Control valve
The control valve consists of a valve spool (1), a sleeve (2), a torsion bar (3) and a pinion (4). The steering column's intermediate shaft is connected to the valve by means of a universal joint. The torsion bar is connected to the upper end of the valve by means of a pin (5). The other end of the torsion bar is a press fit in the pinion. The sleeve is secured to the pinion by a pin (6) and follows the rotation of the pinion exactly. There is also a fail-safe connection between the valve spool and the pinion.
The sleeve has three radial grooves (7), the power steering fluid being pumped to the middle one. When the steering wheel is in the straight-ahead position, the control valve is open and the fluid flows up through the valve and back to the power steering pump via the chamber above the sleeve.
The upper end of the pinion is mounted in a needle bearing while the lower end is mounted in a ball bearing. A spring-loaded plunger presses the rack against the pinion.
The valve housing, which is part of the steering gear housing, has four connections for the flow of power steering fluid: Servo delivery from the power steering pump (A), return to the power steering fluid reservoir (B), to the servo cylinder when turning right or from the servo cylinder when turning left (C) and to the servo cylinder when turning left or from the servo cylinder when turning right (D).
When the steering wheel is turned, the movement is transferred via the torsion bar to the pinion. Since the torsion bar is somewhat elastic, there will be a difference between the degree of rotation of the valve spool (which follows the rotation of the intermediate shaft) and the sleeve which is fixed to the pinion. As a result, the fluid can no longer flow through the control valve and back to the power steering fluid reservoir directly. Instead, delivery and return passages open for the servo cylinder.
When turning left, power steering fluid is pumped to the left-hand side of the servo cylinder via the upper radial groove of the sleeve. At the same time, the right-hand side of the servo cylinder is emptied via the lower radial groove of the sleeve. Power steering fluid is led up through the valve to the chamber above the spool and on back to the power steering fluid reservoir. When turning right, the process is reversed. As long as the torsion bar is twisted, power steering fluid presses on the rack so that servo effect is obtained. The difference between the valve spool and the sleeve reduces when the power steering fluid actuates the rack in the same direction as the pinion. When there is no longer a difference, the valve opens the passage returning power steering fluid to the reservoir. Some power steering fluid continually circulates in the valve except when the steering wheel is turned to an end position. This makes it possible for the control valve in the power steering pump to work while circulation cools the power steering fluid.