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8/7/2019 vinayak bindu hydraulicsvspneumatics

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Hydraulics Versus Pneumatics

By Clifford Mayhew

Fluid Power

Lesson Plan
http://../Esslingen%20Lesson%20Plan.dochttp://upload.wikimedia.org/wikipedia/commons/9/91/Kettenbagger_CAT_325C_LN.jpeghttp://../Esslingen%20Lesson%20Plan.doc


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Pneumatic and Hydraulic Control

Comparison

Pneumatic Control Is:

Clean

Fast

Intrinsically Safe

Overload Safe

Inexpensive for Individual Components

Hydraulic Control:

Is Infinitely Controllable

Produces Extremely Large Forces

Requires High Pressures

Requires Heavy Duty Components


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Pneumatic and Hydraulic

DangersThe dangers of the use of compressed air include:

Air Embolism

Hose/Pipe Whipping

Noise

Crushing/Cutting

The dangers of working with high pressure oil can be

infinitely more drastic:

High Pressure Oil Injection

Oil Burns

Crushing/Cutting

Carcinogens

This injury is a result of placing the hand in front

of a jet of leaking hydraulic fluid at around 180

Bar


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Differences in Symbols

Symbols Reminder

Hydraulic Pump Compressor

Pumps and Motors

differ only by filling in

the direction arrow or

leaving it white.

Supply and Pilot

arrows are also filled

in or left white.

Pneumatically Actuated

and supplied 3/2 Pilot

Spring

Hydraulically Actuated and

supplied 3/2 Pilot Spring
http://../Notes/Symbols%20Reminder.dochttp://../Notes/Symbols%20Reminder.doc


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Differences in SymbolsCylinders and other actuators also differ with respect to supply and direction

arrows.

Hydraulic Double Acting Cylinder Pneumatic Double Acting Cylinder

Hydraulic Filter Pneumatic Filter

Many symbols do not change, for example the Filtersymbol.

Remember however that the physical construction is completely different.

For example, hydraulic filters can be eitherSuction Strainers (suction side of the

pump), Pressure Filter(pressure side of the pump) orReturn Filter(in the return

to tank line). Each filter requires different properties.


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Differences in Symbols

Hydraulic valves have a crossover to Tank.

Pneumatic valves tend to have two

Exhaust outlets to Atmosphere.


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Differences in Medium

Hydraulic Fluid is viscous and can be various types with varyingViscosities including:

Hydraulic fluid can have many additives including:

Hydraulic Oil is Hygroscopic and can be easily Oxidised at high temperatures.

Air can hold Moisture which can turn into Condensation at the Dew Point.

The pneumatic components must remove the Condensation from

the air and provide Lubrication.

Air is safe under a wide range of operating temperatures.

Flame RetardantMineral OilSynthetic OilWater Glycol

Anti OxidantsLubricity ImprovementAnti Foaming AdditiveAnti Wear Additives


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Differences in Principles and

Properties

Air is Compressible.

Oil is considered Incompressible.

Actuator demand is measured in m3 per houror operationCompressor output is measured in m3 per hourFree Air Delivery (FAD)

Gas laws such as Boyles and Charless Laws govern medium behaviour

Both Hydraulics and Pneumatics are described with Pascals Law and F=PA

Bernoullis and otherFluid Flow Laws govern medium behaviour

Actuator demand is measured litres per minute for a specific speed

Pump output is measured litres per minute

Pneumatic systems rely on a supply ofCompressed air flowing through Pipes to

Actuators. The Force for work is produced due to the Pressure of the Airacting

on the Area of the actuator.

Hydraulic systems rely on a supply ofincompressible fluid flowing through Hoses

to Actuators. The Force for work is produced due to the Pressure of the Oil acting

on the Area of the actuator.


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Differences in Pressure and

ForcePneumatic Pressures and Forces

Hydraulic Pressures and Forces

Force Calculator

Force

Pressure

Area

Produced at 10BarUsed at 0~6 BarForces up to 5000Kg

Produced and used at 200~400BarForces up to Thousands of tonnes
http://../Force%20Calculator/Force%20Calculator.htmhttp://../Force%20Calculator/Force%20Calculator.htm


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Differences in ConstructionProduction

The hydraulic Power

Pack contains the

Pump, Tank

(Reservoir), Filters

and commonly a

Relief Valve for

protection of the

system. The unit isusually local to the

machine that is using

it. Hydraulic pumps

are usually Positive

Displacement

devices which

means they displace

all the oils theypump.

The Pneumatic

Compressorinstallation usually

includes a Dryer

and Receiver. The

unit is usually

remote from the

machine that is

using it.


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Differences in ConstructionValves and Actuators

Pneumatic valves and actuators are

generally of light construction as they need

to deal with pressure up to a maximum of

10 Bar.

The cost of these components is cheap

when compared to the much more heavily

constructed hydraulic components.

Hydraulic valves and actuators are much more

heavily constructed than pneumatic

components. This is because the components

must deal with pressures up to 400 Bar+.

Hydraulic actuators can be very large whencompared with common pneumatic actuators.

Hydraulic components are much more

expensive than standard pneumatic

components. A standard hydraulic DCV is in the

region of hundreds of Euro, a standard

application pneumatic valve would typically cost

tens of Euro.


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Differences in ConstructionHoses, Pipes and Connectors

Braid

(Reinforcement)

Hose

Fittings

(Connectors)

Hydraulic hoses and

connectors are heavily

constructed to hold the

higher pressures.

Rubber hoses are

steel Reinforced(Braided) to

Strengthen them.

Pipe

Y

Tee

Straight

Coupling

Fittings

(Connectors)

Pneumatic Pipes and

Fittings are of light

construction.

Pneumatic Pipe is

made from nylon and

generally connects to

the fittings using

Push Fit connectors.


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Differences in Application

Hydraulic systems are

used where large forces

are required such as in

earth moving equipment,

heavy cutting, Pressing

and Clamping

Pneumatic systems are

used for relatively lightmoving, Clamping and

Process operations
http://upload.wikimedia.org/wikipedia/commons/9/91/Kettenbagger_CAT_325C_LN.jpeg


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Application Example Video

Moving and light clamping of

components is easily, cleanly and

quickly achieved using a pneumatic

control system.

Lifting a car on a Car Ramp does

not require high speed or cleancontrol systems. Large forces are

required to lift the heavy car. This

application is particularly suited to

the use of hydraulics.


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Differences in Circuit DesignPneumatic

The operator places a circuit board into an

open drawer on the machine.

The operator then closes the drawer, pushing

in the Slide Cylinder.

The drawer immediately locks shut (Shot Bolt)

and the Press Cylinder extends to cut the

edges off the circuit board.

After a short delay, (Timer) the press cylinder

returns.

Once the press cylinder has returned, the shot

bolt retracts and the drawer, with the Cut ToSize circuit board is opened for the operator.

Timer

This is a typical pneumatic Schematic. The

Sequence Of Operation of the machine is as

follows:


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The highlighted timer is a Sub Circuit of the main schematic. The timer

contains a Accumulator, Restrictor, Check Valve and 3/2 DCV

Restrictor

Check Valve

Accumulator3/2 DCV

All the components on the main schematic represent components that

control Sequencing, Timing and Positioning


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Differences in Circuit DesignHydraulic

This schematic

is from a

system that

manufactures

Castings. The

system is

controlled via a

computer.


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This circuit shows

components that are used

to control heavy loads in

the hydraulic system. Thesystem is called a

Counterbalance Circuit

because it hydraulically

counterbalances the load.

Counterbalance Valves

A pneumatic system

would not normally need

to control loads of this

magnitude, therefore thistype of circuit would not

be seen in a pneumatic

schematic.


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Simple Application ExampleHydraulic

Pump

Tank

DCV

Actuator

Filter


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Simple Application ExamplePneumatic

FR

L3/2DCV

5/2DCV

Actuator


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Electro-Pneumatic and Hydraulic

Systems

Control of Electro-Pneumatic and Hydraulic systems using Electrical control

systems is similar for both media types.

Both systems would use Solenoid actuated valves, either Directly

Actuated orIndirectly Actuated.

An Electronic system would commonly incorporate Push Button Switches

(for human input), Reed Switches (to detect cylinder position), Proximity

Sensors and Photocells (to detect machine/component position).

An Electronic system would also commonly incorporate Relays and

computer controlled systems such as Programmable Logic Controllers.


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Electro-Pneumatic and Hydraulic

Systems

Push Button

Switch

Reed Switch

Proximity

Sensor

Solenoid

RelaySolenoid

Programmable

Logic

Controller

(PLC)

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