What is Over-Running Clutch Drive? Applications Of Over-Running Clutch.


The Overrunning Clutch is a device used as a freewheel unit in the starter motor and comes to action when the pinion gear is needed to disengage from engine flywheel. The figure shows the over-running clutch drive. The starter lever is linked to a starter pedal which extends into the driver’s compartment and is operated by foot pressure.

When the starter motor pedal is pressed, the shift lever presses the driveshaft and spring which then pushes the over-running clutch and pinion gear assembly towards the flywheel. The starter switch is closed by the shift lever when the starter motor 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.

Over Running Clutch Drive
Over-Running Clutch Drive

When the engine starts, the over-running clutch comes into action, the unit is so designed that, as the starting motor turns, the pinion is driven through the overrunning clutch. But as soon as the engine starts, the pinion turns much faster than the starting motor, due to which it slips back into the overrunning clutch. When the self-starter switch is opened, the engaging lever releases the pinion gear from the flywheel.

over running clutch
over-running clutch

The Figure shows the construction of an over-running clutch. It consists of an outer shell, collar, and pinion assembly. The outer shell has four hardened steel rollers fitted into four notches, The notches are not concentric, but are smaller in the end opposite to the plunger springs. When the clutch shell is turned by the armature shaft, the rollers are wedged in the notches to force the collar to run with the shell. Since the collar drives the pinion gear, these actions enable the armature to rotate the pinion, cranking the engine.

Roller type Over-Running Clutch Drive
Roller type Over-Running Clutch Drive

After the engine starts, it turns the pinion gear fastest than the armature, so that the rollers are rotated into the larger sections of the notches, and 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 engagement.

Working animation of Over-Running Clutch Drive.


Different types of switches are used to connect the starting motor with the battery. A heavy-duty foot-operated starter switch was used in some early models. A magnetic switch, known as solenoid switch is used in many present-day models. It consists of a plunger, contact disc, pull-in-winding, terminal, and necessary connecting cables. The switch is connected between the starter motor and the battery.

Starter Motor Magnetic Switch
Starter Motor Magnetic Switch

The current from the battery passes through the pull-in-winding to form a strong electromagnet when the switch is on and the circuit is completed to ground. The electromagnet attracts the plunger against the spring by magnetic force, which causes the two terminals connected by the contact disc. This causes the circuit complete between the starting motor and the battery.

When the switch is cut off, the circuit through the electromagnet winding is broken. The spring moves the plunger back to open the connection between the battery and the starting motor. The manual switch, as shown in Figure, is operated by hand. When the plunger is pressed, it makes the contact between the two terminals to connect the battery from the starting motor.

Over-Running Clutch
Over-Running Clutch

Several types of vacuum switches are also used in some early models. A rotary type switch is operated by the accelerator linkage, which completes the circuit when the pedal is pressed down. The natural safety switch is used in vehicles equipped with automatic transmissions. It is in series with the accelerator vacuum switch in the relay circuit, or in series with the starter switch in the direct control solenoid circuit. It makes the starter circuit inoperative unless the transmission shift control is in neutral or park.

Applications Of Over-Running Clutch

The Over-Running Clutches is used in heavy-duty applications such as:

  1. Agriculture.
  2. Mining.
  3. Aerospace.
  4. Industrial metal processing.

More information on Over-Running Clutch.

Also, Read These Posts

  1. What is Folo-Thru drive?
  2. What is Starter Motor? Construction and Operation of Starter Motor.


Creative Blogger And Automobile Engineer.

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