A static o-ring is used between two mating surfaces that

• A. Move relative to each other
• B. Do not move relative to each other
• C. Rotate
• D. Vibrate

Answer: (B)

A static o-ring is used when the two mating surfaces stay in place relative to each other, so the seal relies on a fixed compression of the o-ring to block leakage. When the surfaces do not move, the o-ring sits in its groove and is squeezed enough to fill gaps, creating a stable seal through deformation. This minimizes wear and maintains a constant seal pressure around the circumference.

If the surfaces moved, the o-ring would experience sliding or flexing, causing shear, extrusion, and wear that can lead to leaks. Dynamic situations require seals designed for motion, with different geometries and materials to handle sliding, friction, and higher wear.