ELECTRICAL EQUIPMENT—Q.12
which now close. Closing of the contacts short circuits
the resistor and allows full current to flow through the
field windings. The armature now rotates under full
power, with the pinion and engine flywheel fully
engaged. This completes the second stage.
In order to avoid premature disengagement of the
pinion before release of the starter button, a ball-locking
device is positioned between the armature shaft and
pinion
sleeve.
This
consists
of
the
following
components:
Four steel balls (16) located in holes in the
pinion sleeve.
A locking collar (17) mounted on the pinion
sleeve.
The collar loading spring (18).
Four recesses (39) in the armature shaft.
The balls, set in holes in the pinion sleeve, are
retained in position by the lock-collar, the inside bore of
which, has a 450 chamfer, causing the balls to be
pressed inwards against the armature shaft.
In the stationary position, the balls hold back the lock
collar. When the pinion moves forward to the fully
engaged position. the balls become opposite the
recesses in the armature shaft sinking into them as the
shaft revolves. Spring pressure pushes the collar over
the top of the balls, locking them in the recesses. Thus
any backward or forward movement of the pinion sleeve
can no longer occur and the pinion is securely held in
the fully engaged position as long as the starter button
remains pressed.
Releasing the starter button cuts off the solenoid
current. Under the combined pressure of the main
contact and plunger springs (14) and (42). the plunger
returns to its normal position. carrying with it the moving
contact plate. Thus both sets of contacts open. With
current cut, the rotation speed of the armature rapidly
drops. The returning plunger pushes back the lock
collar, releasing the balls and freeing the pinion sleeve.
The engine flywheel speed now rapidly over-runs the
pinion speed. This action, combined with the pressure
of the return spring (4) throws the pinion out of mesh
with the flywheel, returning the pinion to the disengaged
position.
Fig. Q.10.
Any tendency of the pinion to wander forward under
vibration resulting in damage to the contact and
armature assembly, is prevented by pressure of the
pinion return spring (4).
On rare occasions, badly worn pinion and flywheel
teeth may meet face to face, preventing engagement
and causing the pinion to remain stationary against the
flywheel when the starter button is pressed. Special
provision is made for overcoming this difficulty. At the
commutator end of the armature, a steel ball thrust
device is fitted.
This consists of a spring (36), guide (33), steel ball (32).
and thrust-washer (30), against which the armature
presses. The spring and ball are held in position by an
end-cap (34) secured to the commutator end shield by a
spring clip (29).
Should a face-to-face contact occur, the armature.
under the influence of the helix, will be turned slightly
and at the same time forced back against the spring:
end movement of approximately 1/16 in (1,6 mm) is
permitted. When the starter button is released the
armature and pinion will come back to the normal
disengaged position, but the radial position of the pinion
will be slightly in advance of the previous position, so
that the next engagement will be made quite smoothly.
In order to maintain the required tolerances, shims (28)
are fitted at the commutator end.
