The best compound fails on a bad surface. Surface requirements depend on whether the seal is static or dynamic, what is being sealed and what the seal is made of.
Typical roughness targets (Ra)
| Surface | Liquid | Gas or vacuum |
|---|---|---|
| Static groove walls and faces | 1.6 micrometres | 0.8 micrometres or finer |
| Static mating face | 0.8 micrometres | 0.4 micrometres |
| Dynamic surface (rod, piston, bore) | 0.2 to 0.4 micrometres | 0.2 micrometres |
| PTFE dynamic counterface | 0.1 to 0.3 micrometres | 0.1 to 0.2 micrometres |
Use Ra with Rz or Rmax as well: peaks cut seals, so the peak height and profile (Rpk) matter. Machine marks should be circumferential on sliding faces, not axial.
Hardness of mating surfaces
- For rubber seals: 35 to 45 HRC minimum; harder prevents scoring.
- For PTFE and filled PTFE: 50 to 60 HRC to resist abrasion by glass or bronze.
- Soft aluminium: anodise (hard anodise) or sleeve with steel.
Coatings
| Coating | Use |
|---|---|
| Hard chrome | Piston rods, long wear, but wear-in microcracks can pump fluid |
| Nitriding | Hard, good against PTFE, thin |
| HVOF tungsten carbide | Harsh duty, subsea |
| Ceramic spray | Corrosion and wear, finish carefully |
| Electroless nickel | Corrosion resistance, softer |
Edges and lead-ins
All edges over which a seal passes need a radius or a 15 to 20 degree chamfer, free of burrs. Cross-drilled ports crossed by a seal need a chamfer or must be located outside the stroke. Threads, keyways and splines need sleeves.
Measuring
Use a stylus profilometer with the correct cut-off (0.8 mm for Ra 0.2 to 2.0). Measure across the lay. Record Ra, Rz and Rpk if possible. Do not rely on touch or comparator plates for critical surfaces.
Cleanliness
Particles in the groove scratch the seal. Specify cleaning and protective packaging for finished housings.
