What kind of seal would you use on a moving part?

• A. O-ring
• B. Gasket
• C. Packing
• D. Sealant

Answer: (C)

The key idea is that sealing a part that moves requires a seal that can tolerate motion and wear. Packing fits that need well: a packing material is placed in a gland (stuffing box) around a shaft or moving surface and is compressed by tightening the gland. As the shaft moves, the packing deforms slightly to fill the small gaps, providing a seal while allowing sliding or rotating motion. When wear reduces sealing efficiency, the gland can be adjusted and the packing replaced, keeping the seal effective without disassembling the entire joint.

O-rings are common dynamic seals too, but they’re best where a groove and a precise cross-section fit are available and where replacement or re-grooving is practical. Gaskets seal static joints, not moving interfaces. Sealants aren’t used for ongoing dynamic sealing around moving parts because they don’t maintain a reliable, adjustable seal under motion and pressure.

So, for a moving part in a gland-sealed area, packing is the appropriate choice.