Recirculating ball
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Recirculating ball, also known as recirculating ball and nut or worm and sector, is a steering mechanism commonly found in older cars and some trucks. Most newer cars use rack and pinion steering instead, but some significant manufacturers (including DaimlerChrysler and General Motors) still use this technology in some models, e.g. the Jeep Wrangler and the Crossfire.
[edit] Mechanism
The recirculating-ball steering gear contains a worm gear. There is a block of metal with a threaded hole in it; this block has gear teeth cut into its outside, that engage a gear which moves the Pitman arm. The steering wheel connects to a threaded rod, similar to a bolt, which sticks into the hole in the block. When the steering wheel turns, it turns the bolt. Instead of twisting further into the block the way a regular bolt would, this bolt is held fixed so that when it spins, it moves the block, which moves the gear, causing the wheels to twist and steering the car.
[edit] Ball bearings
Instead of the bolt directly engaging the threads in the block, all of the threads are filled with ball bearings that recirculate through the gear as it turns. The balls actually serve two purposes: firstly, they reduce friction and wear in the gear; and secondly they reduce slop in the gear. Slop would be felt when changing the direction of the steering wheel — without the balls in the steering gear, the teeth would come out of contact with each other for a moment, making the steering wheel feel loose.
[edit] Power assistance
Power steering in a recirculating-ball system works similarly to in a rack-and-pinion system. Assistance is provided by supplying higher-pressure fluid to one side of the block.
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