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(v) measurement of friction at the commencement of motion of the shaft.
The general layout of the apparatus used is shown in fig. 2. The apparatus itself
basically consists of a shaft running inside two O-ring seals located within a con-
tainer able to withstand a pressure of up to 10 MPa. The force required to draw the
shaft through the pair of seals would then be measured at different pressures. This
idea is schematically shown in fig. 3. However, the problem with this particular
design is that the friction force measured is not for one but for two seals. By consid-
ering the frictional force to be split equally between the two O-rings, a degree of approximation was brought into the results. Because the O-ring is made from an
elastomeric material it tends to extrude between the shaft and the housing contain-
ing it. Obviously, moving the shaft against the direction of extrusion would require
a higher force than moving the shaft in the same direction as the extrusion. This
simply means that in reality the frictional forces on the two seals would not be the
same. The shaft was made of a mild steel and had surface finish of 0.2 #m. Its diameter
was 25 mm and the length 250 mm. The shaft, for safety reasons, had a 50 mm
flange located 60 mm from one end. Both ends of the shaft were chamfered to allow
for easy assembly into the O-ring seals.
The apparatus was equipped with two inductive transducers acting as a sensor,
to measure the axial displacement of the shaft.
2.2. GLAND DESIGN
The O-ring type seal has to be housed in a rectangular gland, schematically
shown in fig. 4. The dimensions of the gland are dependent on the application for
which the seal is to be used. The primary decision to be made is the degree of squeeze
required. Squeeze is defined as the amount by which the seal cross section is larger
than the space available between the shaft and the gland as a proportion of the
shaft. Thus,
squeeze = [(d-b)/d] x 100 (%),
where d is the seal cross-sectional diameter and b denotes the gap between the shaft
and gland. The degree of squeeze required is governed by the nature of the applica-
tion of the seal. High-pressure applications require a considerable amount of
squeeze in order to maximise the sealing action. This, however, results in the
increase in the frictional force which, in turn, produces other undesirable effects
such as power loss, heat generation and wear of the elastomer.
The width of the groove, A, should be larger than the cross section of the seal to
allow for the seal expansion under operating conditions. If the groove is not suffi-
ciently wide the elastomer can become too large for the gland and begins to extrude
between the sealed surfaces. Radial clearance should be kept to a minimum to pre-
vent extrusion of the elastomer.
The tests were carried out at pressures of up to 10 MPa, which represents the limit for the O-ring to operate without extrusion rings. For this reason, the radial
clearance was kept to a minimum on the low-pressure side of the seal.
2.3. O-RING SEALS TESTED
The tests reported here are primarily concerned with O-rings of special design.