Gears transfer rotational motion and power between components inside mechanical devices like driveshafts, clocks, and automated machinery. They have teeth that mesh with other parts to transmit torque, alter speeds, and change direction. Spur gears transfer linear motion without creating axial force. Helical gears transfer heavy loads and high speeds. Gear racks are elongated shafts with teeth that spur or helical gears ride to convert rotary motion into linear motion. Miter gears relay 1:1 momentum at right angles. Bevel gears connect right-angle shafts and are used to change speeds and torque. Screw gears transmit motion between offset shafts. Worm gears connect non-parallel or non-intersecting shafts. Internal gears have teeth in their inner surface that mesh with spur gears and gear couplings to form planetary gearsets. Ratchet gears and pawls prevent backward rotation.