Backup Rings: The Anti-Extrusion Support for High-Pressure Sealing Systems

A backup ring, also called an anti-extrusion ring, is a rigid auxiliary sealing component commonly assembled together with O-rings and other elastomeric seals inside a gland. It is not a sealing element itself; its core mission is to stop the soft O-ring from being squeezed into the clearance gap between mating metal parts under high fluid pressure, a destructive failure mode known as extrusionISO. Even a small extrusion notch will quickly expand under cyclic pressure, causing nibbling, tearing, leakage and early seal breakdown.

Elastomer O-rings such as NBR, FKM, EPDM and HNBR are flexible and reliable for general service, yet they have natural pressure limits. Under standard clearance, a typical 70 Shore A O-ring can only work reliably around 100 bar without backup support. When system pressure rises, or the diametral clearance becomes larger, the O-ring material is forced toward the gap. The backup ring sits on the low-pressure side of the O-ring and forms a solid barrier, filling the gap and containing the elastomer within the gland space. Adding one or two backup rings can greatly lift the pressure rating of the sealing assembly, often up to 350 bar or higher depending on material and groove design.

ISO 3601‑4 classifies backup rings into five main types: spiral‑cut (T1), angle‑cut (scarf‑cut, T2), solid uncut (T3), concave angle‑cut (T4) and concave solid (T5) stylesISO. Each type fits different assembly and operating conditions. Spiral-cut PTFE backup rings can be opened and wrapped over pistons or rods without full gland disassembly, which saves installation time for field maintenance. Scarf-cut rings are widely used for general static and dynamic hydraulic service. Solid uncut rings deliver the best anti-extrusion performance for heavy-duty and aerospace applications, covered under SAE AS5782 and AS5781 standardsSAE Mobilu....

PTFE is the most popular material for backup rings. It offers low friction, broad chemical compatibility, and stable performance across a wide temperature range. Filled PTFE grades with glass, carbon or bronze fillers improve compression strength and wear resistance. For ultra-high pressure and demanding wear scenarios, PEEK backup rings are selected for their superior rigidity and creep resistance, though they cost more than PTFE alternatives.

The selection of backup rings depends on four key factors: working pressure, operating temperature, media compatibility and gland clearance. For unidirectional pressure, a single backup ring is placed on the downstream side of the O-ring. For bidirectional pressure, two backup rings are fitted, one on each side of the O-ring, so the anti-extrusion protection works no matter which direction pressure comes from. Improper installation, wrong ring size, oversized hardware clearance or overheating will still lead to backup ring failure, such as crushing, cracking or cold flow of PTFE.

Backup rings are widely used in hydraulic cylinders, valve manifolds, oil and gas equipment, aerospace actuators, heavy machinery, and chemical processing systems. Many engineers underestimate this simple annular part until they face repeated O-ring extrusion failures. In sealing design, the O-ring takes the sealing responsibility, while the backup ring acts as the safety defender. Together, they extend service life, reduce unplanned downtime and maintain stable sealing integrity in tough high-pressure environments.

In short, a backup ring is a cost-effective, essential upgrade whenever an O-ring system crosses its safe pressure-clearance boundary. It turns a vulnerable seal arrangement into a robust high-pressure sealing solution.