Pressure is a double-edged sword. In most O-ring sealing applications, a moderate amount of system pressure is desirable because it aids in effecting the seal. Since the rubbery O-ring is essentially a highly viscous fluid, moderate pressure forces this thick material firmly against the mating surfaces, and a positive seal is created. If an inadequate amount of squeeze is employed, or if the seal material has poor compressive modulus, low pressure (below 100 psi) may not be enough to help ensure adequate contact between the O-ring and the mating surfaces. A leak path may remain and failure can result.

High pressure can be even more problematic. Pressure increases may expand the mating components, often enlarging the clearance gap between parts. The larger the gap, the greater the likelihood that part of the O-ring will be forced (extruded) into it. Extrusion becomes most likely as pressure approaches or exceeds 1,500 psi. Constant high pressure will cause the surface of the extruded seal to rupture. Even if pressure occasionally drops (as during cycling or system fluctuations), the extruded portion of the seal is still vulnerable. The O-ring’s elastic memory enables it to regain its original shape, but it may not recede out of the retracting gap quickly enough. A small chunk of its material may be torn (nibbled) away. Both extrusion and nibbling ultimately create a leak path, and seal failure inevitably results (see Figures 75 and 76).

The amount of extrusion to be expected in a given application thus depends on three main factors: 1) the pressure imposed on the seal, 2) the amount of clearance between mating surfaces, and 3) the resistance of the seal material to deformation. The first two factors are not easy to adjust. The inherent needs of the application often dictate that system pressure cannot be lowered. Reducing the clearance gap is possible but can often be very expensive. Due to other variables (such as wear and misalignment of mating parts), a redesigned gap still may not adequately prevent extrusion. This leaves the third factor, resistance of the seal to deformation, as the most viable avenue for reducing extrusion concerns.

Hardness and Young’s Modulus (modulus of elasticity, or stiffness) chiefly determine an O-ring’s resistance to deformation. Materials with both high modulus and hardness tend to have fewer extrusion problems. Temperature-induced changes in modulus and stress relaxation behaviors (such as creep/cold flow) can, however, put even the most stalwart O-ring in danger. For more on extrusion, see also Diagnosing O-Ring Failure: Extrusion & Nibbling.

ENVIRONMENT MAIN PAGE