Counter-surface finish properties
The surface properties of the cylinder bore and the piston rod have a great influence on the function and service life of the seal. Parameters for specifying a surface finish are defined by ISO 4287:1997. The most common surface roughness parameter specified is Ra (in units μm or μin.), i.e. the arithmetic mean deviation of the surface profile. This value does not, however, completely describe how the surface can be expected to affect the seal. The reason for this is that two surfaces with the same values of Ra but with different surface profile characteristics can lead to different lubrication film thickness, resulting in varying seal performance and level of wear.
The material ratio curve (Abbott-Firestone curve) provides more information about the surface profile characteristics. It describes the ratio of the material-filled length to the evaluation length at a given cutting depth expressed in percent (fig. 1). The slope at the beginning of the curve represents the peaks in the profile, which are causing initial wear on the seal. The slope at the end of the curve represents the valleys in the profile, which serve as lubricant reservoirs.
Table 1 shows some surface profiles. To ensure a surface finish with a suitable profile and characteristics for effective dynamic sealing, SKF recommends a defined combination of the following surface parameters:
- Ra – arithmetic mean roughness
- Rz – mean peak-to-valley height
- Rmax – maximum peak-to-valley height
- Rmr – material ratio at a given depth
SKF recommends evaluating Rmr at a cutting depth of 0,5 Rz based on cref = 0% (diagram 1).