Page 81 - Plastics Engineering

64 Mechanical Behaviour of Plastics

effect will be very small. Thus the above analysis is perfectly acceptable for
engineering design purposes.
Example 2.7 A nylon ring with a nominal inside diameter of 30 mm, an
outer diameter of 50 mm and a width of 5 mm is to be made an interference fit
on a metal shaft of 30 mm diameter as shown in Fig. 2.17. The design condition
is that the initial separation force is to be 1 kN. Calculate (a) the interference
on radius needed between the ring and the shaft and (b) the temperature to
which the nylon must be heated to facilitate easy assembly. What will be the
maximum stress in the nylon when it is in position on the shaft? The coefficient
of friction between nylon and steel is 0.25. The short-term modulus of the nylon
is 1 GN/m2, its Poisson's ratio is 0.4 and its coefficient of thermal expansion
is 100 x IO-~"C-'.

Im* ----1
D=50 q=mmm

Namlnal
Namlnal diameter
diameter
Fig. 2.17 Inte&rence fit of nylon ring on metal shaft

solution

(a) Force, F = pN
where N = PA
= pnDW
SO, F = xppDW