Inertia-drive Starter Motor
The installation of an inertia drive on a starter motor permits it to attain sufficient speed and consequently power before engaging with the engine flywheel starter-ring gear. Since starter motors are relatively small, they produce insufficient torque to rotate the engine directly. The starter drives the engine through a toothed pinion, attached to the end of the motor armature shaft and a ring gear, fitted around the engine flywheel (Fig. 15.18). To transmit the drive from the starter to the engine, the pinion has to mesh with the ring gear. A gear reduction of around
Fig. 15.18. Inertia-drive starter-motor operation.
10:1 between pinion and ring gear multiplies the starter torque sufficiently to crank the engine at least at a minimum starting speed speed of about 100 rpm.
The internally threaded pinion screws on to an externally threaded sleeve that is splined on to the armature shaft. When the starter is first energized, the shaft and sleeve suddenly rotate, but the pinion due to its inertia, doesn’t get time to revolve. Therefore, the armature shafts threaded sleeve screws the pinion in towards the armature until it engages with the flywheel ring gear. A strong compression spring is attached at the end of the armature shaft, which cushions the shock loading during initial meshing of the pinion with the ring gear. Once the pinion and ring gear are fully engaged, power is transmitted to the stationary flywheel, which cranks the engine from rest to a speed sufficient for the engine to develop its own power. With further increase in engine speed, the flywheel ring gear runs faster than the starter pinion, and it now screws the pinion all the way back along the screwed sleeve until the pinion is out of mesh with the rings gear.