922 Rack and pinion power assisted steering gear power cylinder and control valve

Rack and pinion gear (Fig. 9.15) This power assisted steering system is comprised of a rack and pinion gear with double acting power (servo) piston mounted on the rack and a rotary valve coaxial with the extended pinion shaft (Fig. 9.15).

Helical teeth are cut on the 3% nickel, 1% chromium case hardened steel pinion shaft, they mesh with straight cut teeth on a 0.4% carbon manganese silicon steel rack which is induction hardened. To accommodate these gears, the axis of the pinion is inclined at 76° to that of the rack.

Fig. 9.15 Integral rack and pinion power assisted steering utilized with independent front suspension

Description of rotary control valve (Fig. 9.16(a, b and c)) The three major components of the rotary type control valve are the rotor shaft, the torsion bar and the valve sleeve (Fig. 9.16(a)).

Slots are milled longitudinally on the periphery of the rotor shaft (Fig. 9.16(b)) and on the inner surface of the valve sleeve of which the rotor is assembled and in which it is free to rotate. The sleeve is rotated by the pinion gear shaft. Limited rotation of the rotor shaft, relative to the pinion gear, occurs when the torsion bar that connects the rotor shaft to the pinion shaft is angularly deflected. Hence when steering effort is applied, the torsion bar twists and the slots on the rotor move relative to those in the sleeve to allow fluid to pass to one side of the double acting piston which operates inside the power cylinder. The direction of rotation of the rotor relative to the sleeve determines which side of the double acting piston the fluid will act.

The rotor shaft of the valve forms part of the 0.5% carbon steel rotor shaft and is connected by a torsion bar to the pinion shaft. The outer case-hardened 0.15% carbon steel sleeve which is coaxial with the rotor floats on the ground surface of the rotor shaft, there being a diametrical clearance of 0.004 to 0.012mm between the rotor and sleeve. The sleeve is connected by a steel trim pin screw to the pinion shaft. This pin, the threaded end of which is in a tapped radial hole in the pinion shaft, has a spherical head that fits with no clearance in a radial hole drilled in the sleeve at the pinion end. The axis of the head of this pin is eccentric to that of the threaded shank. Hence, when the pinion shaft and rotor shaft have been locked in the central position of rotation, the correct angular position of the sleeve, relative to that of the rotor, can be set by rotating the pin. This permits the valve assembly to be trimmed, so that the division of fluid flow between rotor and sleeve slot edges is balanced. The amount of opening between the rotor and sleeve control slots is equal to the angular deflection of the torsion bar. Four square section Teflon seals are assembled into annular grooves in the periphery of the sleeve. Between these seals are three wider annular grooves, again on the periphery of the sleeve, from which fluid enters or leaves the valve assembly.

Rotor shaft to pinion shaft coupling (Fig. 9.16(a)) Splines at one end of the rotor shaft register in an internally splined recess in the pinion shaft. The width of the splines is such that the torsion bar can twist a total of seven degrees before the splines contact one another. Manual steering effort is transferred from the rotor shaft through the 1% chromium-molybdenum-steel torsion bar, which has an approximate waist diameter of 5 mm, to the pinion shaft. The splined coupling between these two members ensures that, in the event of failure of the power assistance, the steering gear can be operated manually without overstressing the torsion bar.

Rotor and sleeve longitudinal and annular grooves (Fig. 9.16(b)) Six equally spaced longitudinal slots are milled on the circumference of the rotor. Three of the rotor slots are longer than the other three and these two lengths are disposed alternately around the rotor periphery. There are also six matching equally spaced groove slots with closed-offends in the bore of the valve sleeve. The angular relationship of these two sets of slots controls the flow of fluid from the pump to the power cylinder and from the cylinder to the reservoir.

The positioning of the ports and annular grooves around the valve sleeve are now considered from the pinion end of the rotor shaft.

The first valve port feeds or returns fluid from the left hand side of the power cylinder, the second port delivers fluid from the pump, the third port feeds or returns fluid from the right hand side of the power cylinder and the fourth port acts as the return passage to the reservoir.

Each of the first three ports (counting from the left) communicates with corresponding annular grooves in the outer periphery of the sleeve, while the fourth port, through which fluid is returned to the reservoir, communicates with an annular space between the right hand end of the valve housing and the end of the sleeve.

Radial holes in the central sleeve groove connect the pump pressurized fluid to the short supply slots on the periphery of the rotor shaft. The sleeve grooves on either side are connected through small radial holes to the three longer slots on the rotor surface. These longer slots provide a return passage to the annular space on the right of the valve which then leads back to the reservoir.

Whenever the pump is operating, fluid passes through the delivery port in the sleeve and is then transferred to the three short rotor slots. Thereafter the fluid either passes directly to the longer slots, and hence to the return port, or flows through one feed and return port going to the power cylinder, while fluid from the other end of the cylinder is returned through the longer slots to the reservoir.

Torsion bar stiffness (Fig. 9.16(a)) When torque is applied at the steering wheel, the grip of the tyres on the road causes the torsion bar to twist. Relative movement between the rotor shaft and sleeve upsets the balanced flow of fluid and pressure at the rotary control valve going to both sides of the power piston. Therefore the deflection rate of the torsion bar is important in determining the torque required to steer the vehicle. To transmit the desired input torque from the steering wheel and rotor shaft to the pinion shaft, a standard 108 mm long torsion bar is used. Its diameter and stiffness are 5.2 mm and 1.273 N/deg and the actual diameter of the torsion bar can be changed to suit the handling requirements of the vehicle.

Rotor slot edge control (Fig. 9.16(b)) If the valve rotor and sleeve slot control edges were sharp as shown in the end view (Fig. 9.16(b)), the area through which the fluid flowed would vary directly with the deflection of the torsion bar, and hence the applied torque. Unfortunately, the relationship between pressure build-up and effective valve flow area is one in which the pressure varies inversely as the square of the valve opening area. This does not provide the driver with a sensitive feel at the steering wheel rim. The matching of the pressure buildup, and hence valve opening area, relative to the driver's input effort at the steering wheel rim has been modified from the simple sharp edge slots on the rotor, to contoured stepped edges (Fig. 9.16(b)) which provide a logarithmic change of area so that the increase of pressure with rim effort is linear. This linear relationship is retained up to 28 bar which is the limit for driving a large car under all road conditions. Above 28 bar the rise of pressure with applied steering wheel torque is considerably steepened to ensure that parking efforts are kept to a minimum. The relationship between the angular displacement of the rotary valve and the hydraulic pressure applied to one side of the power cylinder piston is shown in Fig. 9.17.

An effort of less than 2.5 N at the rim of the steering wheel is sufficient to initiate a hydraulic pressure differential across the double acting power piston. During manoeuvres at very low speeds when parking, a manual effort of about 16 N is required. When the car is stationary on a dry road, an effort of 22 N is sufficient to move the steering from the straight ahead position to full lock position.

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