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What Is a Sprag Clutch?

A sprag clutch is employed in various types of helicopter applications.
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  • Written By: T. L. Childree
  • Edited By: R. Halprin
  • Last Modified Date: 24 September 2014
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A sprag clutch is a type of rolling element bearing that rotates freely in one direction and locks in place when turned in the opposite direction. This clutch uses spring loaded sprag rollers that allow one element to freewheel around the other until the direction is reversed. A sprag clutch is often utilized in the automatic shift transmission of a motor vehicle. It is also employed in various types of helicopter, motorcycle, and hoist applications. Sprag clutches are sometimes used as a backstopping device in certain kinds of conveyor systems.

A sprag clutch has the general appearance of a roller bearing, but only operates as one in a single direction. When turned in the opposite direction, it operates as a clutch rather than a bearing. When reverse torque is applied, the sprag rollers located between the two elements tilt slightly to lock into position and instantly stop the second element from spinning. The sprag rollers in this device are spring loaded to prevent backlash from occurring during clutch operations.

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Car manufacturers sometimes utilize a sprag clutch in automatic transmissions to provide smooth gear changing when a vehicle is under load. In this application, a separate clutch is employed to prepare the transmission for the gear change by preventing it from rotating faster than the gear that is currently engaged. During this process, torque is released from the sprag clutch to allow the transmission to rotate in a controlled but freewheeling manner. To complete the gear change, the higher gear is then engaged by causing the sprag to convert to its clutch or driving mode when torque is reapplied. The entire process is then repeated for each successive gear change.

These devices are sometimes used to transfer torque from the engine of a helicopter to its main rotor. In case of an engine failure, this type of clutch will allow the rotor to go into auto-rotation by continuing to spin faster than the engine is turning. Many types of motorcycles also utilize a sprag clutch as the primary drive mechanism between the engine and rear wheel. The use of this type of clutch allows the rear wheel to maintain traction with the road surface during rapid downshifting maneuvers. Sprag clutches are also employed as a backstop in many types of industrial hoist equipment to prevent their load from being dropped if a catastrophic mechanical failure should occur.

A sprag clutch also comes into play as a backstopping mechanism in inclined conveyors. Loaded conveyors operating in an inclined position frequently attempt to run backward when the driving mechanism is stopped. In this application, the outer element attaches to the conveyor by means of a torque arm. The inner element of the clutch then turns freely with the forward motion of the conveyor. If the conveyor attempts to run backward, the sprag rollers are engaged and the motion is halted.

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