Transcript Fluid Power - lehrerfortbildung

Fluid Power
Hydraulics Versus Pneumatics
Lesson Plan
By Clifford Mayhew
Pneumatic and Hydraulic Control
Comparison
Pneumatic Control Is:
Hydraulic Control:
Clean
Is Infinitely Controllable
Fast
Produces Extremely Large Forces
Intrinsically Safe
Requires High Pressures
Overload Safe
Requires Heavy Duty Components
Inexpensive for Individual Components
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
This injury is a result of placing the hand in front of
a jet of leaking hydraulic fluid at around 180 Bar
Differences in Symbols
Symbols Reminder
Compressor
Hydraulic Pump
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.
Hydraulically Actuated and
supplied 3/2 Pilot Spring
Pneumatically Actuated
and supplied 3/2 Pilot
Spring
Differences in Symbols
Cylinders and other actuators also differ with respect to supply and direction
arrows.
Hydraulic Double Acting Cylinder
Pneumatic Double Acting Cylinder
Many symbols do not change, for example the Filter symbol.
Hydraulic Filter
Pneumatic Filter
Remember however that the physical construction is completely different.
For example, hydraulic filters can be either Suction Strainers (suction side of the
pump), Pressure Filter (pressure side of the pump) or Return Filter (in the return
to tank line). Each filter requires different properties.
Differences in Symbols
Hydraulic valves have a crossover to Tank.
Pneumatic valves tend to have two
Exhaust outlets to Atmosphere.
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.
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 Boyle’s and Charles’s 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.
Bernoulli’s 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 Pascal’s Law and F=PA
Differences in Pressure and Force
Pneumatic Pressures and Forces
Produced at 10Bar
Used at 0~6 Bar
Forces up to 5000Kg
Hydraulic Pressures and Forces
Produced and used at 200~400Bar
Forces up to Thousands of tonnes
Force
Force Calculator
Area
Pressure
Differences in Construction
Production
The hydraulic Power
Pack contains the
Pump,
Tank
(Reservoir), Filters
and commonly a
Relief
Valve
for
protection of the
system. The unit is
usually local to the
machine that is using
it. Hydraulic pumps
are usually Positive
Displacement
devices
which
means they displace
all the oils they
pump.
The
Pneumatic
Compressor
installation usually
includes a Dryer
and Receiver. The
unit
is
usually
remote from the
machine that is
using it.
Differences in Construction
Valves 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 when
compared 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.
Differences in Construction
Hoses, Pipes and Connectors
Hydraulic hoses and
connectors are heavily
constructed to hold the
higher
pressures.
Rubber hoses are
steel
Reinforced
(Braided)
to
Strengthen them.
Fittings
Hose
Pipe
(Connectors)
Braid
(Reinforcement)
Fittings
(Connectors)
Straight
Coupling
Y
Tee
Pneumatic Pipes and
Fittings are of light
construction.
Pneumatic Pipe is
made from nylon and
generally connects to
the
fittings
using
‘Push Fit’ connectors.
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 light
moving, Clamping and
Process operations
Application Example Video
Lifting a car on a Car Ramp does
not require high speed or clean
control systems. Large forces are
required to lift the heavy car. This
application is particularly suited to
the use of hydraulics.
Moving and light clamping of
components is easily, cleanly and
quickly achieved using a pneumatic
control system.
Differences in Circuit Design
Pneumatic
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.
Timer
The highlighted timer is a Sub Circuit of the main schematic. The
timer contains a Accumulator, Restrictor, Check Valve and 3/2 DCV
Restrictor
Accumulator
Check Valve
3/2 DCV
All the components on the main schematic represent components that
control Sequencing, Timing and Positioning
Differences in Circuit Design
Hydraulic
This schematic
is
from
a
system
that
manufactures
Castings. The
system
is
controlled via a
computer.
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.
Counterbalance Valves
Simple Application Example
Hydraulic
DCV
Filter
Tank
Simple Application Example
Actuator
Pneumatic
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.
Electro-Pneumatic and Hydraulic
Systems
Reed Switch
Push Button
Switch
Programmable
Logic
Controller
(PLC)
Relay
Solenoid
Solenoid
Proximity
Sensor