HARDWARE Surface Finish
For Teflon®/PTFE Seals

Hardware Surfaces

The seal jacket material is affected to a high degree by the surface finish over which the seal must slide.  Mating surfaces that are too rough can create leak paths and be abrasive to the seal.

The transfer of a thin film of PTFE from the seal jacket to the mating dynamic surface will improve seal life.  Dynamic surfaces with relatively rough finishes wear the jacket material too rapidly.  Extremely smooth dynamic surfaces result in material transfer insufficient to form a thin film.  The graph below illustrates the effect of surface finish on seal wear.

Hardware Surface Finish Recommendations

Dynamic Hardware Sealing Surface Finish Hardness

As a general rule, the lower the sealing surface finish, the better the overall seal performance that can be expected.  Lower finish value reduces wear and increases seal life.  A 40 Rockwell C hardness or greater is recommended for slow to moderate reciprocating motion.

The ideal hardness is 55 to 65 Rockwell C.  Hardness in this range is recommended for moderate- to high-speed linear or rotary motion.  Hard anodized surface finishes must be polished after anodizing. 

Testing has shown that grinding of the shaft is the best method of machining, providing lower friction and heat generation than polishing the surface with a polishing cloth.  Spiral grinding marks may cause helical pumping, leading to leakage; therefore, plunge grinding is recommended. When plunge grinding, make sure to avoid even ratios between shaft and wheel speed.

Static Hardware Surface Finish

In most static sealing applications, better overall sealing performance can be achieved with a smoother sealing surface finish.  The recommended optimum surface finish for most static sealing applications is 32 RMS or better.  Polishing or machining surfaces should be circular.

Gland Design

Consideration of proper gland geometry in the early stages of design can eliminate unnecessary installation problems.

The use of split or separable glands in piston and rod seal applications is always desirable to eliminate the need to stretch or compress the seal during installation into the gland.  Split glands also eliminate the need for special installation tools.

To minimize stretching or distortion during assembly in non-split glands, the gland side wall on the pressure side can be reduced to provide a partial shoulder to retain the seal. 

If stretching into a full groove is unavoidable, consult proper procedures and tools. Assembly of the seal over sharp corners, threads, keyways, etc., should be avoided, or protective tooling should be used when these conditions exist.