Spec'ing roughness suitable for an O-ring

Hello,
I am attempting to write a process standard for checking the surface finish of a tube surface. An o-ring is used to seal between the tube and mating components. The typical check for surface roughness is Ra, the average distance between the peak and valley. The o-ring can handle smooth transition between peaks and valleys, but does not do well for dramatic surface variations. With this noted, is there a check for the distance between these two features, peak and valley?

Hi, George.

I can't directly answer your question but can provide some possible food for thought. Platers have problems like the fact that nickel plating can provide leveling and shine to a roughly polished surface, but will only emphasize rather than fill a significant scratch. So some traditional roughness standards like your Ra don't help them a lot, but they sometimes use a roughness measure called Ry or Rmax which deals with the deep scratches.

Regards,

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

The smoothness of the surface for an o-ring seal normally would depend on the application.

Water tight seal?

Air tight seal?

Vacuum seal?

Anyhow, all of this information should be readily available in an engineering handbook, or if you come back with a little more info, I can get probably come up with an answer for you.