Rotary Valve

The key to the operation of power steering is a "rotary valve" that directs power steering fluid under pressure to either side of the rack piston. The rack piston converts hydraulic power to mechanical force. Friction from the wheel causes a torsion bar in the rotary valve to deflect. This changes the position of the valve spool and sleeve, directing fluid under pressure to the proper end of the power cylinder. The difference in pressure on either side of the piston (which is attached to the rack) helps move the rack to reduce turning effort.