Abstract: A mechanism is described for selectively engaging a handwheel with a shaft to manually rotate the shaft with the handwheel. The mechanism automatically disengages the handwheel from the shaft when the shaft is rotated by a power source above a predetermined speed. Further, when the shaft is thusly rotated by a power source, the mechanism does not allow an operator to engage the handwheel with the shaft. The mechanism includes a complementary groove system and a cam, one being on the shaft and the other on the handwheel. The groove system comprises at least two laterally-spaced grooves encircling the axis of the shaft, and a third groove spiralling about the axis between the first and second grooves. In one embodiment, the cam is mounted on the handwheel and rides in the grooves in the shaft. When the handwheel is axially depressed, to engage the shaft, the cam rides in the innermost encircling groove.