Our O-Ring Design Reference series covers static sealing up to about 1500 psi. Above that region — or sooner when clearance is generous or the compound is soft — pressure starts pushing the O-ring into the clearance gap between piston and bore. The standard engineering answer is simple and inexpensive: a back-up ring. This article explains how they work and how to design them in.
A back-up ring is a rigid or semi-rigid ring seated in the groove next to the O-ring, on the low-pressure side. It does not seal itself — it closes the extrusion path, acting as a zero-clearance wall behind the elastomer. When pressure spikes, the O-ring is energized against the sealing surface while the back-up ring absorbs the pressure trying to squeeze rubber into the gap.
| Arrangement | Configuration | Used when |
|---|---|---|
| O-Ring only | O-ring centered in groove | Pressure below the extrusion limit for your hardness and clearance |
| Single back-up | Back-up ring on the low-pressure side of the O-ring | Pressure acts in one direction |
| Double back-up | One back-up on each side of the O-ring | Pressure alternates direction, or direction is unknown |
| Back-up material | Strengths | Typical service |
|---|---|---|
| Filled PTFE (glass, carbon, bronze-filled) | The default: wide temperature range, chemical resistance, low friction, forgiving installation | −50°C to +200°C general hydraulics; most industrial back-ups |
| PEEK | Very high strength at high temperature and pressure | Demanding oil-and-gas and high-temperature systems |
| Hard elastomer (90 Shore A) | Low cost, elastic, easy to fit | Moderate pressures and temperatures; reusable on disassembly |
| Polyurethane | Tough and abrasion-resistant | Mobile hydraulics with moderate temperature |
| Leather / specialty fabrics | Legacy and certain oilfield designs | Specific legacy applications |
Adding a back-up is not free in geometry terms:
The decision is a three-variable check — pressure, diametral clearance, and O-ring hardness — detailed with numbers in Part 5 of our O-Ring Design Reference. The conservative engineering rule is: plan for back-ups above roughly 1500 psi unless your clearance and durometer calculations prove the O-ring alone survives the worst case, and always use them where pressure spikes or shock loads are expected. Retrofitting back-up rings is also the standard remedy for the nibbled-edge failure pattern in our O-ring failure analysis.
The cheapest high-pressure upgrade in fluid power is a plastic ring in the existing groove. When extrusion is the failure mode, do not buy a harder or fancier O-ring first — block the gap.
We supply PTFE, PEEK and elastomer back-up rings matched to standard AS568 and metric O-ring sizes, including single and double-acting sets. Send us your pressure, clearance and groove dimensions through the O-ring product page and our engineers will confirm size and arrangement.