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Pressure-control valves are found in virtually every hydraulic system, and they assist in a variety of functions, from keeping system pressures safely below a desired upper limit to maintaining a set pressure in part of a circuit.
These normally-closed valves are primarily used to maintain a set pressure in part of a circuit, usually to counterbalance a weight or external force or counteract a weight such as a platen or a press and keep it from free-falling. The valve’s primary port is connected to the cylinder’s rod end, and the secondary port to the directional control valve, Figure 8. The pressure setting is slightly higher than that required to keep the load from free-falling.
When pressurized fluid flows to the cylinder’s cap end, the cylinder extends, increasing pressure in the rod end, and shifting the main spool in the counterbalance valve. This creates a path which permits fluid to flow through the secondary port to the directional control valve and to reservoir. As the load is raised, the integral check valve opens to allow the cylinder to retract freely.
If it is necessary to relieve back pressure at the cylinder, and increase the force at the bottom of the stroke, the counterbalance valve can be operated remotely.
Counterbalance valves are usually drained internally. When the cylinder extends, the valve must open and its secondary port is connected to reservoir. When the cylinder retracts, it matters little that load pressure is felt in the drain passage because the check valve bypasses the valve’s spool.
Over center valves
Overcenter valves resemble counterbalance valves in that their purpose is to maintain a set pressure opposite a load, to keep it from free-falling. The main difference is that an overcenter valve uses a pilot signal, usually from the inlet of the actuator, to assist in opening the spool. This pilot assist makes the overcenter valve more efficient, and reduces the horsepower requirement and heat generation within the system.
As pumps and actuators become more advanced, with negative or positive load sensing and unloading features, and as directional control valves become more sophisticated, controlling a load smoothly using overcenter valves has, in turn, become more challenging. New advancements in overcenter valve technology are making control easier.
These valves are normally used to unload pumps. They direct pump output flow (often the output of one of the pumps in a multi-pump system) directly to reservoir at low pressure, after system pressure has been reached.
The force exerted by the spring keeps the valve closed, Figure 9. When an external pilot signal acting on the opposite end of the valve spool exerts a force large enough to exceed that exerted by the spring, the valve spool shifts, diverting pump output to reservoir at low pressure.
High-low circuits which use two pumps for traverse and speed, or clamping, depend on unloading valves to improve efficiency. Output from both pumps is needed only for fast traverse. During feed or clamping, output from the large pump is unloaded to tank at low pressure.
Piloted unloading valves —Unloading valves are also made with a pilot to control the main valve, Figure 10. A port through the main valve plunger allows system pressure to act on both ends of the plunger. A light spring plus system pressure acting on the larger area at the spring end of the plunger holds the valve closed. A built-in check valve maintains system pressure. When system pressure drops to a preset value, the pilot valve closes. Pump flow through the port in the main valve spool closes the valve.
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For more on pressure-control valves, visit our eBook chapters on the subject:
Fluid Power Basics, Ch. 9: Relief & Unloading Valves
Fluid Power Basics, Ch. 14: Pressure Control Valves (Except Relief & Unloading Valves)
Fluid Power Circuits, Ch. 16: Reducing Valves
Fluid Power Circuits, Ch. 18: Pressure Relief Valves
Fluid Power Circuits, Ch. 20: Sequence Valves