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