CONTROL VALVES:Pilot-Controlled Pressure-Reducing Valves

Pilot-Controlled Pressure-Reducing Valves

Figures 7-16A and B illustrate the operation of a pilot -controlled valve. This valve consists of an adjustable pilot valve that controls the operating pressure of the valve, and a spool valve that reacts to the action of the pilot valve.

The pilot valve consists of a poppet (1), a spring (2), and an adjusting screw (3). The valve spool assembly consists of: a valve spool (10) and a spring (4). Fluid under main system pressure enters the inlet port (11) and under all conditions is free to flow through the valve and the outlet port (5).

Figure 7-16, view A, shows the valve in the open position. In this position, the pres­ sure in the reduced-pressure outlet port (6) has not reached the preset operating pres­ sure of the valve. The fluid also flows through passage 8, through smaller passage 9 in the center of the valve spool, and into chamber 12. The fluid pressure at outlet port 6 is therefore distributed to both ends of the spool. When these pressures are equal, the spool is hydraulically balanced. Spring 4 is a low-tension spring and applies only a slight downward force on the spool. Its main purpose is to position the spool and to maintain opening 7 at its maximum size.

FLUID POWER DYNAMICS-0334

As the pressure increases in outlet port 6, this pressure is transmitted through pas­ sages 8 and 9 to chamber 12. This pressure also acts on the pilot valve poppet (1). When this pressure increases above the preset operating pressure, it overcomes the force of the pilot valve spring 2 and unseats the poppet. This allows fluid to flow through the drain port (15). Because the small passage 9 restricts flow into chamber 12, the fluid pressure in the chamber drops. This causes a momentary differential in pressure across the valve spool 10, which allows fluid pressure acting against the bot­ tom area of the valve spool to overcome the downward force of spring 4. The spool is then forced upward until the pressure across its ends is equalized. As the spool moves upward, it restricts the flow through opening 7 and causes the pressure to decrease in the reduced pressure outlet, port 6. If the pressure in the outlet port continues to increase to a value above the preset pressure, the pilot valve will open again and the cycle will repeat. This allows the spool valve to move up higher into chamber 12, thus further reducing the size of opening 7. These cycles repeat until the desired pressure is maintained in outlet 6.

When the pressure in outlet 6 decreases to a value below the preset pressure, spring 4 forces the spool downward, allowing more fluid to flow through opening 7.

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