Torque Limiters and Overload Release Clutches

Torque Limiters

"A torque limiter is a frictional device whose primary function is to transmit continuous power until a preset limit is achieved. Upon achieving the preset torque limit the frictional device will slip, thereby transmitting only partial power."

Carlyle Johnson torque limiters are basically clutches that are always engaged. A torque limit is preset (and usually adjustable over some range). When the preset torque setting is reached, the device slips, allowing only the transmittal of torque up to the "slip torque" value.

The torque limiter is used to couple two separate bodies of rotation. The two separate bodies may consist of shafts, gears, sprockets, a prime mover or motor, or any combination of these. The components are usually pumps, fans, Power Takeoffs, compressors, motors, gearboxes and generators, but it is usually the shaft of the component itself that is used to transfer power whether a driven or driving element.

Whenever the torque being transmitted is below the "slip torque" value preset in the torque limiter, the full torque will be transmitted. The preset torque level must be exceeded in order for the torque limiter to slip. When the torque is reduced below that preset level, the device resumes transmitting full torque. The torque transmitted during the slip period is a function of the speed, duration of slip, the frictional materials selected and the type of lubrication - if lubrication is utilized.

Some torque limiters are designed to tolerate being run in a continuous slip condition – that is the torque from the driving element may consistently exceed the "slip torque" value, and only the preset level of torque is transmitted.

Other torque limiters are not designed to run in continuous slip environments, and when the torque limit is approached – for example in the case of an overload – the output of the driving element must be reduced after a momentary slip condition – or it must then be decoupled from the driven device until the overload is corrected. Consult the factory for additional information on your specific application.

Overload Release Clutches

Overload Release Clutches are clutches that have the additional feature of self-disengagement when a preset torque is reached. Upon reaching the preset torque value the clutch is fully disengaged by an internal cam mechanism and no power or torque will be transmitted. The disengagement point is usually adjustable over some range for each size clutch.

Once disengaged, the clutch must then be reengaged manually before power can be transmitted. This feature also allows the clutch to establish rotational positioning (indexing) between the two rotating bodies. When used in indexing, the re-engagement point of rotation can be defined in degrees, with 1, 2, 4 or more re-engagement points when the clutch is reset. A clutch which can be engaged every 90° will have four re-engagement points for each complete rotation of the driving element. This assures that the driving elements are always synchronized with the driven devices when timing is critical.

Unlike a torque limiter, an overload release clutch will not tolerate even momentary overload. This is especially useful in applications where safety considerations and protection of sensitive downstream equipment must take precedence over continuous operation.

Considerations in Selecting Torque Limiters vs. Overload Release Clutches

Type	Operation	Precision	Reset	Feature
Torque Limiter	Continuous	Relative	Automatic	Slip Torque
Overload Release	Safety Stop	Maximum	Manual	Rotational Positioning

The nature of the application and the types of driving and driven bodies will determine the proper device selection. Consider, for example, a conveyor being driven by an electric motor. When a jam or backup occurs which prevents the conveyor from moving, use of a torque limiter may present a safety hazard to operating personnel. In this case, if a jam is manually cleared, the torque required to move the conveyor will be immediately reduced, and the motor, which is not disengaged from the conveyor even in a slip condition, will instantly begin moving the conveyor, even while personnel are still clearing the jam, potentially causing injuries.

When a torque limiter is used in a power transmission application, the design engineer must recognize that there will always be 'stored energy' in the system when the driving element is energized, even if the driven device is not moving. Careful consideration to safety factors must be given in this type of environment.

An overload release clutch, in contrast, will positively disengage the conveyor in our example, and will not restart it again until the jam is cleared, and until operating personnel clear the area, positively reset the clutch, and allow the conveyor to resume operation.

Torque limiters are useful in applications where the driven device cannot absorb the full output torque of the driving element. An electric motor driving a bottle capping machine is an example of this kind of application. Sufficient torque must be transmitted to twist the cap into position, but when the cap is tightly fit to the bottle, additional torque would damage the product, the capper, or both. In this case, the torque limiter assures a tight fit of the cap, but not so much torque that any damage occurs, assuring a high level of efficiency with minimum down time in the capping operation.