Getting up to speed with wrap-spring clutch/brakes

No-slip clutch/brakes keep loads and drives synced up. A tape nip uses a motor and belt drive to power the input hub of a wrap-spring clutch/brake. The tape accurately pays out in increments determined by the number and location of stops on the stop collar. Wrap-spring clutch/brakes work at speeds to 1,800 rpm and torque to 5,000 lb-in. A stopping accuracy of ±0.5° is not cumulative cycle to cycle. Single-revolution clutches use a two-tang spring to provide limited braking capacity. The control tang attaches to the surrounding stop collar (not shown), while the output-hub tang affixes to the output hub. Stopping the rotating stop collar also halts the output hub. Such clutches have a stop-load rating roughly one-tenth that of the start load. A typical wrap-spring clutch/brake (top) brings a load up to input speed in about 3 msec and stops it in 1.5 msec. These times are nearly constant, repeatable, and independent of inertia or frictional load, provided the unit runs within its rated torque capacity. Conversely, start and stop times of a frictional clutch or brake (bottom) directly depend on load inertia and friction Machines that move loads to specified positions and hold them within a few thousandths of an inch need highly controllable, precision brakes. Engineers typically spec friction brakes for these applications. But a wrap-spring clutch/brake can also do the job. It combines in a single package a clutch and brake plus an actuator (solenoid or pneumatic cylinder) to engage and disengage the unit. Wrap-spring clutch/brakes work best for machines that start and stop multiple times during a single revolution. Printing presses, postage and packaging machines, and food and material-processing equipment are some examples. Wrap-spring clutch/brakes operate on the same basic principle as wrap-spring clutches, though the latter is suitable only for applications that don't need high