Control valve

Fig 1: Illustration For Steps 1-7

Courtesy of SAAB-SCANIA OF AMERICA, INC.

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 supported in a needle bearing and the lower end in a ball bearing. A plastic bush is fitted between the valve sleeve and pinion.

A spring loaded plunger presses the rack against the pinion.

The valve housing, which is part of the rack housing, has four hydraulic 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).

Fig 2: Cross Sectional View Of Power Steering Rack

Courtesy of SAAB-SCANIA OF AMERICA, INC.

When the steering wheel is turned, the movement is transferred via the torsion bar to the pinion. Since the torsion bar twists slightly, 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.

Fig 3: Identifying Servo Fluid Flow Path

Courtesy of SAAB-SCANIA OF AMERICA, INC.

During a left turn, servo fluid is pumped to the left side of the servo cylinder via the radial groove in the upper sleeve. The servo fluid passes through the valve to the area above the spool and back to the power steering fluid reservoir. During a right turn, the procedure is the opposite.

As long as the torsion bar is twisted, power steering fluid will act on the rack to provide power assistance. The difference between the valve spool and the sleeve will be reduced when the power steering fluid acts on the rack in the same direction as the pinion. When this difference no longer exists, the return passage through the valve is opened again and the servo fluid can flow directly back to the power steering fluid reservoir.

A certain flow of power steering fluid through the valve is always present, except when the steering wheel is turned to full left or right lock. This is necessary for the control valve in the power steering pump to operate and it also helps to cool the fluid.