Flow Control Pneumatik

8/3/2019 Flow Control Pneumatik

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Flow Control Circuits

To control the speed of an actuator, most designers use flow controls. Air circuits normally need controlled flow becausethe plant air compressor is greatly oversized for almost any given circuit. Hydraulic circuits usually have a dedicatedpower source sized to meet the cycle time so flow restrictors are unnecessary.

Flow controls always generate some heat in hydraulic circuits, so consider some other method of controlling actuatorspeed where possible. The circuit examples in this chapter explain the types of flow-control systems and how to applythem.

Figures 10-1 and 10-2 show symbols for fixed orifices, rudimentary components that will control flow. A fixed orifice canbe a simple restriction in a l ine or a factory-preset control with pressure compensation and a bypass. Their low cost andthe fact that they are tamper-proof are two main reasons for using fixed orifices.

Fig. 10-1: Fixed orifice.

Fig. 10-2: Pressure- and temperature-compensated fixed orifice.

Use the needle valve shown in Figure 10-3 when control of fluid flow in both directions is necessary. Add the check valvearrangement shown in Figure 10-4 when a needle valve needs pressure compensation in both directions. These checkvalves, sometime referred to as bridge rectifiers, force fluid to flow through the needle valve in the same directionregardless of actuator movement. (Remember, pressure compensation only works in one direction of flow.)

Fig. 10-3: Needle valve.


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When talking about flow-control hardware, some manufacturers use different terminology. Normally the

term flow controlrefers to an adjustable needle valve with an integral bypass, as pictured in Figure 10-5. This type of

flow control meters flow in one direction and allows free flow in the opposite direction. However, some companies

identify the flow control in Figure 10-5 as a throttle valve. These companies say a flow control must have a bypass and

be pressure-compensated as shown in Figure 10-6.

Fig. 10-5: Flow control with bypass (or throttle valve).

When a hydraulic actuator needs accurate speed control, use a pressure-compensated flow control. System

pressure fluctuations or load changes will cause actuator velocity to change. Regardless of the cause of the pressure

differences, flow across the orifice will change unless the flow control is pressure compensated. Only use a pressure-

compensated valve when very accurate speed control is needed because its cost is as much as six times that of a non-

compensated valve.

Fig. 10-6: Pressure- and temperature-compensated flow control.

Fig. 10-4: Pressure- and temperature compensated needle valve.


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There are three types of flow control circuits from which to choose. They are: meter-in, meter-out, and bleed-off (orbypass). Air and hydraulic systems use meter-in and meter-out circuits, while only hydraulic circuits use bleed-off types.Each control has certain advantages in particular situations.

Figure 10-7 shows a meter-in flow-control circuit for a cylinder. Notice that a bypass check valve forces fluid through anadjustable orifice just before it enters the actuator. Figure 10-8 shows the circuit while the cylinder is extending with thepressures and flows indicated. With a meter-in circuit, fluid enters the actuator at a controlled rate. If the actuator has a

resistive load, movement will be smooth and steady. This is because hydraulic fluid is almost incompressible.

Fig. 10-7: Meter-in flow-

control circuitat rest.

In pneumatic systems, cylinder movement may be jerky because air is compressible. As air flows into a cylinder, asdepicted in Figure 10-9, pressure increases slowly until it generates the breakaway force needed to start the loadmoving. Because the subsequent force needed to keep the load moving is always less than the breakaway force, the airin the cylinder actually expands. The expanding air increases the cylinder speed, causing it to lunge forward. The pistonmoves faster than the incoming air can fill the cylinder, pressure drops to less than it takes to keep the cylinder movingand it stops. Then pressure starts to build again to overcome breakaway force and the process repeats. This lungingmovement can continue to the end of the stroke. A meter-out circuit is the best control to avoid air-cylinder lunging.

Fig. 10-8: Meter-in flow-control circuitwith cylinder


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extending.

Figure 10-7 shows the components in a meter-in flow-control circuit. Notice that a bypass check valve forces fluidthrough an adjustable orifice just before it enters the actuator.

Figure 10-8 shows an extending hydraulic cylinder and indicates the pressures and flows in various parts of the circuit.With a meter-in circuit, fluid enters the actuator at a controlled rate. If the actuator has a resistive load, movement will besmooth and steady with a hydraulic circuit. This is because oil is almost non-compressible.

Fig. 10-9: Pneumatic meter-in

flow-control circuitwith

cylinder extending

In the case of an air system, pressure builds slowly and cylinder movement may be jerky. This jerky movement comesfrom compressibility of the air. As air enters the cylinder, Figure 10-9, pressure builds slowly until it generates thebreakaway force to start the piston moving. Because moving force is always less than breakaway force, air in thecylinder expands. The expanding air speeds up cylinder movement, causing it to lunge forward. This increased speedmoves the piston faster than the incoming air can fill the space behind it, so pressure drops to less than it takes to keepit moving and the cylinder stops. After the cylinder stops, pressure starts to build again to develop breakaway force andthe process repeats. This lunging movement can continue to the end of the stroke. A meter-out circuit is the best controlfor an air cylinder.


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control circuit for overrunning

loadwith cylinder extending.

If the actuator has an overrunning load, such as in Figure 10-10, a meter-in flow control will not work. When thedirectional valve shifts, the vertical load on the cylinder rod makes it extend. Because fluid cannot enter the cylinderscap end fast enough, a vacuum void forms there. The cylinder then free falls, regardless of the setting of the meter-inflow adjustment. The pump will continue to supply metered fluid to the cap end of the cylinder and will eventually fill thevacuum void. After the vacuum void fills, the cylinder can produce full force.

3-speed meter-in circuitThe schematic diagram in Figure 10-11 shows a 3-speed, meter-in flow-control circuit using modular valves. Energizingdifferent combinations of solenoids changes cylinder speed at will. To get additional speeds, add more tandem-centerdirectional valves and flow-control modules like station DV01. The limiting factor would be pressure drop through thevalves tandem centers. Using a bar manifold and modular valves eliminates many fittings and possible leak sources. Asin all meter-in circuits, the pressure-compensated pump shown here generates less heat than a fixed-volume pump.

Fig. 10-11: Three -speed

meter-in flow-control circuit

using modular valves mounted

on bar manifoldat rest with

pump running.

To extend the cylinder at fast speed, shift the valves as shown in Figure 10-12. Energizing solenoid A2on directionalvalve DV02sends fluid through the meter-in flow-control module directly under it to the cylinders cap end. This conditionis always set for the fastest extension speed. Solenoid A2stays energized for all extension speeds.


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on bar manifoldextending at

fast speed.

Energizing solenoid B1 on directional valve DV01, Figure 10-13, sends pump flow through the right-hand flow control in

the module underneath it. This will produce a slower speed -- here called middle speed. Either solenoid A1 or B1 could

produce middle speed, making the opposite solenoid produce slow speed. As with fast speed, the cylinder speed is

variable, but never faster than fast speed.




on bar manifoldextending at

middle speed.

By actuating solenoid A1 on directional valve DV01, fluid passes through the left-hand flow control in the moduleunderneath it. This will produce a different speed here called slow speed.

The cylinder can retract rapidly or at any of the same slower-flow settings as above. By energizing solenoid B2ofdirectional valve DV02, flow will pass through the opposite meter-in flow control. This means fast-speed retracting canbe different from the extending speed. The middle and slow speeds will be at the same flow rate as extension. Cylinderspeed during these reduced flows will be somewhat faster due to the decreased rod-end area.

A simple manifold can give multiple speeds inexpensively, while eliminating potential plumbing leaks.

Note: Select a valve for DV01 that can withstand tank-line backpressure.


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Speed changes with this meter-in circuit will be smooth because the cylinder can coast while slowing down. (It also ispossible that the cylinder could cavitate when slowing down, so an anti -cavitation check valve may be needed.)




on bar manifold

extending atslow speed.

Meter-in flow control of a running-away load

Figures 10-15 and 10-17 show a running-away load controlled by a meter-in circuit and counterbalance valves. Themeter-in flow control works exactly as explained previously, while a counterbalance valve makes cylinder movementresistive. (See Chapter 5 for an explanation of counterbalance valves.) Figure 10-16 pictures a bleed-off flow controlcircuit that gives the same results as a meter-in circuit -- without most of the heat generation.


control circuit for vertical

cylinder extending with over-

running load. Counterbalance

valve prevents load from


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falling.

A meter-in flow control circuit for an over-running load is not the normal design but it may be necessary when the circuithas a pressure switch or a sequence valve.

In any meter-in circuit with a fixed-volume pump, the wasted energy will heat the fluid. In the circuit in Figure 10-15,almost 95% of the power used by the system becomes heat. In this circuit, f luid from the pump enters the cylinder as fastas the meter-in flow control allows. A counterbalance valve at the rod-end port keeps the cylinder from running away asit extends.

Fig. 10-16: Bleed-off or bypass

flow-control circuit for vertical

cylinder extending with

overrunning load.

Counterbalance valve prevents

load from falling.

.

To save energy while using a fixed-volume pump, the circuit in Figure 10-16 works well. A bleed-off or bypass flowcontrol greatly reduces the amount of wasted energy. With a bleed-off circuit, excess pump flow goes to tank at thepressure required to move the cylinder. In the circuit in Figure 10-16, pressure would be approximately 50 psi as thecylinder extends. The extension-stroke speed is still infinitely variable, while pressure in the cylinder cap end l ine nevergoes higher than that caused by load resistance.


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control circuit with load-

sensing pressure-compensated

pump and vertical cylinder

extending with over-running

load. Counterbalance valve

prevents load from falling.

Figure 10-17 shows another way to reduce energy loss and heat generation using as load-sensing, pressure-compensated pump in conjunction with a meter-in flow-control circuit. A sensing line, teed into the cylinder cap-end lineafter the meter-in flow control, transmits pressure information to the pump. With a load-sensing, pressure-compensatedpump, pressure at the pump outlet stays 150 to 200 psi higher than the load until it tries to go above the compensatorspressure setting. The only energy loss here is the 150- to 200-psi pressure drop across the flow control at the volumeset. (Heat generation within a load-sensing pump circuit is explained later in this Chapter.)

When meter-in circuits are necessaryIn some cases a meter-in circuit is the only way to control the speed of an actuator -- even for pneumatic devices.Figures 10-18 through 10-21 show several instances requiring meter-in circuits.


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control circuit with pressure

switch for end-of-stroke

indication.

Many machine circuits use pressure switches to indicate when an actuator meets resistance. If pressure in the actuatorbuilds prematurely, the machine cycle gets out of phase. With the meter-in circuit shown in Figure 10-18, pressure in thecylinder will be just enough to move the cylinder and its attachments until it reaches the load. With a meter-out circuit,pressure in the cylinder cap end would build as soon as the directional valve shifts, tripping the pressure switch longbefore the cylinder contacts the load.

Fig. 10-19: Meter-out flow-

control circuit with pressure

switch for end-of-stroke

indication.

However, it is possible to use a pressure switch with a meter-out flow control circuit. In Figure 10-19, notice that thepressure switch is on the cap-end line to the cylinder. It passes a signal when the cylinder cannot extend farther. Noticealso that the pressure switch setting is very low (50 psi). While the cylinder is moving, oil flowing from the cylinder head-


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end port remains pressurized by the meter-out flow control. When the cylinder contacts the load, pressure in the head-end port drops, actuating the pressure switch and sending a signal. (Use a normally open, 3-way, pilot operated, spring-return directional valve in place of the pressure switch to produce an air or hydraulic pilot signal.)

The pressure-decaying circuit in Figure 10-19 works well in pneumatic circuits because metering air flow out providesgood control. Several companies furnish air logic elements designed specifically for this type of circuit.

Fig. 10-20. Meter-in flow-

control circuit with sequence

valve for end-of-stroke

indication.

Sequence valves often are used to start a second actuator after a cylinder meets resistance and builds tonnage. Withthe meter-in flow control shown in Figure 10-20, pressure in the cylinder cap end increases when -- but not before -- the

cylinder contacts the work. Pressure at the sequence valves inlet stays lower than its spring setting while the actuato r ismoving. (With a meter-out circuit, pressure in the cylinders cap end would go to system pressure when the directionalvalve shifts. Because of this, flow to the secondary circuit would take place prematurely.

The vertical single-acting, weight-return cylinder shown in Figure 10-21 needs meter-in control as it extends. This will bethe case even if it is a pneumatic cylinder where meter-out control works best. For a different retraction speed, use asecond meter-out flow control (as shown in Figure 10-21).

Fig. 10-21. Meter-in flow-

control circuit for a single-


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acting cylinder extending.

Note: A pressure switch or a sequence valve will only indicate that pressure has reached a certain level. What causedthe pressure build-up is unimportant. If the actuator positively has to be at a certain position before the next functionstarts, do not use a pressure-sensing device. Always use a limit switch. (Use a pressure switch or sequence valve inseries with the limit switch if cylinder position and force are both important.)

Action of a meter-in air circuit with a varying loadWhen using a meter-in circuit on an air cylinder with a variable load, movement will not be consistent. Depending on therequired range of forces, movement may be smooth, the cylinder may over-speed, or it may even stop. In Figure 10-22,the cylinder is moving smoothly at a pressure difference of 30 psi. (It takes 30 psi in the bore size being used to generatethe force to move the load.) If the load remains constant, the cylinder can -- and probably will -- advance smoothly.

Fig. 10-22. Meter-in pneumatic

flow-control circuit for loadedcylinder extending slowly and

smoothly toward a second

load.


flow-control circuit as cylinder

contacts second equal load.

Cylinder stops while cap-end

pressure builds to produce


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force required.

When the load doubles, as in Figure 10-23, 30 psi is not enough to keep the cylinder moving. At this point the cylinderwill stop until pressure in the cap end reaches 60 psi. (The meter-in flow setting determines how long this takes.)


flow-control circuit moving

both loads after cap-end

pressure reaches load

requirement.

Once pressure in the cylinder cap end reaches 60 psi, Figure 10-24, the cylinder starts moving again. If the higher loadstays constant, movement is steady.

When the second load is reduced, as diagrammed in Figure 10-25, 60 psi in the cap end is more pressure than needed.This high pressure will cause the cylinder to lunge forward and, as a result, pressure in the cap end will start to decay.The amount of lunge is in direct proportion to the total volume of air in the cylinders cap end and the piping leading to it.Next, as Figure 10-26 shows, once decompression reaches 30 psi, the cylinder slows to its original speed.


flow-control circuit as second

equal load drops off. Cylinder

lunges forward as pressure in

cap end decreases.


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If this stop/lunge/over-speed problem is intolerable and air is the required power source, add some method of oil controlto the circuit. (See Chapter 3 on air-oil circuits.)

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Flow Control Pneumatik

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