Guides / Sourcing

Choosing and Machining PEEK Backup Rings for Seal Glands

September 26, 2026

Sources: manufacturer datasheets and published standards. See editorial standards.

A backup ring has one job and no room to be wrong about it. It sits on the low pressure side of an elastomer seal and closes the clearance gap the seal would otherwise extrude into. Get the grade or the fit wrong and the ring does not fail gracefully. It lets the seal behind it fail instead, usually at the worst possible moment in a pressure cycle.

Why the ring exists

Push an elastomer seal against enough differential pressure and it behaves like a fluid looking for a way out. The way out is almost always the clearance gap between a piston and its bore, or a rod and its gland, wherever that gap is largest. Once the seal starts extruding into that gap it nibbles away with every cycle, and a seal that held perfectly on day one leaks by the tenth.

A backup ring closes the gap before the elastomer reaches it. It does not seal on its own. It gives the seal a solid wall to push against instead of an open clearance, and it does that job at whatever temperature and pressure the tool sees downhole. That is a stiffer requirement than it sounds, because the ring itself sits right at the extrusion gap and has to resist the same forces trying to push the elastomer through it.

Grade choice comes down to the gap

Unfilled PEEK is the default when the clearance gap is small and well controlled and the ring runs at moderate temperature. It is tougher than any filled grade, resists impact and thermal cycling well, and costs less. A tight gap does not give the ring much room to extrude into, so the extra stiffness of a filled grade buys less than it costs.

PEEK CF30 earns its place when the gap is wider than ideal, pressure is high, or the ring runs hot and dynamic against a moving surface. Carbon fibre raises the modulus and cuts creep at temperature, both of which matter directly here: a ring that creeps under sustained pressure opens its own extrusion path over time, even if it survived the first cycle. See CF30 vs GF30 for the full property comparison, though GF30 is rarely the right call here since a backup ring usually does not need to insulate.

Bearing grade blends show up too, mainly on rings that also slide against a mating metal surface during tool actuation rather than sitting static. That is a different failure mode from extrusion and it changes the spec, so say explicitly whether the ring moves in service or just seals against pressure.

What the gland actually demands

A backup ring is only as good as its fit in the groove that holds it. Three things matter more than anything else on the drawing:

Outside and inside diameter. These set how well the ring centres in the gland and how tightly it closes the extrusion gap. This is where the tight tolerance belongs, not spread evenly across every dimension.

Surface finish where the ring meets the elastomer and the bore. A rough surface either side of the ring gives pressure a path around it instead of through the gap it is meant to close. Specify the finish, do not assume the shop will guess the right one.

Edge condition. Sharp corners on the ring or the groove concentrate stress exactly where a ring under pressure is most likely to crack or notch. A small radius or chamfer, called out on the drawing, costs nothing and removes a common failure start point.

Ring geometry itself varies. A solid ring gives the cleanest seal against extrusion but has to be assembled into a closed gland before the elastomer goes in. A scarf cut or single turn ring installs into an assembled gland but has a joint that is, by definition, a weaker point in the ring. Which one you can use is usually decided by the tool’s assembly sequence, not by the ring material, so settle that before the drawing goes out.

What can still go wrong

Two failure modes account for most backup ring problems that are not simple undersizing.

Explosive decompression. A ring that sits in a gas charged environment and sees a fast pressure drop can blister or crack from the inside, independent of how well it fit the gland. This is covered in more depth in explosive decompression and PEEK, and it is worth qualifying for separately from the fit and grade decision above.

Thermal fit mismatch. PEEK expands roughly four times more than the steel gland around it. A ring fitted tight at room temperature can bind or distort at service temperature, and one fitted loose to allow for expansion can open a gap at room temperature during handling and testing. Calculate the fit at the actual service temperature, not at assembly temperature, and check both directions of the cycle.

What to put on the RFQ

Grade and the reason for it, the gland dimensions with the tolerances that actually matter, working and peak pressure, service temperature range, the media, whether the ring is static or dynamic in service and the ring geometry, solid or split. For more background on how downhole PEEK parts are specified as a set, see PEEK parts for oil and gas.

Send the drawing with the gland dimensions and service conditions and we match you with shops that treat a backup ring as a pressure component, not a plain washer.

See also: PEEK backup rings and anti-extrusion seal parts, the reference page with grade tables by duty.

Frequently asked questions

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