Machinery's Handbook, 31st Edition
O-RINGS
2679
(O-Ring Durometer Shore A)
Fig. 4. Extrusion Potential of O-Rings as a Function of Hardness and Clearance Gap Clearance Gap.—When there is relative motion between two parts enclosing an O-ring, there must be a clearance gap between them (see Fig. 2 and Fig. 3). This broadly applies to piston and rod seals. Axial face seal applications also may have a clearance gap, but generally they do not. For a piston gland (see Fig. 3), the radial clearance gap is the distance between the piston surface and the bore wall. For rod glands (again, see Fig. 3), the gap is the distance between the rod surface and the bore wall. The radial clearance gap on one side is at its maximum value when the piston or rod is side-loaded and touches the bore (zero clearance gap) on the other side. This gap often is expressed in terms of diametral clearance, which is twice the nominal radial clearance; diametral clearance is the same, regardless of piston or rod position. If the radial clearance gap between the sealing surface and the gland edge is too large, or if the fluid pressure exceeds the deformation limits of the O-ring, it will extrude into the gap, reducing the effective life of the seal. This is especially problematic for soft sili- cone O-rings, where a 50 percent reduction in the standard clearance gap may be recom- mended. To prevent O-ring extrusion, reduce the clearance gap by modifying the gland dimensions, lower the system operating pressure, or install backup rings. A harder O-ring material may be used but could result in higher drag, increase the tendency of the seal to leak at low pressures, and necessitate additional dimensional changes. Fig. 4 indicates effective diametral clearance values that may be used, depending on fluid pressure and O-ring hardness. If conditions of use fall to the right of the curve,
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