Introduction

O-Rings are torus-shaped (i.e. doughnut-shaped) objects made from elastomeric compounds such as natural or synthetic rubber, and are used to seal mechanical parts against fluid movement (air or liquid). O-Rings perform their sealing action by deforming to take the shape of their cavity, after being oversized to guarantee an predetermined interference fit.

O-Rings are inserted into cavities defined as glands, and are typically used in one of two seal designs, axial or radial. These seal designs along with their gland geometries are shown in the following schematic:

An O-Ring is specified by its inner diameter, its cross-section diameter, its material hardness/durometer (typically defined by the Shore A hardness), and its material composition.

In order for an O-Ring to seal against the movement of fluid, it must be compressed when seated inside the gland. A standard set of design guidelines exist to determine the proper O-Ring dimensions for radial and axial seals of a given dimension.

O-Ring Symbol Definitions

Parameters used in the discussion of O-Rings are defined in the following table:

O-Ring Parameters

SymbolParameterDescription

IDInner DiameterDiameter of the inside edge of the cross-section.

CSmaxMaximum Cross-Section DiameterUpper bound on the cross-section diameter for a given set of input requirements.

CSminMinimum Cross-Section DiameterLower bound on the cross-section diameter for a given set of input requirements.

CStolCross-Section ToleranceManufacturing tolerance on the O-Ring cross-section diameter.

CmaxMaximum CompressionUpper bound for the cross-section compression (in %) when the O-Ring is seated in the gland; used as a design input.

CminMinimum CompressionLower bound for the cross-section compression (in %) when the O-Ring is seated in the gland; used as a design input.

Radial Gland Symbol Definitions

Parameters used in the discussion of glands for radial seals are defined in the following table:

Gland Parameters for Radial Seals

SymbolParameterDescription

BdBore DiameterInner diameter of the bore which confines the outer diameter of the O-Ring.

BtolBore Diameter ToleranceManufacturing tolerance on the bore diameter.

GdGroove DiameterMinimum diameter of the gland which confines the inner diameter of the O-Ring.

GtolGroove Diameter ToleranceManufacturing tolerance on the groove diameter.

GWGroove WidthGroove length in the axial direction; must be large enough to accommodate O-Ring axial expansion.

Design Guidelines for Radial Seals

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 PointO-Ring Stretch Value

%S

minimum1%0.01

maximum5%0.05

recommended2%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),

INCLUDEPICTURE "http://www.efunda.com/DesignStandards/oring/images/eqn_c_limit.gif" \* MERGEFORMATINET 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 TypeRecommendedMaximum Compression

%C

static40%0.40

dynamic30%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,

Design Guidelines for Axial Seals

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.