The ratio of the areal material ratio parameters, Spk/Sk (Reduced Peak Height to Core Roughness), Svk/Sk (Reduced Valley Depth to Core Roughness), and Spk/Svk (Reduced Peak Height to Reduced Valley Depth Ratio) indicate the symmetry of the bearing ratio curve. Two surfaces with indistinguishable roughness average (Sa) may be easily distinguished by a ratio such as Svk/Sk, which shows the relationship between the valley material and core material.
Svk/Sk (Reduced Valley Depth to Core) can provide information about how well the surface will maintain lubricant and/or trap debris. Svk by itself tells us about the magnitude of the valleys, but it doesn’t give information about the overall surface amplitude distribution. Svk/Sk can show whether the surface amplitude symmetry is consistent with what we know to be a good part.
In the example below, the upper surface in this image has high peaks, while the lower surface has deep valleys. Both surfaces have the same Sa value, but with vastly different Spk/Sk values. In a sealing application, a surface with a large Spk/Sk value may rip into the mating seal and cause leakage or failure.
Two surfaces with the same Sa but different Spk/Sk values.
The ratios may be further thought of as a measure of the texture amplitude distribution normalized by the overall roughness magnitude and thus may be used to characterize the texture amplitude symmetry.