Steering Overview

Steering

Overview










Mechanical
The image shows a left-hand drive vehicle with a V8 engine.
Mechanical steering wheel movements are transferred to the wheels via the steering column, steering shaft, steering gear (5) and tie rods. The wheel deflection is limited by a stop in the steering gear.
The oil level is checked using the min/max mark on the servo reservoir (8).
The outer seal is clamped between the steering gear and the floor. The inner seal is secured by plastic mounting elements against the cowl panel from inside the passenger compartment. The seals prevent sound from the engine compartment being transferred into the passenger compartment. They also seal against water, dirt and fire.
The cooling loop (7) reduces the temperature of the oil as the air passes the cooling fins.

The power steering system
The image above shows speed dependent power steering. The steering gear (5) is of the rack and pinion type and grease lubricated. It is installed with two screws in the subframe. The system uses a belt driven servo pump located on the engine. Location varies according to engine type.
The speed dependent power steering, adjusts the power steering assistance according to the driving conditions. Steering becomes lighter when maneuvering at low speeds, for example, when parking the vehicle. The power steering assistance reduces in relation to the speed increase, and is non existent at approximately 80 km/h.
The driver can further adjust the power steering assistance to their own driving style. There are three modes with different settings for degree of power assistance: low, medium and high.

Mechanical servo pump









The mechanical servo pump is controlled by speed.

Design
The shaft in the pump housing is joined to the rotor (5).
The ten vanes (6) are located in the radial grooves on the rotor. Through the ducts in the rear and front control plates there are connections between the suction duct (3) and suction zones as well as the pressure duct (7) and the pump chamber's pressure zones.

Function
From the pressure chamber, the pressurized oil passes through the choke (8) to the pressure line and the integrated relief valve (4)/flow valve (9).
The rotation of the input shaft, and rotor (5) causes the vanes (6), through centrifugal force, to move radially on the groove on the secured outer ring. This motion is reinforced by the pressurized oil that flows from the pressure chamber via ducts to the inner surfaces of the vanes.
Ten independent pump cells are created between the ten vanes, which draw oil into the two half moon shaped pump chambers when the volume increases and force it out into the pressure chamber when the volume decreases.
The combined function of the choke (8) and flow valve (9) make it possible to achieve an increased flow level.
The upper limit of the oil pressure that the power steering mechanism is supplied with is determined by the relief valve (4).

Adaptive steering column, design




The adaptive steering column is a complete unit.
The steering column contains the following components:





The fixed bracket (1) is mounted against the cowl panel. The steering column is adjustable for both height and reach. It has an electrical steering lock with a mechanical locking function.
The steering shaft is divided into two parts and has two joints. A joint between the upper and lower steering shaft and a joint as the connection to the steering gear.

Adaptive steering column, function










Deformation in the event of a collision
An impact of approximately 2 kN is required for the steering column to start to deform. Deformation takes place in three stages:
1. The upper steering (3) is pushed into the running bracket (2) equivalent to the remaining potential adjustment in the adjustment mechanism (4).
2. The tear plate (6) deforms. The running bracket (2) is pushed into the fixed bracket (1). The groove for the running bracket controls the length of deformation.
3. The steering shaft (5) with its telescopic design slides together.
The maximum movement (deformation) of the steering column is approximately 95 mm.
The adaptive steering column allows a relatively gentle deformation process in the event of a collision (only applies to drivers wearing seat belts, USA/CDN).

Collision process (applies to USA/CDN)
Driver not wearing seat belt:
The pin (7) is engaged. This results in the tear plate using its entire structure to absorb energy. The deformation takes place stiffly and slowly.
Driver wearing seat belt:
The explosive charge (8) is activated and pushes the pin (7) down out of the tear plate (6). The tear plate is weakened and the kinetic energy of the collision is transferred on in the system. This produces a relatively soft deformation process. The collision protection system only works if the driver is wearing a seat belt and the airbag deploys.

Collision process (does not apply to USA/CDN)
The steering column does not have an explosive charge. The pin (7) is therefore always engaged. This means that the tear plate (6) uses its entire structure to absorb energy, irrespective of whether or not the driver is wearing a seat belt. The deformation takes place stiffly and slowly.

Electrical steering lock with mechanical locking function
The steering lock (9) is secured in the steering column with two security bolts.
The mechanical locking function for the locking mechanism consists of:
- a lug on the steering wheel lock
- a pipe and catch on the steering column
- a pin for locking the lug.
The lug is pushed into the tube with the catch, locking the steering wheel. In the event of damage to the steering lock cover, the pin blocks the lug in the locked position.

Steering gear, design









This description refers to the steering gear on left-hand drive vehicles. The gear in right-hand drive vehicles is a mirror image of the left-hand drive version. There is also a speed dependent steering gear, for further information see Design and Function, Central electronic module (CEM).
The steering gear is of the rack and pinion type with the mechanical and servo assisted elements combined in one module.
The mechanical element consists of the gear drive (8), steering rack (12) and tie rods (14).
The gear drive is carried by a journal bearing (7) and a roller bearing (10) in the steering gear housing.
The steering rack is carried by a journal bearing on the right end and in the housing is guided by the gear drive (8) and the sprung actuating piston (9).
A cylinder pipe (13) is secured to the steering gear housing (11). This pipe functions as the power steering working cylinder for the hydraulic piston fastened to the steering rack.
The oil flow to the hydraulic cylinder is regulated in the valve housing which is integrated into the steering gear housing.

Valve
The gear drive is driven by the steering gear input shaft (1) which has exterior lugs. There is radial play between the lugs.
A torsion bar (2) which joins the input shaft to the gear drive holds the lugs in a position where the play is equal on each side when there is no steering input on the shaft.
The torsion bar is pressed into the gear drive. The gear drive is fastened to the input shaft with a pressed in lock pin.
A rotary valve (5) is located in the valve assembly and fixed to the gear drive with a lock pin. The valve operating range is radial between four narrow vertical channels in the rotary valve and four milled grooves in the input shaft.
The valve is balanced as a complete unit and cannot be rebalanced.

Steering gear, function









The steering gear power steering function is schematically displayed in the illustration above and under "Valve".
The force of oil pressure built up in the power steering pump affects the piston on the steering rack. The oil flow to the right or left-hand side of the piston is regulated in the valve housing. External pipes lead to both sides from the valve housing.
The rotary valve has three radial grooves, a small one that is fed by servo pump and an upper and a lower groove that are connected with the working cylinders via the outer pipes.

Valve

Neutral




- 1. To the right-hand side of the piston
- 2. To the left-hand side of the piston
- 3. Supply
- 4. Return oil
- 5. Servo oil, free flow
The valve is open when the vehicle's engine is running and there is no steering input. In the open position, none of the ducts to the operating cylinders are blocked. The servo oil circulates freely through the valve.

Steering left




- 6. Servo oil, low pressure
- 7. Servo oil, high pressure
When the steering wheel is turned to the left and the wheel resistance is so great that the torsion bar inertia is overcome, the steering gear input shaft moves to the left in relation to the worm screw, within the play between the lugs.
With this movement the input shaft stops the free flow through the valve and sends the fluid through the upper feed pipe to the right-hand side of the piston.
As long as the torsion bar is affected by steering input, the oil pressure continues to push the steering rack to the left and servo assistance is obtained. If the steering input reduces, the torsion bar springs back. The valve then reverts to the center position so that the oil can circulate freely through the valve housing.

Steering right




- 6. Servo oil, low pressure
- 7. Servo oil, high pressure
The function when turning right is, in principle, the same as to the left. The only difference is that the steering gear input shaft stops the flow of high pressure oil through the return and down through the groove on the inside of the valve and out through the low pressure pipe to the left-hand side of the piston.