3. STARTER (CRANKING) MOTOR CONSTRUCTION. The vehicle cranking motor has only
one job to do. That is to turn the crankshaft at a speed fast enough to start the
engine. Since there are many different types and sizes of engines, there are many
types and sizes of cranking motors. The common starter motor -used on military
vehicles consists of the following five main assemblies: armature, field and frame,
commutator end head, drive end housing, and drive mechanism. The field windings,
frame, and armature are almost the same as in the generator which you have already
studied, except that in the starter motor the windings are much heavier in order to
carry a lot of current. The commutator end head houses the brush holders, brushes,
and a bearing. The drive end houses the drive mechanism and usually the mounting
flange to mount the starter to the engine.
4. STARTER DRIVES. The starter usually drives the engine through a pinion
(small) gear mounted on the starter motor armature shaft. When the starter motor
is running, the pinion gear engages (meshes) with a large gear mounted on the rim
a. Two types of starter drive mechanisms are in common use.
These are the
Bendix drive and the over-running clutch drive.
b. The Bendix drive (fig 4) consists of a threaded sleeve, which is
fastened to the armature shaft by means of a drive spring, and a drive pinion which
is threaded on the sleeve. The pinion has a weight on one side to make it
unbalanced. Think of the sleeve as a bolt and the pinion as a nut threaded to the
bolt. A weight is attached to the nut. If we spin the bolt, the nut, because of
the weight, tries to stand still. However, the spinning bolt would force the nut
to move forward or backward on its threads, depending on which way the bolt was
(1) Now look at A of figure 4. The armature has started to turn and the
pinion, which is not turning because of the weight on one side, is moving toward
the flywheel ring gear. In view B the teeth on the pinion gear have meshed
(engaged) with the teeth on the ring gear. The pinion has reached its stop and
cannot move any further on the threaded sleeve. It is now locked to the sleeve and
must turn with it. The now rotating pinion turns the flywheel gear, which in turn
rotates the flywheel ring gear and engine crankshaft.
(2) As soon as the engine starts, its speed of rotation is faster than
that of the pinion. The ring gear now drives the pinion because it is turning
faster. The pinion then moves back on the threaded sleeve and disengages from the
c. In the over-running clutch type of starter drive (fig 5) the pinion is
shifted into engagement with the flywheel with a lever.
(1) The drive for the over-running clutch has internal (inside) splines
which fit external splines on the starter armature shaft. The drive pinion is
attached to a rotor which forms the inner half of the over-running clutch.
(2) Now look at the end view of the over-running clutch, which is really
a one-way clutch. It can drive in one direction, but not the other. The outer
shell is part of the splined sleeve, so it rotates when the starter armature
rotates. The only connection between the shell and the rotor is the four spring-
loaded rollers between them. Notice the rollers are in slots in the sleeve. They
can move back and forth in the slots. The slots are tapered slightly. When the
sleeve starts to rotate the rollers move in their tapered slots to a point where
they become wedged (jammed) between the sleeve and the rotor. Then the whole
clutch turns as a single unit. When the engine cranks, the rollers are forced to
move the other way in their slots, because the pinion and rotor are now traveling
faster than the over-running clutch sleeve.
OS 010, 6-P5