Design Guide - Solvay Plastics

Since the plastic is constrained, the unit elongation,
combining both thermal expansion and strain, of both
the steel bolt and the plastic, will be as follows:
where
α S
(T F
− T O
) +
F
A S
E S
= α P
(T F
− T O
) −
A S
= cross sectional area of the bolt
A P
= cross sectional area of the washer
E S
= modulus of steel
E P
= modulus of plastic
F = (α P − α S ) (T F − T O ) A S E S
A S
E S
1+
AP E P
F
A P
E P
α S
= coefficient of thermal expansion of steel
α P
= coefficient of thermal expansion of plastic
Solving for F
F = increase in the tensile force of the bolt
and the increase in compressive stress on the plastic
will be:
σ =
F
A P
Loss of Bolt Tightness Due to Creep
When threaded metal fasteners are used to retain or
secure plastic parts to an assembly, and the assembly
is subjected to changes in temperature, the difference
between the thermal expansion coefficients of the metal
and the plastic can cause problems. When a threaded
fastener is tightened, the fastener is elongated slightly and
a compressive stress is generated on the substrate. This
compressive stress maintains the tightness of the bolt.
When the assembly is heated, both the plastic part and
the metal fastener will expand. The plastic part, however,
is constrained by the metal fastener, and cannot expand.
This results in increased compressive stresses in the
plastic, and a corresponding increased tendency for
compressive creep or stress relaxation to occur. The
relaxation of the compressive stress will result in
reduced torque retention in the bolts.
Figure 73: Bolt Torque Retention
Torque, N-m
7
6
5
4
3
2
1
0
Temperature, °F
50 75 100 125 150 175 200 225
A-1133 HS
33% GR PA 6,6
10
30% GR PA 4,6
0
20 40 60 80 100 120
Temperature, °C
To evaluate this tendency, 6.4 mm (0.250 in.) thick
plaques of Amodel A-1133 HS resin, 33% glass
reinforced PA 6,6 and 30% glass reinforced PA 4,6
were bolted to a metal surface with steel machine bolts
tightened to 6.8 N-m (60 in-lb) of torque with a torque
wrench compressing the plastic plaque under the bolt
face. The temperature of the bolted assemblies was
then raised to the indicated temperatures shown in
Figure 73, held for one hour, and then cooled to room
temperature. The torque required to loosen the bolts
was then measured.
Figure 73 compares the amount of torque retained
versus temperature. The coefficient of linear thermal
expansion of Amodel resin is lower than those of the
other materials tested, and therefore closer to that of
the steel.
The smaller difference in thermal expansion results in
lower induced stress due to the compressive strain
caused by the thermal excursion in the constrained
part. This translates to lower creep and therefore better
torque retention for such bolted assemblies.
60
50
40
30
20
Torque, in-lb
72 Amodel ® PPA Design Guide
www.SolvaySpecialtyPolymers.com