Jerry Shaff
Polyclutch of Custom Products Corp.
North Haven, Conn.

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 sewerpipe 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.

Clutch operation

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.

Overload protection

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.