Worm gears are the best gear choice when high drive reduction is required. Worm gear wheels are similar to helical gears with the addition of a throat cut into the outer diameter of the wheel. The throat allows the worm gear to completely encircle the teeth of the worm gear. By adjusting the number of worm threads, different ratios can be obtained without changing the mounting arrangements. A unique feature of worm and wheel sets is that they prevent back driving.