Steering Rack Operation

Steering Rack Operation

Rotary movement of the steering wheel is transferred via the steering column to the valve unit which is mounted on the steering rack. The rotary movement of the steering is converted into linear movement of the steering rack through the rack and pinion. With the engine running and the PAS pump operating, pressurized fluid is available to the steering rack for power assistance.

Neutral Position

With no movement of the steering wheel being applied, fluid flows from the pump to the valve unit. The slots in the outer sleeve and the rotor are so aligned that the fluid pressure flows across the valve unit. Some pressure is applied to the feed and return ports which in turn is felt at either side of the piston in the hydraulic cylinder. With the forces equal on each side of the cylinder, the steering remains in the neutral position. The majority of the fluid delivered from the pump is returned from the valve unit, and via the fluid cooler, back to the reservoir. With the majority of the fluid being returned to the reservoir, the pressure available at each side of the piston is very low.

Left steering

Anti-clockwise rotation of the steering wheel in turn rotates the rotor and the torsion bar in the same direction. The slots in the rotor move out of their neutral alignment and block the return flow of fluid to the reservoir. Fluid pressure from the pump now flows from the pump into the newly aligned slots in the rotor and outer sleeve. The pressure is directed by the alignment of the slots into the left hand side of the hydraulic cylinder where the pressure increases. The right hand side of the hydraulic cylinder is opened to return and allows fluid displaced from the right hand side of the piston to flow to the reservoir. The pressure difference on the left and right hand sides of the piston in the cylinder gives the power assistance to turn the steering.

Right steering

Clockwise rotation of the steering wheel in turn rotates the rotor and the torsion bar in the same direction. The slots in the rotor move out of their neutral alignment and block the return flow of fluid to the reservoir. Fluid pressure from the pump now flows from the pump into the newly aligned slots in the rotor and outer sleeve. The pressure is directed by the alignment of the slots into the right hand side of the hydraulic cylinder where the pressure increases. The left hand side of the hydraulic cylinder is opened to return and allows fluid displaced from the left hand side of the piston to flow to the reservoir. The pressure difference on the left and right hand sides of the piston in the cylinder gives the power assistance to turn the steering.

Progressive Assistance

Progressive power assistance is dependant on the amount of road wheel resistance opposing the turning of the front wheels. When the steering wheel is turned to the left or right, the rotary movement is transferred through the steering column to the rotor shaft which rotates the same amount as the steering wheel. The rotary movement is also transferred from the rotor to the torsion bar. If the road wheel resistance is high, (when parking), the torsion bar will twist. The twisting of the torsion bar means that the rotary movement of the pinion and the sleeve is slightly less than that of the rotor.

The twisting of the torsion bar moves the slots in the rotor and the torsion bar out of alignment, resulting in the gaps between both sets of slots becoming larger. The greater the resistance of the road wheels to the steering rotary movement, the greater the misalignment of the slots in the rotor and torsion bar. As the misalignment becomes greater, the fluid pressure passing to the applicable side of the hydraulic cylinder increases.

When the road wheel resistance reduces or less effort is applied to the turning of the steering wheel, the reduced torque applied to the rotor allows the torsion bar to unwind. This reduces the misalignment between the slots in the rotor and the torsion bar, reducing the fluid pressure applied to the applicable side of the hydraulic cylinder.