Discovery 2. Manual - part 590
STEERING
DESCRIPTION AND OPERATION
57-9
Principle of operation
Movement of the input shaft is transferred through the pin to the torsion bar and valve rotor on the input shaft. As the
input shaft turns, the spline of the torsion bar turns the worm gear. This action causes the roller to rotate on its bearings
and move. As the roller is located by a pin to a yoke on the output shaft, the output shaft rotates in the steering box
housing. As the amount of torque acting on the input shaft increases the torsion bar starts to twist. As the torsion bar
twists the valve rotor turns in the valve sleeve. When the ports in the valve rotor and valve sleeve are turned, hydraulic
fluid is directed to chamber 'A' or 'B' in the power cylinder.
With hydraulic fluid in one chamber under high pressure, the piston moves. The return line ports in the rotary valve,
aligned by the movement of the valve rotor, allow the fluid in the opposite chamber to flow to return. The teeth of the
rack move and transfer the force from the piston to the output shaft, giving assistance to move the drop arm. As the
output shaft rotates the torsion bar load is decreased. The rotor on the input shaft will return as the torsion bar
unwinds, the rotary valve will then be in a neutral position and the pressure in chambers 'A' and 'B' will equalise. With
no high pressure acting on the piston, force on the piston and rack is released.
To prevent heat accumulation at full steering lock due to excessive pressure, a relief valve inside the steering box is
opened as the box approaches full lock. The relief valve pins are located in the cylinder cover and housing and are
not to be adjusted.
The steering box design ensures a mechanical link through the course spline on the control valve rotor, the spline will
become engaged if:
l
The hydraulic pressure fails.
l
The steering box rotary control valve fails.
The course spline may also engage in some full lock situations if sufficient torque is applied to the input shaft.
Rotary control valve
The rotary valve assembly comprises of three parts. 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 course spline on the end of the valve rotor is loosely engaged in the worm gear, the course 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 course spline will drive the worm gear and enable the vehicle to be steered and
driver control maintained.