Page 46 - Plastics Engineering

General Properties of Plastics 29

shows typical primary wear rates for different plastics, the mechanism of wear
is complex the relative wear rates may change depending on specific circum-
stances.
In linear bearing applications the suitability of a plastic is usually determined
from its PV rating. This is the product of P (the bearing load divided by the
projected bearing area) and V (the linear shaft velocity). Fig. 1.9 shows the
limiting PV lines for a range of plastics - combinations of P and V above
the lines are not permitted. The PV ratings may be increased if the bearing
is lubricated or the mode of operation is intermittent. The PV rating will be
decreased if the operating temperature is increased. Correction factors for these
variations may be obtained from materialhearing manufacturers. The plastics
with the best resistance to wear are ultra high molecular weight polyethylene
(used in hip joint replacements) and PTFE lubricated versions of nylon, acetal
and PBT. It is not recommended to use the same plastic for both mating surfaces
in applications such as gear wheels.

Rubbing velocity, (V(ds)
Fig. 1.9 Qpical P-V ratings for plastics rubbing on steel