BCIT: Millwright Level 3 (Hydraulics: Valves 2)

When you close down a flow control to reduce the flow from the pump to the actuator, where does the flow from the pump go that does not go to the actuator?

The excess flow goes through the relief valve and returns to the tank.

What is the purpose of using more than one stack of valves on a multiple station subplate?

Multiple stacks are used to control multiple cylinders or actuators.

List two (2) advantages of using a cartridge valve assembly.

Cartridge valve assemblies are compact and have fewer leak points.

Why is the area ratio of a slip-in cartridge poppet important?

The area ratio determines the pressure at which the valve will open.

List two (2) main advantages of stack valves over externally plumbed valves.

Stack valves are more compact and have fewer external leak points.

What must you prevent when assembling stack valves?

You must prevent cutting the O-rings and ensure the bolts are properly torqued.

Which stack valve must be installed at the top of the stack?

The directional control valve (DCV) must be installed at the top of the stack.

Which valve must be installed closest to the supply to the subplate in a stack valve assembly?

The relief valve must be installed closest to the supply.

Will a stack valve handle a flow of 50 gpm?

Yes, a stack valve can handle a flow of 50 gpm.

Which type of cartridge valve is used for low flows?

A screw-in cartridge valve is used for low flows.

List two (2) specifications that determine whether a slip-in cartridge valve will open.

The pressure on the port and the area ratio across the spool determine whether the valve opens.

What must always be done to a manifold block received from the machine shop before installing cartridge valves?

The manifold block must be thoroughly cleaned.

If a screw-in cartridge fails, should you repair it or throw it away?

A failed screw-in cartridge valve should be replaced rather than repaired.

What is the spool movement proportional to in a proportional valve?

The spool movement is proportional to the electrical input signal.

What is the purpose of the notches on a proportional valve spool?

The notches help control and meter the flow.

What six (6) devices provide input signals to the amplifier card for a proportional valve?

Input signals can come from a PLC, potentiometer, tach generator, temperature sensor, pressure transducer, or joystick.

What is the main reason for using a servo valve instead of a proportional valve?

Servo valves provide much greater accuracy, approximately 0.001 inch.

What is the main maintenance concern with servo valves?

Servo valves require extremely clean fluid because particles as small as 2 microns can jam the spool.

What is the difference between the operation of a conventional solenoid-controlled directional control valve and a proportional valve?

A conventional valve shifts the spool fully, while a proportional valve can position the spool anywhere within its range.

Which extra function can a proportional directional valve perform?

A proportional valve can also perform flow control.

List three (3) advantages of a proportional valve over a servo valve.

Proportional valves are more tolerant of contamination, more rugged, and less expensive.

What is the spool movement proportional to in a proportional valve?

The spool movement is proportional to the electrical input signal.

Besides speed, what else can a proportional valve control?

A proportional valve can also control acceleration and deceleration.

Which type of proportional valve spool is best for controlling a motor?

A restricted spool center is best for motor control.

What is the main function of an amplifier card?

The amplifier card increases the input signal to drive the solenoid.

List four (4) functions an amplifier card can provide besides increasing signal power.

An amplifier card can provide gain setting, dither signal, ramp setting, and deadband compensation.

What does the ramp setting control?

The ramp setting controls acceleration and deceleration.

What does the closed inner loop system control on a proportional valve to provide more accurate actuator control?

The closed inner loop system controls the spool position.

Which device does a proportional valve use to sense spool position?

An LVDT (Linear Variable Differential Transformer) senses spool position.

What does the summing junction in an amplifier do?

The summing junction compares the actual spool or actuator position to the set position and sends an error signal to correct it.

What does a closed outer loop system on a proportional valve correct?

It corrects the actuator position.

What is the operating principle of proportional pressure controls?

A proportional solenoid replaces the spring in a conventional pressure control valve to set the pressure.

List three (3) types of proportional pressure controls.

The three types are poppet, nozzle, and plate.

How are the three (3) types of proportional pressure controls used?

They are used to control the main stage of conventional pressure control valves.

What are the differences between the parts of a servo valve and the parts of a proportional valve?

Servo valves use a torque motor and internal mechanical feedback, while proportional valves use solenoids.

Which is more accurate, a servo valve or a proportional valve?

A servo valve is more accurate.

List three (3) types of servo valves.

The three types are flapper nozzle, staged, and jet pipe.

What does hysteresis indicate?

Hysteresis indicates the accuracy of the valve.

What is the purpose of keeping the lines from the valve to the actuator as short as possible on servo and proportional valves?

Shorter lines improve system accuracy.