Having the correct surface texture and the correct hardness are important. These two characteristics are linked in relation to influencing seal performance. We know that the optimum surface finish is required to control seal wear whilst regulating leakage. However, it is the capability of being able to maintain this surface throughout the operation of the seal that is important.
If the hardness of the running surface isn’t sufficient then a modified surface texture will be achieved after only a short running time. This modified surface texture has the effect of increasing seal wear, increasing leakage and causing damage to the shaft. Damage to the shaft generally means that the contact band achieved between the seal and the shaft is no longer within the expected design envelope, further compromising seal performance. The attached is an extract from a project completed some years back, which offers further explanation.
If the counterface is smooth, then the wear may result from adhesion between the surfaces, and involve deformation only in the surface layers of the polymer. If the counterface is rough then its asperities will cause deformation in the polymer to a significant depth; wear then results from abrasion associated with plastic deformation of the polymer, or from fatigue crack growth in the deformed region.
In general polymers sliding on highly polished counterfaces will experience adhesive wear, while turned or ground surfaces promote cohesive wear. The transition between the two wear characteristics can lead to a pronounced minimum in wear rate at a certain surface roughness, as illustrated in figure 1.0 which shows ultra-high molecular weight polyethylene sliding against stainless steel counterfaces at different surface finishes. The roughness at which the transition occurs can also be influenced by environmental factors, i.e. lubrication and product ingress.
Figure 1.0 - Relationship between wear and surface roughness
Important surface finish measurements for sealing surface. There are many different methods of measuring the surface finish of sealing surface but the most important ones and the ones that FTL would recommend are Ra, Rt and Rz. Typically, for a PTFE rotary shaft seal, the surface texture requirements added to a proposal drawing would be:
Ra - arithmetical mean roughness value
Ra measures the average length between all peaks and valleys across the sampling length. An algorithm is used to calculate the Ra value and during the process ignores some extreme points so that these extreme points do not have any significant influence on the final Ra value. Ra is a common and simple measurement for ensuring consistency of surface finish however on its own does not paint a tue picture of the surface requirement for sealing applications.
Rz - mean roughness depth
Rz is measured across a sampling length (ln) and considers the vertical distance between the highest peak and the lowest valley within five sampling lengths (Ir). These distances are averaged to achieve the Rz value. Only the five highest peaks and the five lowest valleys are used for the calculation and so extremes have a more significant influence on the calculated Rz value. In the below figure the value of Rz would be calculated using;
Rz = (Rz1+Rz2+Rz3+Rz4+Rz5)/5
Rmax is calculated by taking one of the five sampling lengths used in the Rz calculation that shows the greatest distance between the highest valley and lowest peak. Rmax is useful to identify any abnormal surface defects that could be detrimental to seal performance. Rmax is sometime referred to as Rz1max. In the above figure the sampling length Rz3 records the greatest vertical distance between peak and valley and this is the Rmax (Rz1max) value. Rt is the total vertical distance between highest peak and lowest valley.
To find out more about the factors affecting seal performance, contact Mark McCormack (Engineering Director, FTL Technology) at email@example.com or call 0113 220 3435.