Directional Control Valve Symbols Pdf
The following will describe how pilot-operated check valves can cause problems in some applications. Adding an externally drained pilot-operated check valve between the counterbalance valve and the cylinder will hold it stationary.
Faster travel speeds give less control. Some check valves have a removable threaded plug in them that may be drilled to allow controlled flow in the reverse direction. Using two exhaust ports makes the valve smaller and less expensive.
The cylinder in this example has a heavy weight pulling against the rod side. Adding an externally drained pilot-operated check valve between the counterbalance valve and the cylinder holds it stationary.
Symbols show the methods of actuation, the number of positions, the flow paths and the number of ports. Other pneumatic components also have schematics or symbols, but these generally do not require as much explanation as those for the valves. Contact with a machine member opens it.
Figure shows the symbol for a standard pilot to open check valve. Now the load drops rapidly until air pressure in the cap compresses to approximately psi.
As shown in these figures, dashed lined boxes show crossover condition. System pressure goes into the external pilot supply port and a plug shuts off the internal pilot port.
Energizing and holding a directional valve solenoid causes the cylinder to move. The weight-to-cylinder force ratio and the rate of cylinder travel speed control the length of pause.
Using a decompression poppet made it easy to open the main check poppet against the high load induced pressure. The circuit in Figure also shows an anti-cavitation check valve for the cylinder with a relief valve to protect it from over pressure.
Releasing the palm button in Figure allows the valve spring to return to the normal stop flow condition. Energize and de-energize all four valves simultaneously to cycle the cylinder and keep from wasting fluid. Maintenance persons always know which manual override to push during trouble shooting or setup. The anti-cavitation check valve has no effect during any other part of the cycle. 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.
Either valve moves the cylinder to its opposite position when activated. Figure shows the start of this condition. Flow from the small decompression poppet is not enough to handle cylinder flow. 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.
Hydraulic motors always have internal leakage so the circuits shown here will not hold them stationary. Putting low pressure on the rod side of the cylinder uses less compressor air without affecting the operation. In the past, to get this configuration, you only had to wire one solenoid of a double-solenoid, three-position valve.
On the circuit in Figure a pilot line from system pressure goes directly to the pilot valve. This means it will take approximately psi at the cap end cylinder port to open the poppet for reverse flow. 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.
BOOK 2 CHAPTER 8 Directional Control Valves
If there is a restriction causing high backpressure in the reverse flow outlet port, a standard valve may not open when applying pilot pressure. The symbols on the next page detail many of the ports, ways, and positions of common pneumatic valves.
This valve is the pilot operator for hydraulically centered directional valves or normally closed slip in cartridge valves. This pressure would have been about psi while the cylinder was retracting, rheology handbook pdf but quickly drops to zero when the directional valve centers.
BOOK 2 CHAPTER 8 Directional Control Valves
However, in the past few years, poppet type slip-in cartridge valves have been operating large bore hydraulic cylinders this way. Sometimes a port usually an exhaust port goes directly to atmosphere and there is no mechanical means for attachment of silencers, flow control valves, or any other accessories.
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