Steering: Description and Operation

Description
General
The major steering components comprise an impact absorbing telescopic steering column, a Power Assisted Steering (PAS) box, a PAS pump, and fluid reservoir, Hydraulic fluid from the fluid reservoir is filtered and then supplied through the suction line to the inlet on the PAS pump. The PAS pump supplies fluid to the steering box through a pressure line routed above the front cross member. Fluid returns to the reservoir along the same route through a return line. On LH drive vehicles the pipe route above the front cross member is still used, the length of pipe acting as an oil cooler.
To minimis driver's injury in the event of an accident the steering system has a number of safety features including a collapsible steering column. An additional safety feature is an air bag located in the steering wheel.
REFER TO RESTRAINT SYSTEMS.

Steering column assembly and intermediate shaft
The steering column central shaft comprises of two shafts, the upper shaft is splined to accept the steering wheel and located in bearings in the column tube. A universal joint is located on the bottom of the upper shaft, the joint allows for angular movement between the upper and lower shafts. The lower shaft is made in two parts, the top section of the lower shaft is located outside of the lower section. The two sections of the lower shaft are connected by two nylon injection moulded shear pins. The lower shaft goes through a lower bearing attached to the bulkhead, the lower shaft is connected by a universal joint to the intermediate shaft in the engine compartment.

Steering column
An upper column tube provides for the location of the steering lock and ignition switch and also the steering switch gear and a rotary coupler. The rotary coupler provides the electrical connection for the steering wheel mounted airbag, switches and horn. The upper mounting bracket has two slots, a slotted metal bracket is held in each slot by four resin shear pins.
The column is mounted on four captive studs which are located on a column mounting bracket. The captive studs pass through the metal brackets, locknuts secure the steering column to the bulkhead. The two lower mountings are fixed and cannot move when loads are applied to them. The upper mounting is designed to disengage or deform when a load is applied, allowing the column to collapse in the event of an accident. The steering column must be replaced as a complete assembly if necessary.
When an axial load is applied to the upper column tube, energy absorption is achieved by the following mechanism:
^ the mounting bracket deforms,
^ the resin shear pins holding the slotted metal brackets shear,
^ the top mounting bracket slides out of the slotted metal brackets.
The slotted metal brackets remain on the captive studs on the bulkhead. If the column mounting moves, injection moulded shear pins retaining the two sections of the lower column shaft will shear. This allows the two sections of the lower shaft to 'telescope' together.
In the event of a collision where the steering box itself moves, two universal joints in the column allow the intermediate shaft to articulate, minimising movement of the column towards the driver. If movement continues energy absorption is achieved by the following mechanism:
^ the decouple joint in the intermediate shaft will disengage,
^ the lower section of the steering column shaft will move through the lower bearing,
^ the injection moulded shear pins retaining the two sections of the lower column shaft will shear.
This allows the two sections of the lower shaft to 'telescope' together reducing further column intrusion. Protection to the drivers face and upper torso is provided by an SRS airbag module located in the center of the steering wheel.
REFER TO RESTRAINT SYSTEMS.

Tilt adjustment
The column tilt adjuster lever mechanism is located on the LH side of the steering column and allows the upper column tube, nacelle and steering wheel assemblies to be tilted up or down a maximum of 7.50 or 47 mm (NAS vehicles have a smaller range of movement than the ROW vehicles).
The pawl of the mechanism is attached to the lower column and is allowed to pivot, a toothed quadrant is fixed to the upper column tube.
When the lever on the LH side of the steering column is raised the mechanism releases the pawl from the toothed quadrant, this allows the column to be moved. When the lever is released two return springs pull the pawl into engagement with the toothed quadrant.

Steering column lock (All except NAS)
The steering column lock houses the ignition switch, ignition illumination light ring, key lock barrel and the alarm passive coil. The steering lock is attached to the upper column with two shear bolts. The bolts are tightened to a torque which shears off the heads of the bolts preventing easy removal of the steering lock,
The steering lock operates by a bolt, which emerges when the ignition key is turned to position "O' and the ignition key removed. The bolt engages in a lock collar located on the upper shaft in the upper column tube. The lock collar is attached to the upper shaft by a 'wave form' interference ring. If a high torque is applied via the steering wheel with the lock engaged, the lock collar will slip on the upper shaft. This prevents damage to the steering lock, yet still prevents the vehicle from being driven.

Steering column lock (NAS only)
The steering column lock houses the ignition switch, ignition illumination light ring, key lock barrel and the alarm passive coil. The steering lock is attached to the upper column with two shear bolts. The bolts are tightened to a torque which shears off the heads of the bolts preventing easy removal of the steering lock.
The steering column lock operates by a bolt, which emerges when the ignition key is turned to position 'O' and the ignition key removed. The bolt engages in a groove machined into the upper shaft in the column tube.

Steering wheel
The steering wheel comprises a cast center and wire frame onto which the soft polyurethane foam is moulded. The steering wheel is located on the upper column shaft by a spline and is secured with a nut. A remote radio control switch (if fitted) is located on the LH side of the steering wheel, a cruise control switch may be located on the RH side. Horn switches are located on each side of the center of the steering wheel and protrude through the airbag module cover. Both switches are connected by wires to the rotary coupler connector.

Intermediate shaft
One end of the intermediate shaft is attached to the steering column lower shaft by a splined universal joint and a bolt, the universal joint is part of a rubber coupling assembly. The rubber coupling assembly is covered by a heat shield and connects to the lower section of the intermediate shaft via a decouple joint. The rubber coupling reduces the shocks felt by the driver through the steering wheel. A second universal joint on the other end of the intermediate shaft is held in by a bolt. The universal joint is splined and engages with the splined rotor (input) shaft of the steering box.
The decouple joint consists of a metal plate that has open ended slots, the plate is bolted through the slots into the other half of the decouple joint. The top half of the decouple joint has a slot that accepts the lower section of the intermediate shaft. The slotted metal plate clamps the lower section of the intermediate shaft to the top section. An indicator clip is installed between the slotted metal plate and the top half of the decouple joint.
If the intermediate shaft is compressed in an accident, the slotted metal plate in the decouple joint will disengage if sufficient force is applied to the front end of the shaft. If the forces involved do not disengage the shaft, the red indicator clip located in the decouple joint will break off if the shaft moves. The intermediate shaft cannot be repaired and must be replaced as an assembly if accident damage occurs.

Reservoir





The fluid reservoir is made of moulded plastic and is located on LH side of the engine compartment, on a bracket which is attached to the inner wing. Dependent on the vehicles specification the reservoir may be a dual PAS/ACE, or PAS only reservoir. Both types of reservoir are similar to each other the dual PAS/ACE reservoir has two chambers, the PAS only reservoir has one chamber of a larger capacity. On both types of reservoir the PAS chamber has its own filler cap and is identified by lettering on the reservoir body.
A filter of fine polyester mesh is moulded into the base of the chamber. The filter removes particulate matter from the fluid before it is drawn into the supply connection and is non-serviceable. Upper and lower level marks are moulded into the reservoir body, the reservoir is fitted with filler cap, a seal in the cap prevents leakage. The filler cap is pushed onto a latch and turned through 90° to lock. A breather hole is incorporated in the cap to allow venting of air due to fluid level changes during operation. The breather hole also allows air that may be in the fluid to separate out and vent to atmosphere.
The reservoir holds hydraulic fluid and allows for expansion and contraction of the fluid due to temperature variations. With the reservoir correctly filled the inlet to the PAS pump will be kept covered at normal operational attitudes. The fluid flowing to the reservoir is cooled by convection from the pipe surfaces, the fluid held in the reservoir also allows convection from the sides of the reservoir to take place. The total capacity of the reservoir with PAS only is 1000 cc (0.264 US gallons), for vehicles fitted with PAS and ACE the total capacity of the reservoir is 500 cc (0.132 US gallons).

Steering box
The steering box is located behind the first cross member of the chassis and is secured to the chassis rail with four bolts. The steering box is of the worm and roller type and has a rotary control valve. The steering box is connected to the steering knuckles of the front road wheels by the drop arm, drag link and track rod. The steering box is lubricated by the hydraulic fluid in the housing. The input shaft is attached to the steering wheel via the intermediate shaft and steering column. The drop arm is secured to the output shaft with a nut and tab washer. A ball joint allows movement between drop arm and drag link, the ball joint is secured with a locknut. The steering box requires approximately 3.5 turns from lock to lock.
As a maintenance aid, an alignment bolt can be used to lock the drop arm at the steering box center position. The bolt fits in a groove in the rear face of the drop arm and screws in to a threaded hole on the bottom of the steering box housing.





Cross section through steering box

Principle of operation
Movement of the input shaft is transferred through the pin to the torsion bar and valve rotor on the input shaft. As the input shaft turns, the spline of the torsion bar turns the worm gear. This action causes the roller to rotate on its bearings and move. As the roller is located by a pin to a yoke on the output shaft, the output shaft rotates in the steering box housing. As the amount of torque acting on the input shaft increases the torsion bar starts to twist. As the torsion bar twists the valve rotor turns in the valve sleeve. When the ports in the valve rotor and valve sleeve are turned, hydraulic fluid is directed to chamber 'A' or 'B' in the power cylinder.
With hydraulic fluid in one chamber under high pressure, the piston moves. The return line ports in the rotary valve, aligned by the movement of the valve rotor, allow the fluid in the opposite chamber to flow to return. The teeth of the rack move and transfer the force from the piston to the output shaft, giving assistance to move the drop arm. As the output shaft rotates the torsion bar load is decreased. The rotor on the input shaft will return as the torsion bar unwinds, the rotary valve will then be in a neutral position and the pressure in chambers 'A' and 'B' will equalize. With no high pressure acting on the piston, force on the piston and rack is released.
To prevent heat accumulation at full steering lock due to excessive pressure, a relief valve inside the steering box is opened as the box approaches full lock. The relief valve pins are located in the cylinder cover and housing and are not to be adjusted.
The steering box design ensures a mechanical link through the course spline on the control valve rotor, the spline will become engaged if:
^ The hydraulic pressure fails.
^ The steering box rotary control valve fails.
The course spline may also engage in some full lock situations if sufficient torque is applied to the input shaft.

Rotary control valve
The rotary valve assembly comprises of three parts. The valve sleeve is fixed inside one end of the worm gear, the valve sleeve has ports through it to allow the passage of hydraulic fluid. The input shaft has a valve rotor machined on one end, the valve rotor also has ports through it and can rotate in the valve sleeve. A torsion bar is attached to the input shaft by a pin, the torsion bar goes through the input shaft and valve rotor and is engaged by a spline into the worm gear.
The course spline on the end of the valve rotor is loosely engaged in the worm gear, the course spline can make contact and drive the worm gear in some full lock and in no pressure conditions. In the event of a torsion bar failure, power assistance will be lost, the course spline will drive the worm gear and enable the vehicle to be steered and driver control maintained.





Rotary control valve at neutral

When there is no demand for assistance the torsion bar holds the ports in the valve sleeve and valve rotor in a neutral relationship to one another. The ports in the valve sleeve and the valve rotor are so aligned to allow equal (low) fluid pressure on each side of the piston. Excess fluid flows through ports in the valve rotor through the valve sleeve and back to the reservoir.





Rotary control valve in demand mode

When the steering wheel and input shaft is turned steering resistance transmitted to the worm causes the torsion bar to be wound up and the valve ports in the valve rotor and valve sleeve to be aligned for a right or left turn. The alignment of the valve ports directs fluid pressure 'A' from the PAS pump to one side of the piston/rack. The other side of the piston/rack is now connected to return 'B' (due the valves port alignment) and displaced fluid returns to the reservoir. The pressure difference in the cylinder on each side of the piston gives the power assistance to move the rack and so turn the steering.
The greater the resistance of the road wheels to the steering rotary movement, the greater torque acting on the torsion bar and input shaft causing greater changes of alignment of the ports in the valve. As the change of alignment becomes greater, the fluid pressure passing to the applicable side of the piston/rack increases.
Only when the steering wheel stops turning and the torsion bar has unwound, will the valve rotor return to the neutral position. In the neutral position the fluid circulates through the ports in the valve rotor and valve sleeve and back to the reservoir where it is cooled.

PAS pump - V8





The PAS pump is located on the auxiliary housing and is attached by two bolts, the bolts go through flanged bushes in the auxiliary housing. A stud passes through the PAS pump and through a flanged bush in the auxiliary housing, the auxiliary housing and PAS pump are secured by a nut. As the two bolts and nut are tightened the bushes move slightly and the flange of each bush clamps the PAS pump. A drive pulley is attached to the pump drive shaft with three bolts, and is belt driven at a ratio of 1.4 crankshaft revolutions to 1 of the drive pulley. Fluid is drawn into the PAS pump inlet from the reservoir through a flexible hose at low (suction) pressure. Fluid at high pressure from the PAS pump outlet is supplied to the rotary control valve on the steering box.
The PAS pump is a roller vane type and has an internal pressure regulator and flow control valve, The roller vanes can move in slots in the pumps rotor and are moved outwards by centrifugal force as the pump rotates, The pump rotor rotates in the pump housing, the internal shape of the housing forms a 'cam' shape. Due to the 'cam' shape the volume of the housing decreases between the inlet and outlet ports.
As the pump rotor rotates towards the pump inlet the volume between the roller vanes and the pump housing increases, this action causes a depression in the chamber between the pump roller vanes and the housing. As the rotation continues the chamber is opened to the pump inlet, and the depression in the chamber causes fluid to be drawn in. The roller vanes continue past the inlet port, closing off the inlet port and trapping the fluid in the chamber between the rollers and the pump housing.
The internal 'cam' shape of the pump housing causes the rollers to move closer together as the pump rotor rotates towards the outlet port. The reduced volume of the chamber between the roller vanes causes the fluid to become pressurized. When the chamber is opened to the outlet port of the pump the fluid escapes at high pressure. The roller vanes continue turning and go past the outlet port, closing off the chamber between the two roller vanes.
As rotation continues the inlet sequence begins again. The inlet and pressurization/outlet sequences continue as the pump rotates, and is repeated between each two roller vanes. The pump is a positive displacement type and the potential pump output increases with engine (drive pulley) speed. The pressure relief and flow control valve regulates flow/pressure by diverting fluid back to the pump inlet through internal recirculation passages in the pump body.

Steering damper
The steering damper is located behind and just below the first cross member of the chassis. The ends of the steering damper have steel 'eyes' welded on, rubber bushes are installed in each 'eye'. The steering damper is attached between brackets on the chassis rail and the drag link. Each end of the steering damper is secured by a bolt and locknut. The hydraulic damper absorbs shocks in the steering, caused by road wheel deflections when operating on rough terrain.

Operation
Hydraulic fluid is supplied to the PAS pump inlet from the PAS reservoir, the PAS pump draws in and pressurizes the fluid. The PAS pump self regulates internal flow rates and operating pressure, and supplies pressurized fluid from the PAS pump outlet to a rotary control valve in the steering box. At neutral the fluid is circulated by the PAS pump and flows around the system at a lower pressure and a constant flow rate. With most of the fluid being returned to the reservoir the pressure inside the system remains very low. When a control input turns the rotary control valve in the steering box, pressure in the system will rise as the control valve directs fluid to give power assistance.
The action of turning the steering wheel turns the steering column and intermediate shaft. The intermediate shaft turns the input shaft of the steering box. The input shaft moves the rotary control valve in the steering box, the rotary valve controls the pressure used inside the steering box for power assistance. The input shaft also turns a worm gear, the worm gear acts on a roller attached to the output shaft. As the worm gear turns the roller, the roller travels along the lands of the worm gear. As the roller is attached to the output shaft the output shaft turns.
As the output shaft of the steering box turns, hydraulic pressure is supplied via the rotary control valve to the steering box. The hydraulic pressure acts on a rack that assists with the movement of the output shaft of the steering box. A drop arm is attached to the output shaft of the steering box. The drop arm is connected to a drag link by a ball joint. The drag link is connected via ball joints to one front steering knuckle and road wheel. A track rod connected to this steering knuckle links the two steering knuckles together. As one steering knuckle and road wheel is turned by the drag link, the other steering knuckle and wheel is moved by the track rod.