Tension control is used on labeling machines, wire winders, and film-processing lines. It's also used for knitting or sewing machine thread control, printing machines, and simple fishing-rod tension.

There are many ways to apply tension. Material being handled might be pulled around one roller or between two. In any case, pulled material must make its rollers turn (and overcome the slip-clutch torque setting) for force F on the material. When the winding machine stops pulling, F drops to zero. In some designs, the clutch even turns slowly in the direction opposite of motion to prevent slack and ensure force always remains at F, reducing the suddenly applied load even further.

Higher machine speeds are only part of the story. Slip clutches also increase tool life with load cushioning. But even more effective is their elimination of tool slippage against product — and moving this slippage to within the clutch itself. A good example is in capping machines, which use a series of elastomer wheel pairs to screw caps onto bottles as they come down the line. When a cap bottoms out, older designs let the wheels slip against the cap, resulting in damage to the product and short life for the elastomer wheels. Newer designs move slippage out to the clutch, letting the wheels stop when the cap bottoms out. In addition to longer tool life and less product wear, more accurate torque is applied to the cap — and can be changed for different products. Also, with pneumatic or electric actuation, torque settings can be easily and repeatedly changed; this reduces setup time and cost.

Indexing

Continuous slipping is used to index tables and conveyors. On a simple indexing mechanism, a pin is held on an index wheel. The clutch slips continuously until a solenoid removes the hold, allowing the wheel to turn. Then, the solenoid returns the hold before the next pin arrives. This allows a single or partial revolution that is changed easily by moving the pins. Uneven indexes are easily programmed — by simply moving pins to uneven positions. This index mechanism type suits relatively low speed, inexpensive indexing of tables, conveyors, and controls. Overload protection is built in. One interesting indexing application is vending machines. Typically, one motor drives all items forward while inexpensive slip clutches and solenoids hold each back. While an item is selected, the motor continues operation, but only the selected solenoid indexes to deliver that item. The motor then turns off until the next selection. This action is instant, for a short duration, and with little wear on the machine.

Force control and hinges

Slipping clutches can also generate pushing forces. In this arrangement, the clutch pushes against a connecting force arm. For example, force control is used on ice machines to push frozen trays into a "harvesting" cycle; when a single revolution completes, it signals a new freezing cycle to begin. This inexpensive mechanism reduces cycle time considerably, which also saves energy.

Pushing forces can also be generated by attaching rack-and-pinion systems, belts, or chains to slip clutches. Then torque control is used as either tension or thrust control. Sometimes slip clutches are installed on conveyor gates. Because all slippage occurs in the clutch, transported products can push against these gates without damaging the product or conveyor.

Installed at a moving component's pivot point, slip clutches can hold lids, doors, windows, display screens, and covers in any position. When combined with a one-way clutch, there's no resistance as the item is raised, though it remains at the desired position once lifted. Just the lightest force lowers the item. In hinge applications where jerky motion must be avoided, smooth-acting cushioning slip clutches are suitable.

Other applications

These clutches can be used in any application where temporary stoppage is randomly applied, for halting of loads with the lightest of forces. One example: a rotary cylinder used to display pastries at restaurants. The transparent or open cylinder rotates slowly to present pastries. Holding this cylinder with the gentle force of a human hand causes it to immediately halt spinning so that pastries can be removed. In fact, any moving mechanism (conveyors, slides, rotary tables) can be temporarily stopped while a clutch does the slipping.

End-of-cycle slip is useful when a motor moves a mechanism into a locked position. This is sometimes used for closing doors or stopping some linear motion. Alternatively, time delays can protect motors from burning up. However, in these situations a slip clutch can do that too — for any length of time.

For more information, call Polyclutch at (800) 562-9522 or e-mail the editor at eeitel@penton.com