General Design Principles for DuPont Engineering Polymers - Module

turntable c. The completed article is removed and
changed while the next one is being welded; this
reduces the total welding cycle.
• If the production run justifies it, a turntable can of
course be used; it may, for instance, have three
positions: welding, removal and insertion.
The above steps allow the piston stroke to be shortened
considerably, thus avoiding the potentially lethal
arrangement of having the rotating mass on a piston
rod which projects too far.
Figure 11.14 Inertia welding, long parts
L1
c
a
Since the welding pressure is fairly high, the clutch
lining and the ball-bearings of the pulley will be under
an unnecessarily heavy load when in the top position.
It is therefore advisable to operate at two different
pressures, although this does involve a more complicated
pneumatic control. Alternatively, a spiral spring
can be incorporated above the piston, to take up some
of the pressure at the top of its stroke.
In any case, the speed of the piston must be reduced
sharply just before contact is made, so as to reduce the
initial acceleration of the flywheel and protect the
clutch lining.
On machines equipped with a turntable the parts are
ejected after being removed from under the spindle. In
such cases, the piston stroke can be much shorter, as,
for example, with the float shown in Figure 11.13.
L
b
X
X
84
It is also possible to produce the pressure by means of
the diaphragm device shown in Figure 11.15. The
rubber diaphragm is under pressure from compressed
air above it and from a spring below. The spring must
be strong enough to raise the flywheel and to apply
sufficient force to engage the clutch. In a production
unit it is best to guide the shaft by means of axial ballbearings.
The advantages of this device over an
ordinary cylinder are lower friction losses and a longer
life. However, the permissible specific pressures on
the diaphragm are limited, so that larger diameters are
needed to achieve predetermined welding pressures.
(The welding head, with flywheel and belt pulley, is
identical with that shown in Figure 11.13.)
The rubber diaphragm mechanism is suitable for a
piston stroke up to 10–15 mm and for specific pressures
of 3 to 4 bar.
Figure 11.15 Welding head with diaphragm
Since, as has already been mentioned, the operating
speed can be altered by changing the motor belt
pulley, a variable speed motor is not essential. In any
production run there will be cases in which some
possibility of limited speed adjustment would seem to
be desirable.
The kinetic energy of the flywheel is a function of the
square of the speed (rpm), so it is important to keep
the speed as constant as possible.