Static O-Ring Sealing

It has been said that O-rings are “the finest static seals ever developed.” Perhaps the prime reason for this is because they are almost human proof. No adjustment or human factor comes into play when O-rings are assembled originally; or used in repairs when the gland has been designed and machined properly. O-rings do not require high bolting forces (torque) to seal perfectly.They are also versatile in saving space and weight. They seal over an exceptionally wide range of pressures, temperatures and tolerances. Once seated, they continue to seal even though some feel that they theoretically should not. In addition, they are economical and easy to use. Therefore, we agree that the O-ring is “the finest static seal ever developed.

Surface Finish for Static O-Ring Seals

• The design charts in the sections below indicate a surface roughness value not to exceed 32 micro-inches (32 rms) on the sealing surfaces for static seals with a maximum of 16 rms recommended for face-type gas seals. 

  These figures are good general guidelines, but they do not tell the whole story. Equally important is the method used to produce the finish. If the surface is produced by turning the part on a lathe, or by some other method that produces scratches and ridges that follow the direction of the groove, a very rough surface will still seal effectively. Some methods such as end milling or routing, however, will produce scratches that cut across the O-ring. Even these may have a rather high roughness value if the profile across them shows rounded “valleys” that the rubber can readily flow into. Usually, these tool marks have sharp, deep, angular valleys that the O-ring material will not penetrate or fill completely. For this type of surface, the recommended roughness values should not be exceeded.