80.Volume Controls

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Insdustrial Hydraulics

Manual

Chapter X

Volume Controls


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Contents

1.Flow control methods

2.Type of flow controls

3.Questions


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Introduction

Volume or flow control valves are used to regulate speed. As

was developed in earlier chapters, the speed of an actuator depends

on how much oil is pumped into it per unit of time. It is possible toregulate flow with a variable displacement pump, but in many

circuits it is more practical to use a fixed displacement pump and

regulate flow with a volume control valve.


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I. Flow Control Methods

There are three basic methods of applying volume control valves

to control actuator speeds. They are meter-in, meter-out and bleed-off.

Meter-In Circuit

Meter-in operation, the flow control valve is placed between the pump

and actuator (Fig. 10-1). In this way, it controls the amount of fluid

going into the actuator. Pump delivery in excess of the metered amount is

diverted to tank over the relief valve.With the flow control valve installed in the cylinder line as shown,

flow is controlled in one direction. A check valve must be included

in the flow control or placed in parallel with it for return flow. If

it is desired to control speed in both directions, the flow control canbe installed in the pump outlet line prior to the directional valve.

The meter-in method is highly accurate. It is used in applications

where the load continually resists movement of the actuator, such as

raising a vertical cylinder under load or pushing a load at acontrolled speed.


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Meter-Out Circuit

Meter-out control (Fig. 10-2) is used where the load might tend to "runaway. " The flow control is located where it will restrict exhaust flow from

the actuator.To regulate speed in both directions,the valve is installed in the tank line

from the directional valve. More often control is needed in only onedirection and it is placed in the line between the actuator and directionalvalve. Here too a bypass check valve would be required for a rapidreturn stroke.

Bleed-Off Circuit

In a bleed-off arrangement (Fig. 10-3), the flow control is teed off thesupply line from the pump and determines the actuator speed by meteringa portion of the pump delivery to tank. The advantage is that the pumpoperates at the pressure required by the work, since excess fluid returns to

tank through the flow control instead of through the relief valve.Its disadvantage is some loss of accuracy because the measured flow is to

tank rather than into the cylinder, making the latter subject to variations inthe pump delivery due to changing work loads.

Bleed-off circuits should not be used in applications where there is apossibility of the load running away.


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II. Type of flow controls

Flow control valves fall into two basic categories: pressure

compensated and non-pressure compensated. The latter being used

where load pressures remain relatively constant and feed rates are nottoo critical. They may be as simple as a fixed orifice or an adjustable

needle valve, although more sophisticated units may even include a

check valve (Fig. 10-4) for free flow in the reverse direction. Use

of non-pressure compensated valves is somewhat limited, since

flow through an orifice is essentially proportional to the square root

of the pressure drop ( P) across it. This means that any

appreciable change in the work load would affect the feed rate.

Pressure compensated flow controls are further classified as

restrictor and by-pass types. Both utilize a compensator or

hydrostatic to maintain a constant pressure drop across an adjustable

throttle.


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The By-Pass Type - combines overload protection with pressure

compensated control of flow (Fig. 10-5). It has a normally closed

hydrostat which opens to divert fluid, in excess of the throttle setting, to thetank. Pressure required by the work load is sensed in the chamber above

the hydrostat and together with a light spring tends to hold it closed.

Pressure in the chamber below the hydrostat increases due to the

restriction of the throttle and causes it to raise diverting any excess flow

to tank when the difference in pressure is sufficient to overcome the spring.This difference, usually 20 psi, is maintained across the throttle providing a

constant flow regardless of the work load. Some horsepower saving is

accomplished in that the pump need operate at only 20 psi above work load

pressure. Overload protection is provided by an adjustable spring loaded

poppet which limits the maximum pressure above the hydrostat, causing it to

function as a compound relief valve whenever work load requirements

exceed its setting. The bypass flow control can only be used in a meter-in

circuit. If used for metering out, exhaust oil which could not get through the

throttle would be diverted to tank permitting the load to run away.


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The Restrictor Type Flow Control - also maintains a constant 20psi differential across its throttle by means of a hydrostat (Fig. 10-

6). In this valve, the hydrostat is normally open and tends to closeoff blocking all flow in excess of the throttle setting. In these units,the work load pressure acts with a light spring above the hydrostatto hold it open. Pressure at the throttle inlet and under thehydrostat tends to close it, permitting only that oil to enter the

valve that 20 psi can force through the throttle. Because of theirtendency to close off when flow tries to exceed the throttle setting,restrictor type valves may be used in meter-in, meter-out and bleed-off circuits. Unlike the by-pass type, two or more restrictor valvesmay be used with the same pump since the excess pump delivery

returns to tank through the relief valve.When placed in cylinder lines an integral check valve is optional

to provide free flow for a rapid return stroke (Fig. 10-7). Onewould not be required for valves placed in the main supply line, thetank line of a directional valve or when they are used in bleed-off

circuits.


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Temperature Compensated Flow Control Valve

Flow through a pressure compensated flow control valve is subject to

change with variations in oil temperature. Later design Vickers valvesincorporate a temperature compensating feature.

Although oil flows more freely when it is hot, constant flow can be

maintained by decreasing the size of the throttle opening as the

temperature rises. This is accomplished through a compensating rod which

lengthens with heat and contracts when cold (Fig. 10-8). The throttle is asimple plunger that is moved in and out of the control port. The

compensating rod is installed between the throttle and its adjuster. This

design also is available with a reverse free-flow check valve.

Remote Flow Control ValvesRemote flow control valves (Fig. 10-9) permit adjustment of the throttle

size by an electrical signal. The throttle spool is linked to the armature of a

torque motor and moves in response to signals to the torque motor. Operation is

otherwise the same as a pressure compensated flow control valve.


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III. Questions

1. Name two ways of regulating flow to an actuator.

2. What are the three methods of applying flow control valves?

3. Under what conditions would you use each?

4. How can the same valve control flow in both directions of

actuator movement?

5. What is the difference between a bypass and restrictor type flow

control?

6. What is pressure compensation?

7. How is temperature compensation indicated in a valve symbol?

8. When might temperature compensation be needed?

9. What is the advantage of the flow control and relief valve

over a conventional flow control?

10. How is the throttle positioned in a remote flow control valve?

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80.Volume Controls