Steering Rack Operation (page 2 of 2)

Left Steering (RHD Models; Right Steering on LHD Models) Anti-clockwise (on RHD, clockwise on LHD) rotation of the steering wheel rotates the input shaft and the torsion bar in the same direction. The slots in the input shaft move out of their neutral alignment with the slots in the outer sleeve, as the torsion bar twists, and restricts the flow back to the reservoir. This restriction causes the supply pressure from the PAS pump to increase. The increased PAS pump pressure is directed to the annulus side of the hydraulic cylinder. Due to the relative displacement of the slots the increased PAS pump pressure is also applied to the full area side of the cylinder. The difference in effective areas on each side of the piston produces a linear output force along the rack which provides the power assistance to turn the road wheels in the appropriate direction.

Progressive Assistance Progressive power assistance is dependent 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 input shaft which rotates the same amount as the steering wheel. The rotary movement is also transferred from the input shaft to the torsion bar. If the road wheel resistance is high, e.g. when parking, the torsion bar will twist. The twisting of the torsion bar means that the rotary movement of the pinion and the outer sleeve is less than that of the input shaft.

The twisting of the torsion bar moves the slots in the input shaft and the torsion bar out of alignment. The greater the resistance of the road wheels to the steering rotary movement, the greater the misalignment of the slots in the input shaft and outer sleeve. As the misalignment becomes greater, the restriction in the return flow of fluid to the reservoir increases and therefore the fluid pressure applied to the hydraulic cylinder also increases. As the fluid pressure increases, so does the amount of assistance provided.

When the road wheel resistance reduces or less effort is applied to the steering wheel, the reduced torque applied to the input shaft allows the torsion bar to unwind. This reduces the misalignment between the slots in the input shaft and the outer sleeve, reducing the fluid pressure applied to the hydraulic cylinder and therefore the amount of assistance provided.