Overrunning Clutch

In all starter designs the rotary motion is transmitted via an Overrunning Clutch. The overrunning clutch allows the pinion to be driven by the armature shaft (or planetary gear set), however it breaks the connection between the pinion and the armature shaft as soon as the accelerating engine spins the pinion faster than the starter.

The overrunning clutch is located between the starter motor and the pinion and prevents the starter motor armature from being accelerated to an excessive speed when the engine starts.

Fig 1: Overrunning Clutch Components

Courtesy of BMW OF NORTH AMERICA, INC.

Fig 2: Magnet Gear Reduction Starter Components

Courtesy of BMW OF NORTH AMERICA, INC.