Static seals are categorized as either axial or radial, depending upon the direction in which squeeze is applied to the o-ring's cross section.

Static Axial Seals

A static axial seal acts similar to a gasket in that it is squeezed on both the top and bottom of the o-ring's cross section. This type of seal is typically employed in the face (flange) type applications, depicted in Illustration 4.1.

When used as a face seal involving either internal or external pressure, the o-ring should always be seated against the low pressure side of the groove (as shown in Illustration 4.1 and Illustration 4.2) so the o-ring is already where it needs to be as a result of the pressure.

Static axial seals tend to be easier to design than static radial seals. Since there is no extrusion gap, there are fewer design steps and you can control the tolerances easier.

Static Axial Seal Gland Dimensions

SAE recommended dimensions for static axial seal glands by ascending AS568 o-ring numbers. For the complete table, see Static Axial Seal Gland Dimensions

AS568 Number O-Ring Dimensions (O.D.) A
Internal Pressure
External Pressure
Groove Width
Gland Depth
I.D. ±Tol. W ±Tol.
Tolerance   +.005
-004 .070 .005 .070 .003 .210 ** .075 .125 .049
-005 .101 .005 .070 .003 .241 ** .106 .125 .049
-006 .114 .005 .070 .003 .254 ** .119 .125 .049
-007 .145 .005 .070 .003 .285 ** .150 .125 .049
-008 .176 .005 .070 .003 .316 ** .181 .125 .049
**O-Ring seal sizes not listed are not recommended for axial seals because the seal ID after installation becomes too small for practical use.


Static Radial Seals

Static Radial Seals are squeezed between the inner and outer surfaces of the o-ring. They are typically employed in cap and plug type applications, as depicted in Illustration 4.3.


Recommended dimensions for static radial seal glands listed in Table C are based on an application pressure limit of 1500 psi. For higher pressure requirements, reference the Operating Environment Factors section, Illustration 5.1 or contact Apple Rubber for technical assistance.

Static Crush Seals

In crush seal applications, the o-ring is completely confined and pressure deformed (crushed) within a triangular gland made by machining a 45° angle on the male cover. Squeezed at an angle to the o-ring's axis, crush seals are used in such simple applications as the one depicted in Illustration 4.4.

Static Seals With Dovetail Glands

O-Rings are sometimes employed in static or slow moving dynamic situations calling for specially machined “dovetail” glands. Because of the angles involved, controlling the tolerances in these glands may be difficult. The purpose of these glands is to securely hold the o-ring in place during machine operation and/or maintenance disassembly. A typical valve seat application is shown in Illustration 4.5.

In this application, o-ring squeeze is primarily axial in direction (as valve operation exerts force on top and bottom seal surfaces). To avoid tearing or nicking, the use of o-ring lubrication is recommended while installing the o-ring into the dovetail gland. Because of the difficulty in creating the groove and tight tolerances required, this type of seal application should only be used when necessary.