As part of this system, AMP Research incorporated an integral overload protection feature, in the event something blocked the motion of the Power Step.

The initial design employed a traditional ratchet device with balls and a spring-loaded pressure plate. This system worked well, but proved complicated to assemble.

Specifically, the design used a long spline on the end of the output shaft onto which a backing plate was pressed. The backing plate had pockets for six hardened steel balls that mated with holes in the gear face. The gear was free to rotate on the shaft, and axial pressure was applied using a pair of belleville disc springs held by a special adjusting nut and locking washer. In all, the system included 11 parts that had to be assembled, not including the shaft and gear.

To simply the process, AMP switched to Tolerance Rings from USA Tolerance Rings (Pennington, NJ), spring steel strips with waves curled into them, so that they act like a slip clutch. Specifically, the rings apply a spring tension between mating components such as a gear and a shaft.

In the event of overload conditions, the rings allow slippage to occur to prevent damage to the mechanism. Once the overload is relieved, the rings allow the mechanism to function as if nothing has happened. In the adjustable running board application, the rings fit within the gear bore to provide both torque drive and the slip at predetermined torque range.

Now, the system's overload protection includes the redesigned shaft with a groove to center the Tolerance Ring, the ring itself and two C-clips to assure gear location-a total of three parts, not including the shaft and gear. The ring is installed via a simple press fit-no adjustment or calibration necessary-to assure torque overload slip range.

For more on torque-sensitive couplings, visit www.usatolerancerings.com or eInquiry 4.