To reduce the risk of such damage, Nexen Group Inc., Vadnais Heights, Minn. (www.nexengroup.com), builds a torque limiter with a Turck proximity sensor that indicates when the limiter is activated.

The proximity sensor is next to a dowel embedded in the unit's air chamber. When set torque is exceeded, creating an overload, the torque limiter drive flange containing the balls rotates relative to the drive ring containing detents. The balls lift out of their detents and push the drive ring, bearing, and cylinder assembly axially away from the drive flange. (Detent and ball size determine torque capacity.)

This moves the dowel out of the proximity sensor range, sending a signal to the control valve to block air to the air chamber. Air leaving the chamber through a quick-exhaust valve and a series of compression springs instantly separates the ball and detent interfaces.

The five ball and detents are arranged in an unequally spaced pattern, so the drive ring and flange cannot be engaged after overloads unless they are in the same synchronized position. The torque limiter is set in the run mode by jogging the shafts, with air pressure applied, until the balls engage in the detents.