Single-Head (Double-Head) Small Modulus Worm

Like a ball screw, the worm in a worm gear may have one or more starts - meaning multiple threads or spirals on the worm. For a single-start worm, each full revolution of the worm advances the gear by one tooth.
The meshing of the worm and gear is a mixture of sliding and rolling actions, but the sliding contact is dominated by high reduction ratios. This sliding action creates friction and heat, which limits the efficiency of the worm gear to between 30% and 50%. To minimize friction, the worm and gears are made of dissimilar metals—for example, the worm might be hardened steel and the gears bronze or aluminum.