Slip Clutches Solve Diverse Design Problems
- Wednesday, 01 April 2009
Slip clutches are commonly used to protect against overloads, but they can solve many other problems as well. Their applications include increasing machine speeds, applying constant tension to webs or wires, indexing a mechanism, holding a hinged object in position, controlling torque on capping or assembly operations, and providing soft starts or cushioned stops.
A continuous slip clutch (see Fig. 1.) can provide a surprisingly long life in a broad range of applications, and at a definite cost advantage when compared to alternative solutions. They are available with torque ranges from oz/inches to more than 1000 lb/inches. Clutch capacity depends on torque, RPM and duty cycle, all of which are interdependent. A reduction in any of these factors will allow an increase in any other. The limiting factor is heat buildup, which is measured in watts, where:
Watts= Torque (inch pounds) x RPM x .011
Excess heat from wattages above the design wattage will shorten the life of the clutch. The friction plate design, when running within design limits, will generally operate for more than 30 million cycles. In most applications, the clutch will outlast the mechanism in which it is installed.
How They Operate
A typical slip clutch (see Fig. 2.) consists of two assemblies: a cartridge and a housing. The cartridge is set screwed or keyed to the input shaft. The housing may be either set screwed or keyed to the output shaft. It also can be attached to the output gear or pulley, with a bronze bearing to allow relative motion between the input shaft and the output gear or pulley.
Torque is transmitted from the flats on the hub of the clutch to the mating flats on the inner plates, through the friction pads to the outer plates, through the torque pins to the housing and the output gear or pulley. The torque level is controlled by compressing the springs with the adjusting nut. For a fixed-torque clutch, a collar is attached to the hub in a fixed position instead of to the adjusting nut. In operation, either the input shaft or the housing can be the input member, with the other member being driven.
Here are some applications where slip clutches can solve motion control problems:
- Overload Protection – This is one of the most common applications of slip clutches). Overload protection can be provided by anything from a shear pin or ball detent to a friction plate clutch, but the first two solutions has limitations. A shear pin will effectively save the mechanism but will operate only once. It must then be replaced. A ball detent will slip at the set torque, with a pulsating torque going from zero to the breakaway setting. Once the impediment is removed, it will again provide overload protection. The friction plate design will slip at the breakaway torque and give a constant, continuous torque at this setting. It will continue to slip until the impediment is removed. The basic design uses axially loaded plates and friction pads to transmit the torque. The higher the axial load, the higher the torque that can be transmitted. The axial load is supplied mechanically by various spring arrangements. The load also can be applied pneumatically or electrically, in which case the torque can be changed during operation by varying the air pressure or the voltage. Servomechanisms also can vary the torque to suit the needs of the mechanism. This feature also simplifies setup, as changes are easy to dial in and repeat as needed.
- Cushioning & Tension Control — the continuous slip capabilities of slip clutches open up many possibilities to cushion loads or control tension. One such example is increasing the speed of a machine or mechanism. Slip clutches can be designed and manufactured so the static friction is lower than the dynamic friction. This results in gradual application of torque, which can cushion loads that are applied suddenly. When the tension on a web of paper or film, a wire or thread is applied gradually in this manner, it is half of the suddenly applied load. This allows the machine to run faster, without overloading and possibly breaking the material.