What is the purpose of a backup ring?

• A. Increases sealing pressure
• B. Acts as lubricant reservoir
• C. Provides electrical insulation
• D. Prevents extrusion of O Ring

Answer: (D)

Backup rings are used to stop the O-ring from being pushed or squeezed into the clearance gap when pressure is high. They provide a hard, wear-resistant barrier that supports the O-ring and shares the load, so the elastomer doesn’t extrude into the groove. This keeps the seal effective even under high-pressure conditions.

They’re usually made of non-elastomeric materials like PTFE or nylon and placed in the groove on the high-pressure side to resist extrusion. This function is different from increasing sealing pressure, storing lubricant, or providing electrical insulation, which is why the other options don’t fit. Proper thickness and material selection are key; an incorrect backup ring can cause leaks or sealing problems rather than solve them.