BRI-STU - Back-up-rings

Back-up rings are used to prevent O-rings from gap extrusion. In the case of the combination of rising pressures and large sealing gaps there is the risk, that the O-ring material will be pressed into the sealing gap on the low pressure side. If this is repeated several times and the pressure continues rising, the O-ring can be damaged irreversible and finally get completely destroyed.

Back-up rings do not provide any sealing function. Through reduction of the sealing gap on the low pressure side they take care, that the O-ring can fulfill it's sealing function without being damaged.


The selection of materials for back-up rings is primarily based upon the pressure and the appropriate extrusion resistance resp. hardness. Additionally parameters like hight of the extrusion gap, media resistance and temperature range have to be taken into consideration. In practice there are various plastics and elastomers like for example PTFE, PA, POM, polyurethane, polyesterelastomer as well as NBR, FKM and EPDM available.

Installation housings

Back-up rings are normally installed in wider designed housings on the low pressure side. Because of the plurality of O-ring housings which are common in the market, the back-up ring dimensions normally have to be adapted to the existing housing geometry. To layout back-up rings the following parameters are required:

  • housing dimensions, including tolerances
  • type of application:
    static / dynamic sealing
    internal / external sealing
  • O-ring dimensions
    pressure, medium, temperature

If back-up rings from existing dimension ranges are used e.g. like NBR 90 the installation housing has to be designed according to the producers guidelines.
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