Coaxial (Sliding Gear) Starter Motor

Similar to the axial starter motor, the coaxial starter motor moves the pinion into engagement under reduced power and when it is fully meshed only then full power is applied. The main difference lies in the method of sliding the pinion into mesh with the ring gear. Instead of the whole armature assembly moving axially, in this type a solenoid mounted coaxially with the shaft in a housing moves only the pinion for engagement.

The main details of the circuit of a coaxial starting motor are shown in Fig. 15.30. The main terminal is connected directly to the battery and the terminal marked ‘sol’ is connected to the battery through the drivers starter switch. When this switch is operated, the two-stage solenoid is energized, which moves the pinion into mesh and also closes the first set of contacts, the second set is kept open by trip lever. During this period, a resistor limits the current to the main field, so the armature rotates slowly. Just before reaching the fully meshed position between the pinion and ring gear, a lever trips the second set of contacts. This action by-passes the resistor, and supplies full current to the main field so that the motor produces its maximum torque.


Fig. 15.31. Actions of coaxial starter.
A. At rest position. B. Cranking position.
C. Engaging position. D. Over-speed position.

The actions of one type of Lucas CAV motor are presented in Fig. 15.31. This design uses four steel balls, which lock the pinion sleeve to the shaft to avoid premature rejection of the pinion during spasmodically firing of the engine. When the engine starts and operates on it own, over-speeding of the motor is prevented by the centrifugal action of a set of steel balls placed adjacent to the locking balls. When a pre-set speed is reached, the centrifugal force on the balls moves the locking collar outward and allows the pinion to disengage. A return spring positioned at the flywheel end of the armature shaft helps pinion disengagement and holds the pinion clear of the flywheel during running of the engine.

In Bosch sliding-gear motors the main solenoid is installed at the opposite end of the motor to the pinion. This solenoid holds the pinion in full engagement position until the driver releases the starter switch. Drive from the armature to the pinion is transmitted by a multi-disc clutch. This clutch limits the torque to be transmitted and hence prevents over speeding of the motor by releasing the plates and slipping when the engine is started. Some motors have a shunt field winding, which limits the no-load speed and others use a brake winding that comes into action when the driver releases the starter switch.




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