Engagement is provided by operation of the ignition switch in the start position which activates a starter mounted solenoid whose plunger is engaged with the hooked end of a pinion shift lever and operating fork.
Solenoid operation moves the operating fork causing the pinion to engage with the ring gear and also causes the plunger contacts to bridge the main starter terminals.
The fork locates in a guide ring on a pinion driver which is coupled to the pinion via a roller type over-running clutch designed to transmit drive in one direction only.
The pinion driver is mounted on a helix, machined on the armature shaft to form a very coarse thread. This allows the pinion driver to rotate slightly when it is moved towards the ring gear and this feature together with a chamfer on the leading edge of the ring gear and pinion teeth, is designed to assist meshing and easy engagement. However, if the pinion teeth butt against the ring gear teeth and engagement is prevented, the guide ring continues its axial movement by sliding over the sleeve of the driver and compressing a meshing spring until the plunger contacts bridge the main terminals and the armature begins to turn.
Slight armature rotation and the force from the meshing spring allows the pinion teeth to drop into mesh with the ring gear assisted by the screw action of the helix. The helix forces the pinion further into the ring gear until the pinion contacts a stop ring on the armature shaft. This prevents further axial movement and the driver and pinion now lock to the shaft via the helix and over-running clutch and transfer the armature rotation to the flywheel.
The pinion has only a small number of teeth compared to the ring gear and this means the armature will rotate several times for each revolution of the flywheel. The gear reduction also multiplies the torque from the starter motor.
As soon as the engine starts, its rotational speed will eventually exceed the speed of the armature. At this instant the over-running clutch breaks the connection between the pinion and the armature shaft and prevents over-speeding and damage of the armature.
The pinion remains meshed as long as the engaging lever is held in the engaged position. Releasing the starter switch allows the solenoid plunger return spring, to return the engaging lever, driver and pinion to their original position.
Source: CDX Global
