The Effects Of Pressure

Differential pressure affects an O-ring by forcing it to the low pressure side of the gland, causing the cross section to distort (See Illustration 5.1). This motion blocks the diametrical clearance gap between the mating surfaces and forms the seal. If the O-ring cannot resist increasingly high pressure, part of the O-ring will be forced (extruded) into the diametrical gap. This condition leads to premature failure, leakage and system contamination. O-rings operate optimally within a certain range of pressure. Differential pressure does aid in sealing potential by compensating for the elastomer's tendency to assume a compression set over time, which reduces O-ring compression and utility.

Methods commonly used to prevent O-ring extrusion under pressure include:

  • increasing the O-ring hardness (durometer) (See Illustration 5.2).
  • the use of back-up rings to block the diametrical clearance gap and provide support for the O-ring.
  • reducing the diametrical clearance gap dimension
  • lowering of system pressure

    • The Use of Back-up Rings

    As shown in Illustration 5.2, the extrusion limit of O-rings under pressure is determined by the size of the diametrical clearance gap and the hardness of the O-ring material.

    If the point representing the intersection of the lines of sealed pressure and diametrical clearance falls to the right of the material's hardness curve, either the material hardness must be increased, or back-up rings will be required.

    Examples

    A) Material hardness of 70 Shore A. Sealed pressure of 1,000 psi. Diametrical clearance of .016".

    Intersection of sealed pressure and diametrical clearance lines falls to the right of material hardness curve. Increase hardness, use back-up rings, or reduce diametrical clearance.

    B) Material hardness of 80 Shore A. Sealed pressure of 1,000 psi. Diametrical clearance of .016".

    Intersection of sealed pressure and diametrical clearance lines falls to the left of material hardness curve. This is acceptable.

    The use of two back-up rings (one on each side of the rings) is preferred. This will help prevent installation errors, assuring that the clearance gap is always correctly blocked, regardless of pressure direction.