Starter Motor (1999-2006): Design: Freewheel

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Fig 1: Freewheel

Courtesy of VOLVO CARS OF NORTH AMERICA.

Functions as a one way clutch (freewheel). The freewheel allows the starter motor to drive the car engine, but not the other way around. This prevents the starter motor from being over-revved. This is carried out by spring-loaded rollers which lock the pinion and gear together when the starter motor operates.

The engine cannot drive the starter motor because the pinion turns in the rollers.

The freewheel clutch is attached to the armature shaft by spiral splines, which facilitate engagement and disengagement of the starter motor pinion / flywheel ring gear.

The left-hand illustration in Fig 1 displays the normal position. The starter motor, connected to the outer section drives the pinion (inner section) by means of the rollers, which then lock.

The right-hand illustration in Fig 1 displays what happens when the starter motor remains connected when the engine is started. The pinion (inner section) cannot drive the starter motor (outer section) because the rollers disengage.