Operation of the rotary control valve and power piston

Neutral position (Fig. 9.16(a)) With no steering effort being applied when driving along a straight

-Splines

Fig. 9.16(a-d) Rack and pinion power assisted steering with rotary control valve

-Splines

Fig. 9.16(a-d) Rack and pinion power assisted steering with rotary control valve

Fig. 9.16 contd

track, the longitudinal lands formed by the slots milled on the rotor periphery angularly align with the internal sleeve slots so that equal space exists between the edges of adjacent rotor and sleeve slots.

Fluid therefore flows from the pump to the delivery port into the short slots in the rotor. Some fluid then passes to the feed/return slots in the sleeve and out to the ports communicating with either side of the power cylinder. The majority of the fluid will pass between the edges of both the rotor and sleeve slots to the rotor long slots where it then flows out through the return port back to the reservoir. Because fluid is permitted to circulate from the pump to the reservoir via the control valve, there is no pressure build-up across the power piston, hence the steering remains in the neutral position.

Anticlockwise rotation of the steering wheel (Fig. 9.16(b)) Rotating the steering wheel anticlockwise twists the rotor shaft and torsion bar so that the leading edges of the rotor lands align with corresponding lands on the sleeve, thereby blocking off the original fluid exit passage. Fluid now flows from the pump delivery port to the short slots on the rotor. It then passes between the trailing rotor and sleeve edges, through to the sleeve slots and from there to the left hand side of the power

Fig. 9.17 Relationship of fluid pressure delivered to the power cylinder from the control valve and the angular deflection of the control valve and torsion bar

cylinder via the feed/return pipe so that the pressure on this side of the piston rises. At the same time fluid will be displaced from the right hand cylinder end passing through to the sleeve slots via the right hand feed/return pipe and port. The flow of fluid continues, passing between the trailing land edges of the rotor and sleeve to the long rotor slots and out through the return pipe back to the reservoir. Thus a pressure difference is established across the double acting piston, to provide power assistance.

Clockwise rotation of the steering wheel (Fig. 9.16(c)) Rotating the steering wheel clockwise angularly deflects the rotor so that the leading edges of the rotor lands overlap with corresponding internal lands on the sleeve. Fluid now flows from the pump delivery port into the short rotor slots and out to the right hand feed/return port to the power cylinder via the gap created between the trailing edges of the rotor and sleeve lands. Pressurizing the right hand side of the power cylinder pushes fluid out from the left hand side of the cylinder, through the feed/return pipe and port into the sleeve slots, through the enlarged gap created between the trailing rotor and sleeve edges and into the long rotor slots. It is then discharged through the return port and pipe back to the reservoir.

Progressive power assistance (Fig. 9.16(b and c)) When the steering wheel is turned left or right, that is, anticlockwise or clockwise, the rotor shaft which is rigidly attached to the steering column shaft rotates a similar amount. A rotary movement is also imparted through the torsion bar to the pinion shaft and the valve sleeve as these members are locked together. However, due to the tyre to ground resistance, the torsion bar will twist slightly so that the rotation of the pinion and sleeve will be less than that of the rotor input shaft. The greater the road wheel resistance opposing the turning of the front wheel, the more the torsion bar will twist, and therefore the greater the misalignment of the rotor and sleeve slots will be. As a result, the gap between the leading edges of both sets of slots will become larger, with a corresponding increase in fluid pressure entering the active side of the power cylinder.

As the steering manoeuvres are completed, the initially smaller sleeve angular movement catches up with the rotor movement because either the road wheel resistance has been overcome or steering wheel turning effort has been reduced. Consequently, the reduced torque now acting on the steering column shaft enables the torsion bar to unwind (i.e. straighten out). This causes the power assistance to be reduced in accordance with the realignment or centralization of the rotor slots relative to the sleeve lands.

9.2.3 Integral power assisted steering gear power cylinder and control valve

Description of steering gear and hydraulic control valve (Figs 9.18 and 9.19) The integral power assisted steering gearbox can be used for both rigid front axle and independent front suspension (Fig. 9.18) layouts.

The rack and sector recirculating ball steering gear, power cylinder and hydraulic control valves are combined and share a common housing (Fig. 9.19(a)). The power piston in this arrangement not only transforms hydraulic pressure into force to assist the manual input effort but it has two other functions:

1 it has a rack machined on one side which meshes with the sector,

2 it has a threaded axial bore which meshes via a series of recirculating balls with the input worm shaft.

The input end of the worm shaft, known as the worm head, houses two shuttle valve pistons which have their axes at right angles to the worm shaft. Since they are assembled within the worm head they rotate with it.

Drive is transferred from the hollow input shaft to the worm shaft through a torsion bar. Movement of the shuttle valves relative to the worm shaft which houses the valves is obtained by the hollow double pronged input shaft. Each prong engages with a transverse hole situated mid-way between the shuttle valve ends.

Fig. 9.18 Integral steering gearbox and power assisted steering utilized with independent front suspension

(a) Neutral position

Worm head Return groove

Return grove land

Shuttle valve piston

Recirculating balls — ^ Piston and n

Exit groove

-Intake groove

Delivery groove

Return passage

Torsion bar

- Reservoir

Sector and shaft

Worm head Return groove

Shuttle valve piston

Recirculating balls — ^ Piston and n

Exit groove

-Intake groove

(a) Neutral position

Return grove land

Worm

Pump Torsion bar

-Shuttle valve body Worm head

Worm

Pump Torsion bar

Double pronged input shaft

-Shuttle valve body Worm head

Fig. 9.19(a-c) Integral power assisted steering gear power cylinder and control valve

Fig. 9.19 contd

When the steering wheel is turned, the tyre to ground reaction on the front road wheels causes the torsion bar to twist according to the torque applied on the steering column shaft. Therefore the relative angular movement of the worm shaft to that of the input shaft increases in proportion to the input torque at the steering wheel, so that the shuttle valves will both be displaced an equal amount from the mid-neutral position. As soon as the steering wheel effort is released, the elastic torsion bar ensures that the two shuttle valves return to the neutral or mid position. The function of these shuttle valves is to transfer fluid under pressure, in accordance to the steering input torque, from the pump delivery port to one or other end of the integral power cylinder whilst fluid from the opposite end of the cylinder is released and returned to the reservoir.

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