Hydraulics Versus Pneumatics

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Fluid Power

• Hydraulics Versus Pneumatics

Lesson Plan
By Clifford Mayhew
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Pneumatic and Hydraulic Control Comparison

• Is Infinitely Controllable
• Produces Extremely Large Forces
• Requires High Pressures
• Requires Heavy Duty Components

• Clean
• Fast
• Intrinsically Safe
• Overload Safe
• Inexpensive for Individual Components

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Pneumatic and Hydraulic Dangers
The 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

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Differences in Symbols
Symbols Reminder
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Differences in Symbols
Cylinders and other actuators also differ with
respect to supply and direction arrows.
Many symbols do not change, for example the
Filter symbol.
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Differences in Symbols
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Differences in Medium
Hydraulic Oil is Hygroscopic and can be easily
Oxidised at high temperatures.
Hydraulic Fluid is viscous and can be various
types with varying Viscosities including

• Flame Retardant
• Mineral Oil
• Synthetic Oil
• Water Glycol

Hydraulic fluid can have many additives including

• Anti Oxidants
• Lubricity Improvement
• Anti Foaming Additive
• Anti Wear Additives

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.
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Differences in Principles and Properties
Pneumatic systems rely on a supply of Compressed
air flowing through Pipes to Actuators. The Force
for work is produced due to the Pressure of the
Air acting on the Area of the actuator.
Air is Compressible.
Gas laws such as Boyles and Charless Laws
govern medium behaviour
Actuator demand is measured in m3 per hour or
operation
Compressor output is measured in m3 per hour Free
Air Delivery (FAD)
Hydraulic systems rely on a supply of
incompressible 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.
Oil is considered Incompressible.
Bernoullis and other Fluid Flow Laws govern
medium behaviour
Actuator demand is measured litres per minute for
a specific speed
Pump output is measured litres per minute
Both Hydraulics and Pneumatics are described with
Pascals Law and FPA
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Differences in Pressure and Force
Pneumatic Pressures and Forces

• Produced at 10Bar
• Used at 06 Bar
• Forces up to 5000Kg

Hydraulic Pressures and Forces

• Produced and used at 200400Bar
• Forces up to Thousands of tonnes

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Differences in ConstructionProduction
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Differences in ConstructionValves and Actuators
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Differences in ConstructionHoses, Pipes and
Connectors
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Differences in Application
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Application Example Video
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Differences in Circuit DesignPneumatic
This is a typical pneumatic Schematic. The
Sequence Of Operation of the machine is as
follows
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 To Size circuit board is
opened for the operator.
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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
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. The
system is called a Counterbalance Circuit because
it hydraulically counterbalances the load.
A pneumatic system would not normally need to
control loads of this magnitude, therefore this
type of circuit would not be seen in a pneumatic
schematic.
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Simple Application ExampleHydraulic
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Simple Application ExamplePneumatic
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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 or Indirectly 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