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