Sprag Clutch

Before we come to “Sprag Clutch” it is necessary to know about a “clutch”. So what is a Clutch? A clutch is a mechanical device that commands the power transfer amid the components of a machine by causing two rotary shafts to engage and disengage (brings into and out of gear). Clutches are available in different types, sizes and shapes catering to the requirements of each individual vehicle or machine. Clutches are used in all sorts of machine that consists of any mechanical gearing. They are used in cars, construction vehicles, copy machines, motor cycles etc. Although they are most commonly used and referred the gearing system of a car, they have other applications as well. A cooling fan that is heat activated may use a clutch.

Clutch mainly falls into two categories: Friction and non-contact. Friction clutches work by engaging rotating gears and non-contact clutches are designed to work without making any contact what so ever.

Frictional clutches

  • Band Clutch
  • Disk Clutch
  • Cone Clutch
  • Expanding Drum Clutch
  • Multiple tooth clutch
  • Positive clutch
  • Single disk clutch
  • Slip clutch
  • Sprag clutch
  • Warp spring clutch.

Non-Frictional Clutches

  • Electromagnetic Clutch

So, what is a sprag clutch? A sprag clutch is a freewheeling device which is able to exert a great deal of torque force. Sprags are one way roller clutches which are used in devices like turngates to prevent them from swinging backwards. The sprag clutch consists of a ring with an inner race that is locked in place, a roller in the middle ring and an outer race that only moves in one direction. The sprag works by spinning freely in one direction and locking in the other. That is why the tailgate only moves in one direction and not in the other.

Since the movements of sprags are very specific, they must be very carefully attached. If a sprag facing the wrong way is fitted inside a device, and then the device is turned on, there is a very real chance that the whole device will be wrecked. Mechanics should therefore be very careful when placing sprag clutches.

Sprags have a great many uses. They are used in the automatic transmission of cars, allowing them to change smoothly from one gear to another. With a one sprag, one can change the gear multiple times. Sprags are also used in the engines of helicopters to transfer power from the engine to the main rotor. If the engine is damaged and starts to fail, the sprag clutch ensures that the rotor continues to spin, and that the speed of rotation is faster than the speed of the engines, allow the helicopter to go into autorotation. Autorotation occurs when the rotors spin because of the air passing through it rather than because of the driving power of the engine. Because of the sprag’s freewheeling ability, the rotors can continue spinning for a long time, which allows helicopters to land slowly and safely. Sprag clutches are vital for their ability to conduct an emergency landing.

Sprags are also used in motorcycles. Modern motorcycles have engines with electric starter motors which contain sprags. Sports and racing bikes now have sprag clutches in the drive between transmission and engine. They prevent the back wheel in the bike from losing traction when the road speed is not equal to the bike speed. Rapid downshifiting in those cases can lead to accidents if the bike has no sprags.

In conveyor drives, sprags are used for overrunning. Overrunning occurs when two motors can run on the same machine, one at low speed and one at high. The sprag keeps one of them still while the other runs.



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