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A sprag by definition is an object used to prevent an object, which is inclined to move in a circular direction, from moving. An example of this would be placing a wedge shaped object (i.e. a sprag) underneath a motor vehicle•s wheel to prevent it from moving!

A sprag clutch or bearing uses a similar object to prevent it from moving in both directions. The clutch or bearing would actually look very similar to a normal bearing but it would have an array of sprag elements within its structure that prevents it from moving in both directions.

As pictured, a sprag would have an outer race or ring as well as an inner race or ring. This is about the extent to which it resembles a bearing. Further bearings, rollers or the sprag elements themselves support the relationship between the inner and outer race. This is done by inserting the elements between the inner and outer race which allows the races to move in different circular directions but only in one way! The sprag elements resist the two circular races to move in the other direction.

The sprag elements themselves are designed in a figure of eight type of set up to allow the movement of the races to take place in one direction and cause a spragging effect in the other direction. So in effect when the unit rotates in one direction the rollers slip or free-wheel, but when a torque is applied in the opposite direction, the rollers tilt slightly, producing a wedging action and binding because of friction. The use of a spring to help the spragging in the lock position causes the unit to experience very little backlash!    

Automatic transmissions

Within some automatic transmissions a sprag clutch is used as a means of allowing the vehicle•s transmission to change gears smoothly under load. This is more prevalent in older transmissions which require a synchronised engagement of one clutch when the next one disengages.

When one uses an overrunning clutch the synchronisation becomes automatic. This happens mainly because the sprag clutch simply "lets go" when its reaction force is no longer needed. Newer electronic "clutch to clutch shifts" do not require sprags.
Helicopter sprags

Helicopter sprags

In many helicopter designs a sprag clutch is used to transfer torque power from the helicopters engine to its main rotor. When a helicopters engine fails the sprag clutch allows the rotors to overrun faster than the engine. This is due to the fact that the rotors would then not be directly mounted on the engine shaft!

As a result of this phenomena the helicopter will allow itself to enter auto rotation. Helicopter pilots are trained to land a helicopter when in auto rotation. This is thanks to a sprag clutch!

Motorcycle starters

To replace Bendix gear starters, common to automobile starters, some modern motorcycle manufactures are beginning to use sprag clutches. This is mainly due to size and weight advantages.

After the electric motor gets the motorcycle•s engine started using the sprag clutch in the driving action, the rotation of the motorcycle•s engine which takes over from the electric motor overruns thanks to the presence of the sprag clutch!

Motorcycle primary drives

Many modern sport and racing bikes use sprag clutches in the primary drive between the engine and transmission.

This prevents the rear wheel from losing traction during rapid downshifts when the engine speed is not matched to road speed. If a sprag clutch is not present, much greater care is required when downshifting because losing rear wheel traction can cause dangerous highside accidents.

Conveyors

On conveyor drives, a sprag clutch is used for indexing and anti-runback

Frequently Asked Questions - All you need to know about Sprag Components, Bearing, Clutches and Bearings