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
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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)