Fig. 6. Pilot-operated, pressure reducing valve has reduced pressure on both ends of the spool. A light spring holds the spool open.

Fig. 7. Sequence valve is a 2-way valve held closed by an adjustable spring and opened by pressure at the inlet port acting on the left of the spool.

Fig. 8. Counterbalance valve stops flow from its inlet port to its outlet port until pressure at the inlet port overcomes adjusting spring force. An integral check valve permits free flow through the valve in the opposite direction.

Fig. 9. Unloading valve is spring-loaded to the closed position. When system pressure, transmitted to the valve though the pilot port, is sufficient to overcome force of the adjustable spring, the valve opens. Pump delivery unloads to tank at low pressure.

Fig. 10. Unloading valve for accumulator circuit opens at a set unloading pressure and closes at a lower pressure. The valve opens when the system reaches a pressure determined by the adjustable spring and pump pressure on the right of the control piston. The valve closes at a lower pressure because force from system pressure on the left of the control spool must only overcome force of the adjusting spring.

Fig. 11. Piloted unloading valve has piston with pump pressure at both ends. When system pressure on the pilot piston overcomes force of the pilot spring the pilot valve opens. Fluid from behind the main valve piston drains to tank, which opens the valve. When system pressure fails, the pilot valve closes, restoring equal pressure to both ends of main valve spool.

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 though the pilot operated relief valve.

Pilot-operated pressure reducing valves generally have a wider range of spring adjustment than direct-acting valves. They generally provide more 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.

Sequence valves

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.

Normally, a sequence valve permits pressure 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, it is desirable to provide an interlock so that sequencing does not occur until the primary actuator reaches a certain position. This is done with remote operations.

Counterbalance valves

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 pressure 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.

Overcenter 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.

Unloading valves

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 reservoir at low pressure.

Unloading valves for accumulator circuits - An unloading valve can be used in an accumulator circuit to unload the pump after the accumulator has been charged, Figure 10. The valve remains closed while the pump is charging the accumulator. After the accumulator is charged, the unloading valve opens, unloading the pump at low pressure while the accumulator supplies pressure fluid to the system. Every time pressure in the accumulator drops below a preset level (controlled by the setting of the spring) the charge/unload cycle repeats.

Piloted unloading valves - Unloading valves are also made with a pilot to control the main valve, Figure 11. 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.

In most pump-unloading valves for accumulator circuits, only the opening pressure is adjustable and the closing pressure is a fixed percentage of it. However, a pilot-operated unloading valve can adjust both pressures.