Specific pitches and leads of the worm do not let the worm gear to generate the worm. This is valuable when the software requirements to lock the output if the software is working in the opposite course. When the helix angle is significantly less than 5°, the worm is self-locking. When the helix angle is higher than 10°, the worm can be driven back. Worm and worm gear assemblies need to be mounted on vertical, non-intersecting shafts.