Power steering gearbox components

Written by chris stevenson
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Power steering gearbox components
The steering wheel sends comands to the gearbox, which has to interpret them by sensing pressure. (Nicolas Loran/Photodisc/Getty Images)

Power steering has the job of making the vehicle turn via the steering wheel with a minimal amount of pressure applied. It does this by using a fluid pump that drives power steering fluid through high pressure lines into a steering gearbox that has valves, seals and gears. The rack and pinion type steering gears comprise most of the systems in use today. Steering boxes can be adjusted to take slack out of the steering wheel. Steering boxes also have internal parts that can be rebuilt and serviced.

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Main Gearbox Body Housing

The main gearbox housing, usually made of cast iron, contains all the internal parts of the gearbox. It has four powering steering line inlets threaded into its casing -- one from the reservoir, one from the pump and two fluid lines.

Rotary Valve

The rotary valve inside the gearbox uses pressure from the steering wheel to sense the force applied to it. The valve actuates only with detected pressure. When driving in a straight line or with hands off the steering wheel, the rotary valves works only with minimal operation.

Torsion Bar

The torsion bar works in conjunction with the rotary valve. The torsion bar consists of a thin rod of metal which turns when it receives torque (turning) pressure. It activates when turning left or right from the commands of the steering wheel. The torsion bar connects to the steering wheel at the uppermost portion -- a small input shaft with splines on it. The other end, or bottom of the bar, fastens to the worm or pinion gear that turns the wheels. The more pressure that is applied to the wheel, the more the torsion bar rod must twist.

Sector Shaft

The sector shaft, which has splines on its end, exits the bottom of the gearbox and attaches to a tie rod on the pitman arm. The pitman arm controls the steering linkage of the wheels. The part of the sector shaft that sits inside the gearbox contains several gear teeth. The teeth mesh with the power piston inside the gearbox. The sector shaft spins in a seat of needle bearings. Several seals and washers surround the shaft and are held in place with a snap ring.

Input Shaft

The input shaft exits the top of the gearbox. It has splines on it that mate with a universal coupler or a rag-joint flange that connects directly to the steering column.

Sector Shaft Adjustment Nut

A plate sits on the top of the gearbox housing that has an adjustment stud and locknut on it. The locknut must be loosened to turn the stud either way, which allows more free play in the steering or takes the excess slack out of it. It has also been referred to "worm gear lash."

Power Piston

The power piston moves back and forth inside the gearbox. The piston has machined spirals that ride on ball bearings within a tube assembly, which allows it to mesh with the worm gear. When the worm gear and input shaft turn, it shoves the piston back and forth. The outside of the piston has teeth that mate with the sector shaft. The power piston's movement causes the sector shaft to rotate, which turns the pitman arm, which turns the steering linkage.

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