Starter Motor Drive Mechanism | Types and Working Principle.

The Starter Motor drive mechanism depends upon the gears attached to the system. There are different types of starter motor drive mechanisms available in the automobile system. 

In this article, I will discuss the working principle and the types of starter motor drive mechanisms. 
Starter Motor Drive Mechanism

Starter Motor Drive Mechanism

The starter motor is linked to the engine flywheel to a set of gears. A pinion gear is attached to the starter armature which drives a ring gear attached to the flywheel. The arrangement is so made that the two gears engage to crank the engine until it starts and then disengage automatically when the engine is running. 
The running engine gear ratio is about 15:1. The armature rotates 15 times to cause the flywheel to rotate once. Thus, the cranking motor requires one-fifteenth as much power as would an electric motor directly coupled to the crank. The armature would revolve at about 2000-3000 R.P.M when the engine motor is operated and hence the flywheel is rotated as high as 200 R.P.M.
When the engine starts, its speed may increase to about 3000 R.P.M. If the pinion is still in mech with the flywheel, it will revolve the armature at about 4500 R.P.M., which is a very high speed. At this speed, the centrifugal force would cause the conductors and commutator segments to be thrown out of the armature damaging the motor. Hence, the pinion must be disengaged from the flywheel, after the engine has started. The automatic engagement and disengagement of the motor with the engine flywheel is obtained with the help of a drive mechanism.

Types of Starter Motor Drive Arrangement

The drive management is of two types. depending upon the engine system and the design these are- 
  1. Inertia Driver.
  2. Over Running Clutch 
The Inertia drive has two types-
  1. Bendix Drive.
  2. Folo-Thru Drive.
I will discuss each of these starter motor drive mechanisms.

Bendix Drive

Bendix Drive


This Fig shows the Bendix drive for the starter motor. It is fastened in an armature shaft of the starter motor. The drive head is keyed to the end of the armature shaft. The pinion gear, having internal threads, is mounted on the threaded sleeve, just like a nut-on bolt. The sleeve is not connected directly to the shaft of the starter motor but is used only as a bearing. A spring is attached to the drive head and also to the sleeve. 

When the starter motor is at rest the pinion gear is not engaged with the flywheel. When the starter motor is switched on, the armature begins to rotate. This causes the sleeve to rotate also because the sleeve is fastened to the armature shaft through a spring. The pinion, because of its inertia of rest and its unbalanced weight, turns very little, but it moves forward on the revolving volt until it engages with the teeth of the flywheel. The slight turning of the pinion gear helps to engage it properly with the flywheel. When the pinion gear strikes the collar, it begins to turn with the sleeve, causing the flywheel to run with it. When the flywheel turns, the crankshaft also turns and the engine starts. The spring between the armature shaft and the threaded sleeve takes the shock of the start. 
After the engine has started, the pinion gear is turned by the engine faster than when rotated by the starter motor. This causes the pinion gear to turn back on the thread sleeve, making it disengage with the flywheel.

Folo-Thru Drive

Folo-Thru Drive


The Folo-Thru drive is very similar to the Bendix drive. The difference is that the Folo-Thru drive keeps the starter motor engaged with the flywheel until a predetermined engine speed is reached, but in the Bendix drive it is not so. In the Folo-Thru drive the threaded sleeve is attached to the armature shaft through a spiral spring. 

A pinion is mounted on the threaded sleeve. The pinon base has two small spring-loaded pins a lock pin and an anti-drift pin. The anti-drift pin is similar to the lock pin but has a stronger spring. The anti-drift pin rides on the anti-drift slope of the threaded sleeve and keeps the pinion from drifting into the ring gear when the starter is not in use. It imposes a fraction drag that holds the pinion in the disengaged position. The lock pin drops into a detent in the sleeve thread as the pinion moves out of the cranking position. This holds the pinion from being disengaged with the flywheel during cranking. It prevents the pinion from being disengaged by a false start, during which the engine might fire a few times and die. The pinion is thus held in the engaged position and cranking continuously until the engine really gets started. 
After the engine has started and the engine speed increases, the centrifugal force in the lock pin moves it out of detent and the opinion disengages from the flywheel. 

Over Running Clutch Drive

Over Running Clutch Drive


The over-running clutch prevents the starting motor from being damaged when the engine speed is greater than the starting motor speed. In this condition, the pinion, still engaged by the lock pin, overruns the threaded sleeve and ratches over the clutch teeth. If the engine speed slows down, the pinion automatically resumes driving the flywheel as soon as the speed of the over-running part decreases to the starter motor speed. This intermediate operation will continue until the engine speed increases to disengage the pinion from the flywheel. 

Working principle

The starter lever is linked to the starter pedal which extends into the driver’s compartment and is operated by foot pressure. When the starter pedal is pressed, the shift lever compresses the drive shaft and spring which ultimately pushes the over-running clutch and pinion gear assembly toward the flywheel. The starter switch is closed by the shift lever when the starter pedal is fully pressed. As soon as the starter switch is closed, the pinion gear will run and engage with the flywheel, thus starting the engine. 
When the engine starts, the over-running clutch comes into action. The unit is so designed that, as the starter motor turns, the pinion is driven through the over-running clutch. But as soon as the engine starts, the pinion turns much faster than the starting motor, due to which it slips backward into the over-running clutch. When the starting switch is opened, the engaging lever releases the pinion from the flywheel. 

Construction of overrunning clutch drive

Construction of over running clutch drive


This Fig shows the construction of an over-running clutch. It consists of an outer shell, pinion, and collar assembly. The outer shell has four hardened steel rollers fitted into four notches. The notches are not concentric but are smaller at the end opposite to the plunger spring. When the clutch wheel is turned by the armature shaft, the rollers are wedged in the notches to force the collar to run with the cell. Since the collar drives the pinion gear, this action enables the armature to rotate the pinion, cranking the engine.
After the engine starts, it turns the pinion gear faster than the armature, so that the rollers are rotated into the larger sections of the notches, where they are free. This allows the pinion to over-run the remainder of the clutch. When the shift lever is released, a spring on the shift lever pulls the pinion back out of the engagement.


The starter motor drive mechanism and the drive arrangement depend on the engine placement and the design of the vehicle. 
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