Flow and pressure control valves

Description of the flow and pressure control valve unit (Fig. 9.22(a and b)) The quantity of fluid discharged from the roller type pump and the build-up in fluid pressure both increase almost directly with rising pump rotor speed. These characteristics do not meet the power assisted steering requirements when manoeuvring at low speed since under these conditions the fluid circulation is restricted and a rise in fluid pressure is demanded to operate the power cylinders double acting piston. At high engine and vehicle speed when driving straight ahead, very little power assistance is needed and it would be wasteful for the pump to generate high fluid pressures and to circulate large amounts of fluid throughout the hydraulic system. To overcome the power assisted steering mismatch of fluid flow rate and pressure build-up, a combined flow control and pressure relief valve unit is incorporated within the cast iron pump housing. The flow control valve consists of a spring loaded plunger type valve and within the plunger body is a ball and spring pressure relief valve. Both ends of the plunger valve are supplied with pressurized fluid from the pump. Situated in the passage which joins the two end chambers of the plunger is a calibrated flow orifice. The end chamber which houses the plunger return spring is downstream of the flow orifice.

Fluid from the pump discharge ports moves along a passage leading into the reduced diameter portion of the flow control plunger (Fig. 9.22(a)). This fluid circulates the annular space surrounding the lower part of the plunger and then passes along a right angled passage through a calibrated flow orifice. Here some of the fluid is diverted to the flow control plunger spring chamber, but the majority of the fluid continues to flow to the outlet port of the pump unit, where it then goes through a flexible pipe to the control valve built into the steering box (pinion) assembly. When the engine is running, fluid will be pumped from the discharge ports to the flow control valve through the calibrated flow orifice to the steering box control valve. It is returned to the reservoir and then finally passed on again to the pump's intake ports.

Principle of the flow orifice (Fig. 9.22(a and b)) With low engine speed (Fig. 9.22(a)), the calibrated orifice does not cause any restriction or apparent resistance to the flow of fluid. Therefore the fluid pressure on both sides of the orifice will be similar, that is P1.

As the pump speed is raised (Fig. 9.22(b)), the quantity of fluid discharged from the pump in a given time also rises, this being sensed by the flow orifice which cannot now cope with the increased amount of fluid passing through. Thus the orifice becomes a restriction to fluid flow, with the result that a slight rise in pressure occurs on the intake side of the orifice and a corresponding reduction in pressure takes place on the outlet side. The net outcome will be a pressure drop of P1-P2, which will now exist across the orifice. This pressure differential will become greater as the rate of fluid circulation increases and is therefore a measure of the quantity of fluid moving through the system in unit time.

Operation of the flow control valve (Fig. 9.22 (a and b)) When the pump is running slowly the pressure drop across the flow orifice is very small so that the plunger control spring stiffness is sufficient to fully push the plunger down onto the valve cap stop (Fig. 9.22(a)). However, with rising pump speed the flow rate (velocity) of the fluid increases and so does the pressure difference between both sides of the orifice. The lower pressure P2 on the output side of the orifice will be applied against the plunger crown in the control spring chamber, whereas the higher fluid pressure P1 will act underneath the plunger against the annular shoulder area and on the blanked off stem area of the plunger. Eventually, as the flow rate rises and the pressure difference becomes more pronounced, the hydraulic pressure acting on the lower part of the plunger P1 will produce an upthrust which equals the downthrust of the control spring and the fluid pressure P2. Consequently any further increase in both fluid velocity and pressure difference will cause the flow control plunger to move back progressively against the control spring until the shouldered edge of the plunger uncovers the bypass port (Fig. 9.22(b)). Fluid will now easily return to the intake side of the pump instead of having to work its tortuous way around the complete hydraulic system. Thus the greater the potential output of the pump due to its speed of operation the further back the plunger will move and more fluid will be bypassed and returned to the intake side of the pump. This means in effect that the flow output of the pump will be controlled and limited irrespective of the pump speed (Fig. 9.23). The maximum output characteristics of the pump are therefore controlled by two factors; the control spring stiffness and the flow orifice size.

is machined on the large diameter portion of the plunger just above the shoulder. A radial relief hole connects this groove to the central spring housing.

With this arrangement the ball relief valve is subjected to the pump output pressure on the downstream (output) side of the flow orifice.

If the fluid output pressure exceeds some predetermined maximum, the ball will be dislodged from its seat, permitting fluid to escape from the control spring chamber, through the centre of the plunger and then out by way of the radial hole and annular groove in the plunger body. This fluid is then returned to the intake side of the pump via the bypass port.

Immediately this happens, the pressure P2 in the control spring chamber drops, so that the increased pressure difference between both ends of the flow control plunger pushes back the plunger. As a result the bypass port will be uncovered, irrespective of the existing flow control conditions, so that a rapid pressure relief by way of the flow control plunger shoulder edge is obtained. It is the ball valve which senses any peak pressure fluctuation but it is the flow control valve which actually provides the relief passage for the excess of fluid. Once the ball valve closes, the pressure difference across the flow orifice for a given flow rate is again established so that the flow control valve will revert back to its normal flow limiting function.

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