Control Valve Principle

Control Valve Principle

Types and Operation

During operation of the engine, the fluid in power steering keeps flowing from the pump to the control valve, then back to the reservoir and both the pressure surfaces are exposed to the same system pressure. In the straight ahead position of the wheel, the fluid flows through the circuit. With the increase of steering effort, the valve shift also increases exerting proportional amount of pressure by directing fluid to one pressure surface and increasing the size of the return passage opening from the opposite pressure surface to the reservoir. Figure 27.66 illustrates the principle of operation of the control valve in neutral and during a turn. The control valve is either located inside or is attached to the exterior of the integral type power steering gear. In the link type power steering gear, the valve may be built into the end of the power cylinder or be a separate unit.

The mechanical input force applied by the steering wheel on one hand and the mechanical-hydraulic resistive force of the steering linkage and tyres on the road on the other hand keeps the control valve balanced. As the steering wheel is turned, its mechanical input force moves the control valve towards the linkage resistive force. In this position, the control valve restricts the pressure area and outlet flow to cause pressure builds-up on the pressure surface in a direction that assists the steering wheel input force to move against the steering linkage resistance. Once the steering wheel reaches the desired position, it is held steady and the linkage finally cataches up, centering the control valve to terminate assist. When no effort is being applied to the system, the centering springs centre the spool position to balance pressure on both pressure surfaces, holding the steering linkage in place. With steering wheels in position, if the front tyres hit an object that tries to deflect them, the control valve adds pressure on the pressure surface that opposes the upsetting force, allowing the driver to maintain vehicle control.

Fig. 27.67. A sliding spool control valve in sectional view.

Fig. 27.68. A torsion bar actuated sliding control valve in sectional view.

Fig. 27.69. A torsion bar actuated rotating control valve.



The control valves may be of sliding spools or rotating spools type. The control action is same for both the types of valve. In the link type units the sliding spool valve is mounted concentric with the worm shaft between the Pitman arm and steering linkage, and in integral type it is placed parallel in housing outside the steering gear case. The rotary spool type valve is always mounted concentric with worm shaft. The actuation of the sliding spool valve in the integral steering gear is carried out by a slight endwise movement of the worm shaft or steering linkage. The front wheels and steering linkages hold the sector so it resists movement. The worm shaft is pushed endwise against one of its bearings as the ball nut tries to move the sector. This slight movement is transmitted to the parallel spool valve with a pivot lever. This type of valve is shown in Fig. 27.67.

The concentric spool valve endwise movement is actuated by a torsion bar shown in Fig. 27.68. The steering wheel input effort twists the torsion bar. The twist changes the relative position of the worm shaft actuator assembly in relation to the input shaft position to move the actuator endwise on the helical splines. The spool valve is held in position on the actuator with snap rings so any actuator endwise movement also moves the spool valve endwise. Endwise movement of the spool valve fully opens the passage for the return of fluid from one pressure surface, where as pressure is built up on the other pressure surface to assist steering effort.

The rotary spool control valve, shown in Fig. 27.69, is also operated with a torsion bar. The spool valve body surrounds the control valve. Steering effort on the input shaft twists the torsion bar, which makes a relative difference between the valve body attached to the worm shaft and the valve spool attached to the input shaft. Repositioning the valve spool within the valve body opens a return passage to one of the pressure areas and directs fluid under pressure to the opposite pressure area.

Power steering gears are designed to allow the driver to feel of the amount of effort he is putting into the steering system. The pivot-lever-actuated type control valve gives driver feel with centering springs and with fluid pressure developed on reaction rings while assist is occurring. The link-type power unit may have a reaction valve that is proportional to the pressure developed in the steering system to provide driver feel. This driver feel is called reaction control.

The link type power steering system has the pressure surfaces or areas on either side of the piston within the power cylinder. In integral power steering gear, the ball nut exterior acts as a piston and is called a ball-nut power piston. Each side of this piston is the power chamber. The ball-nut power piston has a rack of gear teeth that rotate a sector in the same way the standard steering does. Steering ratios for power steering gear are usually numerically lower than the standard steering gears, so they have a quicker response to steering wheel movement.

27.8.4.

Power Steering Pumps

The types of power steering pumps, in common use, are the vane type, the slipper type, and the roller type (Fig. 27.70), and principle of operation and design of these pumps are very similar. A power steering pump consists of a belt driven rotor that is turned within an elliptically shaped cam insert ring. Vanes, slippers, or rollers are installed in the rotor slots, grooves, or cavities. Pressure thrust plates on each side of the rotor and cam seal the pump. The assembly is placed in a housing containing rotor bearings and oil passages. The pump housing is usually surrrounded by an oil reservoir, and the pump and reservoir are sealed with O-rings.

Fig. 27.70. Power steering pumps.

These are positive displacement pumps. Each revolution delivers the same amount of fluid irrespective of speed of the pump. The pump capacity must be large enough to supply the fluid at required quantity and pressure during idling of the engine.

During operation, the rotor spins, causing centrifugal force to throw the vanes, slippers, or rollers outward due to which their outer surface maintain contact with the cam. Slippers usually have a back up spring to aid in maintaining cam contact. The cam fits the rotor closely at one or two opposite locations. The spaces between the vanes, slippers, or rollers gradually move outward as the rotor turns them past the close fitting point and the pump fluid flows from the reservoir into these expanding space through inlet passage. As the vanes, slippers, or rollers reach the widest part of the cam insert, they close the inlet passage from the reservoir and make contact with the pressure passage. Continuous turning decreases the volume between the vanes, slippers, or rollers, forcing the pump fluid into the pump pressure outlet passage. The power steering pump is connected to the power steer­ing gear control valve with one high pressure hose and one low pressure return hose.

Power assist requirements become very high as the wheels are turned with the vehicle at rest. The engine is idling during this time, so the engine driven power assist pump is also running slowly. Power assist requirements are very low when driving at highway speeds and when pump speeds are fast. To compen­sate for high pump volumes at cruising speeds, a flow control valve and a pressure relief valve are used.

Fig. 27.71. Principle of flow control

Flow Control Valve Principle and Operation.

The passage from the pump outlet contains a restricting orifice. The difference in pressure drop across the- orifice increases with the in­crease of oil flow. High oil pressure from the upstream side of the orifice is directed at one end of the flow control valve and low oil pressure from the downstream side of the orifice is

directed at the other end of the valve. A calibrated spring is also located on the low pressure side of the flow control valve. The flow control valve, shown in Fig. 27.71, is operated by small pressure difference along with the calibrated spring.

With the increase of the engine speed from idle, both the pump flow and pressure in­crease. When flow reaches the maximum re­quired value, the flow control valve moves towards its low pressure end and a passage between the pump outlet and pump inlet is opened, bypassing a portion of the fluid back to the pump inlet. With the increase of pump speed, the opening becomes larger, keeping the flow at the maximum required rate. Re­circulating the oil back through the pump reduces the pump power requirement and keeps the oil temperature low.

Pressure Relief Valve.

Flow is restricted by the control valve when it sends fluid to one of the power cham­bers causing the pressure to rise. When the driver turns the wheels against the steering linkage turning stops and continues to hold the steering wheel in the full turn position, pressure builds to a maximum.

Fig. 27.72. Principle of pressure regulator

Fig. 27.73. Power-assisted steering layout. 

In order to avoid damage to the power steering unit seals and hoses, the pressure must be limited to a safe value and the pressure relief valve limits this pressure by opening a passage between the pump outlet chamber and the pump inlet or the pump reservoir. The pressure regulator normally is built into the flow control valve as shown in Fig. 27.72. If the pressure on the low pressure side of the control valve reaches a predetermined point, the pressure relief valve opens to allow oil to flow from the low pressure side of the flow control valve to the pump inlet. This drops the pressure in the low pressure side allowing pump output to flow freely into the inlet, thus lowering the pressure at the pump outlet.

27.8.5.

Rack and Pinion Power Assisted Steering

Figure 27.73 shows a rack and pinion power steering system. The engine-driven hydraulic pump is integral with the reservoir, and supplies oil to a control valve installed in the housing in which the pinion shaft is also mounted. The shaft in the steering column through a torsion bar provides motion of the control valve. The movement of the control valve directs oil to one side or the other of the ram piston attached to the steering rack.

           Fig. 27.74. Spool control valve.

Figure 27.74 illustrates a simplified view of a rotary-type spool valve controlled by a torsion bar, placed between the steering shaft and the pinion of the steering box. The spool valve is a shaft having six flutes and is encased by a sleeve having six internal axial grooves. Oil from the supply line flows through the radial ports in the sleeve and shaft to the lines connected to the ram chambers. A series of splines between the shaft and sleeve limit the twist of the torsion bar to about 7 degrees in each direction. Therefore, the torsion bar is capable of transmitting the torque applied by the driver to the steering box pinion only up to 7 degrees on either side from central position. This fall-safe feature provides a mechanical drive from the steering shaft to the pinion during a failure of the power assisted system.

The amount of the torsion bar twist and the spool valve movement are proportional to the effort applied by the driver. Power assistance starts at about 0.5 degree deflection of the bar and this assistance rises till the bar moves to about 4 degrees, the point of maximum power supply. When the valve takes the no-power position (Fig. 27.74) all the ports open, and oil returns to the reservoir through the valve.

If the steering wheel is turned against a resistance encountered at the road wheel, the torsion bar is deflected so that the spool rotates relative to the sleeve. This cuts off the oil flow both to the reservoir and one side of the ram, but the other side of the ram is subjected to oil pressure. When this pressure builds up sufficiently, it moves the road wheel and brings back the torsion bar to the no-torque position. During this period, the oil displaced from the unpressurised side of the ram is returned to the reservoir. In case the resistance to road wheel movement is excessive, the oil pressure builds up to its maximum so that a relief valve, fitted adjacent to the pump, opens and allows oil to return to the pump inlet.