POWER CYLINDER (hydraulic cylinder formed around the rack)- The power cylinder is precisely machined to accept the power piston. Provisions are made for the hydraulic lines. The power cylinder bolts to the vehicle frame, just like the rack of a manual unit.

POWER PISTON (a double-acting hydraulic piston formed on the rack)- The power piston is formed by attaching a hydraulic piston to the center of the rack. A rubber seal fits around the piston to prevent fluid from leaking from one side of the power cylinder to the other.

HYDRAULIC LINES (steel tubing that connects the control valve and power cylinder).

CONTROL VALVE (a hydraulic valve which regulates hydraulic pressure entering each end of the power piston)- There are two types of control valves- rotary and spool. Using a torsion bar connected to the pinion gear operates the rotary valve, whereas the spool valve is operated by the thrust action of the pinion shaft.

Other components of the power rack and pinion are similar to those that are found on manual rack-and-pinion steering system.

Power rack-and-pinion operation is fairly simple. When the steering wheel is turned, the weight of the vehicle causes the front tire to resist turning. This resistance twists a torsion bar (rotary valve) or thrusts

Figure 8-28.- Power steering gearbox.

the pinion shaft (spool valve) slightly. This action moves the control valve and aligns the specific oil passages. Pump pressure is then allowed to flow through the control valve, the hydraulic line, and into the power cylinder. Hydraulic pressure then acts on the power piston and the piston action assists in pushing the rack and front wheels for turning.