mem 341_chapter 9 hydraulic circuit

8/12/2019 MEM 341_Chapter 9 Hydraulic Circuit

http://slidepdf.com/reader/full/mem-341chapter-9-hydraulic-circuit 1/22

MEM341_Chapter 9_Hydraulic circuits

9.0 Industrial hydraulic circuit

In this chapter we will discuss basic hydraulic circuits that used the most basic

components that make up a hydraulic system.

Basic electrical control devices (push button switches, pressure switches, solenoids,relays, timers and basic electrical control devices (ladder diagram) will not be included.

A hydraulic circuit is a group of components such as pumps, actuators, control valves and

conductors (pipes) are arranged so that they will perform a certain task.

Three important factors when analyzing or designing hydraulic circuit:

o safety of operation

o performance of desired function

o efficiency of operation

Circuit drawing has to be done using standards that are recognized worldwide.

Standard are important for the following reasons:

o interchangeable of components

o Symbols and circuit diagrams can be read by everyone. So that the hydraulic

circuit can be interpreted and install correctly

The standards organizations in this field are:

o BS (British Standards)

o ANSI (American National Standards Institution)

o ISO (International Standards Organization)




Example of industrial hydraulic circuit




9.1 Graphic symbols

Symbols are used in all parts of the world for designating components in fluid power

circuits.

Example of symbols of in fluid power circuits

The intent of a graphic symbol is to represent the:

o Type

o Functions

o Operation

o External connections

o Does not show the actual construction of the unit

Fluid power graphic symbols consist of basic figures:

o Lines

o Circles

o Squares

o Triangles

o Dots

o Arrows




A number of mechanisms are used to control the valves in a fluid power system.

Control mechanisms are shown on the end of the general valve graphic symbol.

Example control mechanism symbols

Joining of lines in a circuit is represented by a graphic symbol, which is a dot.

Lines that cross without joining are shown as an arc crossing without a dot.




Symbols for pressure control valves are shown as normally open or normally closed

Normally open valve is open at rest, allowing flow through the valve

Normally closed valve is closed at rest, blocking flow through the valve

Symbols for normally open and normally closed pressure control valves

Simplified symbol for an adjustable flow control valve shows an orifice with an arrow

drawn diagonally across it.

Arrow indicates the orifice is adjustable

When appropriate, symbols also indicate pressure and temperature compensation

Symbol for an adjustable flow control valve

Symbols for pressure- and temperature-compensated flow control valves; symbols

may be combined




Circuit diagrams provide a variety of information about fluid power systems for use

during system assembly, operation, and testing.

o Schematic diagrams

o Component lists

o Sequence of operation

Circuit diagram schematics must:

o Include all components and connections

o Be logically arranged so it is possible to easily follow the operating cycle of the

system

For more hydraulic circuit symbols, refer to the appendix section at the end of the notes.




9.2 Basic circuit design and example of circuit applications

Hydraulic circuit involve a group of components that closely work together to produce a

desired performance

i.

Pressure control

ii. Flow control

iii. Motion control

iv. Miscellaneous functions

A hydraulic circuit typically includes several of these segments

Control of a single-acting hydraulic cylinder

Control of single-acting cylinder

Figure shows a two-position, three-way, manually actuated, spring-offset DCV usedto control the operation of a single-acting cylinder.

In the spring offset mode, full pump flow goes to the tank via the pressure reliefvalve.

The spring in the rod end of the cylinder retracts the piston as oil from the blank end

drains back to the tank. When the valve is manually actuated into its left position, pump flow extends the

cylinder.

At full extension, pump flow goes through the pressure relief valve.

Deactivation of the DCV allows the cylinder to retract as the DCV shifts into itsspring offset mode.




Control of a double-acting hydraulic cylinder

Control of a double-acting hydraulic cylinder

When the four-way valve is in its spring-centered position, the cylinder is

hydraulically locked. Also the pump is unloaded back to the tank at essentially

atmospheric pressure.

When the four-way valve is actuated into the flow path configuration of the left

position, the cylinder extends against its load force F load as oil flows from port P

through port A. Also, oil in the rod end of the cylinder is free to flow back to the tank

ia the four-way valve from port B through port T.

When the four-way valve is deactivated, the DCV returns to its center position, and

the cylinder is once again hydraulically locked.

When the four way valve is actuated into the right position configuration, the cylinder

retracts as oil flows from port P through port B. Oil in the blank end is returned to the

tank via the flow path from port A through port T.

At the end of the stroke, there is no system demand for oil. Thus pump flow goes

through the relief valve back to the tank.




Regenerative hydraulic circuit

o Used to speed up the extending speed of a double acting cylinder

o Unfortunately, load carrying capability of the cylinder during extension is reduced.

Operation:

The operation of the cylinder during the retraction stroke is the same as that of a

regular double-acting cylinder. Fluid flows through the DCV via the right position

during retraction.

When DCV is shifted into its left position, the cylinder extends. The speed of

extension is greater than that for a regular double-acting cylinder because flow form

the rod end regenerates with the pump flow higher total pump flow.

Basic regenerative hydraulic circuit

Application of regenerative hydraulic circuit, can be seen in Drilling Machine

Drilling machine application




In center position, cylinder extends rapidly.

In left position, cylinder extension speed reduces; this is when the drill starts to cut

into the workpiece.

The right envelope mode retracts the cylinder.

Why does the center position result is the cylinder to rapidly extend?

Double pump hydraulic circuit

- In a double pump hydraulic circuit, a high and low pump is used in tandem.

- Provides high-volume flow during low-pressure demand.

- Provides low-volume flow during high-pressure demand.

- Lowers equipment costs by reducing prime mover size and cuts ongoing system energy

consumption.

- Typical application is a sheet metal punch press in which the cylinder must extend

rapidly over great distance with very low pressure but high flow-rate requirements.

Operation of a sheet metal punch press

- In the left position, rapid extension occurs under no external load as the punching to

which is connected to the end of the cylinder approaches the sheet metal strip to be

punched.

- During the short motion portion when the punching operation occurs, the pressure

requirements are high due to the punching load. During the punching operation, the

cylinder travel is small and thus the flow-rate requirements are low.

- When the punching operation begins, the increased pressure opens the unloading valve

to unload the low-pressure pump. The purpose of the relief valve is to protect the high-

pressure pump from overpressure at the end of the cylinder stroke and when the DCV is

in its center position.

- The check valve protects the low-pressure pump from high pressure, which occurs

during the punching operation, at the ends of the cylinder stroke, and when the DCV is inits center position.




Sequencing hydraulic circuits

o sequencing circuits operate actuators in a specific order

o specially constructed pressure control valves can be used to sequentially block fluid

flow to actuators

– When a preselected pressure is reached, these valves open to allow fluid flow tothe actuator.

– This provides the order of operation.

– Check valves allow reverse flow around these pressure control valves.

Operation:

– Figure below shows a hydraulic sequence circuit where two sequence valves are

used to control the sequence of operations of two double-acting cylinders.

– When the DCV is in the left position, the left cylinder extends completely, and

then the right cylinder extends.

– If the DCV is in the right position, the right cylinder retracts fully first, and then

the left cylinder retracts.

– The sequence of the cylinder operation is controlled by the sequence valves.




– When in the center position of the DCV, both cylinders are locked in place.

– Example of application can be seen in production operation:

o Where the left cylinder could extend and clamp a workpiece via a power

vise jaw. Then the right cylinder extends to drive a spindle to drill a holein the workpiece.

o The right cylinder then retracts the drill spindle, and then the left cylinder

retracts to release the workpiece for removal.




Cylinder synchronizing hydraulic circuits

o Figure below shows two identical cylinders which is synchronized by piping them in

parallel.

o The loads on the cylinders must also be the same in order for them to extend in exactsynchronization. If the loads are not the same, the cylinder with the smaller load will

extend first because it will move at a lower pressure level.

o After this cylinder has fully completed its stroke, the system pressure will increase to

the higher level required to extend the cylinder with the greater load.

o However, in most cases, no two cylinders are really the same preventing cylinder

synchronization of the circuit.

o Therefore, to ensure cylinders movement is synchronize, the two cylinders must be

connected in series.

o The figure below shows that connecting two-cylinders in series is a simple way to

synchronize the two cylinders.

o During the extending stroke of the cylinders, fluid from pump is delivered to the

blank end of cylinder 1 via the flow path shown in the upper position of the DCV. As

cylinder 1 extends, fluid from its rod end is delivered to the blank end of cylinder 2.

o Fluid returns to the oil tank from the rod end of cylinder 2 as it extends, via the DCV.




Motor powered hydraulic circuits

Two factors that are basic to motor operation are:

i. Fluid flow rate establishes motor speed

ii. Pressure establishes torque output

o Figure below shows a circuit where speed control of a hydraulic motor is

accomplished using a pressure-compensated flow control valve.

The operation is as follow:

i. In center position, the motor is hydraulically locked.

ii. In left position, the motor rotates in one direction. Its speed can be varied

by adjusting the setting of the throttle of the flow control valve.

iii. When returned to the center position, the motor stops suddenly and

becomes locked.

iv. In right position, the motor turns in the opposite direction. The pressure

relief valve provides overload protection if, for example, the motor

experience an excessive torque.




Hydraulic motor braking system

o When using hydraulic motor in fluid power system, consideration should be

given to the type of loading that the motor will experience.

o A hydraulic motor may be driving a machine having large inertia (requireshigher work output to ensure motion).

o This would create a flywheel effect (continuous motion even after loading is

removed) resulting in build-up pressure at the pump when flow of fluid to the

motor is stopped.

o The figure below shows a hydraulic motor braking circuit which would stop

the motor rapidly but without damage to the system.




Tutorial Questions

1. When analyzing or designing a hydraulic circuit, what are three important considerations?

2. What is the purpose of a regenerative circuit?

3. List down, sketch and describe the operation of a single-acting hydraulic cylinder.

4. List down, sketch and describe the operation of a double-acting hydraulic cylinder.

5. Name one application of a regenerative cylinder circuit, list down, sketch, and describe its

operation.

6. List down, sketch and describe the operation of a sheet metal punch press machine.

7. List down, sketch and describe the operation of a hydraulic cylinder sequencing circuit.

8. List down, sketch and describe the operation for the hydraulic cylinder synchronizing circuits.

9. How is the speed of a hydraulic motor accomplished? List down, sketch, and describe the

circuit.

10. Describe the hydraulic motor breaking system.

mem 341_chapter 9 hydraulic circuit

Documents