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Orifices — A simple orifice in the line, Figure 1(a), is the most elemen-tary method for controlling flow. Note that this is also a basic pressure control device. When used for flow control, the valve is placed in a series with the pump. An orifice can be a drilled hole in a fitting, in which case it is a fixed orifice; or it may be a calibrated needle valve, in which case it functions as a

Eight basic configurations of valves can be used to control flow rate according to fluid volume, weight, or mass.

Flow-control valves

(b) (a)

Variable orifice

Fixed orifice

Inlet

Bypass Regulated flow Handwheel

Fig. 1. Simple fixed orifice (a) and vari-able orifice (b) flow controls.

Fig. 2. Flow regulator adjusts to variations in inlet and output

pressures. Fig. 3. Bypass flow regulator returns excess flow from pump to

tank.

conditions to accuracy’s of 0.5%, Fig-ure 4. PressFig-ure-compensated, variable flow control valves are available with integral free-reverse-flow check valves and integral overload relief valves.

Pressure- and temperature-com-pensated, variable flow valves — Be-cause viscosity varies with tempera-ture, so does the clearance between a valve’s moving parts. For this reason, output of a flow control valve may tend to drift with temperature changes. An attempt has been made to compensate not only for such temperature varia-tions, but pressure variations as well, Figure 5. Temperature compensators adjust the control orifice setting to off-set the effects of viscosity changes caused by temperature fluctuations of the fluid. Pressure compensators adjust the control orifice for pressure changes, as described above.

Demand-compensated flow con-trols — Flow concon-trols are available to bypass excess system flow to a sec-ondary circuit, Figure 6. Controlled flow rate is ported to the primary cir-cuit. Bypass fluid can be used for work functions in secondary circuits without affecting the primary one. There must be flow to the primary one. There must be flow to the primary circuit for this type of valve to function: if the primary circuit is blocked, the valve will cut off flow to the secondary circuit.

Priority valves — A priority valve, Figure 7, is essentially a flow control valve which supplies fluid at a set flow rate to the primary circuit, thus func-tioning like a pressure-compensated flow control valve. Flow in excess of that required by the primary circuit by-passes to a secondary circuit at a pres-sure somewhat below that in the pri-mary circuit. Should inlet or load

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Drain

Compensator piston

Adjustable orifice

Temperature-sensitive element

Fixed orifice

Secondary circuit Primary circuit

Pilot pressure

Pilot pressure Circuit 1 Circuit 2

Fig. 4. Pressure-compensated, variable flow control valve adjusts to varying inlet and load pressures.

Fig. 5. Pressure-, temperature-compensated, variable flow con-trol valve adjusts concon-trol orifice settings to offset effects of vis-cosity changes.

Fig. 6. Demand-compen-sated flow control by-passes full pump output to tank during idle portion of work cycle.

Fig. 7. Priority valve sup-plies fluid at a set rate to a primary circuit.

Adjustable bypass orifice

Solenoid

Fig. 8. Deceleration valve slows load by be-ing gradually closed by action of cam mounted on cylinder load.

pressure (or both) vary, the primary cir-cuit has priority over the secondary as far as supplying the design flow rate is concerned.

Deceleration valves — A decelera-tion valve, Figure 8, is a modified 2-way, spring-offset, cam actuated valve used for decelerating a load driven by a cylinder. A cam attached to the cyl-inder rod or load closes the valve grad-ually. This provides a variable orifice which gradually increases backpres-sure in the cylinder as the valve closes.

Some deceleration valves are pres-sure-compensated.

This force-balance concept of con-trol function also applies to flow rate control valves.

Flow control methods

There are three basic ways to control flow: meter-in, meter-out, and bleed-off.

Meter-in control — The circuit in Figure 9 illustrates meter-in control.

The flow control valve is placed in

se-ries with the directional control valve in the cylinder’s high pressure line.

Thus, flow control valve A meters the amount of fluid entering the cap end of the cylinder. This type of control is best suited for resistive loads, where it is es-sential to control the speed at which a cylinder extends.

Meter-out control — Here, the flow control valve is placed in the cylinder return line, Figure 10. The valve con-trols the rate of fluid flow from the cyl-inder to tank and is best used with over-running loads. The valve controls the rate at which fluid leaves the head end of the cylinder. Thus, it controls the speed of the piston rod and load. Also, because it is placed in the return line, the overrunning load cannot force the piston rod to move at higher speed than that set by the flow control valve.

Bleed-off control — This flow con-trol device is placed in parallel with the cylinder, bypassing a part of the pump output flow to tank over the flow con-trol valve, Figure 11. The flow concon-trol valve can be sized to handle bleed-off flow only, rather than entire pump out-put. Because the bleed-off valve is mounted in parallel (not in series) with the active elements, this flow control valve does not introduce a pressure drop into the active part of the circuit.

Inlet pressure will be actual load pres-sure rather than the prespres-sure of the re-lief valve setting. Figure 11 illustrates how a typical bleed-off circuit might be installed.

Other flow controls

Flow-dividers — A flow-divider valve is a form of pressure-compen-sated flow control valve which receives one input flow and splits it into two output flows. The valve can deliver equal flows in each stream or, if neces-sary, a preset ratio of flows. The circuit in Figure 12 shows how a flow divider could be used to roughly synchronize two cylinders in a meter-in configura-tion. Note that like all pressure- and flow-control devices, flow dividers op-erate over a narrow bandwidth rather than at one set point. Thus, there are likely to be flow variations in the sec-ondary branches, and for this reason, precise actuator synchronization can-not be achieve with a flow-divider valve alone.

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Load V_p

Load V_p

Load V_p

Fig. 9. In meter-in control circuit, flow control valve is connected in series with direc-tional flow control valve.

Fig. 10. In meter-out control circuit, flow control valve is installed in cylinder return line.

Fig. 11. In bleed-off control circuits, flow control valve is mounted in parallel with cylinders.

Flow dividers can also be used in meter-out circuit configurations.

Bleed-off does not affect the perfor-mance of a flow divider valve. Flow di-viders can also be “cascaded,” that is, connected in series, to control multiple actuator circuits, Figure 13.

Rotary flow dividers — Another technique for dividing one input flow into proportional, multiple-branch out-put flows is with a rotary flow divider, Figure 14. It consists of several hy-draulic motors connected together me-chanically in parallel by a common shaft. One input fluid stream is split into as many output streams as there are motor sections in the flow divider.

Since all motor sections turn at the same speed, output stream flow rates are proportional and equal to the sum of displacements of all the motor sections.

Rotary flow dividers can usually han-dle larger flows than flow divider valves.

The pressure drop across each motor section is relatively small because no

energy is delivered to an external load, and is usually the case with a hydraulic motor. However, designers should be aware of pressure intensification gener-ated by a rotary flow divider. If, for any reason, that load pressure in one or more branches should drop to some lower level or to zero, full differential pressure will be felt across the motor section(s) in the particular branch(es).

The sections thus pressurized will act as hydraulic motors and drive the maining section(s) as pump(s). This re-sults in higher (intensified) pressure in these circuits branches. When specify-ing rotary flow dividers, system de-signers must be careful to minimize the potential for pressure intensification.

Rotary flow dividers can also integrate multiple branch return flows into a sin-gle return flow.

Pump control of flow rate — Pump control of flow rate presupposes the use of a variable-displacement pump. Non pressure-compensated pumps require an auxiliary control to stroke the

pump-ing element to vary the pump’s dis-placement. These auxiliary controls are available in hydraulic, pneumatic, me-chanical, and electrical versions to match the needs of most control appli-cations. Though pressure-compensated pumps are usually considered to be pressure control devices, designers must remember that flow control is achieved by reducing the displacement of the pump when a set pressure level is reached. Thus, a change in flow rate is involved. If this change occurs while the actuator is still moving, it will result in a change in actuator speed.

The purpose of flow control is speed control. All the devices discussed in this section control the speed of the ac-tuator by controlling the flow rate.

Flow rate also determines rate of en-ergy transfer at any given pressure. The two are related in that the actuator force multiplied by the distance through which it moves (stroke) equals the work done on the load. The energy transferred must also equal the work

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Combining ports

Control orifice Restriction area

Dividing port Flow divider Cylinder

1

Cylinder 2

Fig. 12. Linear type flow divider splits input flow into two out-put flows.

Fig. 13. Flow dividers can be cascaded in series to control multiple actuator circuits.

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