valves 3

Chapter 18

Controlling a Pneumatic System

Pressure, Direction, and Flow

Permission granted to reproduce for educational use only.3© Goodheart-Willcox Co., Inc.

Objectives

Explain the function of the three general types of control valves used in pneumatic systems.

Describe the methods used to control air pressure in a pneumatic system.

Compare the design and operation of pneumatic pressure regulator valves.

Describe the design and operation of pneumatic system directional control valves.


Objectives

Name and compare the various types of construction used to assure sealing of the internal passages found in pneumatic directional control valves.

List and compare the methods used to position control spools in pneumatic directional control valves.


Objectives

Describe the structure and operation of flow control valves used to control actuator speed in pneumatic circuits.

Explain how flow control valves are used in pneumatic circuits to control actuator speed.


Primary Control Functionsin a Pneumatic Circuit

Control valves are used to establish the level of force and type of motion needed to operate a machine

These valves control:– Air pressure– Direction of airflow– Rate of airflow



Pressure control valves:– Limit maximum system pressure– Regulate the force or torque an actuator generates



Directional control valves control airflow direction to establish the direction of actuator movement

Flow control valves regulate the rate of airflow to control actuator operating speed


Basic Control ValveDesign and Structure

The body of pneumatic system valves may be formed from:– Metal casting– Standard bar stock

Precision bores and passageways are machined into the body– Hold component parts– Allow airflow that controls the operation of the

valve



Cast valve body



Bar stock valve body



The operation of a control valve depends on a series of control elements in the valve

The elements used depend on:– Function of the valve– Operating techniques preferred by the valve

manufacturer



Control elements include:– Fixed orifices– Spools– Pistons– Diaphragms– Precision-machined sliding plates– Needle and poppet valves



Fixed orifice



Needle valve



Spool



Diaphragm



Precision-machined sliding plate

Rotary Directional Control Valve



Varying the position of control elements within a control valve determines:– Setting of the valve– Accuracy of system control



The force used to position the internal elements of a control valve is provided by:– Internal springs– Internal air pressure– External pilot pressure– Electromagnets– Manually applied force



Internal spring force



Electromagnetic force (solenoids)


Pressure Control Methods

and Devices Pressure control is provided in three specific

areas of a pneumatic system:– Air distribution system– Workstation areas– Portions of circuits operating within the workstation

areas



and Devices The pressure in the distribution lines of a

pneumatic system is controlled by:– Bypass– Compressor-capacity control system



and Devices Bypass pressure control limits maximum

system pressure by bleeding excess air to the atmosphere through a relief-type valve– The compressor in this system operates on a

continuous basis– The compressor continuously functions at the

maximum pressure setting of the system



and Devices Compressor-capacity control limits maximum

pressure using one of several different control methods– Compressor start-stop– Inlet valve unloading– Compressor speed control– Control of the cross-sectional area of the inlet (in

dynamic compressors)



and Devices Start-stop compressor capacity control



and Devices Control of air pressure at the workstation is

provided by a pressure regulator It is often a part of the filter/pressure

regulator/lubricator (FRL) unit commonly used at the workstation



and Devices Pressure regulator of an FRL unit

IMI Norgren, Inc.



and Devices Pressure regulator is required at the workstation

due to:– Air from the distribution line is at a higher pressure

than desired– Varying distribution line pressure caused by the

compressor-capacity control system



and Devices A variety of pressure regulator designs are

used to control workstation pressure– Direct operated– Basic, diaphragm chamber– Relieving– Balanced poppet– Pilot operated



and Devices When sections of a circuit require maximum

air pressures lower than that provided by the FRL unit, additional pressure regulators may be inserted in the circuit to provide the needed lower pressures



and Devices Additional workstation

regulator provides lower pressure



and Devices Subcircuit is provided with lower pressure



and Devices When sections of a circuit require a pressure

exceeding that available from the system distribution line, an air-to-air or air-to-oil booster unit can be used to produce the higher pressure



and Devices Air-to-air booster provides higher pressure



and Devices A safety valve prevents excessive air pressure

in distribution lines due to failure of compressor capacity control components

A positive-acting, relief-type valve large enough to exhaust the output capacity of the compressor is used for this application



and Devices Safety valve on a small compressor


Directional Control Methods and Devices

Directional control valves can be grouped into four categories:– Shut-off valves– Check valves– Three-way valves– Four-way valves



Shut-off valves used in pneumatic systems include:– Globe– Gate– Ball– Spool– Needle

These valves are used to allow or block airflow in circuit lines



Check valves automatically allow the free flow of air in one direction and block airflow in the opposite direction

They are commonly used to block or allow air to flow around control valves during different phases of circuit operation



Check valve allows unrestricted retraction



Spring-assisted check valve



Three-way directional control valves:– Pressurize a single-acting cylinder during extension– Vent air during retraction to allow the cylinder to be

returned to the initial position by external forces



Operation of a three-way valve



Four-way directional control valves direct compressed air to power a cylinder during both extension and retraction

They also power air motors during both directions of rotation



Four-way directional control valves are available in four-port and five-port configurations– A common exhaust is used in the four-port design– The five-port design uses separate exhaust ports for

the extension and retraction positions of the valve



Four- and five-port, four-way valves



In most directional control valves, resilient materials are used to seal the space between the bore and the spool– In packed-spool designs, sealing materials are

attached to the valve spool lands– In packed-bore designs, sealing materials are

attached to the bore in the valve body



Packed-spool directional control valve

IMI Norgren, Inc.



Packed-bore directional control valve



Four-way directional control valves are normally available providing two or three positions of operation– Two-position valves power an actuator in one

direction in the first position and in the opposite direction in the second position

– Three-position valves provide additional operating options using the middle valve position



Center position provides additional options



Three center position configurations are commonly available for pneumatic three-position, five-port valves– Blocked center– Open center– Pressure center



Blocked-center configuration



Open-center configuration



Pressure-center configuration


Flow Control Methodsand Devices

The most common methods of controlling pressurized airflow in pneumatic circuits are:– Fixed-size orifices– Needle valves



Needle valve with integral check valve



Preferred method of flow control in pneumatic circuits is the meter-out design– Involves the placement of the flow control valve in

the outlet line of the actuator– Favored due to the compressibility of air



Meter-out and meter-in circuits



Meter-out circuits with four- and five-port valves


Special-Purpose Control Valves and

Other Devices Quick exhaust valves allow exhaust air from a

cylinder to be vented directly to the atmosphere without passing through the directional control valve

This reduces backpressure in the exhaust line, increasing the efficiency of circuit operation



Other Devices Circuit with a quick-exhaust valve



Other Devices Structure of a quick-exhaust valve



Other Devices Shuttle valves automatically select the higher

pressure of two sources Allows a circuit to always be connected to the

highest pressure



Other Devices Structure of a shuttle valve



Other Devices Timing-volume reservoirs are used to time the

shifting of pilot-controlled pneumatic valves The rate of airflow and the volume of the

reservoir determine the time it takes to shift the piloted valve



Other Devices Circuit with timing-

volume reservoir



Other Devices Exhaust mufflers:

– Reduce the level of air exhaust noise– Prevent the entrance of dirt and liquids into the

exhaust ports of directional control valves and actuators



Other Devices Typical exhaust mufflers

(Norgren, Inc.)


Selecting and Sizing Pneumatic Valves

A great deal of technical information is available about pneumatic components from manufacturer catalogs– Pressure ranges– Flow rates– Physical dimensions– Graphs illustrating performance under various

conditions



Physical dimensions



Graphs illustrate performance



Several approaches are available to determine the size of the control valves needed in a specific pneumatic circuit

Procedures, formulas, and online calculators are available from manufacturers that can be used to select an appropriate valve



Typical data required for valve selection:– Cylinder piston area– Cylinder stroke– System compression– System pressure drop– Cylinder stroke time


Review Question

List the three factors control valves regulate to produce a desired performance in a pneumatic circuit.

1) Air pressure2) Direction of airflow3) Rate of airflow


Review Question

What is the most common method used to control the maximum operating pressure in the distribution line of a pneumatic system?

A compressor-capacity control system.


Review Question

List five designs of air pressure regulators.

Direct operated; basic, diaphragm chamber; relieving; balanced poppet; and pilot operated.


Review Question

Name the two basic functions of the center position of a three-position, four-way, directional control valve.

1) Stop actuator movement 2) Provide additional operating characteristics.


Review Question

List five primary types of information available about pneumatic components found in manufacturer catalogs.

1) General valve description2) Pressure, flow, and temperature operating ranges3) Drawings providing dimensions and fitting sizes4) Schematic symbols for valve variations5) Graphs showing performance at various pressures

and flows.


Review Question

List four sources of internal and external force that are used to operate the control elements in pneumatic control valves.

1) Springs2) Air pressure3) Manual force4) Electromagnets.


Review Question

The two most common basic flow control devices used in a pneumatic system are fixed-sized orifices and _____ valves.

needle


Review Question

The _____ flow-control circuit is the preferred method to use for controlling the operating speed of cylinders in pneumatic circuits.

meter-out

valves 3

Documents

control spools

machinethese valves

basic control valvedesign

educational use

primary control functionsin

series of control elements

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