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 nonetheless it is similar to a screw thread. The worm wheel, coloured yellow, is like a normal worm gear and worm wheel equipment wheel or spur equipment. The worm usually drives the worm wheel round, it is never the opposite way circular as the system tends to lock and jam