Back-Up Ring

A washer like device of a relatively hard, tough material installed in the gland on the downstream side of the O-ring to prevent O-ring extrusion into the diametrical gap while under pressure.

U-Cup

Used extensively throughout the pneumatic and hydraulic industries because of low breakaway force and dynamic friction. Also used for sealing large extrusion gaps. Designed for low speeds/pressures and can be energized with springs or O-rings.