Pumps used in power steering systems are positive displacement designs. That is with each revolution they displace a given amount of fluid. Therefore as pump speed increases so does the flow rate. This, unfortunately, is the reverse of what is required as most power steering assistance is required when parking and at low engine RPM. Pumps are selected to provide sufficient flow rates at idle speeds to give satisfactory steering speed. As engine speed increases so does the pump flow and where a pump was delivering, say, 10 I/min at 600 rpm it could be producing a flow rate of 50 I/min at 3000 rpm. This flow rate, if directed through the steering gear, would generate excessive heat. Therefore a valve arrangement is required to limit the flow rate to, say. 15 I/min. In addition to flow control a pressure relief valve is usually incorporated within the pump.

Figure 1 - Pump Pressure and Flow Control

The flow control spool has primary pump pressure acting on one side of the spool and pressure taken from the down stream side of the flow control orifice on the other. At low flow rates the restricting orifice causes no pressure build up and consequently pressure on each side of the spool is the same. This pressure balance allows the spring to hold the spool so that "spill" port is closed. Full pump flow is directed towards the steering gear. As the flow rate increases with engine rpm pressure on the pump side of the orifice increases to the point where the spool is moved against the spring and uncovers the "spill" port which dumps excess flow back to the reservoir or inlet side of the pump.

It will be noticed that there is a pressure relief valve in the circuit which is pre set to the maximum pressure required by the steering gear. In most cases this is the only pressure relief point in the steering system.

It is not proposed to discuss the design and operation of the power steering gear in detail in this paper, however a sufficient understanding of it's operating principals will be given as an aid to fault diagnosis. At the outset there are three important facts which must be remembered when dealing with power steering. These are:

To achieve steering assistance a valve must be employed to direct oil flow to either end of the power steering gear's piston. If this valve's movement was unopposed there would be no resistance felt at the steering wheel. Such a system would work but there would be no steering feel and the vehicle would be very difficult to drive. Therefore power steering systems use some form of spring to resist valve movement. The rate of this spring will determine the degree of effort which must be input and the degree of hydraulic assistance for that input effort. The spring rate will also determine how much road feel will be relayed to the driver. Firm valve springing will require greater steering inputs to resist cornering forces generated by caster and changes in road camber. The valve springs are also required to centralise, or neutralise, the valve after input effort has been removed to allow the self aligning effect of caster to centralise the steering. If the valve did not automatically neutralise the steering gear would oppose any self entering effect of caster.

The minimum spring load on power steering valves is determined by the degree of friction in the steering column and universal jointed shaft connecting the column to the steering gear. As caster aligns the steering system the input shaft of the steering gear must rotate the steering column/shaft assembly. If friction in the steering column is sufficient to cause the input shaft of the steering gear to displace the power steering valve then hydraulic pressure may be applied to the appropriate end of the steering gear's piston and self aligning will be opposed.

For the purpose of trouble shooting an understanding of valve operation is necessary and Fig. 2 is a simplistic representation of a radial valve/torsion bar spring combination.

Some steering gears, such as Sheppard's 92 series, use a spool valve with adjustable flat washer like valve springs at each end of the valve. These springs are adjusted by factory set valve adjusting nuts. Without special equipment it is virtually impossible to correctly set valve adjustment. When dismantling these systems great care should be taken to maintain the initial adjustment. Refer to Sheppard's service manual for the procedure.

Figure 2 - Radial Valve Operation

	If the valve is rotated clockwise and lands "A" close, oil is diverted to port "E" and then to the appropriate end of the piston. Oil on the opposite side of the piston is able to escape to the reservoir through port "F" via port "D". Partial movements of the valve will cause pressure imbalances at each end of the piston
	During highway operation only small amounts of
pressure are used, typically 1750 kpa (250 PSI). During parking close to full system pressure may be used.

Toe in / toe out is the attitude of the front edge of the tyres in relation to the rear edge of the tyres. The toe dimension is used to counteract the effect camber has on a tyre where, due to the cone effect, the unrestrained tyre would roll in a circle. ( See the paragraph on Camber ) For example, the front wheels on a vehicle with negative camber would try to roll towards the centre of the vehicle. Whereas if the front wheels were set to positive camber the wheels would try to roll away from the vehicle centre line. Naturally both wheels are restrained by the steering linkage and must follow a similar path and in maintaining their position relative to each other a degree of slip over the road surface results. To reduce tyre wear the toe in/toe out is adjusted to counteract tyre slip over the road surface. If there were positive camber on both front wheels a degree of toe in would be introduced to counteract the tendency of the wheels to roll out thereby reducing the tyre slip angle.

Toe in used to be introduced to counteract the effect of wear in the tie rod ends as there is a natural tendency for wheels to toe out or turn about the king pin because of the rolling resistance of the tyre on the road if there is slight wear in both tie rods then a small amount of toe in could be introduced to compensate for the toe out caused by rolling resistance. Since the introduction of spring loaded ball joints and tie rod ends any of these components with apparent free play should always be replaced.

The effect of excessive toe in/out settings is to make the steering feel "mushy" or less direct due to the increased tyre slip angles.

Whereas caster is the tilting of the steering axis in side elevation, KPI (king pin inclination) is the tilting of the steering axis in front elevation. Refer to FIG. 3. There are two schools of thought employed when

designing in KPI on front axles which result in great variations in KPI angles in vehicles used in Australia. These two schools of thought are as follows:

A. Small KPI angles are employed which result in large scrub radii. The purpose being to allow the tyre to roll in an arc about the king pin thereby reducing static steering effort. The small KPI angle also reduces vehicle lift when the wheels are turned from lock to lock (see illustration). Smaller KPI angles are often considered when vehicles are manufactured with power steering as an option.

B. Large KPI angles are employed to reduce the scrub radius in an effort to reduce the moment about the king pin resulting from wheel impacts. These large KPI angles tend to generate more vehicle lift when the steering is turned on to lock and thereby increasing steering effort.

KPI also aids the steering in it's ability to self centre as the vehicle is raised slightly when the steering is turned off centre, the vehicles weight acting on the front axle provides a self centering force additional to that of caster. See FIG 3, arc aa.