Which statement correctly defines a static o-ring?

• A. Used between moving surfaces
• B. Used between surfaces that do not move relative to each other
• C. Used as a belt drive
• D. Used to measure rotation

Answer: (B)

A static o-ring is used to seal a joint where the two mating surfaces do not move relative to each other. In this setup, the o-ring sits in a groove or on a face and is compressed by the clamped surfaces. The compression deformes the o-ring just enough to fill the small gaps, creating a tight barrier that prevents fluid or gas from leaking. Because there’s no sliding motion, the seal relies on the elastic squeeze of the material rather than friction, which is why this arrangement works reliably for stationary joints.

This is distinct from dynamic seals, where movement occurs between surfaces (like a rotating shaft or reciprocating piston) and would require different design considerations to handle wear, extrusion, and lubrication.