worm gear and worm wheel

The arrangement of gears seen above is called a worm and worm wheel. The worm, which in this example is brown in colour, only has one tooth but it is like a screw thread. The worm wheel, coloured yellowish, is like a standard gear wheel or spur gear. The worm generally drives the worm wheel circular, it is never the opposite way circular as the worm gear and worm wheel system tends to lock and jam