What is Overrunning Clutch or Freewheel used in Automobiles

What is Overrunning Clutch Or Freewheel :

Overrunning clutches, sometimes called freewheel clutches, are used to freewheel in one direction while driving in another direction of rotation. When the driven shaft is rotating faster than the driveshaft, the clutch mechanically disconnects the driveshaft from the driven shaft.

A freewheel or overrunning clutch is a device in a transmission that disengages the driveshaft from the driven shaft when the driven shaft rotates faster than the driveshaft. An overdrive is sometimes mistakenly called a freewheel, but is otherwise unrelated.

The condition of a driven shaft spinning faster than its driveshaft exists in most bicycles when the rider stops pedaling. In a fixed-gear bicycle, without a freewheel, the rear wheel drives the pedals around.

Construction Of Freewheel : 

The simplest freewheel device consists of two saw-toothed, spring-loaded discs pressing against each other with the toothed sides together, somewhat like a ratchet. Rotating in one direction, the saw teeth of the drive disc lock with the teeth of the driven disc, making it rotate at the same speed. If the drive disc slows down or stops rotating, the teeth of the driven disc slip over the drive disc teeth and continue rotating, producing a characteristic clicking sound proportionate to the speed difference of the driven gear relative to that of the (slower) driving gear.

A more sophisticated and rugged design has spring-loaded steel rollers inside a driven cylinder. Rotating in one direction, the rollers lock with the cylinder making it rotate in unison. Rotating slower, or in the other direction, the steel rollers just slip inside the cylinder.

Most bicycle freewheels use an internally step-toothed drum with two or more spring-loaded, hardened steel pawls to transmit the load. More pawls help spread the wear and give greater reliability although, unless the device is made to tolerances not normally found in bicycle components, simultaneous engagement of more than two pawls is rarely achieved.

Overrunning Clutch
Overrunning Clutch

Overrunning clutches are used to prevent a reverse transmission of motion in a kinematic loop, such as motion from the driving wheel of a bicycle to the pedals.

They are used to convert a rocking motion to a rotary motion, for instance, in the pulse type of continuously variable transmissions, or to impart to a slowly rotating shaft a faster rotation in the same direction, as, for example, in mechanisms for high-speed shifting in metal cutting machines.

Other applications include winding mechanisms and arresting devices, where reverse shaft rotation must be prevented.

Advantages of Freewheel :

  • A freewheel also produces slightly better fuel economy on carbureted engines (without fuel turn-off on engine brake) and less wear on the manual clutch, but leads to more wear on the brakes as there is no longer any ability to perform engine braking. This may make freewheel transmissions dangerous for use on trucks and automobiles driven in mountainous regions, as prolonged and continuous application of brakes to limit vehicle speed soon leads to brake-system overheating followed shortly by total failure.
  • By its nature, a freewheel mechanism acts as an automatic clutch, making it possible to change gears in a manual gearbox, either up- or downshifting, without depressing the clutch pedal, limiting the use of the manual clutch to starting from standstill or stopping. The Saab freewheel can be engaged or disengaged by the driver by respectively pushing or pulling a lever. This locks or unlocks the main shaft with the freewheel hub.

Application of Freewheel :

They are used in heavy-duty applications, such as agriculture, mining, aerospace, industrial metal processing, indexing, and more. They are often used when backstops, multiple-speed, dual- and one-way drives are required.

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