Worm Gear

A "worm gear" is a shaft with very coarse thread, which is designed to operate or drive another gear or a portion of a gear. The special shape of the gear allows the rotation direction to be turned when the gears engage with minimal friction. An example is found inside the steering box, where the steering shaft turns a worm gear that is screwed into a large nut. The nut moves back and forth on friction-reducing ball bearings, which are continuously recirculated by dropping into the nut's bored channels and emerging at the opposite side. Power to move the nut comes from pressurized fluid entering from a pump through rotary valves that open in response to the steering wheel. The worm gear engages the cross shaft through a roller or by a tapered pin.