How OEM's can make their parts better, faster, and more efficient.

NASA Tech Briefs

Innovations developed by NASA and its industry partners in a wide array of fields.

Salary Expectation

8 things to know about the interview question "What's your salary expectation"?

Metal 3D Printing Design Guide

Direct Metal Laser Sintering (DMLS) 3D printing for parts with reduced cost and little waste.

more free magazines
In radial seals, the gland is defined by the Bore Diameter on the outside radius, the Groove Diameter on the inside radius, and the Groove Width in the axial direction (see schematic).

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.

Operating Point O-Ring Stretch Value
% S
minimum 1% 0.01
maximum 5% 0.05
recommended 2% 0.02
The O-Ring inner diameter ID can be found from the recommended stretch Srec and the Groove Diameter Gd,

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 Bd and the Groove Diameter Gd. 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%.

Seal Type Recommended
Maximum Compression
% C
static 40% 0.40
dynamic 30% 0.30
Typically, compression is a design input assigned by the design engineer. In this case, CS is found by inverting the above compression equation,

To account for manufacturing tolerances, a range of cross-section diameters (CSmin to CSmax) 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,

O-Ring design for axial seals is similar to that for radial seals, with the important points summarized below:

  • 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.

Post-Design Selection
Once the O-Ring's cross-section diameter and inner diameter have been calculated, specific O-Rings can be selected from the AS 568A Standard. If the desired O-Ring is not available in this standard, then manufacturer catalogs can be consulted for a larger selection. If there is still no match, then the only remaining options are to change the design of the bore or groove (or both); OR request that a custom O-Ring be manufactured.

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.