SEAL RINGS - ELASTOMERIC

Seal rings of the O-ring type are used as both static and dynamic seals. Static seals
serve the same purpose as gaskets; that is, they provide a seal between two members
that are not intended to undergo relative motion. Dynamic seals, however, are used
where rotating or reciprocating motion is intended to occur.
O-rings are molded to the size of the elastomeric material with a circular cross
section, as shown in Fig. 17.1«. The size is designated by the cross-sectional diameter
w and the nominal inside diameter (ID). The standard sizes specified in SAE J120a
are summarized in Table 17.!.The first size number in a group is associated with the
minimum inside diameter, and the last size number is associated with the maximum
inside diameter. Some manufacturers provide additional sizes that extend the range
of inside diameters for a particular cross-section size. The nominal inside diameters
were selected to provide dynamic seals in cylinder bores dimensioned in inches and
common fractions of inches. Either SAE Jl2Oa or manufacturers' recommendations
should be consulted to obtain the recommended compression of the cross section.
The compression is different for static and dynamic applications.
The rectangular-section ring in Fig. 17.Ib is manufactured by cutting lengths from
a tube of molded material. The standard sizes listed in SAE J120a are summarized in
Table 17.2. The first size number in a group is associated with the minimum inside
diameter, and the last size number is associated with the maximum inside diameter.
Rectangular-section rings are suitable for static applications with pressures up to
1500 psi [10.3 newtons per square millimeter (N/mm2)].
The standard shape of groove for sealing rings is shown in Fig. 17.2. The actual
groove dimensions depend on the type and size of the seal ring cross section and the
nature of the application. Recommended groove dimensions are provided in SAE
J120a and in the manufacturers' literature. Because elastomeric materials are almost
incompressible, it is necessary to provide sufficient volume for the seal ring in the
groove. The recommended groove dimensions do so.
For static seals, a finish on surfaces contacted by the seal ring that is rougher than
32 uin (0.8 um) may lead to leakage. Because rough finishes accelerate seal wear in
dynamic seals, a surface finish of 5 to 16 uin (0.13 to 0.4 um) is preferred. Friction is reduced with the smoother finish, but surfaces smoother than 5 uin (0.13 um) may
not be satisfactory for reciprocating motion.
A static seal ring application in which the joint is subject to internal pressure only
is shown in Fig. 17.3«. The groove design in Fig. Il3b is for a joint subject to external
pressure or internal vacuum only. It is generally advisable in these applications
to use as large a seal ring cross section as possible because the tolerance on the
groove depth is greater with larger cross sections. This requires less precise machining
and tends to reduce manufacturing costs.
O-rings are also used as static seals for hydraulic tube fittings that are screwed
into tapped holes. Recommended machining dimensions are provided in SAE J514
(June 1993).
Elastomeric sealing rings are most commonly made of nitrile (Buna N) compounds.
These compounds are low in cost and are compatible with alcohol, gasoline,
hydraulic fluids, lubricating oils, and water. They also are suitable for temperatures
ranging from -67 to 2570F (-55 to 1250C). For resistance to higher temperatures
or compatibility with other fluids, other compounds are employed. Among these
compounds are butyl, ethylene propylene, neoprene, fluorocarbon, silicone, and
polyurethane.