Scheme 1
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 equalize. 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
- The hydraulic pressure fails.
- The steering box rotary control valve fails.
The coarse spline may also engage in some full lock situations if sufficient torque is applied to the input shaft.
OPERATION
Hydraulic fluid is supplied to the Power Assisted Steering (PAS) pump inlet from the PAS reservoir, the PAS pump draws in and pressurizes the fluid. The PAS pump self regulates internal flow rates and operating pressure, and supplies pressurized fluid from the PAS pump outlet to a rotary control valve in the steering box. At neutral the fluid is circulated by the PAS pump and flows around the system at a lower pressure and a constant flow rate. With most of the fluid being returned to the reservoir the pressure inside the system remains very low. When a control input turns the rotary control valve in the steering box, pressure in the system will rise as the control valve directs fluid to give power assistance.
The action of turning the steering wheel turns the steering column and intermediate shaft. The intermediate shaft turns the input shaft of the steering box. The input shaft moves the rotary control valve in the steering box, the rotary valve controls the pressure used inside the steering box for power assistance. The input shaft also turns a worm gear, the worm gear acts on a roller attached to the output shaft. As the worm gear turns the roller, the roller travels along the lands of the worm gear. As the roller is attached to the output shaft the output shaft turns.
As the output shaft of the steering box turns, hydraulic pressure is supplied via the rotary control valve to the steering box. The hydraulic pressure acts on a rack that assists with the movement of the output shaft of the steering box. A drop arm is attached to the output shaft of the steering box. The drop arm is connected to a drag link by a ball joint. The drag link is connected via ball joints to one front steering knuckle and road wheel. A track rod connected to this steering knuckle links the two steering knuckles together. As one steering knuckle and road wheel is turned by the drag link, the other steering knuckle and wheel is moved by the track rod.