Subject: Automobile Engineering

Topic: Electrical System, Starting Systems & Alternator

Difficulty: Medium

Bendix drive is used to engage starter pinion with flywheel gear. For fastening this unit with the armature starting motor shaft the drive heads is keyed to the end of armature shaft while the other parts are free from armature shaft & drive head. They are only connected through the spring which is attached to the head & steeve carrying pinion. When the current passes into the starting motor the armature begin to turn at full speed causing the transmission of turning effect to the threaded steeve.

Although the pinion is turned very little, yet it is moved forward by the spiral thread till it engaged with the teeth of the flywheel of pinion is prevented by a collar attached on sleeve, because of this pinion start rotating as it is mesh with engine flywheel. The flywheel rotates & engine starts.

The starter pinion & flywheel gear do not reunion mesh but are automatically disengaged by the Bendix drive as soon as engine start because in comparison to the rotation of the starting motor,the engine rotate the pinion much faster after it starting causing damage to the starter motor. This disengagement is affected due to pinion rotation much faster by the engine in comparison to the rotation of starting motor at the time of the engine actually start. Therefore, the starting motor will be also protected due to pinion meshing out of the flywheel gear because of its screwing back on the spiral threaded sleeve when the starter switch is released by the operator.

Folo through drive:
The Construction of this similar to the standard Bendix drive. The armature shaft is connected to the threaded sleeve through a spring a & over running clutch. The inside of the pinion barrel fits into the sleeve threads. Towards the ends of the sleeve is provided a defend as shown in fig:6.1. Two spring loaded pins the lock pin & the anti-drift pin are also provided. The anti-drift pin provided only a frictional contact with the sleeve teeth , whereas the lock pin action is in connection with the detent.

The engagement of the pinion with the flywheel takes place in the similar manner as in case of standard Bendix drive. However, towards the end of pinion travel, the lock pin drops into the detent & could not let the pinion disengage prematurally due to false start eg weak or intermitted firing. The pinion thus continuos driving the flywheel till the engine really gets started & attains a speed of about 400 rpm, when the lockpin came out of the detent due to centrifugal force which overcomes the spring force. The pinion then gets disengaged.

The anti-drift pin is loaded with a spring which is stiffer than the spring of lock pin. This prevents the drifting of the piston to engage with the flywheel teeth accidently. The over running clutch serves to avoid damage to the starting motor in case of due to any reason, the pinion remains engaged with the flywheel ever after starting of engine.