The Sprag Clutch Design & Functionality
A sprag clutch resembles a roller bearing, but instead of allowing the elements to roll freely in both directions the rolling elements lock when rolled in the opposite direction.
A Sprag clutch has three main stopping options:
- Overrunning
- Indexing
- Backstopping
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 one direction but provides instantaneous locking when force is applied in the opposite direction.
Torque is passed from one raceway to the other by the wedging action of the sprags, consistently providing safe and reliable performance.
A sprag clutch is designed to fit the maximum number of sprags that will enable the greatest amount of torque in comparison 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 in along the engagement curve until an equal amount of force between the torque, tension and mating parts is achieved. 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 force of reaction to the expansion of the mating parts.
The engagement angle creates an angle of twist between outer and inner ring which will remain equal with constant operating conditions and should not be considered slippage or intermittent slippage.
Precise Indexing & Long Operating Life
The logarithmic spiraled engagement curve is the prerequisite for precise indexing and long operating life.
To achieve these factors it is 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 special 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 position angles of the sprags.
Spring System
In order to provide instant torque transmission, the sprags have to be kept in friction contact all the time reliably with a spring system. We provide two spring systems:
| 1. Meander Spring A special three-dimensional meander shaped spring was developed for our Free-wheel Clutch model FE 400, also called FE 400 M. This special shaped spring provides spring loading to each single sprag. This spring systems is recommended for every precise and fast indexing application. | 2. Z-type spring Model FE 400 Z and shows a simple spring which is slung around all sprags led through a groove in each sprag. The lower spring loading results in less wear in overrunning and backstopping applications. |
The Standard design of model FE 400 includes the meander spring system even when the suffix “M” has not been added. For Z-type spring please add suffix “Z”, e.g. FE 442 Z, FR 422 Z etc. Please note that our ball bearing sized clutches are available with Z-type spring system only.