Page 295 - The Tribology Handbook

B27 Soft piston seals

SELECTION AND DESIGN

Table 27.1 Guidance on the selection of basic types

l&e name Distributor ‘u’ CUP ‘0’ rinz
External-fitted to piston, sealing in bore

COMPRESSION
Internal-fitted in housing, sealing on piston or rod

Simple housing design Good Good Poor Very good
Low wear rate Very good Good Good Poor

High stability (resistance to roll) Good Fair Very good Po01
Low friction Fair Fair Fair Good
Resistance to extrusion Good Good Good Fair

Availability in small sizes Fair Good Poor Very good
Availability in large sizes Good Fair Good Good
Bidirectional sealing Single-acting only. Use in pairs back-to-back for Effective but
double-acting. ‘Non-return’ valve action can usually used in
be useful pairs

Remarks Do not allow heel to touch mating surface Avoid parting
except under high pressure. line flash on
Use correct fits and guided piston, etc. the sealing

If seal too soft for pressure, lip may curl away Unsuitable for
from surface rotational
movement

Application notes
acetal resin, nylon, PTFE, glass fibre/PTFE or metal
Long lips take up wear better and improve stability but bearings.
increase friction. Use plastic back-up rings to reduce To prevent mixing of unlike fluids, e.g. aeration of oil,
extrusion at high pressures. The use of a thin oil will use two seals and vent the space in between to atmosphere.
reduce wear but may increase friction. For pneumatic
assemblies use light grease which may contain colloidal
graphite or MoS2. Choose light hydraulic oil for mist
lubrication.
Avoid metal-to-metal contact due to side loading or
piston weight. If seals will not maintain concentricity use VENT TO
ATMOSPHERE
B27.1