Description
General
The major steering components comprise a shock absorbing telescopic steering column, a Power Assisted Steering (PAS) rack, a PAS pump, a fluid reservoir, a fluid cooler and fluid pipes and hoses.
Steering Column
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The steering column design incorporates an energy absorbing mechanism to reduce driver impact loads in the event of a collision. The column is mounted on four captive studs that are located in an extension 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 is adjustable to allow the steering wheel to be moved vertically up or down to the desired position. The steering column is not serviceable and must be replaced as a complete assembly if necessary.
The upper column tube is telescopic and can slide over the lower column tube. The 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 and the switches for the horn and, where fitted, In-Car Entertainment (ICE) and cruise control.
The central shaft comprises of two parts and is located in bearings in the upper and lower column tubes. The upper shaft is located inside the lower shaft. Mating cross holes in each shaft are connected by nylon injection molded shear pins.
The upper shaft is splined to accept the steering wheel. The lower shaft is connected by a universal joint to the intermediate shaft. A second universal joint on the opposite end of the intermediate shaft is attached to a split adaptor. The adaptor is splined and mates with the splined input shaft from the steering rack.
The upper mounting bracket has two open slots with a PTFE coated, metal "U" clip over each slot. Two soft metal "curling" plates are riveted to the mounting bracket and cover the slots.
The mounting studs pass through the "U" clips and the curling plates. The lock nuts that retain the upper mountings to the bulkhead extension are tightened to between 12 and 15 Nm. This torque figure controls the breakout load required to move the upper mounting brackets from the studs in the bulkhead extension and is critical to the crash performance of the column.
Energy absorption is achieved by the following mechanism: When an axial load is applied to the steering column, the mounting bracket deforms or slides out of the "U" clips and the curling plates deform. The "U" clips remain captive on the studs. When the column mounting moves, the upper column tube slides on the lower column tube and allows approximately 63 mm (2.5 inch) maximum of axial movement. The nylon shear pins holding the upper and lower shafts together shear, allowing the central shaft to telescope the same amount as the column tubes.
In the event of a collision where the steering rack itself moves, the two universal joints in the column allow the intermediate shaft to articulate, minimizing movement of the column towards the driver. If movement continues after articulation of the intermediate shaft, the nylon shear pins retaining the two halves of the central shaft shear causing the shafts to "telescope" together reducing further column intrusion.
The steering wheel comprises a cast center and wire frame onto which the soft polyurethane foam is molded. A horn switch is located at each side of the wheel. On models with a high specification ICE system and/or cruise control, additional switches are located on the steering wheel for control of one or both features. All switches are connected by wires to the rotary coupler connector.
Protection for the drivers face and upper torso is provided by an SRS airbag located in the center of the steering wheel under a plastic cover.
The column adjuster is located on the left hand side of the steering column and allows the steering column and wheel to be tilted up or down over a range of approximately 3.5° or 30 mm (1.2 inch) of vertical movement of the steering wheel. The column adjuster comprises a bolt that passes through each side of the column upper mounting bracket. The column adjustment lever is attached to a two-start left hand threaded nut that screws onto the bolt.
When the lever is lowered the nut is slackened and allows the column to be moved up or down to the desired position.
Lifting the lever upwards tightens the nut, clamping the mounting bracket to the column, retaining the steering column in the selected position. The nut tightening torque is 10.5 Nm (7.7 ft. lbs.) that gives the correct feel to the lever operation.
The steering lock houses the ignition switch, ignition illumination light ring, key lock barrel and the security system transponder coil.
The steering lock is attached to the upper column with a clamp plate and 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 0 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, which allows the lock collar to slip on the upper shaft if a torque of 200 Nm (147.5 ft. lbs.) or higher is applied. This prevents the bolt being sheared, by someone forcibly turning the steering wheel while the steering lock is engaged, yet still effectively locks the steering. The steering lock is also designed to be resistant to slide hammer and shock retraction.
Steering Rack
The steering rack is unique in having the track rods mounted at a central location on the rack. The track rods are very long and combined with the optimized steering geometry provide low levels of "bump steer".
The steering rack is mounted in the engine compartment on the lower part of the bulkhead. The rack is retained with two bolts through cast lugs near the valve unit and two bolts which secure a clamp over the opposite end of the rack. The lower bolt through the cast lug is fitted with a large washer. The steering rack is handed for left and right hand drive vehicles.
The rack requires 3.2 turns from lock to lock and the rack and pinion has an overall ratio of 19.6:1.
A valve unit is fitted to one end of the steering rack and connects, via a splined input shaft, to the adaptor of the steering column. The valve unit has four hydraulic connections; a pressure feed from the PAS pump, a return line to the reservoir and two pressure lines to the annulus and full area of the hydraulic cylinder.
An input shaft, installed through the valve unit, is connected to a pinion gear which drives on a rack which, in turn is connected to the piston rod in the hydraulic cylinder.
At a central position on the rack are two threaded holes that allow for the attachment of the track rods. Rubber bellows cover the movement area of the rack.
Two hydraulic ports, one at each end of the hydraulic cylinder of the rack, are connected by metal pipes to the valve unit. The ports supply hydraulic pressure to the annulus and full area of the cylinder. The annulus end of the cylinder is supplied with PAS pump outlet pressure and the full area end of the cylinder is supplied with pressure regulated by the valve unit. The cylinder end of the piston rod is fitted with a piston that houses a piston seal.
The track rods are fitted with rubber bushes that are hard enough to give positive feel to the steering but at the same time reduce unwanted feedback through the track rod. Each track rod is fitted with a track rod end. The track rod end is a ball joint that locates on an extended bracket on the suspension strut and secured with a lock nut. A turnbuckle is screwed into the track rod and the track rod end and allows for the adjustment of the steering alignment.
The track rods are located on a bushed slider that locates on the rack. Two spacers are located between the slider and the track rods. Two flanged bolts secure the track rods to the rack.
Valve Unit
The valve unit is an integral part of the steering rack. The principal function of the valve unit is to provide maximum power assistance (i.e. when parking) with minimum effort required to turn the steering wheel.
The cast outer casing of the valve unit has four ports that provide the connections for pressure feed from the PAS pump, return to the fluid reservoir and pressure feeds from the valve unit to the annulus area and the full area of the cylinder. A non-return valve and seal is fitted in the pressure feed port from the PAS pump.
The valve unit comprises an outer sleeve, input shaft, torsion bar and a pinion shaft. The valve unit is coaxial with the pinion shaft that is connected to the steering column via the input shaft. The valve unit components are located in a housing that is attached to a mating casting on the steering rack main body and secured with screws.
The outer sleeve is located in the main bore of the valve unit. Three annular grooves are machined on its outer diameter. PTFE rings are located between the grooves and seal against the bore of the valve unit. Holes are drilled radially in each annular groove through the wall of the sleeve. The bore of the outer sleeve is machined to accept the input shaft. Six equally spaced slots are machined in the bore of the sleeve. The ends of the slots are closed and do not continue to the ends of the outer sleeve. The radial holes in the outer sleeve are drilled into each slot.
The input shaft is splined at its outer end. The inner end of the input shaft forms a dog-tooth which mates with a slot in the pinion shaft. The fit of the dog-tooth in the slot allows a small amount of relative rotation between the input shaft and the pinion shaft before the dog-tooth contacts the wall of the slot. This ensures that, if the power assistance fails, the steering can be operated manually without over stressing the torsion bar. The central portion of the input shaft has equally spaced longitudinal slots machined in its circumference. The slots are arranged alternately around the input shaft.
The torsion bar is fitted inside the input shaft and is an interference fit in the pinion. The torsion bar is connected to the input shaft by a drive pin, fitted after hydraulic balancing. The central diameter of the torsion bar is machined to a smaller diameter than at each end. The smaller diameter allows the torsion bar to twist in response to torque applied from the steering wheel in relation to the grip of the tires on the road surface.
The pinion shaft has machined teeth on its central diameter that mate with the teeth on the steering rack. A slot machined in the upper end of the pinion shaft mates with the dog-tooth on the input shaft. The pinion shaft locates in a cast housing that is part of the steering rack and rotates on ball and roller bearings.
Power Assisted Steering (PAS) Pump
The vane type PAS pump supplies hydraulic pressure to the steering rack valve unit. The PAS pump is driven by a Poly Vee belt from the crankshaft pulley. A self-adjusting tensioner is fitted to maintain the correct belt tension.
The PAS pump comprises a body and cover that house the internal components of the pump. A pressure relief valve assembly is installed in the body. The relief valve also incorporates a flow control valve. The pressure relief valve limits the maximum pressure to between 90 and 100 bar (1305 and 1450 inch lbs.) on K1.8 models or 120 and 127 bar (1740 and 1842 inch lbs.) on KV6 and TD4 models. The flow control valve limits the maximum flow to between 5.0 and 6.0 l/min (1.32 and 1.58 US gal/min).
A shaft runs longitudinally through the pump. One end of the shaft has a drive flange that accepts the drive pulley. The opposite end is closed by the cover. The shaft runs in bearings located in the body. Oil seals at each end of the shaft prevent the leakage of hydraulic fluid.
An oval cam ring is located in the body. Ten roller vanes are housed in a carrier and rotate within the cam ring. The carrier is mounted in the center of the shaft and receives positive drive from the shaft via a drive pin. The carrier is seated against an end plate that is located in the cover. The front of the carrier is covered by a port plate that is located against a seal plate in the body. The port plate controls the fluid flow into and out of the roller vanes during their cycle.
Fluid Reservoir
The fluid reservoir is mounted in the right hand side of the engine compartment on a bracket attached to the inner wing.
The reservoir comprises a body, a cap and a filter assembly. The reservoir has a capacity to the upper level mark of 335 cc.
An O-ring seal in the cap prevents leakage. The cap is pushed onto the latch and turned through 90 to lock. A breather hole is incorporated in the cap to allow changes in fluid level during operation.
A filter assembly is fitted into the bottom of the reservoir. The filter is made from fine nylon mesh that is molded into the frame of the filter. The filter removes particulate matter from the fluid before it is drawn into the supply connection and is non-serviceable.
The primary function of the fluid reservoir is to hold a surplus of hydraulic fluid in the system to allow for expansion and contraction of the fluid due to temperature variations. The fluid level ensures that the supply connection is covered with fluid at all operational attitudes. Any air that may be present in the hydraulic system will be exhausted from the fluid at the reservoir.
High Pressure Hose
The high pressure hose connecting the PAS pump to the valve unit incorporates an attenuator. The attenuator comprises a bullet shaped valve that is located between two spiral wound springs. The valve operates as a restrictor to damp pressure pulses from the PAS pump, reducing noise and strain on downstream components. The attenuator is integral with the hose and cannot be serviced separately.