'O' Ring Guide

General design notes
How ‘O’ rings work
Rubber has a very high bulk modulus and
is therefore virtually incompressible. This
means that an ‘O’ ring must be deformed
on the diameter section to give it an initial
sealing force within a housing (Figure 1).
G
2
G
2
Figure 1
Pressure
When system pressure is applied, the
‘O’ ring deforms further (Figure 2). But
Pressure
because of the initial squeeze, the sealing
force always exceeds the force exerted on
the ‘O’ ring by the system pressure.
Figure 2
At higher pressures, back-up rings may be
required to prevent ‘O’ ring extrusion (see
page 12). Leakage problems often arise
due to lack of initial squeeze, or the ‘O’ Pressure
ring diameter section being too large for
Pressure
its housing.
Reciprocating applications
‘O’ rings marked with an asterisk (*) on
our charts can be used for both static and
light/medium reciprocating duties. Other
sizes are not recommended for these
dynamic applications.
Rotary applications
‘O’ rings may be used for rotating
applications where peripheral speeds are
low. Please contact our Technical Support
Team for recommendations.
G
2
Non-standard housing diameters
Individual Grings 2
can be stretched or
squeezed very slightly to fit housing
diameters Gthat do not match the
dimensions 2
specified on our ‘O’ ring
size charts.
The amount of allowable deformation
varies according to the application, as
follows:
l Groove in cylinder application:
A maximum of 3 per cent squeeze
is acceptable on an ‘O’ ring outside
diameter G to fit a shaft diameter C
that is not 2 covered by JW chart sizes.
(See Figs G 5 & 6, page 29.)
2
l Groove in piston application:
A maximum of 4 per cent stretch
is acceptable on an ‘O’ ring inside
diameter to fit a cylinder diameter
D that is not covered by JW chart sizes.
(See Figs 7 & 8, page 29.)
l Flange and triangular groove
applications: A maximum of 2 per
cent stretch is acceptable on an
‘O’ ring inside diameter when the seal
is used on an external pressure flange,
or housed in a triangular groove.
(See Fig 9, page 29.)
Likewise, a maximum of 1 per cent
squeeze is acceptable on an ‘O’ ring
outside diameter when the seal is used
on an internal pressure flange.
(See Fig 10, page 29.)
Cylinder and piston housing tolerances
It is important that tolerances on housing
diameters for cylinders and pistons meet
the requirements of the formulae given
on page 29 (ie, dimensions C and D in
Figures 5-8).
Surface finish of metal parts
For maximum seal life the surface finish
of metal parts in contact with an ‘O’ ring
should not exceed:
l 0.8µm (32µin) CLA or Ra for static
parts.
l 0.4µm (16µin) CLA or Ra for moving
parts.
A finish finer than 0.15µm (6µin) should
be avoided in dynamic applications as a
lubricating film may not be retained. For
details of these finishes, please refer to
BS 1134: Assessment of surface texture.
Diametral clearance G
Under no circumstances should the
maximum total diametral clearance
(G max) indicated on our housing tables
(pages 30-31) be exceeded. This is
to ensure that, if complete shaft offset
occurs, the maximum extrusion gap at
any point on the ‘O’ ring does not
exceed G.
To order or get further details, call your local contact shown on rear cover or listed at www.jameswalker.biz 27