Torque overload devices ensure operator safety and protect machines from excessive loading by disengaging drives from driven components when torque exceeds an upper limit. There are several designs. On the simplest, a pin shears when the limiter is overloaded; then an operator physically opens the device to replace the pin. Friction plates are another option; their drawback is inaccuracy and wear. In common hub designs, a single roller in a pocket pops out when over-torqued. Though these are capable of automatic one-position resets, they exhibit backlash and also tend to skew.
Another torque overload device, the Browning Torq/Pro X, transmits torque through several points to maintain parallelism. When engaged, five steel balls are pressed into five V-shaped holding channels on the driven flange by a spring-loaded pressure plate. In between flange and pressure plate, another hub spaces the balls out — not in a perfect pentagon about the center axis, but in an asymmetrical array.
During overload, all five balls are torn from their pockets and roll freely on the flange surface — effectively disconnecting the drive from driven components, and allowing operators to remove jams or make repairs. Once torque again declines to a transmissible value, the limiter can automatically reset; the array of balls drops back into the mating pattern of channels, to resume transmission at the reset position.
Suitable for indexing applications, the torque limiter has a trip-torque precision of ±3%. So, for a torque setting of 10 lb-ft, the device trips between 9.7 and 10.3 lb-ft. How? The balls are constrained at five points by three pieces, so accuracy is greatly increased and there is virtually no backlash. In contrast, the average precision of friction-pad torque limiting devices is ±25%.