Indirect Control Using Pilot-Operated Valves...Indirect Control Using Pilot-Operated Valves 4-4 Figure 4-1. Double-pilot, 4-way, 2-position Directional Control Valve. The operation

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Exercise 4-1

Indirect Control Using Pilot-Operated Valves

EXERCISE OBJECTIVE

C To learn about pilot-operated valves;C To show the advantages of indirect control of a single acting cylinder.

DISCUSSION

The main difference between pilot-operated directional control valves and direct-operated valves is in how their spools are shifted. On a pilot-operated valve, an airsignal replaces the mechanical force used to shift the spool in a direct-operatedvalve. Other than this, the housings and spools of both valve types are so similar thatthese parts are often interchangeable.

The greatest advantage of a pilot-operated valve is that it permits theremote-actuation of large valves with inexpensive pilot lines. The more expensiveworking lines of the larger valves can then be kept short to save money. Cheaperpilot-lines can be run for some distance without any loss of circuit performance.Since pilot-operated valves do need not be manually actuated, they can becontrolled with outside devices or systems. This makes process automation possible.Also, because pilots require minimum pressures and volumes to shift against theworking pressures, they reduce the delays caused by compressibility of air andfriction in long tubing lines.

Pilot-operated valves can be 3-way and 4-way (4 ports or 5 ports). They can beeither 2-position or 3-position. Usually, 3-way pilot valves are used to remotelycontrol linear or rotary actuators in one direction and then exhaust their workinglines. The 4-way pilot valves are used to remotely control linear and rotary actuatorsin two directions, as well as exhausting their working lines.

Pilot-operated valves may move their spools using one or two pilots along with areturn spring. If a 2-position valve uses only one pilot, the pilot moves the valve spoolagainst a spring and into the housing opposite the pilot. The spring will return thespool when pilot pressure is removed.

Double-piloted valves have a pilot at each end of the housing. Opposing pilots areused to shift the spool back and forth, but the circuit must exhaust one pilot beforethe other pilot can shift the spool. The lack of return springs in double-piloted valves,allows the spool position to be maintained, or memorized, without maintaining pilotpressure.

The pilot-operated valve supplied with your Trainer is a double-piloted, 4-way, 5-port,2-position directional control valve. It is called a 4-way valve instead of a 5-way valvebecause one of the exhaust ports is usually not used in a given valve position. It isillustrated in Figure 4-1.


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Figure 4-1. Double-pilot, 4-way, 2-position Directional Control Valve.

The operation of a double-piloted, 4-way, 2-position directional control valve isillustrated in Figure 4-2. When pilot port A is pressurized, ports 1 and 2 areinterconnected through the valve, supplying the branch circuit with compressed air.Ports 3 and 4 are also interconnected through the valve, connecting the branchcircuit to atmosphere. When the spool is shifted by pressurizing pilot port B, ports 1and 4 are interconnected to supply the branch circuit with compressed air.Compressed air is exhausted from the branch circuit to atmosphere throughinterconnected ports 2 and 3. Pilot-operated valves can have manual overrides tomove the spool without pilot pressure for system setup and troubleshooting.

Figure 4-2. Operation of a Double-pilot, 4-way, 2-position Directional Control Valve.

The circuit shown in Figure 4-3 shows that a 3-way, 2-position pilot-operated valveallows the use of shorter expensive working lines. The cheaper pilot-lines can be runfor some distance without any loss of circuit performance while minimizing delays.


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Figure 4-3. Indirect Control Using a 3-way, 2-position Piloted Directional Control Valve.

REFERENCE MATERIAL

For additional information on directional control valves, refer to the chapter entitledDirectional Control Valves in the Parker-Hannifin manual Industrial PneumaticTechnology.

Procedure summary

In this exercise, you will verify the operation of a 4-way, 5-port, 2-position directionalcontrol valve by operating a double-acting cylinder.

In the second part you will use the long line device to verify that pilot lines requireminimum pressure and volume to shift the spool.

You will verify that priority can be maintained on a position when the pilot portremains pressurized. You will also verify that indirect control reduces delays causedby compressibility of air and friction in long tubing lines.

EQUIPMENT REQUIRED

Refer to the Equipment Utilization Chart, in Appendix A of this manual, to obtain thelist of equipment required to perform this exercise.


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PROCEDURE

G 1. Verify the status of the trainer according to the procedure given inExercise 1-2.

G 2. Retract the piston rod of the Double-Acting Cylinder and connect the circuitshown in Figure 4-4.

Figure 4-4. Schematic Diagram of a Pilot-Operated Circuit.

G 3. From the schematic diagram shown in Figure 4-4, predict which of thevalves DCV1 or DCV2 controls the extension and/or the retraction of thepiston rod?

G 4. Open the main shutoff valve and the branch shutoff valves at the manifoldand set the pressure regulator at 100 kPa (or 15 psi) on the regulatedPressure Gauge.

G 5. Push down the button on the directional control valve DCV2 to set the spoolin the pilot-operated valve as illustrated in Figure 4-4. The piston rod shouldbe retracted.


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G 6. Actuate the cylinder using the directional control valves DCV1 and DCV2.Do the valves DCV1 and DCV2 control the extension and retraction of thecylinder piston rod as predicted? If not, explain why.

G 7. Push down the button on the directional control valve DCV2 and maintainthis position. With your other hand, push down the button on the directionalcontrol valve DCV1. Does the piston rod extend?

G Yes G No

G 8. Release the button on the directional control valve DCV2, then push downthe button on the directional control valve DCV1 and maintain this position.With your other hand, push down the button on the directional control valveDCV2. Does the piston rod retract? Explain why.

G 9. Close the shutoff valves and turn the regulator adjusting knob completelycounterclockwise.

G 10. Modify your circuit as shown in Figure 4-5.

Figure 4-5. Schematic Diagram of a Circuit Using a Long Tubing Line.

G 11. Open the shutoff valves and set the pressure regulator at 100 kPa (or15 psi) on the regulated Pressure Gauge.


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G 12. Push down the button on the directional control valve while observing thetime taken by the rod to extend fully. Estimate the time taken by the rod toextend fully and record your result in Table 4-1. Repeat your observationthree times, then calculate the mean value.


G 14. Modify your circuit as shown in Figure 4-6.

Figure 4-6. Schematic Diagram of an Indirect Control Circuit.

G 15. Open the main shutoff valve and set the pressure regulator at 100 kPa (or15 psi) on the regulated Pressure Gauge.

G 16. Push down the button on the directional control valve DCV2 to set the spoolin the pilot-operated valve as illustrated in Figure 4-6. The piston rod shouldbe retracted.

G 17. Push down the button on the directional control valve DCV1 while observingthe time taken by the rod to extend fully. Estimate the time taken by the rodto extend fully and record your result in Table 4-1. Repeat your observationthree times, then calculate the mean value.


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READINGEXTENSION TIME OF THE PISTON ROD

DIRECT CONTROL INDIRECT CONTROL

First Reading

Second Reading

Third Reading

Mean Value

Table 4-1. Extension Time of The Piston Rod

G 18. Compare the results shown in Table 4-1. Does the rod extend faster whenthe valve is controlled indirectly?

G Yes G No

G 19. Since the same Long Line was first used to power the cylinder and then topilot the directional control valve, what can you conclude?

G 20. Does the piston rod retract when the button on the directional control valveDCV1 is released? Explain why.

G 21. On the Conditioning Unit, close the shutoff valves and turn the regulatoradjusting knob completely counterclockwise. You should read 0 kPa (or0 psi) on the regulated Pressure Gauge.

G 22. Disconnect and store all tubing and components.

CONCLUSION

In this exercise, you verified the operation of a 4-way, 5-port, 2-positionpilot-operated directional control valve. You have seen that the piloted valve suppliedwith your trainer can be used to remotely control linear and rotary actuators in twodirections.

You have seen that when a pilot port is maintained pressurized, priority is maintainedon that position although the other port becomes pressurized.


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You have seen that pilots require minimum pressures and volumes to shift againstthe working pressures, minimizing delays caused by compressibility of air and frictionin long tubing lines.

REVIEW QUESTIONS

1. What is the greatest advantage of pilot-operated valves over manually operatedvalves?

a. They require a high pressure to operate.b. They permit the remote actuation of large valves.c. They can be made smaller than other valves.d. They can be made larger than other valves.

2. What is the purpose of the 4-way piloted-operated valves?

a. To control linear actuators in one direction remotely.b. To control rotary actuators in one direction remotely.c. To control linear and rotary actuators in one direction remotely.d. To control linear and rotary actuators in two directions remotely.

3. What is the main difference between pilot-operated and direct-operated controlvalves?

a. Pilot-operated control valves are smaller.b. The way their spools are shifted.c. Pilot-operated control valves can work in both directions.d. Pilot-operated control valves cannot be spring return.

4. What is the purpose of the manual override on a pilot-operated valve?

a. To bleed off excess compressed air.b. To reverse the direction of the valve.c. To reverse pilot operation.d. To manually duplicate the operation of the valve.

5. Give the reason why double pilot-operated valves can memorize a position?

a. They need a pilot signal to shift the spool.b. They do not need a pilot signal to shift the spool.c. It is a characteristic of pilot-operated valve.d. Because they are remotely-controlled.

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Exercise 4-2

Pneumatic Motor Circuits

EXERCISE OBJECTIVE

C To describe the design and operation of a pneumatic motor;C To learn how to control the direction and speed of pneumatic motors.

DISCUSSION

Pneumatic motors convert fluid energy into mechanical rotational energy. When apressure differential is created within a pneumatic motor, the higher pressure airexpands. This expanding air acts upon the internal surfaces of the motor to causethe motor output shaft to turn. Figure 4-7 shows a pneumatic motor and its symbol.

Figure 4-7. Pneumatic Motor and Symbol.

Types of Pneumatic Motors

There are three basic types of pneumatic motors: vane, piston and turbine.

C The vane motor is the type of motor supplied with your Trainer. It is relativelysimple in construction: a slot rotor is located off-center inside a vane housing asshown in Figure 4-8.


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Figure 4-8. Vane Type Pneumatic Motor.

As compressed air enters the inlet port, it expands inside one of the air chambersbetween two vanes. The rotor turns to create an increasing volume between thevanes to accommodate the expanding air. When air reaches the outlet side of themotor, it is expelled into the atmosphere through the exhaust port. Vane motorsmay be unidirectional or bidirectional.

C The operation of a piston pneumatic motor is shown in Figure 4-9. As air entersthe piston compression chambers, the pistons move inside the cylinders toaccommodate the increasing volume of air. This linear piston movement isconverted into rotational motion by piston connecting rods that rotate a driveshaft.Piston pneumatic motors are used as low speeds, and can generate high startingtorque.

Figure 4-9. Piston Type Pneumatic Motor.

C The operation of a turbine pneumatic motor is shown in Figure 4-10. It uses aturbine wheel to convert the kinetic energy of moving air into rotary motion.Turbine pneumatic motors drive devices such as small drilling and grinding tools


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used by dentists. This motor type also drives heavy commercial rotary saws anddrills. Turbine motors convert fluid energy into rotary motion in one direction only.

Figure 4-10. Turbine Type Pneumatic Motor.

The main advantages of pneumatic motors are: their speed can be easily adjusted,they can be used in environment which may be unfavorable for electric motors, theycan be operated in flammable or corrosive atmospheres, they can be stalled withoutdamage, and they are simple in design and construction.

The main disadvantages of pneumatic motors are: low efficiency, motor speed goesdown when the torque goes up and vice versa, noise level is higher than thatproduced by an electric motor.

Motor Displacement

The displacement of a pneumatic motor is the volume of air required for the motorshaft to turn one complete revolution. It is expressed in cubic centimeters perrevolution (cm3/r) (or cubic inches per revolution (in3/r)). Due to mechanical frictionand internal leakages, the displacement varies with air pressure.

Motor Speed

The theoretical motor speed of a pneumatic motor is calculated by dividing the flowrate by the motor displacement. The speed is directly proportional to the flow ratethrough the motor, and inversely proportional to the motor displacement. The formulafor calculating the theoretical speed of a motor is:


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Due to internal leakages and compressibility of air, the actual motor speed will beless than the theoretical speed given by the formula.

Motor Torque

The motor torque is the turning effort, or rotary force, generated at the motor shaft.For example, the force applied to the end of a wrench to tighten a bolt is calledtorque. Torque is usually expressed in Newton-meter (N@m) (or pound force-inch(lbf@in)). A rotary force of 22.2 N (or 5 lbf) applied to a shaft 0.0508 m (or 2 in) fromthe center of the shaft would be expressed as a torque of 1.13 N@m (or 10 lbf@in).

The resistance of the load connected to the motor shaft determines the amount ofsystem pressure developed at the motor inlet, and, therefore, the amount of torquegenerated at the motor shaft. No torque will be generated if there is no load on theshaft.

Motor Output Power

The amount of power in W (or hp) generated by a pneumatic motor is equal to thetorque developed at the motor shaft multiplied by the speed of the shaft. Therefore,it is zero when the speed or the torque becomes zero. In equation form:

SI units:

English units:

Due to internal leakage, mechanical friction and compressibility of air, the actualamount of power generated by a pneumatic motor will be less than the theoreticalvalue given by the formula.

Cylinders vs Motors

Pneumatic cylinders are linear actuators, whereas pneumatic motors are rotaryactuators. As with cylinders, however, the speed of a motor is a function of flow rate,while its output force, or torque, is a function of pressure. Therefore, increasing thesystem pressure available to a motor increases the torque output capability.Conversely, increasing the flow rate through a motor increases the speed of rotation.

REFERENCE MATERIAL

For additional information, refer to the chapter entitled Check Valves, Cylinders, andMotors in the Parker-Hannifin manual Industrial Pneumatic Technology.


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Procedure summary

In the first part of this exercise, you will verify the torque vs pressure relationship.

In the second part you will verify the speed of rotation vs flow relationship.

In the third part you will set up a circuit that allows control of both the direction andthe speed of rotation of the motor.

EQUIPMENT REQUIRED

Refer to the Equipment Utilization Chart, in Appendix A of this manual, to obtain thelist of equipment required to perform this exercise.

PROCEDURE

Torque vs Pressure Relationship

G 1. Verify the status of the trainer according to the procedure given inExercise 1-2.

Note: Be very careful when using pneumatic motors. Refer to thesafety rules in Exercise 1-1.

G 2. Connect the circuit shown in Figure 4-11. Facing the motor shaft, connectthe directional control valve to the left port of the motor.

Note: Because of high levels of noise generated by pneumaticmotors, you should always exhaust compressed air from yourcircuit through the muffler at the exhaust port of the main shutoffvalve on the Conditioning Unit.

Figure 4-11. Schematic Diagram of Pneumatic Motor Circuit


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G 3. Referring to the schematic diagram shown in Figure 4-11, indicate in whichdirection the motor will rotate?

G 4. Open the main shutoff valve and the branch shutoff valve at the manifoldand set the pressure regulator at 100 kPa (or 20 psi) on the regulatedPressure Gauge.

G 5. Push the button on the directional control valve while observing the directionof rotation of the motor shaft. Does the motor rotate as predict in step 3? Ifnot, explain why.

G 6. Release the pushbutton and indicate which modification must be done toreverse the direction of rotation of the motor shaft.

Note: Wear protective gloves when conducting this part of theexercise.

G 7. Hold the shaft of the motor with your hand while opening the directionalcontrol valve. Is it difficult to stall?

G Yes G No

G 8. Release the button and set the pressure regulator to 600 kPa (or 80 psi).Retry to stall the shaft of the motor. Is it less difficult or more difficult to stallthe shaft with this setting?

G 9. Which motor characteristic do you observe when you try to stall the shaft ofthe motor?

G 10. Close the shutoff valves.


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Speed of Rotation vs Flow Relationship

G 11. Replace the directional control valve by a Flow Control Valve to control theflow rate through the motor and connect a Flowmeter to the outlet port of themotor.

G 12. Open the Flow Control Valve by turning the control knob fullycounterclockwise.

G 13. Open the shutoff valves and set the pressure regulator at 500 kPa (or70 psi).

G 14. Adjust the Flow Control Valve to obtain a flow rate of 100 R/min (or4.0 SCFM) on the Flowmeter. At each setting of the Flow Control Valve,ensure that the regulated pressure is set at 500 kPa (or 70 psi). Adjust ifnecessary.

G 15. Use a tachometer to measure the rotational speed of the motor. Recordyour result in the appropriate cell in Table 4-2. Repeat your measurementsfor the flow rate values indicated in Table 4-2.

PRESSURE FLOW RATE SPEED OF ROTATION

100 R/min (or 4.0 SCFM)



Table 4-2. Rotational Speed Values.

G 16. From the values indicated in Table 4-2, what can you conclude from therotational speed vs flow rate relationship?


Directional and Speed Control of Pneumatic Motors

G 18. Modify your circuit as shown in Figure 4-12. Facing the motor shaft, connectthe Flow Control Valve FCV1 to the left port of the motor.


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Figure 4-12. Schematic Diagram of a Circuit Using Speed Control Both Directions

G 19. Open the Flow Control Valves FCV1 and FCV2 by turning the control knobsfully counterclockwise.

G 20. Referring to the schematic diagram shown in Figure 4-12, determine inwhich direction the motor will rotate when pressurizing the circuit. Explainwhy.

Note: Wear ear protectors when conducting this part of theexercise.

G 21. Open the shutoff valves and set the pressure regulator at 400 kPa (or60 psi) on the regulated Pressure Gauge.

G 22. Use the required directional control valve to set the motor rotation in thecounterclockwise direction.

G 23. Adjust the Flow Control Valve FCV1 to obtain a rotational speed of3000 r/min.

G 24. Record the flow rate in the appropriate cell in Table 4-3.


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G 25. Push down the button on the directional control valve DCV2 to set the motorrotation in the clockwise direction.

G 26. Adjust the Flow Control Valve FCV2 to obtain a rotational speed of2000 r/min.

G 27. Record the flow rate in the appropriate cell in Table 4-3.

DIRECTIONOF ROTATION SPEED FLOW RATE

Counterclockwise

Clockwise

Table 4-3. Pneumatic Motor Operating Parameters.

G 28. From the values indicated in Table 4-3, what can you conclude about theoperation of the circuit?

G 29. On the Conditioning Unit, close the shutoff valves and turn the regulatoradjusting knob completely counterclockwise. You should read 0 kPa (or0 psi) on the regulated Pressure Gauge.

G 30. Disconnect and store all tubing and components.

CONCLUSION

In this exercise, you saw that pneumatic motors convert fluid energy into mechanicalrotational energy. You also learned about the three basic types of pneumatic motors:vane, piston and turbine.

You learned that motor speed is proportional to the flow rate through the motor andinversely proportional to the displacement. You also learned that the torque of apneumatic motor is directly proportional to the system pressure at the motor inlet andto displacement.

You experienced how to control the direction of rotation, speed and torque of apneumatic motor.


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REVIEW QUESTIONS

1. Torque refers to

a. the linear force generated by a single-acting cylinder.b. the speed of a pneumatic motor.c. the rotational force generated by a motor.d. the lifting force generated by an air bearing.

2. The speed of rotation of a motor is directly proportional to

a. the flow rate.b. the displacement.c. the pressure.d. the load.

3. To increase the torque output capability of a motor

a. the flow rate must be decreased.b. the flow rate must be increased.c. the displacement must be increased.d. the pressure must be increased.

4. The three basic types of pneumatic motors are

a. vane, piston and turbine.b. rotary, vane and piston.c. turbine, rotary and vane.d. piston, turbine and rotary.

5. The output power of a motor is

a. equal to the torque developed at the motor shaft multiplied by the flow rate.b. zero when the speed or the torque becomes maximum.c. maximum when the speed or the torque becomes zero.d. equal to the torque developed at the motor shaft multiplied by the speed of

the shaft.

Documents

Indirect Control Using Pilot-Operated Valves...Indirect Control Using Pilot-Operated Valves 4-4 Figure 4-1. Double-pilot, 4-way, 2-position Directional Control Valve. The operation