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Starter Motors -Why it doesn't stay in mesh

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Update time : 2016-11-17 13:54:05
Starter Motors -Why it doesn't stay in mesh
 
The starter motor engages with a gear on the flywheel of the engine, to crank the engine, in order to make it fire. 
 
To explain further - the gear on the starter has for instance, ten teeth. The flywheel gear has 100. The gear ratio is therefore ten turns of starter to one of the engine. 

If the gear on the starter itself turns at 2,500 rpm, this will relate to an engine speed of 250 rpm. If the starter stayed engaged once the engine had fired, the internals of the starter would not be able to cope with the centrifugal forces generated by the engine now turning the starter! 

The average engine will routinely rev to 4,000 rpm and many are capable of well in excess of this. The net result of a starter staying in mesh is that, with a gearing of 10:1 it would rev at 40,000 per minute (well in excess of the designed speed) and almost certainly cause a complete failure.

So the first requirement of the starter (apart from spinning the engine) are that it disengages.