Why extrusion occurs
Pressure pushes the elastomer toward the clearance gap between mating parts. If the gap, pressure and temperature are high enough, the O-ring enters the gap and develops nibbling, peeling or permanent damage. Softer compounds and pressure cycling generally reduce the available margin.
A back-up ring is a harder, low-deformation support installed beside the O-ring on the low-pressure side. It bridges the clearance and limits how far the elastomer can enter it.
One ring or two?
For pressure from one direction, place one back-up ring on the downstream or low-pressure side of the O-ring. If pressure can reverse, use support on both sides unless the selected seal system provides another controlled solution. Drawings should show pressure direction explicitly; “fit one back-up ring” is incomplete without placement.
Common constructions
Solid rings provide continuous support but require access for installation. Split or scarf-cut rings can be fitted into closed glands but introduce an assembly orientation and gap to control. Spiral designs are easier to install in some hardware and provide overlapping support. PTFE and engineered thermoplastics are common, but temperature, fluid, creep and wear still require grade-specific selection.
The groove must change
Do not add a back-up ring to an existing O-ring groove without checking width. The gland must accommodate the O-ring, back-up ring thickness, tolerances, thermal expansion and any expected swell while preserving working clearance. A groove that becomes overfilled can trade extrusion risk for friction, heat and assembly damage.
Recalculate squeeze and fill after changing the gland. The extrusion calculator helps screen the combination of pressure, maximum clearance, hardness and temperature; it does not replace the dimensional requirements of the selected back-up-ring supplier.
Assembly and inspection
Protect the back-up ring from sharp edges and excessive bending. Orient split designs exactly as specified. After assembly, verify that the O-ring has not slipped under the support ring and that neither component crosses a port or housing split.
When a back-up ring is not enough
At very high pressure, rapid gas decompression, high speed, large eccentricity or severe temperature, the problem may require a smaller clearance, harder or decompression-resistant compound, different gland, pressure staging or a different seal architecture. Treat the back-up ring as one element of a pressure-control design—not a universal add-on.