Pressure control valves are found in virtually all hydraulic systems and support a variety of functions from safely maintaining system pressure below a desired upper limit to maintaining a set pressure in some part of a circuit.
exhaust valves
Most fluid power systems are designed to operate within a predefined pressure range. This range is a function of the forces that the system's actuators must generate in order to do the required work. Without controlling or limiting these forces, hydraulic power components (and expensive equipment) can become damaged. Relief valves avoid this danger. These are protective devices that limit the maximum pressure in a system by diverting excess oil when the pressure is too high.
opening pressure and pressure relief.The pressure at which a relief valve first opens to allow fluid to flow is referred to asbursting pressure. When the valve bypasses its full rated flow, it is in a state offull flow pressure.The difference between total flow and set pressure is sometimes referred to asdifferential pressure,also known aspressure release.
Direct acting relief valves.A direct-acting valve can consist of a seat or ball that is subjected to system pressure on one side and a spring with a preset force on the other side. On a normally closed adjustable relief valve(Illustration 1), the force of the compression spring exceeds the force of the system pressure acting on the ball or seat. The spring holds the ball or seat firmly in place. A reservoir port on the spring side of the valve returns spilled fluid to the tank.
1. Direct-acting adjustable relief valves block flow through the valve until the system pressure force on the seat exceeds the adjustable spring force and downstream pressure.
When the system pressure begins to exceed the valve spring setting, the fluid brakes the ball or seat, which allows a controlled amount of fluid to be diverted to the reservoir, thereby maintaining the system pressure at the valve spring setting. The spring reinserts the ball or seat when enough fluid is released (bypassed) to lower the system pressure below the valve spring setting.
Most relief valves are adjustable, usually with a spring-loaded adjusting screw. The operator compresses or relaxes the spring by turning the screw in or out. The valve can be set to open at any pressure within a desired range.
Pilot operated relief valves.For applications that require valves that must unload large flows with a small pressure difference, pilot operated unloading valves are often used.(Figure 2). The pilot operated relief valve works in two stages. A pilot stage consisting of a small spring-loaded unloader valve (usually built into the main unloader valve) acts as a poppet to control the main unloader valve. However, the pilot can also be remotely located and connected to the main valve by tubing or tubing.
The main relief valve is normally closed when the inlet pressure is below the main valve spring setting. HoleBno main valve(Figure 2)allows the system pressure fluid to act on a larger area on the spring side of the seat so that the sum of this force and the force of the main spring keeps the seat seated. At this time, the pilot valve is also closed. step pressureBequal to the system pressure and less than the pilot valve spring setting.
2. The pilot operated relief valve has an orifice in the piston that is held closed by light spring force and system pressure acting on a larger area of the piston at the end of the spring.
As the system pressure increases, the pressure in the passage increasesBalso increases and when it reaches the pilot valve setting, the pilot valve opens. The oil is drained through the passage behind the main valveBthrough the drain hole. The resulting pressure drop across the orificeAat the main relief valve, it opens and excess oil flows into the tank, preventing an increase in inlet pressure. The valves close again when the inlet oil pressure falls below the valve setting.
Since these valves only begin to open when the system reaches 90% of full pressure, system efficiency is protected as less oil is released. These valves are best suited for high pressure, high volume applications. Pilot operated unloading valves, while operating more slowly than direct acting unloading valves, maintain a system at a more constant pressure during unloading.
pressure reducing valves
The most practical components for maintaining a lower secondary pressure in a hydraulic system are pressure reducing valves. Pressure reducing valves are normally open 2-way valves that close when there is sufficient downstream pressure. There are two types: direct acting and pilot operated.
Direct Action.A pressure reducing valve limits the maximum available pressure in the secondary circuit, regardless of pressure changes in the main circuit. This assumes that the workload does not cause backflow into the reducing valve port, in which case the valve closes.(Abb. 3). The pressure detection signal comes from the downstream side (secondary circuit). This valve actually works in reverse to a relief valve (which senses inlet pressure and is normally closed).
3. Direct operated pressure reducing valve is held open by spring force. Increased pressure at the outlet port moves the piston to the right, closing the valve.
When the pressure in the secondary circuit increases(Abb. 3), the hydraulic force acts on the surfaceAvalve, partially closing it. The spring force counteracts the hydraulic force, allowing just enough oil to flow through the valve to supply the desired pressure to the secondary circuit. Spring configuration is adjustable.
When the outlet pressure reaches the valve setting, the valve closes, except for a small amount of oil that flows from the low pressure side of the valve, usually through a hole in the spool, through the chamber from the spring to the valve. Valve. Reservoir.
When the valve closes fully, leakage through the spool can cause pressure build-up in the secondary circuit. To prevent this, a vent passage to the reservoir keeps the reservoir slightly open, preventing the outlet pressure from rising beyond the valve setting. The drain channel returns the leakage flow to the tank. (Valves with built-in relief are also available to eliminate the need for this orifice.)
Constant and fixed pressure reduction.constant pressureto reduceThe valves deliver a preset pressure independent of the main circuit pressure as long as the main circuit pressure is presenthigherthan in high school. These valves balance the secondary circuit pressure against the force exerted by an adjustable spring trying to open the valve. When the pressure in the secondary circuit drops, the spring force opens the valve enough to increase the pressure and maintain a constant reduced pressure in the secondary circuit.
firm pressureReducing valves provide afixed amount of pressure reductionindependent of the pressure in the main circuit. Suppose a valve is set to provide a 250 psi reduction. If the main system pressure is 2750 psi, the reduced pressure is 2500 psi; if the main pressure is 2000 psi, the reduced pressure is 1750 psi.
This valve works by balancing the force exerted by the pressure in the main circuit against the sum of the forces exerted by the pressure in the secondary circuit and the spring. Because the pressure areas are the same on either side of the seat, the fixed reduction is that applied by the spring.
Pilot operated pressure reducing valves.The spool in a pilot operated pressure reducing valve is hydraulically balanced at both ends by downstream pressure(Figure 4). A light spring keeps the valve open. A small pilot relief valve, usually built into the main valve body, bleeds fluid to the tank when the reduced pressure reaches the pilot valve spring setting. This flow of liquid causes a pressure drop in the coil. The pressure differential then moves the spool to its closed position against a slight spring force.
4. Pilot operated pressure reducing valve has reduced pressure at both ends of the spool. A light spring keeps the spool open.
The pilot valve delivers just enough fluid to position the main valve spool or seat so that flow through the main valve meets the flow requirements of the reduced pressure circuit. When no flow is required in the low pressure circuit during part of the cycle, the main valve closes. High pressure fluid that leaks from the vacuum portion of the valve is then returned to the reservoir through the pilot operated relief valve.
Sequencing valves
In circuits with more than one actuator, it is often necessary to B. cylinder to control in a defined order or order. This result can sometimes be achieved by sizing the cylinders according to the load to be moved. However, in many installations, space constraints and power requirements dictate the cylinder size required for 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 order in which different functions in a circuit occur.(Figure 5). They are similar to direct-acting unloader valves, except that their spring-loaded chambers generally drain externally to the sump rather than internally to the exhaust port like in an unloader valve.
5. The sequence valve is a 2-way valve held closed by an adjustable spring and opened by pressure on the inlet port acting to the left of the spool.
A sequence valve will normally only flow pressurized fluid to a second function after a previous priority function has been completed and satisfied. When normally closed, a sequence valve allows fluid to flow freely into the primary circuit to perform its primary function until the valve's set pressure is reached.
When the main function is fulfilled, the pressure in the main circuit increases and is recorded in the pressure sensing line.A. This will pressurize the spool and overcome the force exerted by the spring. The spring compresses, the spool moves, and oil flows into the secondary circuit.
balancing valves
These normally closed valves are used primarily to maintain a set pressure on part of a circuit, usually to counteract an external weight or force, or to counteract a weight such as a free-falling plate or press. The main port of the valve is connected to the rod end of the cylinder and the secondary port is connected to the directional control valve.(Figure 6). The pressure setting is slightly higher than necessary to prevent the load from falling freely.
6. The counterbalance valve stops flow from its inlet port to its outlet port until the pressure at the inlet port exceeds the force of the adjustment spring.
When hydraulic fluid enters the head end of the cylinder, the cylinder extends, increasing pressure at the end of the rod and displacing the main piston in the balance valve. This creates a path that allows fluid to flow through the secondary port to the directional control valve and into the reservoir. As the load increases, the built-in check valve opens to allow the cylinder to retract freely.
If the back pressure needs to be relieved by increasing the cylinder force at the bottom of the stroke, the counterbalance valve can be operated remotely. Counterbalance valves are usually drained internally. When the cylinder is extended, the valve must open and its secondary port is connected to the reservoir. When the cylinder is retracted, it doesn't matter what load pressure is felt in the discharge line because the check valve bypasses the valve spool.
exhaust valves
These valves are typically used to unload pumps. They direct the output flow from the pump (usually the output from one of the pumps in a multi-pump system) directly to the reservoirlowPrint,aftersystem pressure is reached.
The force of the spring keeps the valve closed.(Figure 7). When an external pilot signal acting on the opposite end of the valve spool applies a force large enough to overcome the force exerted by the spring, the valve spool shifts, diverting the pump output to the low-pressure accumulator.
7. The dump valve is spring loaded to the closed position. When the system pressure exceeds the adjustable spring force, the valve opens.
High-low circuits that use two pumps for displacement and speed or clamp feature unloader valves to improve efficiency. The flow rate of both pumps is only required for rapid displacement. When feeding or clamping, the large pump output is discharged into the tank with low pressure.
Pilot exhaust valves.Blow-off valves are also made with pilot to control the main valve.(Figure 8). A port through the main valve spool allows system pressure to act on both ends of the spool. A light spring plus system pressure acting on the larger area at the spring end of the plunger keeps the valve closed. An integrated check valve maintains the system pressure. When the system pressure drops to a predetermined level, the pilot valve closes. Pump flow through the port in the main valve spool closes the valve.
8. The pilot operated relief valve has a piston with a pressure pump at both ends.