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All O-ring seals tested had the same dimensions, i.e. inside diameter 24.77 mm (to
be used with 25 mm shaft diameter) and cross section diameter 5.33 mm. However,
a number of other seal designs have also been tested for comparison of perfor-
mance. A special design of seal is a PTFE-encapsulated O-ring. The central core is a
solid O-ring moulded in silicone rubber. This is surrounded by a tube of PTFE. The
seal design aims to combine the sealing performance of an elastomer with the chem-
ical resistance and low friction of PTFE. The core provides the sealing action and
the elasticity required for continuous reset of the seal after compression, although
not quite as much as a standard elastomer O-ring. Over long periods of time the
PTFE tube protects the seal from hardening and thus the elasticity is maintained.
Despite the apparent simplicity of the seal, a number of precautions have to be
taken to ensure correct operation of the O-ring seal. The dimensions of the seal and
its housing are critical. Poor surface finish of the mating element can also lead to
excessive wear of the seal. In general, it is important that the seal comes into contact
with no sharp machined edges.
The O-ring materials tested are given in table 1.
2.4. INSTRUMENTATION
The studies required accurate measurement of axial displacement of the shaft
(not greater than a fraction of a millimetre) as the rubber materials used had a ten-
dency to creep before large movement of the shaft occurred. The axial displacement
of the shaft was measured using two inductive transducers; one was active and the
other one passive. The transducers were able to detect any change in distance
between the transducer and a ferromagnetic material through a variation in the
inductance in the coils within the probe. The output from the two probes was sent 3. Discussion of results
3.1. PTFE-ENCAPSULATED SEAL
The results of testing are shown in fig. 5 and refer to the seal tested over the entire
10 MPa operating pressure range at 9% squeeze. As expected, increasing the pres-
sure differential across the seal increased the friction force considerably. Indeed,
the frictional force at 10 MPa was over five times that at atmospheric pressure. This
increase in friction force is undoubtedly related to the increase in contact area
between the seal and the shaft, resulting from inevitable seal extrusion due to the
pressure differential.
The curve representing the relationship between friction and pressure is not lin-
ear. It shows an initial period of increase in friction followed by a second period
where the gradient is less and finally a third period where there is again a rapid rise
in friction. A possible reason for this behaviour could be asigned to the fact that the
seal is made of two materials with entirely different mechanical properties.
A series of measurements were taken for the PTFE-encapsulated O-ring at a
value of 16% squeeze. They are shown in fig. 6 together with those found at 9%
squeeze. The graph shows that at 16% squeeze the frictional force values are consis-