The parameters Spk, Sk, Svk, SMr1, and SMr2 are all derived from the Areal Material Ratio curve based on the ISO 13565-2:1996 standard. The Reduced Peak Height, Spk, is a measure of the peak height above the core roughness. The Core Roughness Depth, Sk, is a measure of the “core” roughness (peak-to-valley) of the surface with the predominant peaks and valleys removed. The Reduced Valley Depth, Svk, is a measure of the valley depth below the core roughness. SMr1, the Peak Material Portion, indicates the percentage of material that comprises the peak structures associated with Spk. The Valley Material Portion, SMr2, relates to the percentage (i.e., 100%-SMr2) of the measurement area that comprises the deeper valley structures associated with Svk.
A large Spk implies a surface composed of high peaks providing small initial contact area and thus high areas of contact stress (force/area) when the surface is contacted. Thus Spk may represent the nominal height of the material that may be removed during a running-in operation. Consistent with Spk, SMr1 represents the percentage of the surface that may be removed during running-in. Sk represents the core roughness of the surface over which a load may be distributed after the surface has been run-in. Svk is a measure of the valley depths below the core roughness and may be related to lubricant retention and debris entrapment. Sk is a measure of the nominal roughness (peak to valley) and may be used to replace parameters such as Sz when anomalous peaks or valleys may adversely affect the measurement.