Most engineers associate slip clutches with their most common application, that of overload protection. But slip clutches also let motors upstream run at a constant rate, to both act as simple drive control and protect motors from excessive wear. Applications are plentiful, ranging from underground sewer-pipe cleaning to fiber-optic cable manufacturing, from truck mirrors to airplane video screens, and from braiding machines to bar code printers and plotters. Generally speaking, slip clutches are used for
- Increasing machine speeds or applying constant tension to paper, film, wire, and threads
- Soft starts and cushioned stops on indexing conveyors
- Holding lids, covers, screens, automatic toilet seats, windows, and robots in position
- Controlling torque for bottle capping, screw assembly, and valve closing.
Depending on the application, even continuous-slip clutches can operate for long life — and at a cost lower than alternate methods of metering power. Continuous-slip clutch torque ratings extend from a few oz-in. to over 1,000 lb-in. Other larger cycling clutches are used for engaging, disengaging, and overload protection. But for all varieties, capacity is defined by torque, rpm, and duty cycle — all of which are interdependent. A reduction in one allows an increase in any other. The limit is based on heat buildup measured in Watts, where
Watts = Torque (lb-in.) × rpm × 0.011
Excess heat from higher-than-design wattage shortens life. However, when running within design limits, friction-plate designs operate for over 30 million cycles. In most cases, these clutches outlast the mechanisms in which they're installed.
Slip clutches, of course, are best known for their overload protection. Employed this way, they include either a shear pin, ball detent, or friction-plate design. A shear pin effectively saves the mechanism from damage, but operates only once and must then be replaced. On the other hand, ball detents slip at a set torque with a pulsating torque from zero to the designed breakaway setting. Once the impediment is removed, ball detents provide overload protection again.
Friction-plate designs slip at the breakaway torque to give constant, continuous torque at this setting. They continue slipping until the impediment is removed. Its basic design uses axially loaded plates and friction pads to transmit torque. The higher the axial load, the higher the torque. This axial load is supplied mechanically by various spring arrangements. Load can also be applied pneumatically or electrically, in which case the torque can be changed during operation by varying the air pressure or voltage. Servomechanisms can also vary torque to meet mechanism needs. This feature simplifies setup, as changes can easily be dialed in and repeated when needed. If properly designed, these friction systems provide long life and accurate torque.
Cushioning and tension control
Continuous-slip clutches suit tension control and cushioning applications. For example, slip clutches might be used to gently increase the speed of a plant line. Slip clutches can also be designed and manufactured so that a mechanism's static friction is lower than its dynamic friction. With unique materials, such clutches transmit torque that (starting at zero) increases with rpm. This eliminates stiction and results in very smooth starts — useful for gradually applying torque and cushioning against suddenly applied loads. Then tension (on paper, thread, wire, or film) can be gradually applied at motion start, so the resulting load is smaller than if applied abruptly; in some cases, gradual tension application with a slip clutch might even halve this initial load. The machine can then run faster without overloading spooled material. This cushioning also reduces impact on gears, pulleys, slides, and chains, thus benefiting entire machine systems and allowing higher speeds. Tension control is accurate, repeatable, and durable.
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.
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.
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 [email protected].