Steering Rack: Notes
The steering gear comprises the mechanical steering rack, the valve and an integrated hydraulic power cylinder.
The steering rack uses a rack with an integrated piston which is guided on plain bearings within the rack housing. The pinion, which is attached to the valve runs in bearings and meshes with the rack teeth. The rack is pressed against the pinion by a spring loaded yoke which ensures that the teeth mesh without any play. The pinion is connected to the valve rotor via a torsion bar.
The rotary motion of the steering wheel is converted into axial movement of the rack by the pinion and is initiated by the valve. This motion is transferred into movement of the wheels by adjustable track rod arms.
The rotary valve is used to control the pressurized fluid required for power assistance. The valve comprises a valve body, a control bush and a torsion bar. The valve body has eight control grooves in its bore. The control bush also has eight radial grooves which are matched to the valve. The control bush is positively attached to the pinion. The torsion bar is the connecting element between the valve body, the pinion and the control bush.
Torque input from the steering wheel is passed to the valve body and causes a rotary motion of the torsion bar. The valve body changes its relative position to the control bush and subsequently the relative positions of the control grooves is also changed. This allows pressurized fluid to pass via the mismatched grooves to the rack piston and provide the required assistance in the selected direction.
The piston is located at one end of the rack housing. Each side of the piston is connected to fluid pressure or fluid return via a metal external pipe which is connected to the valve housing.
Each end of the rack has a threaded hole which provides for the fitment of an inner track rod joint. See Fig 1. The external ends of the rack are sealed with gaiters which prevent the ingress of dirt and moisture. The inner track rod joints has a long outer threaded shank which screws into the outer track rod. The steering toe can be adjusted using the threaded end of the inner track rod. When the correct toe is achieved, a locknut on the inner tie rod is tightened to prevent further inadvertent movement.