A worm gear pair can be designed in such a way that only the drive along the worm is possible, i.e. that applying any torsionial force on the shaft of the wheel brings no movement of the worms. To obtain this the lead angle should be smaller than the angle of friction or (γ m≤f). Caution should be taken into account for choosing the angle of friction “f”; its value is essential depending on the condition (rest or movement) in which the drive is located. Practically, one should assume that a “worm-worm wheel” gear pair grinds to a halt at the disappearance of the driving force to the worm when the helix angle is below the 2°.
The inertial forces of the moving elements of course influence the span in which the pair grinds to a halt.