Gears have evolved over the last decade. Worm gears have sliding surfaces but they can have higher friction. Spur gears have rolling surfaces, but because the teeth are cut straight across on a face; one or two teeth at a time are in contact with another gear, which can create high noise. Worm and spur gears are less expensive to manufacture, but they typically last about three years before they need to be replaced.
Helical gearing is machined to have angled teeth, then hardened and ground to achieve an efficient gear mesh. The teeth angle enables the gears to gradually mesh. Thus, two or three teeth of each gear are always in contact with other gears. This configuration lessens the load on each tooth and creates a smooth transition of forces from one tooth to the next. The result: less vibration, wear, noise, and a longer life.