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 of the engine flywheel.
Two types of starter drive mechanisms in common use are the Bendix drive and the overrunning
FIGURE 20. BENDIX STARTER DRIVE.
The Bendix drive 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 spinning.