Page 298 - The Tribology Handbook

Soft piston seals 627

Table 27.6 ‘0‘ ring fits

Hydraulic Pneumatic

SMALL
INTERFERENCE INTERFERENCE
SMALL
LARGE
CLEARANCE LARGE
CLEARANCE

Dimensions tQ BS 1806 No standard available
High friction Low friction
Rapid wear Slow wear

Extrudes into ports Will pass small ports
Small radial clearance Large clearance possible
Seal supports piston Piston unsupported
Moderate bore and housing tolerances Close tolerances on ‘0’ ring dimensions, bore and groove width
-~
Tolerant to material swell and shrinkage Sensitive to swell and shrinkage
Seals at zero pressure drop Seals gas at low pressure-under 1 p.s.i. with 0.003 in
clearance on width
Unsuitable for liquids at any pressure

FAILURE

Table 27.7 Types and causes of failure
TVpe Usual symptom Cause
~
~~~
Channelling (fluid cutting) Small, straight grooves across the sealing Fluid leaking across seal at high velocity
surface
~ ~~
Abrasive wear Flat on ‘0’ ring Pressure too high or abrasive mating surface
Circumferential groove on lip seal
Sharp sealing edge on lip seal
~-
Extrusion Surface broken Pressure too high or too much clearance
Slivers of rubber
-~
Chemical attack Softening or hardening-may break up Incompatible fluid
Temperature effects Hardening and breaking up Too hot and/or excess friction
Breaking up Too coId
~___
~~~
Notes: Symptoms ofcontamination by solid particles are similar to channelling but the grooves are less regular. Uneven distribution of wear
suggests eccentricity or side loading. ‘0’ ring rolling produces variation in shape and size of section.

B27.4