A pressure-control valve may limit or regulate pressure, create a particular pressure condition required for control, or cause actuators to operate in a specific order. All pure pressure-control valves operate in a condition approaching hydraulic balance. Usually the balance is very simple: pressure is effective on one side or end of a ball, poppet, or spool and is opposed by a spring. In operation, a valve takes a position where hydraulic pressure balances a spring force. Since spring force varies with compression, distance and pressure also can vary. Pressure-control valves are said to be infinite positioning. This means that they can take a position anywhere between two finite flow conditions, which changes a large volume of flow to a small volume, or pass no flow.
Most pressure-control valves are classified as normally closed. This means that flow to a valve’s inlet port is blocked from an outlet port until there is enough pressure to cause an unbalanced operation. In normally open valves, free flow occurs through the valves until they begin to operate in balance. Flow is partially restricted or cut off. Pressure override is a characteristic of normally closed-pressure controls when they are operating in balance. Because the force of a compression spring increases as it lowers, pressure when the valves first crack is less than when they are passing a large volume or full flow. The difference between a full flow and cracking pressure is called override.
a. Relief Valves.
b. Pressure-Reducing Valves.
c. Sequence Valves.
d. Counterbalance Valves.
e. Pressure Switches.
A counterbalance valve allows free flow of fluid in one direction and maintains a resistance to flow in another direction until a certain pressure is reached. A valve is normally located in a line between a directional-control valve and an outlet of a vertically mounted actuating cylinder, which supports weight or must be held in position for a period of time. A counterbalance valve serves as a hydraulic resistance to an actuating cylinder. For example, a counterbalance valve is used in some hydraulically operated fork lifts. It offers a resistance to the flow from an actuating cylinder
when a fork is lowered. It also helps support a fork in the up position.
Figure 5-9 shows a counterbalance valve. The valve element is balance-spool valve 4 that consists of two pistons which are permanently fixed on either end of the shaft. The inner piston areas are equal; therefore, pressure acts equally on both areas regardless of the position of the valve, and has no effect on the movement of the valve, hence, the term balanced. A small pilot piston is attached to the bottom of the spool valve.
When the valve is in the closed position, the top piston of the spool valve blocks discharge port 8. If fluid from the actuating unit enters inlet port 5, it cannot flow through the valve because discharge port 8 is blocked. However, fluid will flow through the pilot passage 6 to the small pilot piston. As the pressure increases, it acts on the pilot piston until it overcomes the preset pressure of spring 3. This forces the spool up and allows the fluid to flow around the shaft of the spool valve and out the discharge port 8.
During reverse flow, the fluid enters port 8. Spring 3 forces spool valve 4 to the closed position. The fluid pressure overcomes the spring tension of check valve 7. It opens and allows free flow around the shaft of the spool valve and out port 5. The operating pressure of the valve can be adjusted by turning adjustment screw 1, which increases or decreases the tension of the spring. This adjustment depends on the weight that the valve must support.
Small amounts of fluid will leak around the top piston of the spool valve and into the area around spring 3. An accumulation would cause a hydraulic lock on the top of the spool valve (since a liquid cannot be compressed). Drain 2 provides a passage for this fluid to flow to port 8.