Worm and Wormwheel
A gear which has one tooth is called a worm. The tooth is in the form of a screw thread. A wormwheel meshes with the worm. The wormwheel is a helical gear with teeth inclined so that they can engage with the thread-like worm. The wormwheel transmits torque and rotary motion through a right angle. The worm always drives the wormwheel and never the other way round. Worm mechanisms are very quiet running.