Transcript DRIVE SYSTEM ENT 271

HYDRAULICS & PNEUMATICS
Introduction
Components
Presented by: Dr. Abootorabi
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Introduction
Three basic methods of transmitting power:
 Electrical
 Mechanical
 Fluid power
In practice, most applications actually use combination of the three
methods to achieve the most efficient overall systems.
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Introduction
 We have to understand the features of each method in order
to get the best result.
 For
example, fluid systems can transmit power more
economically over greater distance compare to mechanical
systems. But fluid systems are restricted to shorter distances
compared to electrical systems.
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Definition
 Hydraulics is the science of
forces
and
movements
transmitted by means of
liquids.
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Comparison of power system
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Comparision of power system
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Applications of hydraulic power
Machine tools
 Machine-tool
typical
area
hydraulics.
machine
construction
of
With
tools,
workpieces
are
is
application
modern
the
a
of
CNC
tools
and
clamped
by
hydraulic means. Feed motions
and the spindle drive can also be
hydraulically powered.
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Applications of hydraulic power
Press with elevated reservoir
 This is an application in which extremely high
forces are required.
 A special feature is the elevated reservoir, which
utilizes the static pressure in the pressure
medium.
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Applications of hydraulic power
Mobile hydraulics: Excavator
 On this hydraulic excavator,
not
only
movements
all
working
(linear
drives)
but also the propulsion of the
vehicle
(rotary
hydraulically
drive)
are
powered. The
primary
drive
excavator
is
an
of
the
internal-
combustion engine.
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Fluid power
 Fluid power is divided into two: hydraulic system (using oils)
and pneumatic system (using compressed air).
 Each system has its own advantages and drawbacks.
 There are many factors to consider to choose a suitable
system.
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Advantages of fluid power can be summarized as
follows:
 Ease and accuracy of control
 Multiplication of force
 Constant force or torque
 Simplicity, safety, economy
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Drawbacks of fluid power
 Hydraulic oils are messy and leakage is impossible to eliminate
completely.
 Hydraulic lines can burst and might result in injuring people and
damaging surrounding objects.
 Prolonged exposure to loud noise can damage hearing.
 Most hydraulic oils can cause fires if there is a leakage.
 Compressive air for pneumatic systems can be dangerous if the
pressure is too high.
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Structure of a hydraulic system
 This simplified block diagram
shows the division of hydraulic
systems into a signal control
section and a hydraulic power
section. This signal control
section is used to activate the
valves in the power control
section.
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Hydraulic power section
 The diagram of the hydraulic power
section is complemented in this case by
a circuit diagram to allow correlation of
the various function groups; the power
supply section contains the hydraulic
pump and drive motor and the
components for the preparation of the
hydraulic fluid. The power control
section consists of the various valves
used to provide control and regulate the
flow rate, pressure and direction of the
hydraulic fluid. The drive section
consists of cylinders or hydraulic motors,
depending on the application in
question.
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Components of a fluid power system: Hydraulic system
A hydraulic system has six basic components:
 A tank to hold the hydraulic oil
 A pump to force the oil through the system
 An electric motor or other power source to drive the pump
 Valves to control oil direction, pressure and flow rate
 An actuator to convert the pressure of the oil into mechanical
force or torque to do useful work
 Piping to carry the oil from one location to another
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Components of a fluid power system: Hydraulic system
•
•
•
•
•
A tank (reservoir) to hold the hydraulic oil (A)
An electronic motor or other power source to drive the pump (B)
A pump to force the oil through the system (C)
Valves to control oil direction, pressure and flow rate (D-G)
An actuator to convert the pressure of the oil into mechanical force
or torque to do useful work (H)
• Piping to carry the oil from one location to another
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Components of a fluid power system: Pneumatic system
A pneumatic system also has six basic components:
 An air tank to store a certain volume of compressed air
 A compressor to compress the air coming from the atmosphere
 An electric motor or other prime mover to drive the compressor
 Valves to control air direction, pressure, and flow rate.
 Actuators, which are similar in operation to hydraulic actuators
 Piping to carry the pressurized air from one location to another
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Components of a fluid power system: Pneumatic system
A pneumatic system also has six basic components:
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Primary functions of a hydraulic fluid
 Transmit power
 Lubricate moving parts
 Seal clearance between mating parts
 Dissipate heat
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Hydraulic fluid
 In order to be safe, hydraulic fluids must also be changed
periodically.
 The frequency of changing depends on the fluid as well the
operating conditions.
 Advice from laboratory analysis could be sought to determine
when the fluid should be changed.
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Fluids: liquids and gases
 A liquid is a fluid which has a definite volume independent of
the shape of its container.
 A liquid is considered to be incompressible so that its volume
does not change with pressure changes.
 This is only approximation but the change in volume due to
pressure change is quite small that it is ignored for most
engineering purposes.
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Fluids: liquids and gases
 A gas is a fluid which is compressible.
 In addition, its volume will vary to fill the vessel containing it.
 A gas is greatly influenced by the pressure to which it is
subjected.
 If the pressure increases, the volume decreases, and vice
versa.
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Fluids: liquids and gases
 Air is the only gas commonly used in fluid power systems
because it is inexpensive and readily available.
 Air has the following desirable features:
1. Fire resistant
2. Not messy
3. Can be released back to atmosphere
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Disadvantages of air
 Due its compressibility, air cannot be used in an application requiring
accurate positioning or rigid holding.
 Because air is compressible, it tends to be sluggish.
 Air can be corrosive since it contains oxygen (about 21%) and water.
 A lubricant must be added to air to lubricate valves and actuators.
 High pressure air (greater than 250 psi = 17 atm) is typically not
used due to the explosive dangers.
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Hydrostatic pressure
 Hydrostatic pressure is the pressure
created above a certain level within a
liquid as a result of the weight of the
liquid mass. Hydrostatic pressure is
not dependent on the shape of the
vessel concerned but only on the
height and density of the column of
liquid.
 Hydrostatic pressure can generally be
ignored for the purpose of studying
hydraulics.
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Pressure propagation
 If a force F acts on an area A of
an enclosed liquid, a pressure p
is
produced
which
acts
throughout the liquid (Pascal's
Law).
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Power transmission
 If a force F1 is applied to an area A1 of
a liquid, a pressure p results. If, as in
this case, the pressure acts on a larger
surface A2, then a larger counter-force
F2 must be maintained. If A2 is three
times as large as A1, then F2 will also
be three times as large as F1.
 Hydraulic
power
transmission
is
comparable to the mechanical law of
levers.
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Displacement transmission
 If the input piston of the hydraulic
press travels a distance s1, a
volume of fluid will be displaced.
This same volume displaces the
output piston by the distance s2.
If the area of this piston is larger
than that of the input piston, the
distance s2 will be shorter than
s1.
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Pressure transfer
 The fluid pressure p1 exerts a force F1 on the
surface A1 which is transferred via the piston rod
to the small piston. The force F1 thus acts on the
surface A2 and produces the fluid pressure p2 .
Since the piston area A2 is smaller than the piston
area A1, the pressure p2 must be larger than the
pressure p1.
 The
pressure-transfer
(pressure-intensification)
effect is put to practical use in pneumatic/hydraulic
pressure intensifiers and also in purely hydraulic
systems
when
extremely
high
pressures
required which a pump cannot deliver.
are
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Types of flow
 A distinction is made between laminar
flow and turbulent flow. In the case of
laminar flow, the hydraulic fluid moves
through the pipe in ordered cylindrical
layers. If the flow velocity of the hydraulic
fluid rises above a critical speed, the fluid
particles at the center of the pipe break
away to the side, and turbulence results.
 Turbulent flow should be avoided in
hydraulic circuits by ensuring they are
adequate sized.
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Types of flow
 Laminar
 Turbulent
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Cavitation
 Motion energy is required for an increase in
the flow velocity of the oil at a restriction.
This motion energy is derived from the
pressure energy. If the vacuum which
results is smaller than -0.3 bar, air dissolved
in the oil is precipitated out. When the
pressure rises again due to a reduction in
speed, the oil bursts into the gas bubbles.
 Cavitation is a significant factor in hydraulic
systems as a cause of wear in devices and
connections.
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Cavitation
 Local pressure peaks occur during cavitation. This causes the erosion
of small particles from the wall of the pipe immediately after the
reduced cross-section, leading to material fatigue and often also to
fractures. This effect is accompanied by considerable noise.
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Components of a hydraulic system
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Components of a hydraulic system
 Power supply section:
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Components of a hydraulic system
 Power supply section:
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Components of a hydraulic system
 Power supply section:
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Components of a hydraulic system
 Hydraulic fluid:
 Valves:
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Components of a hydraulic system
 Types of valves:
 1. Directional control valves:
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Components of a hydraulic system
 2. Pressure valves:
 00
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Components of a hydraulic system
 3. Flow control valves:
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Components of a hydraulic system
 4. Non-return valves:
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Components of a hydraulic system
 Cylinders (linear actuators):
 1. Single-acting Cylinders:
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Components of a hydraulic system
 Cylinders (linear actuators):
 2. Double-acting cylinders:
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Components of a hydraulic system
 Motors (rotary actuators):
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Components of a hydraulic system
 Tank (reservoir) for a hydraulic system:
 The function of a tank is to store the fluid used. However, it serves
functions other than storage and is actually a working part of the
system.
 A hydraulic reservoir or tank has the following functions:
 Stores the hydraulic fluid of the system, including some reserve
 Protects the stored fluid from outside contamination
 Provides means to check the amount of fluid in the system
 Provides means to add or change the fluid
 Cools the fluid as it returns from the actuators, and
 Removes contaminants such as water, dirt, pieces of metal, or
chemicals from the fluid
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Components of a hydraulic system
Tank (reservoir) of a hydraulic system
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Components of a hydraulic system
Tank (reservoir) of a hydraulic system
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Components of a hydraulic system
 Most tanks are of welded construction with supports for
mounting for easy access to the drain plug and also to permit
cooling air to circulate underneath.
 A tank must be totally enclosed and should have a filtered air
breather to screen out particles from the surrounding air.
 The fluid that flows in the hydraulic system must be cleaned.
Contaminant are screened out using a strainer and a filter. Some
reservoirs have magnetic plugs to trap iron and steel particles
carried by the fluid.
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Components of a hydraulic system
 A strainer blocks the relatively large solid particles from
entering the system. It is attached to the pump inlet line and
may immersed in the oil near the bottom of the tank. Particles
stuck to the strainer are cleaned off later and the strainer is
ready for reuse.
 A filter is used to remove smaller particles by absorbing them.
Fluid is allowed to flow through but fine particles are trapped
and absorbed. When the filter becomes clogged, it is replaced
by a new one.
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Components of a hydraulic system
 Strainers and Filters:
 Strainers are constructed of a fine wire screen that usually has
openings more than 150 micron or μm.
 A strainer only moves the larger particles.
 The condition of the strainer can be monitored by installing a
pressure gage between the pump and the strainer. A pressure drop
shown by the gage indicated that the strainer is becoming clogged.
If the strainer is not cleaned, the pump can be starved, resulting in
cavitations and increased pump noise.
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Components of a hydraulic system
 Basically, filters and strainers are similar. However, the size of
particles that can be removed by a strainer is normally greater
than 150 μm. On the other hand, a filter can remove much
smaller particles, down to 1 μm.
 Even particles as small as 1 μm can produce a damping effect
on hydraulic systems and can also accelerate oil deterioration.
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The end.
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