Recirculating Ball and Nut Steering Gear

Recirculating Ball and Nut Steering Gear

The larger and heavier the car, the more difficult it is to steer. Most large American cars are equipped with a recirculating ball-type steering gear. This type of steering gear is very low in friction and provides a good mechanical advantage for a heavy vehicle.

The recirculating ball and nut steering gear consists of several parts contained in a steering gear housing. The steering gear shaft is connected to the steering wheel either directly or through some type of flexible joint. There is a worm gear on the end of the steering gear shaft. A cross (Pitman) shaft is mounted in the housing in a position 90 degrees to the worm gear. A ball nut rides on the worm gear and a gear on the cross (Pitman) shaft, called the cross shaft sector, is engaged with this nut.

Ball or roller bearings are used to support both ends of the worm gear and are adjustable to remove end or side play from the worm gear. The cross (Pitman) shaft is supported by bushings, needle bearings, or a combination of the two, and provision is made to control the worm and cross shaft clearance. All parts are enclosed in a cast housing that is partly filled with lubricant. Seals are used to prevent the entry of dirt or the loss of lubricant. Provision is made to bolt the steering gear housing to a rigid area, usually the frame.

The ball nut has internal threads that are meshed to the threads of the worm with continuous rows of ball bearings between the two. The ball bearings are recirculated through two outside loops, called ball guides.

The sliding ball nut has tapered teeth cut on one face that mate with teeth on the sector. As the steering wheel is rotated, the nut is moved up or down on the worm. Because teeth on the nut are meshed with the teeth on the sector, the movement of the nut causes the sector shaft to rotate and swing the steering linkage connected to it.

The recirculating ball construction results in a friction-free contact between the nut and the worm. When the steering wheel is turned to the left, the ball bearings roll between the worm and the nut and work their way upward in the worm groove. When the ball bearings reach the top of the nut, they enter two ball guides and are directed downward into the worm groove at a lower point. When the steering wheel is turned to the right, the ball bearings circulate in the opposite direction.