When pressure in the head end of the cylinder reaches about 15 psi, as shown in Figure 8-56, the cylinder starts to move. An inching circuits repeatability is usually not closer than 1 in. 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. Want to start the conversation? A double-acting cylinder only needs one 4-way directional valve to extend and retract it. 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. https://www.linkedin.com/company/11091630, Volvo Sees Continued Growth Opportunity in Electrification, This Week in Power & Motion: Continental Building New Hydraulic Hose Factory in Mexico, Industry Provides Insight on Powering the Future of Electric Vehicles, Bosch Rexroth Launches Range of Electrification Products for Off-Highway Equipment, This Week in Power & Motion: igus Introduces Lubrication-Free Two-Component Ball Bearing, Poclain Makes New Investments to Accelerate Electromobility and Connectivity Development, Hydrogen-Battery Hybrid System Drives World's Largest Mine Haul Truck, Embark Trucks Advances Autonomous Driving Tech with Winter Testing and New Industry Partnership. A 4-way valve pressurizes and exhausts two ports interdependently. Pump flow to the cylinder rod end builds pressure in the pilot line to the cap end of the pilot-operated check valve, causing it to fully open. Using 4-way valves
A pilot-operated check valve with the decompression feature would not help in this circuit. Figure 8-25 shows a weight-returned, single-acting cylinder powered by a 2-way in the at rest condition. The pilot-operated check valve in the line to the rod end opens by pump flow like any check valve. Shut-off valves are the only option for lines that flow out of the tank to a pump or other fluid using device. Figure 8-12 shows a single solenoid, spring-centered valve. If there are outside forces on the cylinder, it will creep when the valve centers. This saves piping time and the cost of flow control valves. The most common limit valve is a miniature 3-way like the one shown in Figure 8-30. The cylinder starts to move almost immediately and continues moving smoothly to the end. It is best to control the cylinder in this example with a counterbalance valve. Any metal-to-metal fit spool valve never fully blocks flow. Figure 8-66 shows the symbol for a standard pilot to open check valve. 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. This valve shifts from an actuator moving flow path to center condition for certain special circuits. 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. Valve center conditions perform different functions in relation to the actuator and pump. The circuit in Figure 8-70 shows a horizontally mounted, non-leaking cylinder, positively locked in place any time the directional centers. The 5-way valve is found most frequently in air circuits. Using this port connection arrangement consistently makes it is easy to wire the circuit because the electrician knows A solenoid extends the cylinder while B solenoid retracts it. Even with some spool type counterbalance valves, the cylinder still drifts. All return lines though, can have a check valve piped as shown in Figure 8-65. The load lowers smoothly and safely without lunging or bouncing, as fast as cap end air exhausts. Use a spool type valve here also. Figures 8-11 to 8-15 show different configurations available in 4-way directional control valves. As shown in Figure 8-69, a blocked center valve can actually cause a cylinder to creep forward. This load-induced pressure holds against the poppet in the pilot-operated check valve, forcing it closed. Placing the pilot-operated check valve in the line after the counterbalance valve would require neither an external drain nor decompression feature. Using directional controls in ways other than normal is a common practice. Using 3-way valves in place of 2-way valves reduces inventory cost and saves time. This valve is the pilot operator for hydraulically centered directional valves or normally closed slip in cartridge valves. Figure 8-37 shows the normal hookup of a 4-way directional valve. Some 3-way valves have a third position that blocks flow at all ports. A tandem center valve lets the pump unload while blocking the cylinder ports. A 5-way valve performs the same function as a 4-way valve. The cylinder would extend with a decompression poppet, but at a very slow rate. In normal condition, fluid in the control circuit exhausts through the exhaust port. Energizing the solenoid on this valve stops fluid flow. Placing a flow control after the pilot-operated check valve causes backpressure against its pilot piston and could keep it from opening at all. Metal-to-metal fit spool valves will not hold a cylinder for any length of time. It is best to control the cylinder shown here with a counterbalance valve. Spool valves normally take pressure at any port without malfunction. To operate a double-acting cylinder with 3-way valves, use the hookup shown in Figure 8-32. When the valve shifts, as seen in Figure 8-60, down force is 720 lb and up force is 800 lb. In the crossover or transition condition it causes very little shock. With the flow control after the pilot-operated check valve, use one with an external drain. 3-way directional control valves
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 pressure would have been about 1200 psi while the cylinder was retracting, but quickly drops to zero when the directional valve centers. A 2-way directional valve has two ports normally called inlet and outlet. This requires a 3-way valve. The reason this might happen is the pilot piston sees backpressure from the reverse flow outlet port. The pilot-operated check valve in the line to the cap end opens by pump flow like any check valve. An inline check valve stops any chance of reverse flow and is useful and/or necessary in many applications. Also check with the manufacturer if there is any doubt about the valves performance in an unusual application. Make sure the valve is capable of pressure in all ports before applying it to some of these circuits. When the valve shifts, flow is fromP through B to system and from A through T to system. Valve operators come in different types. The cylinder sits still unless there is an outside force trying to move it. In figure 8-59 to 8-61, the cylinder strokes smoothly and quickly in both directions with dual-pressure valve. Palm-button-operated 3-way diverter valve.4-way directional control valves
Deenergizing the solenoid or retracting, lets the valve shift to home position, and the cylinder retracts from outside forces. When the directional valve returns to normal, as shown in Figure 8-58, down force quickly changes to 1240 lb. Many of the circuits in this manual show standard check valves in use. The symbol in Figure 8-64 shows how to represent this in a symbol. With this circuit, system shock very quickly damages piping, cylinders, and valves. At the moment the valve shifts to extend the cylinder, down forces are up to 1240-lb while up force is only 800 lb. Directional control valves perform only three functions: These three functions usually operate in combination. 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. 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. All rights reserved. When the valve shifts to retract the fully extended cylinder, there is another problem. An external drain indicates there is internal leakage, so the drift problem may decrease -- but would not go away. In the at-rest condition there is no flow through the valve. See chapter five for the different types of counterbalance circuits. This air savings results in lower operating cost and leaves more air to run other actuators. This center condition allows pilot pressure to drop and the pilot-operated check valves to close. This move eliminates the need for externally drained pilot-operated check valves. By itself, a 2-way valve cannot cycle even a single acting cylinder. Contact with a machine member opens it. A 2-way valve stops flow or allows flow. Fixed volume pumps use this center condition. If the reverse flow outlet port backpressure cannot be eliminated, then specify a pilot-operated check valve with an external drain. With the head end regulator set at 15 psi, down force from air pressure and the load is almost offset by up force. A water faucet allows flow or stops flow by manual control. Figure 8-65 shows some other applications for check valves. Figures 8-22, 8-23, and 8-24 show some uses for 2-way directional control valves. Except for bleeder type control circuits, a limit valve requires at least a 3-way function. The all-ports-blocked center condition valve of Figure 8-42 appears to block the cylinder ports. It takes about 120 psi on the 10-in.2 area to slow the cylinders rapid retraction. (Some suppliers call their 5-way valves, 5-ported 4-ways.") In the shifted condition there is flow from inlet to outlet. To duplicate the 2-way function, block the exhaust port of the 3-way valve. Shown are circuits that require a pilot-operated check valve to have external drain and/or decompression capabilities. The pilot piston on most pilot-operated check valves has an area that is three to four times that of the poppet. The regeneration center position of the valve in Figure 8-45 pressurizes and connects both ports of a cylinder to each other. Figure 8-41 shows an all-ports-open center condition valve. A water faucet is a good example of a 2-way valve. As discussed before, reducing air pressure at the cylinder uses less compressor horsepower. Figures 8-47 to 8-48 show what is commonly referred to as the crossover or transition condition of a spool. Heat exchangers, filters, and low-pressure transfer pumps often need a low-pressure bypass or relief valve. To hold a cylinder stationary, it must have resilient continuous non-leaking seals, no plumbing leaks, and a non-leaking valve. The exhaust ports often have speed control mufflers to reduce noise and control the amount of exhaust flow. Four 2-way valves may seem to be a complex and expensive way to operate a cylinder. 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. Pump output is available for other valves and actuators with this center condition. The heavier the weight and the slower the cylinder speed, the longer the pause. Usually, force required to return a cylinder is minimal, so lower pressure at the rod port saves energy. Many valves use the two exhaust ports for speed control mufflers. Notice the pipe between the pilot-operated check valve and the counterbalance valve is at zero psi while the cylinder is held retracted. They depict air cylinder movement with conventional hookup. A standard pilot-operated check valve circuit usually has minimum backpressure at the reverse flow outlet port. Backpressure from the flow controls can push the pilot piston closed and stop the cylinder, then pressure would drop and it would start again. Palm-button-operated 3-way diverter valve. The longer the valve-to-cylinder lines are, the greater the air waste. Some manufacturers use dual 3-way valves to conserve air. Figure 8-6 depicts an all-ports-blocked, 3-way, 3-position valve. If both inlet pressures are too low to operate the valve, plumb an external pilot supply from the main air system. A vertical, up-acting air cylinder, with a heavy load, gives sluggish and jerky operation when valved conventionally. Speed-control mufflers in the direct-mounted 3-way valves independently control the extend and retract speed of the cylinder. The 5-way selector valve and shuttle valve in Figure 8-50 works where a 3-way selector may not. Figure 8-3 shows a solenoid pilot operator using solenoid-controlled pressure from the inlet port to move the working directional spool. As will be explained later, dual exhausts used for speed-control mufflers or as dual-pressure inlets make this configuration versatile. On most air circuits the cylinder does little or no work on the retract stroke. Also use dual inlet piping to make an air cylinder operate quickly and smoothly. All spool valves are five ported, but hydraulic valves have internally connected exhaust ports going to a common outlet. However, in the past few years, poppet type slip-in cartridge valves have been operating large bore hydraulic cylinders this way. The problem comes when the 2-way returns to normal at the end of cycle. Energize and de-energize all four valves simultaneously to cycle the cylinder and keep from wasting fluid. Figure 8-51 shows a pair of 5-way valves piped to act like a three way light switch. Adding an externally drained pilot-operated check valve between the counterbalance valve and the cylinder holds it stationary.
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