Page 45 - Plastics Engineering

28 General Properties of Plastics
1.43 Wear Resistance and Frictional Properties
There is a steady rate of increase in the use of plastics in bearing applications
and in situations where there is sliding contact e.g. gears, piston rings, seals,
cams, etc. The advantages of plastics are low rates of wear in the absence of
conventional lubricants, low coefficients of friction, the ability to absorb shock
and vibration and the ability to operate with low noise and power consumption.
Also when plastics have reinforcing fibres they offer high strength and load
carrying ability. Qpical reinforcements include glass and carbon fibres and
fillers include PTFE and molybdenum disulphide in plastics such as nylon,
polyethersulphone (PES), polyphenylene sulfide (PPS), polyvinylidene fluoride
(PVDF) and polyetheretherketone (PEEK).
The friction and wear of plastics are extremely complex subjects which
depend markedly on the nature of the application and the properties of the
material. The frictional properties of plastics differ considerably from those of
metals. Even reinforced plastics have modulus values which are much lower
than metals. Hence metalkhennoplastic friction is characterised by adhesion
and deformation which results in frictional forces that are not proportional to
load but rather to speed. Table 1.7 gives some typical coefficients of friction
for plastics.

Table 1.7
Coefficients of friction and relative wear rates for plastics
Coefficient of friction
Relative
Material Static Dynamic wear rate

Nylon 0.2 0.28 33
Nylodglass 0.24 0.3 1 13
N y lodcarbon 0.1 0.11 1
Polycarbonate 0.31 0.38 420
Polycarbonate/glass 0.18 0.20 5
Polybutylene terephthalate (PBT) 0.19 0.25 35
PBT/glass 0.11 0.12 2
Polyphenylene sulfide (PPS) 0.3 0.24 90
PPS/glass 0.15 0.17 19
PPS/carbon 0.16 0.15 13
Acetal
0.21
-
m 0.2 0.05 -
0.04
The wear rate of plastics is governed by several mechanisms. The primary
one is adhesive wear which is characterised by fine particles of polymer being
removed from the surface. This is a small-scale effect and is a common
occurrence in bearings which are performing satisfactorily. However, the other
mechanism is more serious and occurs when the plastic becomes overheated
to the extent where large troughs of melted plastic are removed. Table 1.7