Operation of control valve and power piston

Neutral position (Fig. 9.19(a)) Fluid from the pump flows into and around an annular chamber surrounding the worm head in a plane similar to that of the shuttle valves where it acts on the exposed end faces of the shuttle valve pistons.

With the shuttle valves in the neutral position, fluid moves through the intake passages on the right hand end of the shuttle valve pistons, to the two annular grooves on the periphery of the worm head. Fluid then passes from the worm head annular grooves to the left hand side of the power piston via the horizontal long passage and sector chamber, and to the right hand piston face directly by way of the short passage. From the worm head grooves fluid will also flow into the shuttle valve return grooves, over each return groove land which is aligned with the exit groove, to the middle waisted region of the shuttle valve and into the torsion bar and input shaft chamber. Finally fluid moves out from the return pipe back to the pump reservoir.

Turning left (anticlockwise rotation) (Fig. 9.19(b)) An anticlockwise rotation of the steering wheel against the front wheel to ground opposing resistance distorts the torsion bar as input torque is transferred to the worm shaft via the torsion bar. The twisting of the torsion bar means that the worm shaft also rotates anticlockwise, but its angular movement will be less than the input shaft displacement. As a result, the prongs of the input shaft shift the upper and lower shuttle valves to the left and right respectively. Accordingly this movement closes both the intake and return passages of the upper shuttle valve and at the same time opens both the intake and return passages of the lower shuttle valve.

Fluid can now flow from the pump into the worm head annular space made in the outer housing. It then passes from the lower shuttle valve intake to the right hand worm head annular groove. The transfer of fluid is complete when it enters the left hand power cylinder via the sector shaft. The amount of power assistance is a function of the pressure buildup against the left side of the piston, which corresponds to the extent of the shuttle valve intake passage opening caused by the relative angular movements of both the input shaft and worm shaft.

Movement of the power piston to the right displaces fluid from the right hand side of the power cylinder, where it flows via the worm head annular groove to the lower shuttle valve return passage to the central torsion bar and input shaft chamber. It then flows back to the reservoir via the flexible return pipe.

Turning right (clockwise rotation) (Fig. 9.19(c)) Rotating the steering wheel in a clockwise direction applies a torque via the torsion bar to the worm in proportion to the tyre to ground reaction and the input effort. Due to the applied torque, the torsion bar twists so that the angular movement of the worm shaft lags behind the input shaft displacement. Therefore the pronged input will rotate clockwise to the worm head.

With a clockwise movement of the input shaft relative to the worm head, the upper shuttle valve piston moves to the right and the lower shuttle valve piston moves to the left. Consequently, the upper shuttle valve opens both the intake and return passages but the lower shuttle valve closes both the intake and return passages.

Under these conditions fluid flows from the pump to the annular space around the worm head in the plane of the shuttle valves. It then enters the upper valve intake, fills the annular valve space and passes around the left hand worm head groove. Finally, fluid flows through the short horizontal passage into the right hand side of the power cylinder where, in proportion to the pressure build-up, it forces the piston to the left. Accordingly the meshing rack and sector teeth compel the sector shaft to rotate anticlockwise.

At the same time as the fluid expands the right hand side of the power cylinder, the left hand side of the power cylinder will contract so that fluid will be displaced through the long horizontal passage to the worm head right hand annular groove. Fluid then flows back to the reservoir via the upper shuttle valve return groove and land, through to the torsion bar and input shaft chamber and finally back to the reservoir.

9.2.4 Power assisted steering lock limiters

Steering lock limiters are provided on power assisted steering employed on heavy duty vehicles to prevent excessive strain being imposed on the steering linkage, the front axle beam and stub axles and the supporting springs when steering full lock is approached. It also protects the hydraulic components such as the pump and the power cylinder assembly from very high peak pressures which could cause damage to piston and valve seals.

Power assisted steering long stem conical valve lock limiter The lock limiters consist of a pair of conical valves with extended probe stems located in the sector shaft end cover (Fig. 9.20(a and b)). Each valve is made to operate when the angular movement of the sector shaft approaches either steering lock, at which point a cam profile machined on the end of the sector shaft pushes open one or other of the limiting valves. Opening one of the limiter valves releases the hydraulic pressure in the power cylinder end which is supplying the assistance; the

Right hand valve closed

Reservoir ands pump

Left hand valve open

Piston and nut

(a) Turning left anticlockwise

Conical valve and profile stem

(b) Turning right clockwise

Right hand valve closed

Reservoir ands pump

Left hand valve open

Piston and nut

(a) Turning left anticlockwise

Conical valve and profile stem

(b) Turning right clockwise

Input hollow shaft

Torsion bar

Fig. 9.20(a and b) Power assisted steering long stem conical valve lock limiter

Input hollow shaft

Torsion bar

Fig. 9.20(a and b) Power assisted steering long stem conical valve lock limiter excess fluid is then permitted to flow back to the reservoir via the control housing.

Turning left (anticlockwise steering rotation) (Fig. 9.20(a)) Rotation of the input shaft anticlockwise applies both manual and hydraulic effort onto the combined power piston and nut of the steering box so that it moves to the right within the cylinder. Just before the steering reaches full lock, one of the sector cam faces contacts the corresponding valve stem and pushes the conical valve off its seat. Pressurized fluid will immediately escape past the open valve through to the return chamber in the control valve housing, where it flows back to the reservoir. Therefore, any further rotation of the sector shaft will be entirely achieved by a considerable rise in manual effort at the steering wheel, this being a warning to the driver that the steering has reached maximum lock.

Turning right (clockwise steering rotation) (Fig. 9.20(b)) Rotation of the steering box input shaft clockwise screws the worm out from the piston and nut and actuates the control valve so that hydraulic pressure builds up on the right hand end of the piston. As the sector shaft rotation approaches maximum lock, the sector cam meets the valve stem, presses open the valve against the valve return spring tension and causes the hydraulic pressure in the right hand cylinder chamber to drop. The excess fluid will now flow back to the reservoir via the right hand end annular chamber in the control valve housing. The driver will immediately experience a considerable increase in manual effort at the steering wheel, indicating that the road wheels have been rotated to near enough maximum lock.

Power assisted steering double ball valve lock limiter This lock limiter consists of a simple double ball valve located in the blank end of the integral piston and nut. To control the stroke of the piston an adjustable stop pin is mounted in the enclosed end of the power cylinder housing, while the right hand piston movement is limited by the stop pin mounted in the end of the worm shaft.

Turning left (anticlockwise steering rotation) (Fig. 9.21(a)) Rotation of the steering input shaft anticlockwise causes both manual and hydraulic effort to act on the combined power piston and nut, moving it towards the right. As the steering lock movement is increased, the piston approaches the end of its stroke until the right hand ball valve contacts the worm shaft stop pin, thereby forcing the ball off its seat. The hydraulic pressure existing on the left side of the piston, which has already opened the left hand side ball valve, is immediately permitted to escape through the clearance formed between the internal bore of the nut and the worm shaft. Fluid will now flow along the return passage leading to the control reaction valve and from there it will be returned to the reservoir. The release of the fluid pressure on the right side of the piston therefore prevents any further hydraulic power assistance and any further steering wheel rotation will be entirely manual.

Turning right (clockwise steering rotation) (Fig. 9.21(b)) Rotation of the steering box input shaft clockwise screws the worm out from the piston and nut. This shifts the shuttle valve pistons so that the hydraulic pressure rises on the right hand end of the piston. Towards the end of the left hand stroke of the piston, the ball valve facing the blind end of the cylinder contacts the adjustable stop pin. Hydraulic pressure will now force the fluid from the high pressure end chamber to pass between the worm and the bore of the nut to open the right hand ball valve and to escape through the left hand ball valve into the sector gear chamber. The fluid then continues to flow along the return passage going to the control reaction valve and from there it is returned to the reservoir. The circulation of fluid from the pump through the piston and back to the reservoir prevents further pressure build-up so that the steering gearbox will only operate in the manual mode. Hence the driver is made aware that the road wheels have been turned to their safe full lock limit.

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