Dynamic sealing is defined as:

• A. A stationary seal on a moving component.
• B. A valve seal at a hydraulic line.
• C. A moving seal such as a piston seal on a hydraulic cylinder.
• D. An external seal on the platform surface.

Answer: (C)

Dynamic sealing means a seal that has to maintain a barrier while there is relative motion between the sealing surfaces. The seal and the moving part slide past each other, so the lip must seal effectively under movement and pressure changes.

A piston seal inside a hydraulic cylinder is a classic example: the piston moves back and forth, and the seal slides against the bore to prevent leakage from the high-pressure side to the low-pressure side during operation. This moving, sliding interface is what characterizes dynamic sealing.

The other descriptions describe seals that are static or not part of a moving hydraulic interface, so they don’t exemplify dynamic sealing in the same way.