra value of 18 μin.
RMS = Rq. The Root Mean Square (RMS) as defined by ISO 4287:1997 and other standards is often defined as Rq. These terms are interchangeable.
Rq = Ra. Confusion has typically surrounded these values, leading to misconceptions that they are interchangeable. Rq and Ra will never be equal on typical surfaces. Another misconception is that there is an approximate 11% difference between the two. Ground and polished surfaces can have Rq values that are 20 to 50 percent higher than Ra. The 11% difference would only occur if the surface being measured took the form of a true sine wave. A series of tests conducted at Parker have shown Rq to be 30% higher than Ra on average.
What's the Significance? Specifications previously based on a maximum surface finish of 16 μin RMS for ground and polished rods should specify a maximum finish of 12 μin Ra.
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