Inner Diameter

In order for the O-Ring to fit snugly in the groove, it is desirable to circumferentially stretch the O-Ring slightly. The recommended amount of stretch S is between 1% to 5%, with 2% as the preferred stretch value.

By stretching the O-Ring, we ensure that the O-Ring will stay in the groove and will not fall out or otherwise twist in some unpredictable manner during assembly.

Cross Section Diameter

The O-Ring is compressed in the radial direction when seated in the gland. Hence, one can think of the O-Ring cross-section as being pinched between the Bore Diameter B_d and the Groove Diameter G_d. In order for the O-Ring to be compressed when in the gland, its cross-section diameter CS must be greater than the total effective depth of the groove,

The difference between CS and the effective gland depth represents the compression C of the O-Ring (a dimensionless quantity),

C is required to be greater than zero in order for the O-Ring to be compressed. The recommended upper limit of C depends on the type of seal. In static seals, where the O-Ring is not in axial motion in the bore, the recommended maximum compression is approximately 40%. In dynamic seals, such as a piston moving inside a cylinder, the recommended maximum compression is somewhat less at 30%.

To account for manufacturing tolerances, a range of cross-section diameters (CS_min to CS_max) can be provided by the following two equations,

where all symbols are defined in these tables. These two equations are implemented in the Radial O-Ring Selection calculator.

Groove Width

When the O-Ring is compressed radially, it will expand axially (since most elastomeric materials are effectively incompressible). The Groove Width GW should therefore be about 1.5 times the O-Ring cross-section diameter to accomodate this axial expansion,

• The O-Ring must be compressed by a predetermined amount, and this compression determines the O-Ring cross-section diameter.

• The O-Ring inner diameter is typically chosen to be close to the groove's inner diameter; by selecting it to be slightly less than the groove's inner diameter, the O-Ring will stretch and hug the groove.

• The Groove Width must be larger than the O-Ring cross-section diameter, to accommodate the radial expansion of the O-Ring when it's axially compressed in the gland.

Refer to the Radial Seal design for equations and discussions.

To complete the O-Ring design process, the O-Ring material needs to be specified based on the seal's anticipated environment. Chemicals, temperature extremes, and pressure are some of the environmental factors to consider.