Slip Clutches Solve Diverse Design Problems
- Wednesday, 01 April 2009
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.
- 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.
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.
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.