Worm 1 rotates about fixed axis A-A and has three sections, a, b and c, with different numbers of threads (starts) and, consequently, different lead angles. Interchangeable worm wheel 2 rotates about fixed axis B. The mechanism can have three different transmission ratios depending on which section, a, b or c, of worm 1 meshes with wheel 2.
$2811$WG,3L$