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Power-assisted and manul recirculating ball steering gears work the same way as manual gears, except that they have a sliding spool valve or a rotating spool valve and torsion bar to route hydraulic pressure to chambers on either side of a power piston that may be located inside or outside the steering box. With the internal assist type, the power piston is part of the ball nut assembly and the hydraulic chambers are on either side of the piston inside the steering box. With the external assist type, hose connect the steering box to a hydraulic cylinder attached to the steering linkage.
The amount of power assist or "reaction control" is determined by the diameter of the torsion bar. Vehicle manufacturers fine tune the steering feel to a particular model of vehicle and powertrain by using different sized torsion bars. Thus, a larger, heavier luxury car would typically have more built-in steering assist than a smaller, sportier car. Yet the steering boxes on both cars might appear to be identical from the outside.
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