The useful life of a PTFE seal is heavily influenced by the surface over which the seal slides or rotates. This includes both the material choice, surface finish, and hardness of the mating surface. In this article, we will look at how the mating surface influences the life and behavior of a PTFE seal and how to account for that effect during the design process.
If the surface finish is too rough, then the seal will wear prematurely. However, the surface finish needs to be rough enough for a thin film of PTFE to be transferred from the seal to the mating surface.
The recommended surface finish for use with PTFE seals ranges between 2 and 16 RMS depending on the type of media and temperatures involved. As you can see in the table below, cryogenic temperatures require a much smoother surface finish.
Plunge grinding is believed to be the best method of polishing the surface as long as you avoid an even ratio between the shaft speed and the wheel speed. Spiral grinding is to be avoided because it can result in helical pumping effect that leads to seal leakage.
When the mating surface has sufficient hardness, adhesion which can be a major problem in seal applications is reduced. This in turn lowers friction and reduces wear, thus extending the life of the seal. The table below shows the suggested Rockwell surface hardness based on the type of motion involved.
Material and Surface Treatment
Part of the material selection will be based on the surface hardness as just discussed. However, most materials appropriate for use as a shaft may not have a sufficiently hard enough surface without subsequent surface heat treatments. For example, stainless steel grades such as 440C and 17-4 possess good corrosion and chemical resistance and can achieve a hardness between 40RC and 60RC with surface hardening. Here are some examples of surface finishes to consider and the Rockwell hardness that can be achieved with them:
- Electroless nickel plating, 50RC as plated with 62RC after subsequent heat treatment
- Hard chrome plating, 65RC
- Dense chrome plating, 70RC
- Gas nitriding, 70RC
- Plasma, 73RC
When specifying a PTFE seal for an application, it is vital to take into consideration the mating surface. It should have the surface finish recommended for the media and temperatures involved, as well as a certain level of hardness depending on the type of motion and speed. In addition, the material choice is going to be constrained by the hardness that can be achieved by available surface finishes. Careful attention to the mating surface can significantly increase the life of a PTFE seal.
Guide to Surface Finish: http://www.iron-foundry.com/machining-surface-finish-chart-comparator-method-degree-ra-rz-rms.html
Surface Finish Conversion Chart: http://www.engineersedge.com/manufacturing/surface-roughness-conversion.htm
Rockwell hardness testing: http://www.mee-inc.com/hamm/rockwell-hardness-testing/
Electroless plating: http://www.corrosion-doctors.org/MetalCoatings/Electroless.htm
Gas nitriding: http://www.totalmateria.com/articles/Art132.htm