The engagement/disengagement mechanism has two functions.
Engages the pinion gear with the ring gear.
Disengages the pinion gear from the ring gear.
When the end faces of the pinion gear and the ring gear come into contact with each other by the pull-in action of the magnetic switch, the drive spring is compressed. Then the main contact is turned on and the armature revolving force increases. A part of the revolving force is converted into a thrust by the helical spline to push out the pinion gear. In other words, pinion gear is engaged with the ring gear by a pull-in force of the magnetic switch, a revolving force of the armature and a thrust of the helical spline.
The pinion gear and the ring gear are chamfered for easy engagement.
When the pinion gear rotates the ring gear, a high pressure is applied to the tooth surfaces of the two gears.
Since the revolving speed of the engine (ring gear) becomes faster than that of the pinion gear when the engine starts, the ring gear begins to rotate the pinion gear. A part of the revolving force is converted into a thrust by the helical spline to disengage the pinion gear. The overrunning clutch mechanism prevents the revolving force of the starter from being transmitted to the pinion gear by making the ring gear to rotate the pinion gear. As a result, the pressures applied to the both gears decrease, and the pinion gear is more easily disengaged.
Since the pull-in force from the magnetic switch is released, a compressed return spring pushes back the pinion gear and disengages it from the ring gear.