PTFE Spring-Energized Seal Design
When designing a component that includes a spring-energized PTFE seal, its not enough just to specify all the seal parameters; another key part of the design is the surface finish of the mating surface. In this article, we are going to review what surface finish is, discuss the effects of surface finish on seal life, and then go over recommended surface finishes for spring-energized PTFE seals.
For more articles on PTFE Spring-Energized Seals from the Advanced EMC Technologies Blog:
- Five Energizers for PTFE Spring-Energized Seals
- Six Kinds of Polymer Jacket Materials for Spring-Energized Seals
- PTFE Energized Seals with PEEK Back-up Ring: Match Made in Heaven for High Pressure Applications
Surface Finish
Surface finish is a measure of the roughness of a surface. If you look at a surface in profile under a microscope, you will see that even a smooth surface is made up of microscopic valleys and peaks. The mean surface height is the average height of all the valleys and peaks of a given surface. The value RA is the average of how much the peaks and valleys deviate from the average surface height, and the RMS is the root mean square of this deviation. The RMS value is more sensitive to extremely high deviations than the RA, and some consider it a better measure of surface finish.
Smoother surfaces have small values for RA and RMS, which makes sense because a smoother surface should have smaller peaks and valleys.
Surface Finish and Sealing
If the surface finish is too rough, not only can is abrade the seal lip but leak paths can be created, which defeats the purpose of a seal.
However, its good thing if a thin film of PTFE is transferred to the mating surface. This thin film reduces friction between the seal and the surface, and increases seal life. This implies something else, though: if the surface is too smooth, then that thin film of PTFE wont form on the mating surface.
Surface Finish Requirements
For a spring energized PTFE seal, the recommended surface finish for the mating surface is typically 2 RMS to 16 RMS, depending on what your application is. For gases and liquids at cryogenic temperatures, you want a smoother finish with an RMS between 2 to 4. For gases at non-cryogenic temperatures, the recommended surface finish is between 6 and 12 RMS. For liquids, a surface finish between 8 and 16 RMS is sufficient.
Conclusion
When specifying a PTFE spring-energized seal, never take for granted the finish of the mating surface.
Need more information? Download a free copy of the High Performance Sealing Solutions Guide from Advanced EMC Technologies.
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