Spool valves normally take pressure at any port without malfunction. The boxes show the function of the main or working spool that controls the actuator. On most schematics, the simplified symbol is sufficient. When the inlet is blocked in the at-rest condition, as shown in Figure 8-1, it is referred to as "normally closed" (NC). The counterbalance valve keeps the cylinder from running away no matter the flow variations, while the pilot-operated check valve holds it stationary when stopped. A two position, single solenoid, spring return valve is sufficient for this operation. A single-acting cylinder needs supply to and exhaust from its port to operate. Most spool-type air valves come in a 5-way configuration. The symbol in Figure 8-64 shows how to represent this in a symbol. When the directional valve shifts, starting the cylinder forward, as shown in Figure 8-74, pressure in the cap end cylinder port starts climbing to 150 psi. Figure 8-67 shows a pilot-operated check with a decompression feature. The reason for this pressure drop is leakage past the counterbalance valve spool, which is the reason for adding the pilot-operated check valve. Palm-button-operated 3-way diverter valve.4-way directional control valves
Every time a cylinder cycles, the lines to both ports fill and exhaust. If both inlet pressures are too low to operate the valve, plumb an external pilot supply from the main air system. Mounting air valves directly to the cylinder ports minimizes air waste. Any metal-to-metal fit spool valve never fully blocks flow. This air savings results in lower operating cost and leaves more air to run other actuators. Note the port hookup is A to cap and B to rod. An all-ports open center condition directional valve unloads the pump and allows the actuator to float as shown in Figure 8-38. A 3-way valve not only supplies fluid to an actuator, but allows fluid to return from it as well. The circuit in Figure 8-65 also shows an anti-cavitation check valve for the cylinder with a relief valve to protect it from over pressure. In figure 8-59 to 8-61, the cylinder strokes smoothly and quickly in both directions with dual-pressure valve. Adding a pilot-operated check valve in front of the counterbalance valve stopped cylinder drifting. Three-position valves come in several styles, including: cylinder ports open as seen in Figure 8-19; all ports blocked as seen in Figure 8-20; and pressure to cylinder ports as seen in Figure 8-21. To unload the pump while blocking the cylinder from moving, use the valve shown in Figure 8-46. See chapter five for the different types of counterbalance circuits. The anti-cavitation check valve has a very low-pressure spring, which requires 1-3 psi to open, so it allows tank oil to fill any vacuum void that might form. In Figure 8-78, rod end pressure is at 3565 psi because pilot pressure continues to climb. After the air exhausts to the lower pressure, PR.1, the shuttle shifts and low pressure holds in the system. Even with some spool type counterbalance valves, the cylinder still drifts. An inline check valve stops any chance of reverse flow and is useful and/or necessary in many applications. The backpressure check valve in the pump line maintains a minimum pilot pressure while the pump unloads. 5-way directional control valves
Heat exchangers, filters, and low-pressure transfer pumps often need a low-pressure bypass or relief valve. The load lowers smoothly and safely without lunging or bouncing, as fast as cap end air exhausts. Notice the pipe between the pilot-operated check valve and the counterbalance valve is at zero psi while the cylinder is held retracted. The 3-way selector does fine when going from low to high pressure, but if there is no air usage to allow expansion, it is almost impossible to go from high to low pressure. The cylinder in this example has a heavy weight pulling against the rod side. In the crossover or transition condition it causes very little shock. Normally a check valve is not thought of as a directional control valve, but it does stop flow in one direction and allow flow in the opposite direction. They range from the simple, two-position, single, direct solenoid, spring-return valve shown in Figure 8-11, to the more complex three-position, double solenoid, pilot-operated, spring-centered, external-pilot supply, external drain valve shown in Figure 8-15. It protects low-pressure devices in case of through flow blockage. The problem is, if the load on the cylinder changes or there is any slight leak in the piping or seals, it will not hold position once it stops. A water faucet allows flow or stops flow by manual control. In the situation shown here, it is obvious the relief valve will open before reaching a pilot pressure high enough to open the pilot-operated check valve. Make sure the valve is capable of backpressure at the tank port. Read Chapter 17 for a full explanation of this regeneration circuit. (See the section on Check Valves as Directional Valves.). Changing the pilot line in the field with assistance from the suppliers catalog is quite easy. Figures 8-70, 8-71, and 8-72 show a typical pilot-operated check valve circuit that prevents cylinder creep. Except for bleeder type control circuits, a limit valve requires at least a 3-way function. A load induced pressure of 1508 psi plus 142 psi from pilot pressure acts against the poppet in the pilot-operated check valve. Both pauses that occur when extending and retracting are eliminated by using the dual-inlet feature of a 5-way valve. Speed-control mufflers in the direct-mounted 3-way valves independently control the extend and retract speed of the cylinder. Energizing the solenoid, or extending, allows flow to move to the cylinder port and it extends. A check valve with a 25-125 psi spring makes an inexpensive, non-adjustable, flow path for excess fluid. 2022 Endeavor Business Media, LLC. In Figure 8-1, the active box shows blocked ports, or a closed condition, while the upper box shows a flow path. Figure 8-10. Placing the pilot-operated check valve in the line after the counterbalance valve would require neither an external drain nor decompression feature. A 2-way valve stops flow or allows flow. An external force can pull against the trapped oil in the cylinder and cause damage or failure without relief protection. The circuit in Figure 8-70 shows a horizontally mounted, non-leaking cylinder, positively locked in place any time the directional centers. Speed control mufflers give individual meter-out speed control in each direction of travel. If this circuit did not have externally drained pilot-operated check valves, the cylinder would operate in jerks or not at all when the directional valve shifts. Poppet design valves normally take pressure at the inlet port only. This site requires you to register or login to post a comment. An inching circuits repeatability is usually not closer than 1 in. In the example cited, a 15,000-lb platen pulling against a 26.51 square inch rod end area gives a 566 psi load-induced pressure. The pause comes from weight pushing down along with force from air pressure on the cylinder rod end. Blocking the exhaust of a 3-way is usually not necessary for most 2-way applications. One (NO) and one (NC) 2-way directional valve piped to the cap end cylinder port allows fluid to enter and exhaust from it. When the directional valve shifts, as seen in Figure 8-55, there is a pause before the cylinder extends. Using a directional valve with blocked A and B ports in center condition, may keep the pilot-operated check valves open and allow cylinder creep. In some actuator applications it is important to know what the valve port flow conditions are as it shifts. To make a high flow 2-way valve from a 4-way valve try the circuit shown in Figure 8-34. When pressure in the head end of the cylinder reaches about 15 psi, as shown in Figure 8-56, the cylinder starts to move. The weight-to-cylinder force ratio and the rate of cylinder travel speed control the length of pause. When the valve shifts, flow is fromP through B to system and from A through T to system. The pilot-operated check valve in the line to the rod end opens by pump flow like any check valve. Use a spool type valve here also. Lowering pressure at the rod end port is another way to save air with dual 3-way valves mounted directly to the cylinder port. This valve shifts from an actuator moving flow path to center condition for certain special circuits. Many valves use the two exhaust ports for speed control mufflers. Figure 8-28 shows a 3-way valve, used to select Pr. In Figure 8-80, a running away load had a drifting problem with only the counterbalance valve installed. Normally, input air goes to the center port of the side with three ports. Here it is in the line feeding the directional valves, other times it is in the tank line. Pilot operated directional valves commonly use a check valve in the tank or pump line to maintain at least 50-75 psi pilot pressure during pump unload. Some 3-way valves select fluid flow paths as in Figure 8-9. For a full time regeneration circuit, pipe the 4-way as shown in Figure 8-35. These are two of the three actions a directional control valve can perform. Pump output is available for other valves and actuators with this center condition. Open crossover stops shock while the spool shifts, while a closed crossover reduces actuator override travel. To avoid running the pump dry, its shutoff should have a limit switch indicating full open before the electrical control circuit will allow the pump to start. It is best to control the cylinder shown here with a counterbalance valve. The anti-cavitation check valve has no effect during any other part of the cycle. The exhaust ports often have speed control mufflers to reduce noise and control the amount of exhaust flow. Use this spring-centered, single solenoid valve in control circuits for special functions. At about 150 psi the poppet in the pilot-operated check valve opens and allows oil from the cylinder rod end a free flow path to tank. However, this void can cause erratic action when the cylinder cycles again, so install an anti-cavitation check valve. When a cylinder retracts to pick up another part, it often has to go too far to make sure it is behind the part. Use a spool-type valve for this operation. As pilot pressure increases, down force and rod end pressure escalates also. A valve rated at 10 gpm is now good for 20 gpm with little or no increase in pressure drop. At first sight it looks as if this circuit might work. This is the normal center condition for the solenoid valve on a solenoid pilot-operated, spring-centered directional valve. Palm-button-operated 3-way diverter valve. As will be explained later, dual exhausts used for speed-control mufflers or as dual-pressure inlets make this configuration versatile. Also use dual inlet piping to make an air cylinder operate quickly and smoothly. Figure 8-10. This requires a 3-way valve. Most air cylinders stroke from one extreme to the other. The pilot piston must have sufficient pressure to open the poppet with 566 psi pushing against it. Both symbols in Figure 8-49 represent the same valve. This pressure would have been about 1200 psi while the cylinder was retracting, but quickly drops to zero when the directional valve centers. Once this normally closed valve shifts, it passes a signal on to continue the cycle. The at-rest box or the normal condition is the one with the flow lines going to and from it. System pressure goes into the external pilot supply port and a plug shuts off the internal pilot port. The higher cycle rate results in greater savings. In normal condition, fluid in the control circuit exhausts through the exhaust port. Figure 8-44 shows a tandem center valve. This move eliminates the need for externally drained pilot-operated check valves. Valve operators come in different types. Using directional controls in ways other than normal is a common practice. Figure 8-2 shows a "normally open" (NO) 2-way directional valve. The speed of exhausting air controls how fast the cylinder moves once it starts. If the valve is solenoid pilot-operated, the supply to the pilot valve usually comes from port #1. The open center condition unloads the pump and allows the actuator to coast to a stop or float. Adding an externally drained pilot-operated check valve between the counterbalance valve and the cylinder holds it stationary. When the directional valve returns to normal, as shown in Figure 8-58, down force quickly changes to 1240 lb. The only difference is an extra tank or exhaust port. The pilot piston on most pilot-operated check valves has an area that is three to four times that of the poppet. A common use for a drilled check valve is as a fixed, tamper proof, flow control valve. As pilot pressure builds to the 500 psi required, pressure against the poppet in the pilot-operated check valve increases at twice the rate. Figure 8-65 shows some other applications for check valves. With the flow control after the pilot-operated check valve, use one with an external drain. The delay could be three to four seconds in extreme cases. Energize and de-energize all four valves simultaneously to cycle the cylinder and keep from wasting fluid. Either valve moves the cylinder to its opposite position when activated. Energizing the solenoid on this valve stops fluid flow. Because air usually exhausts to atmosphere, the extra exhaust port is no problem. The center condition of a 3-position valve can unload a pump, open actuator ports to tank for free movement, block actuator ports to stop movement, give regeneration, or work in combinations of these functions. Figure 8-49 shows an all ports blocked center condition, solenoid pilot-operated valve, as a simplified and complete symbol. If the crossover condition is important to the circuit or machine function, show it on the schematic drawing. The three sequences show a 4-way valve in action. Following are schematic symbols for commonly used directional control valves. Make sure the valve is capable of pressure in all ports before applying it to some of these circuits. Energizing and holding a directional valve solenoid causes the cylinder to move. This oscillating movement would continue until the cylinder competes its stroke. All return lines though, can have a check valve piped as shown in Figure 8-65. The 5-way valve is found most frequently in air circuits. Shifting the 2-way valve, or extending, sends fluid to the cylinder cap end and it extends. The cylinder immediately runs away, pressure in cylinder cap port drops, the pilot-operated check valve closes fast and hard, and the cylinder stops abruptly. if travel speed is slow. A 3-way valve has three working ports. With a 3-way directional valve at both ports, both extend and retract strokes of a double-acting cylinder have force. The first four account for about 90% of all 3-position hydraulic valves in use. If the reverse flow outlet port backpressure cannot be eliminated, then specify a pilot-operated check valve with an external drain. Pilot operated check valves positively lock the cylinder but are invisible to the electric control circuit. Figure 8-62 shows the symbol for a plain check valve. A moving machine member usually operates this type valve. The following will describe how pilot-operated check valves can cause problems in some applications. Shown are circuits that require a pilot-operated check valve to have external drain and/or decompression capabilities. For air valves, atmosphere is the tank, so exhaust piping is usually unimportant. Most hydraulic directional control valves are 3-position. Also check with the manufacturer if there is any doubt about the valves performance in an unusual application. Connect pump flow to the normal inlet port and its outlet port, then connect the other outlet port to the normal tank port and on to the system. The float center valve of Figure 8-43 allows the actuator to float while blocking pump flow. Another example later in this section shows dual exhaust ports piped with different pressures to save air. Directional control valves perform only three functions: These three functions usually operate in combination. A water faucet is a good example of a 2-way valve. The pilot-operated check valve in the line to the cap end opens by pump flow like any check valve. When solenoid B on the directional valve shifts, as seen in Figure 8-72, the cylinder retracts. Placing a flow control after the pilot-operated check valve causes backpressure against its pilot piston and could keep it from opening at all. Pilot-operated check valves
When it is necessary to lock out one of two circuits while the other one operates, the hookup in Figure 8-29 works well. A 2-way valve makes a blow-off device or runs a fluid motor in one direction. If the cylinder extends with only one valve actuated, it would be slow and waste a lot of air. With the head end regulator set at 15 psi, down force from air pressure and the load is almost offset by up force. This means it will take approximately 141-188 psi at the cap end cylinder port to open the poppet for reverse flow. A tandem center valve lets the pump unload while blocking the cylinder ports. 2-way directional control valves
If it is only necessary to keep the cylinder from moving in one direction, one pilot-operated check valve will suffice. A cylinder with these conditions falls and stops all the way to the work unless it meets enough resistance to keep it from running away. It also works well for pilot-operated check valve locking circuits or with counterbalance valves. Figure 8-25 shows a weight-returned, single-acting cylinder powered by a 2-way in the at rest condition. When outside forces move the cylinder, fluid from the rod end goes to the cap end, but is not enough to fill it. As long as down forces exceed up force, the cylinder will not move. In Figure 8-53, the 5-way has a dual inlet instead of dual exhaust. One use is the blow-off function shown in Figure 8-22. This sets a pressure differential across the piston before the valve shifts. shows a circuit that operates a single-acting cylinder with 2-way valves. When using an on-off type solenoid valve, a fast moving cylinder stops abruptly when the directional valve centers. Using a decompression poppet made it easy to open the main check poppet against the high load induced pressure. Some 3-way valves have a third position that blocks flow at all ports. Metal-to-metal fit spool valves will not hold a cylinder for any length of time. Figures 8-41 to 8-46 show several commonly used 4-way hydraulic valve center conditions. When the cylinder moves slowly, a repeatable mid stroke position of plus or minus an inch might be possible. Figure 8-13 shows another unusual 4-way configuration. After shifting the valve, or extending, air flows from #1 port through #2 port to the cylinder cap end. Flow from the small decompression poppet is not enough to handle cylinder flow. This requires a high pilot pressure to open the pilot-operated check valve. Figures 8-54 to 8-61 show another reason for using dual pressure inlets. A check valve with a low-pressure spring, called an tank isolation check valve, on each return line allows free flow to tank, while blocking flow out of it. The exhaust port on a 3-way valve lets fluid in the cylinder escape to atmosphere. When there is much backpressure on the outlet of a pilot-operated check valve, it is best to use one with an external drain. This flow control valve is not pressure compensated. The circuit in Figure 8-24 works well for electrically unloading a pump for easy start up and/or reduced heat generation. If the pilot-operated check valve did not have an external drain, backpressure from the counterbalance valve can force it shut when the cylinder starts moving. It moves up smoothly and steadily as long as the load remains constant. A pair of 2-way valves at each cylinder port gives a power stroke in both directions. Use a spool type valve for this hookup, since it takes pressure at any port without malfunction. A 3-position, 4-way valve stops an actuator or allows it to float. To stop an air cylinder in mid-stroke, use the 3-position valve shown in Figures 8-19 through 8-21. The cylinder would extend with a decompression poppet, but at a very slow rate. Flow from the cylinder rod end goes to #4 port and exhausts through #5 port. The cylinder would only return if the valve, cylinder seals, or pipe connections leak. Figure 8-52 shows the normal hookup of a 5-way valve. Adding a flow control between the cylinder and pilot-operated check valve is one way to keep it from running away. When the valve shifts, as seen in Figure 8-60, down force is 720 lb and up force is 800 lb. Want to start the conversation? Because oil must return to tank, it is convenient to connect the dual tank ports to a single return port. Vertically mounted cylinders with down acting loads always creep when using a metal-to-metal fit spool valve. A 5-way valve performs the same function as a 4-way valve. This saves piping time and the cost of flow control valves. The heavier the weight and the slower the cylinder speed, the longer the pause. Figures 8-47 to 8-48 show what is commonly referred to as the crossover or transition condition of a spool. Most hydraulic valves are a metal-to-metal fit spool design, so do not depend on the cylinder setting dead still with a tandem center spool. See Figures 8-34 to 8-36 for some uncommon uses of 4-way directional control valves. With externally drained pilot-operated check valves, the cylinder is easy to control at any speed. In the shifted condition there is flow from inlet to outlet. All rights reserved. However, the reason for installing the pilot-operated check valve was to stop drifting. The cylinder sits still unless there is an outside force trying to move it. With the pilot-operated check valve after the counterbalance valve, the counterbalance valve must have an external drain. An external drain indicates there is internal leakage, so the drift problem may decrease -- but would not go away. Many of the circuits in this manual show standard check valves in use. About 90% of air circuits use this type of valve. When the pilot-operated check valve closes, pressure at the cap end cylinder port again builds to 150 psi, opening the check valve, and the process starts again. The reason this might happen is the pilot piston sees backpressure from the reverse flow outlet port. Adding an externally drained pilot-operated check valve between the counterbalance valve and the cylinder will hold it stationary. Most solenoid pilot-operated valves take air from the normal inlet port to operate the pilot section. There are some circuits that need the positive shut off of a check valve but in which reverse flow is also necessary. It is difficult if not impossible to accurately stop an air cylinder any place other than at end of the stroke. Check valves as directional valves
Even with some spool type counterbalance valves, the cylinder still drifts. With a dual inlet pressure circuit shown in Figure 8-59, the cap end port has 80 psi while the rod end port is only 15 psi. Shut-off valves are the only option for lines that flow out of the tank to a pump or other fluid using device. This cylinder has nearly twice the area on the cap side as the rod side, so every 100 psi on the cap side gives about 200 psi on the rod side. Figures 8-11 to 8-15 show different configurations available in 4-way directional control valves. Deenergizing the solenoid, or retracting, lets the valve spring return to its normal condition causing the cylinder to retract. Usually, force required to return a cylinder is minimal, so lower pressure at the rod port saves energy. Figure 8-66 shows the symbol for a standard pilot to open check valve. Figure 8-41 shows an all-ports-open center condition valve. A 2-way directional valve has two ports normally called inlet and outlet. Maintenance persons always know which manual override to push during trouble shooting or setup. This load-induced pressure holds against the poppet in the pilot-operated check valve, forcing it closed. Since the example selector valve is solenoid pilot-operated, it is important to determine which port has the higher pressure. When an operator shifts the valve, it is the same as sliding the upper box down to take the place of the lower box. The problem comes when the 2-way returns to normal at the end of cycle. Figures 8-76 and 8-78 show another possible problem using a pilot-operated check valve to keep a vertical down-acting cylinder from drifting. Figures 8-5 through 8-10 show schematic symbols for 3-way directional control valves. To operate a double-acting cylinder with 3-way valves, use the hookup shown in Figure 8-32. Figures 8-22, 8-23, and 8-24 show some uses for 2-way directional control valves. This figure shows weight, cap and head end areas, and pressures at both cylinder ports. The 5-way and shuttle valve arrangement gives an exhaust path for high-pressure air when shifting to low pressure. A lot of air valve manufacturers call this #1 port. Some manufacturers make a check valve with an adjustable spring, for pressures up to 200 psi or more. It takes about 120 psi on the 10-in.2 area to slow the cylinders rapid retraction.
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