What is in this article?:
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.
Constant and fixed pressure reduction —
Constant-pressure-reducing valves supply a preset pressure, regardless of main circuit pressure, as long as pressure in the main circuit is higher than that in the secondary. These valves balance secondary-circuit pressure against the force exerted by an adjustable spring which tries to open the valve. When pressure in the secondary circuit drops, spring force opens the valve enough to increase pressure and keep a constant reduced pressure in the secondary circuit.
Fixed pressure reducing valves supply a fixed amount of pressure reduction regardless of the pressure in the main circuit. For instance, assume a valve is set to provide reduction of 250 psi. If main system pressure is 2750 psi, reduced pressure will be 2500 psi; if main pressure is 2000 psi, reduced pressure will be 1750 psi.
This valve operates by balancing the force exerted by the pressure in the main circuit against the sum of the forces exerted by secondary circuit pressure and the spring. Because the pressurized areas on both sides of the poppet are equal, the fixed reduction is that exerted by the spring.
Pilot-operated pressure reducing valves —The spool in a pilot-operated, pressure-reducing valve is balanced hydraulically by downstream pressure at both ends, Figure 6. A light spring holds the valve open. A small pilot relief valve, usually built into the main valve body, relieves fluid to tank when reduced pressure reaches the pilot valve’s spring setting. This fluid flow causes a pressure drop across the spool. Pressure differential then shifts the spool toward its closed position against the light spring force.
The pilot valve relieves only enough fluid to position the main valve spool or poppet so that flow through the main valve equals the flow requirements of the reduced pressure circuit. If no flow is required in the low-pressure circuit during a portion of the cycle, the main valve closes. Leakage of high-pressure fluid into the reduced-pressure section of the valve then returns to the reservoir through the pilot operated relief valve.
Pilot-operated pressure reducing valves generally have a wider range of spring adjustment than direct-acting valves and provide better repetitive accuracy. However, oil contamination can block flow to the pilot valve and the main valve will fail to close properly. Pilot-operated valves with built-in reduced pressure system relieving capability also are available.
In circuits with more than one actuator, it is often necessary to drive the actuators, such as cylinders, in a definite order or sequence. One way to do this is with limit switches, timers, or other electrical control devices.
Sometimes, this result can also be achieved by sizing cylinders according to the load they must displace. The cylinder requiring the least pressure to move its load extends first. At the end of its stroke, system pressure increases and extends the second cylinder. This continues until all cylinders are actuated.
However, in many installations, space limitations and force requirements determine the cylinder size needed to do the job. In this case, sequence valves can be used to actuate the cylinders in the required order.
Sequence valves are normally closed, 2-way valves. They regulate the sequence in which various functions in a circuit occur, Figure 7. They resemble direct-acting relief valves except that their spring chambers are generally drained externally to reservoir, instead of internally to the outlet port, as in a relief valve.
A sequence valve usually permits pressurized fluid to flow to a second function only after an earlier, priority function has been completed and satisfied. When normally closed, a sequence valve allows fluid to flow freely to the primary circuit, to perform its first function until the pressure setting of the valve is reached.
When the primary function is satisfied, pressure in the primary circuit rises and is sensed in pressure-sensing passage A. This pressurizes the spool and overcomes the force exerted by the spring. The spring is compressed, the valve spool shifts, and oil flows to the secondary circuit.
Sequence valves sometimes have check valves, which permit reverse flow from the secondary to the primary circuit. However, sequencing action is provided only when the flow is from the primary to the secondary circuit.
In some applications, an interlock can prevent sequencing occurring until the primary actuator reaches a certain position. This is done with remote operations.