Engineers know the way to torque down screw caps is to run the bottle top past multiple pairs of rotating elastomer wheels. Each pair imparts a twist on the cap until it finally bottoms out. Then, the tight cap slips past as the elastomer wheels keep turning.
To spare undue wear on the cap and the wheels especially, some machine designers install slip clutches that permit the wheels to overrun once the cap hits a finished torque. Elegant in their simplicity, humble slip clutches can last a long time. Properly designed, they'll go 30 million cycles, says Jerry Shaff, president of Polyclutch. The secret is in limiting the heat they build up.
Shaff says the formula,
Watts = torque (in.-lbs.) * rpm * 0.011
determines the heat a system generates. Staying under that number when specifying the clutch ensures its long life—one that often outlasts the mechanism it's in, Shaff says.
That's for a continuous duty clutch, of course. Intermittent duty clutches, such as those used for overload protection—the popular application—can be sized more leniently.
Capping is just one of a host of intriguing applications. Shaff, a former machine designer himself, says the devices are particularly adept at letting machines run faster than they ordinarily would. The device can add a soft-start feature to a winder, say, which allows it to run twice as fast because no shock is introduced to the wire when starting up. Electrical devices do this all the time, of course, but the inexpensive slip clutch does it with little fanfare.
Anatomy of a Slip Clutch
Multiple friction discs transfer torque between a motor shaft and a pulley at a torque set by spring pressure upon the stack. A fixed torque clutch substitutes a plate for the adjusting nut. Polyclutch slip clutch torques are calibrated to ±20 percent but can be held to tigher tolerances, the company says.
Contact: Jerry Shaff or Phil Bacon, Polyclutch Tel: 800-562-9522