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.
When tension is controlled this way, it is accurate, repeatable, cushioned and durable. The cushioning effect extends throughout the machine or mechanism, reducing the impact on gears, pulleys, slides or chains and allowing even higher speeds.
Tension control applications range from labeling machines, wire winding, film processing, thread control on knitting or sewing machines, and printing machinery to simple tension control on items such as fishing reels. Tension can be applied by pulling the material around a roller or between two rollers, or many other ways. As the material is pulled, it must make the roller turn and overcome the torque setting in the slip clutch, which applies a force to the material. When the machine stops pulling, the force drops to zero. In some cases, the clutch can be turning slowly in the opposite direction so that there is no slack and the force always remains constant. This reduces the suddenly applied load even further.
Increased tool life is another benefit of slip clutches. Their cushioning effect will prolong tool life significantly in applications such as bottle capping or turning screws. Even more effective is eliminating slippage of the tool against the product and instead handling this slippage within the clutch. For example, capping machines use several pairs of elastomer wheels to screw caps onto bottles as they come down the line. When the cap bottoms out, previous machines let the wheels slip against the cap, often damaging the product and shortening the life of the elastomer wheels. Newer machines incorporate a clutch to take up the slip, so the wheels stop when the cap bottoms out. Clutch life can exceed 30 million cycles.
Besides longer tool life and reduced product wear, torque is applied more accurately to the cap and can be changed for different products. With electric or pneumatic actuation, the torque settings can be changed easily and repeatedly, with corresponding reductions in setup time and costs.
Similar applications include turning screws automatically to a torque limit, closing valves, setting controls and similar torque control applications.
Indexing — a clutch’s continuous slip characteristic — also can be used to index tables, conveyors and other mechanisms. A simple, relatively inexpensive index mechanism for low speed applications consists of a device that holds a pin on an index wheel. The clutch slips continuously until a solenoid removes the hold, allowing the wheel to turn. The solenoid then returns the holding component to position before the wheel indexes to the next pin. Pins can be located to allow a full or partial revolution, and the cycle can be changed easily by moving the pins. Uneven indexes also are easy to program by spacing the pins unevenly. The clutch also provides overload protection in these applications.
Vending machines are one interesting application of indexing. One motor can drive dispensers for all the items, with an inexpensive slip clutch and solenoid holding back each item individually. When an item is selected, the motor turns on, but only the solenoid for that item turns on to allow indexing and deliver the item. The motor then turns off until the next selection. The action is instant, for a short duration, and with little wear on the machine.
- Force Control — pushing forces also can be generated by allowing a slipping clutch to push against a connecting force arm. This force control is used on ice machines to push a frozen tray into a “harvesting” cycle. When a single revolution is completed, it signals a new freezing cycle to begin. This inexpensive mechanism reduces cycle time considerably, which also saves energy.
On conveyor applications, the transported product can push against a gate without damage to the product or the conveyor. Slippage occurs only in the clutch, which also provides overload protection.
- Friction Hinge — a popular and effective application for a slip clutch is to use it as a friction hinge. Installed at the pivot point, it will hold a lid, cover, door, window, display screen or similar hinged object at any position. When combined with a one-way clutch there will be no resistance, since the cover is raised but remains at the desired position. Fingertip force is all that is required to move the cover. The slip clutch provides a smooth, cushioned action in applications where jerky motions must be avoided.
- More Applications — a slip clutch can be used in any application where temporary stoppages occur when a mechanism is held at random. An example is a rotary cylinder used to display pastries in many restaurants. The cylinder is either transparent or open and rotates slowly so customers can view and select the pastries or other items. By simply holding the cylinder, a person can remove the desired item. Similarly, a conveyor, slide, rotary table or other moving mechanism can be stopped temporarily while the clutch does the slipping.
When a motor moves a mechanism into a locked position, such as closing a door or reaching a linear motion stop, it may be useful to apply slip at the end of the cycle. Rather than using a time delay to prevent the motor from burning up, a slip clutch can remove the load from the motor for any length of time.
Slip clutches have been used in a vast range of other applications, including holding robotic elements in a desired position. They also have been used on applications as diverse as automatic toilet seats, underground sewer pipe cleaning equipment, move-able airplane video display screens, braiding machines, plotters, fiber-optic cable systems, automatic doors, bar code printers and truck mirrors.