Torque Limiters and Slip Clutches Information
Torque limiters and slip clutches automatically slip or disengage the clutch at a predetermined torque. They are generally used to protect equipment from damage or mechanical overload, although some varieties also protect the work upon which the mechanical device is working. Torque limiters may prevent damage by uncoupling the load, or as in the case of a slip clutch, by limiting slippage.
A freewheeling clutch is a unidirectional torque transmitter that spins freely in the direction opposite the intended drive rotation.
Backstop clutches are a type of freewheeling clutch that transmit torque in one direction only; they are specifically used for preventing drives from "backing up" or driven systems such as inclined conveyors from going down.
An overrunning clutch is used as a safety mechanism. The drive element stops and the load winds down freely.
Shear torque limiters have mechanical fuses which shear at specified torque limits to prevent damage to the driveshaft, transmission, axle, or other drivetrain component.
Slip clutches or disengagement torque limiters have a threshold torque that causes them to disengage the drive in case of jamming or overload. They are primarily used for personnel safety and equipment protection.
Clutch or Drive Engagement
There are several choices for clutch engagement.
Friction torque limiters use the friction between contact surfaces to transmit power. This is the most common configuration.
Noncontact technologies use magnetic fields and eddy currents.
Toothed clutch engagement uses toothed contact surfaces to transmit power without slipping. Although toothed clutches do not generate heat, they can be engaged only when stopped at slow speeds (< 20 rpm).
In wrap spring engagement systems, torque is transmitted from input to output by a coiled spring that wraps around the output element. The clutch is disengaged when the spring is uncoiled via a control tang at its end.
With oil shear systems, drive engagement is achieved via the viscous shear of transmission fluid between the clutch plates.
In plate/disc systems, torque level is controlled by compression springs that force plates together.
A ball detent torque limiter is a slip mechanism in which, upon overload, balls ride up out of their seats and overcome springs or air pressure engagement.
Roller detent systems use rollers, held in place by springs, which wedge between inner and outer races to engage clutch.
Pawl detent clutch slip mechanisms are those in which, upon overload, a pawl overcomes spring or air pressure engagement and rotates out of its detent.
Sprag engagement systems use steel wedges that tip in one direction to wedge between inner and outer races. These torque limiters and slip clutches can be configured with either the inner or outer race as the input or output. Too much torque makes the sprags tip too much and contact is no longer held. Sprag clutches can often transmit more torque than other designs of slip or overrunning clutches.