Rotary Control Valve
The rotary valve assembly comprises of three parts. See Fig 1. The valve sleeve is fixed inside one end of the worm gear, the valve sleeve has ports through it to allow the passage of hydraulic fluid. The input shaft has a valve rotor machined on one end, the valve rotor also has ports through it and can rotate in the valve sleeve. A torsion bar is attached to the input shaft by a pin, the torsion bar goes through the input shaft and valve rotor and is engaged by a spline into the worm gear.
The coarse spline on the end of the valve rotor is loosely engaged in the worm gear, the coarse spline can make contact and drive the worm gear in some full lock and in no pressure conditions. In the event of a torsion bar failure, power assistance will be lost, the coarse spline will drive the worm gear and enable the vehicle to be steered and driver control maintained.
When there is no demand for assistance the torsion bar holds the ports in the valve sleeve and valve rotor in a neutral relationship to one another. The ports in the valve sleeve and the valve rotor are so aligned to allow equal (low) fluid pressure on each side of the piston. Excess fluid flows through ports in the valve rotor through the valve sleeve and back to the reservoir.