Prefill Valve Animation
Quiz on Chapter 10 CHAPTER 10: Directional Control Valves, part 1 CHAPTER 10: Directional Control Valves, part 5 CHAPTER 10: Directional Control Valves, part 3 CHAPTER 10: Directional Control Valves, part 4 Table of Contents Answers to Quiz 10
Pre-fill valves operate similarly to pilot-operated check valves, but they are usually much larger. Some pre-fill valves can handle flows in excess of 6000 gpm at pressure drops of less than 4 to 8 psi. Their normal function is to fill and exhaust a large bore cylinder as it travels to and from contact with the work piece. Large, high-tonnage presses -- both vertical and horizontal -- use pre-fill valves to reduce pump size while maintaining cycle time.
The cutaway view and symbol in Figure 10-8 show the construction of a typical poppet-type pre-fill valve. A large main-flow poppet seals the path between the tank and the cylinder ports. As the piston advances, vacuum in the void behind it allows atmospheric pressure to push the main-flow poppet open so fluid from the tank can fill this void. On the retraction stroke, a signal to the pilot piston pushes the main-flow poppet open so fluid can return to tank. While a pilot-operated check valve’s pilot piston is larger than the poppet it opens, the main-flow poppet in a pre-fill is much larger in diameter than the pilot piston. Thus it is impossible to open the main-flow poppet against high backpressure. This keeps decompression shock from damaging pipes and components.
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Decompression shock occurs when large volumes of fluid at high pressure are released suddenly. Because all hydraulic oil has some entrained air (bubbles so small they cannot be seen without magnification), there is a 0.5 to 1% compressibility that must be dealt with when using large-bore cylinders. On top of fluid compressibility, the cylinder tube may stretch diametrically and longitudinally. In addition, the framework that is resisting the tonnage produced also can stretch. Summing all these factors, a 50-in. bore cylinder with a 72-in. stroke can contain more than 25 gal of extra fluid at 3000 psi. If this trapped fluid suddenly has a large open path to atmosphere, its velocity at first release is such that it can break fittings, blow hoses, straighten tubes or pipe bends with relative ease. Releasing this same trapped fluid in a controlled manner over a few seconds dissipates the excess energy and no damage is seen.
The plain pre-fill valve might be used on smaller cylinders or circuits that have other means for decompressing. The pre-fill valve with decompression has a small poppet in the large poppet that is easy to open at high pressure but will not allow the high flow that causes decompression shock. This decompression poppet usually has a means to adjust how fast the cylinder decompresses.
Another pre-fill valve design is the sleeve type that must be externally shifted open and closed. Both designs give the same results even though their operation is different. (See Chapter 4 for a cutaway view and symbol of a sleeve type pre-fill valve.)
Breather Valve Working Principe
The circuit in Figure 10-9 operates a vertical single-acting hydraulic ram press with pullback cylinders for the retraction stroke. The press has a poppet-type pre-fill and gets a fast stroke from only filling the pullback cylinders during the approach stroke. A sequence valve keeps pump flow from going to the ram until pressure reaches a preset level.
During the approach part of the stroke, atmospheric pressure pushes fluid into the large-bore ram through the pre-fill valve because there is vacuum behind the extending ram. When it contacts the work, the ram stops and the pre-fill valve closes. Pressure starts to rise and when it is high enough to open the sequence valve, pump flow goes to the pullback cylinders and the ram. Extension speed slows and tonnage increases to do the work required.
A signal that the work is complete shifts the directional control valve to send pump flow to the rod ends of the pullback cylinders and to the pilot signal of the pre-fill valve. The pre-fill valve’s pilot piston moves forward and contacts the decompression poppet. This lets trapped fluid flow out at a controlled rate. Pressure in the ram drops quickly and smoothly. When pressure is low enough, the pilot piston opens the main poppet to let fluid from the ram return to tank. When the ram loses pressure, the pullback cylinders can raise the platen and push fluid from the ram back to tank.
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Directional control valves are specified generally by the number of ports or ways (lines attached to the symbol’s box) and the number of positions (boxes or envelopes in the symbol) they have. Other information about them includes whether they are normally closed (not passing fluid), normally open (passing fluid), how they are operated (solenoid, manual, or spring) and other features such as manual overrides, drain ports, pilot ports, etc.
(According to this method of specifying, check valves and pre-fill valves would be 2-way valves because they have two ports. However, because these valves are basically single function and have infinitely variable flow paths, their symbols and terminology do not follow general directional control valve rules.)
Figure 10-10 shows the symbol for a 2-way directional control valve and how it could function in a circuit. Notice the symbol has two boxes (or envelopes) to indicate two positions. Each position is a flow path. The box with flow lines coming to it is the normal or at-rest position of the valve. The normal or at-rest position is usually at the spring end of a spring-return valve as seen in the figure.
Prefill Cum Exhaust Valve
The circuit at rest in Figure 10-10 illustrates how a schematic drawing shows the component symbols for the system builder or troubleshooter. Valves, actuators, flow paths and line connections are all shown according to the ANSI or ISO graphic symbols that were explained in Chapter 4. To understand how the circuit operates, a person must be able to read the symbols and know how they represent a piece of hardware. The valve in this circuit is 2-way, 2-position, direct solenoid-operated, spring return, normally closed. The diagrams to the right of the circuit at rest show how the directional control valve shifts to its second position and ports fluid to the cylinder. In the real world, this is done in a person’s imagination . . . and can be confusing when several valves are working simultaneously. In the diagram it is easy to see that with the solenoid energized, the normally open box moves in line with the input flow and sends fluid to the cylinder. The arrow in the normally open box shows flow from inlet to cylinder port, causing the piston to extend. If the solenoid is de-energized, the spring returns the valve to the circuit at rest condition and the cylinder stops in its last position.
Two-way valves cannot have more than two positions because they can only stop or allow fluid flow. It is easy to see that a 2-way directional control valve will not operate a single-acting cylinder. These valves are only good for operations that require an on-off supply. As shown in the bottom half of Figure 10-10, two 2-way valves are needed to control a single-acting cylinder. A double-acting cylinder needs four 2-way valves to control it. There are both normally closed and normally open valves in these circuits.
Figure 10-11 shows how 3-way valves can replace 2-way valves and make a machine simpler. This circuit at rest has a cylinder powered by a 3-way, 2-position, solenoid pilot-operated, spring-return, normally closed directional control valve. Because this valve has a flow path from the pressure port to the cylinder port and from the cylinder port to atmosphere, it can control a single-acting cylinder. The diagrams to the right show that when the solenoid is energized, the cylinder extends under power. The next schematic diagram shows the cylinder retracting from external forces with the solenoid de-energized.
Atrial Fibrillation And Atrial Flutter
Two 3-way valves are needed to power a double-acting cylinder as shown in Figure 10-11. The double-acting palm button activates this circuit. The valve on the cap end is normally closed and the valve on the head end is normally open. This is a simple anti-tie down circuit, but is not OSHA safe because one palm button can be depressed before the second one and the cylinder will move. OSHA requires that both buttons be operated concurrently to make the cylinder extend. It does meet the anti-tie down requirement because the cylinder will not retract until both palm buttons are released.
The double-acting inching circuit in Figure 10-11 uses two 3-way, 3-position, spring-centered valves to make it possible to stop the cylinder at any point in its stroke. A 3-way valve can have a third position to perform another function. The pictured center condition has all ports blocked, which stops flow at all ports. This is the center condition normally found on a 3-way valve.
Note: pneumatic inching circuits cannot stop and hold a load consistently. Any change in speed, load, or pressure can produce a different stopping position. About plus or minus
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