The Design & Functionality of a Sprag Clutch
A sprag clutch looks like a roller bearing, but instead of allowing the elements to roll freely in both directions they lock when rolled in the opposite direction.
A sprag clutch has three main stopping options:
- Overrunning
- Indexing
- Backstopping
The design of the sprag clutch consists of a series of cam shaped steel wedges (or sprags). Each sprag is placed at an angle to allow rotation in only one direction and instantly locks when force is applied in the opposite direction.
The wedging action of the sprags allows torque to be passed from one raceway to the other. This provides the consistent, reliable performance of a sprag clutch.
A sprag clutch is designed to fit the maximum number of sprags that will enable the greatest amount of torque (compared to its size and weight).
Technical definition of the Function: The sprag clutch design is a sophisticated geometrical shape in which the engagement curve activates when a certain pitch angle gets in contact with the round mating parts.
The Engagement Curve
When torque is applied to the clutch, the sprags roll along the engagement curve. This creates an equal amount of force between the torque, tension and mating. This part of the engagement curve is called the operating zone.
The engagement curve has to meet the operating condition:
- (? <= µ) (µ = coefficient of friction)
The size of the engagement angle is determined by the applied torque and the reaction force to the expansion of the mating parts.
The engagement angle creates an angle of twist between the outer and inner ring. This remains equal with constant operating conditions and should not be considered slippage or intermittent slippage.
Precise Indexing & Long Operating Life
Precise indexing and a long operating lifespan is the result of the logarithmic spiraled engagement curve.
To achieve these two factors it’s important that all sprags engage simultaneously with the same angle for torque pickup. Equal force acting on all sprags is required from the instant of engagement to full torque transfer.
The geometric shape of the logarithmic spiral, along the engagement curve, guarantees equal pitch angles between the sprags and round mating parts in each position of the engagement curve. This angle remains equal, even with different positions and angles of the sprags.
Sprag Clutch Spring System
In order to provide instant torque transmission, the sprags need constant contact at all times. One of the most reliable ways to do that is with a spring system. Our sprag clutch product line offers two types of spring systems:
- Meander Spring: A special 3D meander-shaped spring we created for our Sprag Clutch FE 400 M series. This special shaped spring provides spring loading to each single sprag. This spring systems is recommended for every precise and fast indexing applications.
- Z-Type Spring: A simple spring which is placed around all sprags led through a groove in each sprag. This produces lower spring loading results and less wear in overrunning and backstopping applications. Our Sprag Clutch FE 400 Z is a good example of this type of spring.
Please note: Our standard sprag clutch FE 400 includes the meander spring system even when the suffix “M” has not been added. If you want the Z-type spring in your quote or order, add the suffix “Z” to the end of the product number: Ex: FE 442 Z, FR 422 Z etc.
Also, if you are interested in our ball bearing sprag clutches, they are only available with a Z-type spring system.
We hope this has answered your questions about how a sprag clutch works. Contact us and let us know what you think!
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