COURSE NUMBER: ME 433
Fluidics
3 credit hour
Course teacher
P f Mahbubur
Prof.
M hb b Razzaque
R
1
Fluid Power Control
Fluid power control deals with the generation, control, and
transmission of ppower usingg ppressurized fluids.
It can be said that fluid power is the muscle that moves industry.
This is because fluid ppower is used to ppush,, ppull,, regulate,
g
, or drive
virtually all the machines of modern industry.
For example, fluid power steers and brakes automobiles, launches
spacecraft, moves earth, harvests crops, mines coal, drives machine
tools, controls airplanes, processes food, and even drills teeth.
In fact, it is almost impossible to find a manufactured product that
has not been "fluid-powered" in some way at some stage of its
production or distribution.
distribution
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Fluid Power
Fluid power is called hydraulics when the fluid is a liquid and is
called pneumatics when the fluid is a gas. Thus fluid power is the
ggeneral term used for both hydraulics
y
and ppneumatics.
Hydraulic systems use liquids such as petroleum oils, synthetic oils,
and water. The first hydraulic fluid to be used was water because it
is readily available. However, water has many deficiencies. It freezes
readily, is a relatively poor lubricant, and tends to rust metal
components Hydraulic oils are far superior und hence are widely
components.
used in lieu of water.
Pneumatic systems use air as the gas medium because air is very
abundant and can be readily exhausted into the atmosphere after
completing the assigned task.
There are actually two different types of fluid systems: fluid
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transport system and fluid power system.
Fluid transport
p
systems
y
have as their sole objective
j
the deliveryy of
a fluid from one location to another to accomplish some useful
purpose. Examples include pumping stations for pumping water to
homes,, cross-countryy ggas/oil lines,, and systems
y
where chemical
processing takes place as various fluids are brought together.
Fluid power systems are designed specifically to perform work. The
work is accomplished by a pressurized fluid bearing directly on an
operating fluid cylinder or fluid motor.
A fluid cylinder produces a force resulting in linear motion, whereas
a fluid motor produces a torque resulting in rotary motion. Thus in a
fluid power system,
system cylinders and motors (which are also called
actuators), provide the muscle to do the desired work.
Of course,
course control components such as valves are needed to ensure
that the work is done smoothly, accurately, efficiently, and safely.
4
Hydraulics and Pneumatics systems
Hydraulics systems use liquids which provide a very rigid medium
for transmitting power and thus can operate under high pressures to
provide huge forces and torques to drive loads with utmost accuracy
and precision.
On the other hand. pneumatics systems exhibit spongy
characteristics due to the compressibility of air. However, pneumatic
s stems are less expensive
systems
e pensi e to build
b ild and operate.
operate These systems
s stems can
be used effectively in applications where low pressures can be used
because the loads to be driven do not require large forces. They
provide
id faster
f t motion
ti but
b t have
h
l positional
less
iti l accuracy.
5
Power transmitting systems
There are three basic methods of transmitting power: electrical,
mechanical, and fluid power.
Most applications actually use a combination of the three methods to
obtain the most efficient overall system.
y
To pproperly
p y determine
which method to use, it is important to know the salient features of
each type.
For example, fluid systems can transmit power more economically
over greater distances than mechanical systems can. However, fluid
systems are restricted to shorter distances compared to the electrical
systems.
The fluid power has greater versatility and manageability.
manageability Fluid
power is not hindered by the geometry of the machine, as is the case
in mechanical systems.
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Also, power can be transmitted in almost limitless quantities
Also
depending on the physical strength of the material used for each
component.
Industry is going to depend more and more on automation in order to
increase productivity. This includes remote and direct control of
production
d ti
operations,
ti
manufacturing
f t i
processes, andd materials
t i l
handling.
Fluid
l id power is
i well
ll suited
i d for
f the
h automation
i applications
li i
b
because
off
advantages in the following four major categories.
1. Ease and accuracy of control. By the use of simple levers and
push buttons, the operator of a fluid power system can readily start,
stop, speed up or slow down, and position forces that provide any
desired horsepower with tolerances as precise as one ten-thousandth
of an inch.
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2. Multiplication of force. A fluid power system (without using
cumbersome gears,
gears pulleys,
pulleys and levers) can multiply forces simply
and efficiently from a fraction of an ounce to several hundred tons of
output.
3. Constant force or torque. Only fluid power systems are capable
of providing constant force or torque regardless of speed changes.
Thi is
This
i accomplished
li h d whither
hith the
th workk output
t t moves a few
f
i h
inches
per hour, or thousands of revolutions per minute.
4. Simplicity, safety, economy. In general,
l fluid
fl id power systems use
fewer moving parts than comparable mechanical or electrical
systems. Thus they are simpler to maintain and operate. This, in turn,
maximizes safety, compactness and reliability.
Additional benefits of fluid power systems include instantly
reversible motion, automatic overload protection, infinitely variable
speed control and the highest power-per-weight ratio.
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Drawbacks of fluid power
Fluid power systems also have some drawbacks. For example,
hydraulic oils are messy, and leakage is impossible to eliminate
completely.
l t l Hydraulic
H d li lines
li
can burst,
b t possibly
ibl resulting
lti in
i injuries
i j i
to people due to high-speed oil jets and flying pieces of metal if
proper design is not implemented.
Prolonged exposure to loud noise, such as that emanating from
pumps, can result in loss of hearing. Also, most hydraulic oils can
cause fires if an oil leak occurs in an area of hot equipment.
In pneumatic systems, components such as compressed air tanks and
accumulators are potentially explosive if the pressure is allowed to
increase beyond safe design limits. Therefore each application must
be studied thoroughly to determine the best overall system to
employ.
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Components of a hydraulic system
There are six basic components required in a hydraulic system:
11. A tank
t k (reservoir)
(
i ) to hold
h ld the
h hydraulic
h d li oil
il
2. A pump to force the oil through the system
3. An electric motor or other power source to drive the pump
4. Valves to control oil direction, pressure, and flow rate
5. An actuator to convert the pressure of the oil into mechanical
force or torque to do useful work. Actuators can either be cylinders
or motors (hydraulic)
6. Piping, which carries the oil from one location to another.
The sophistication and complexity of hydraulic systems will vary
depending on the specific applications. This is also true of the
individual components that comprise the hydraulic system.
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11
12
Components of a Pneumatic system
Pneumatic systems have components that are similar to those used in
hydraulic systems. Essentially the following six basic components
are required
i d for
f pneumatic
i systems:
1. An air tank to store a given volume of compressed air
2. A compressor to compress the air that comes directly from the
atmosphere
3. An electric motor or other prime mover to drive the compressor
4. Valves to control air direction, pressure, and flow rate
5. Actuators which are similar in operation to hydraulic actuators
6. Piping to carry the pressurized air from one location to another
In pneumatic systems after the pressurized air is spent driving
actuators, it is then exhausted back into the atmosphere. On the other
hand, in hydraulic
y
systems
y
the spent
p
oil drains back to the reservoir
and is repeatedly reused after being repressurized by the pump as
needed by the system.
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Hydraulic fluids
The single most important material in a hydraulic system is the
working fluid itself. Hydraulic fluid characteristics have a crucial
effect
ff
on equipment
i
performance
f
andd life.
lif It
I is
i important
i
to use a
clean, high-quality fluid in order to achieve efficient hydraulic
system operation.
Most modern hydraulic fluids are complex compounds that have
been carefully prepared to meet their demanding tasks. In addition to
having a base fluid, hydraulic fluids contain special additives to
provide desired characteristics. A hydraulic fluid has the following
four primary functions:
1. Transmit power
2. Lubricate movingg pparts
3. Seal clearances between mating parts
4. Dissipate heat
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Properties of hydraulic fluids
To accomplish properly the four primary functions and be practical
from a safety and cost point of view, a hydraulic fluid should have
the
h following
f ll i properties:
i
1. Good lubricity
2. Ideal viscosity
3. Chemical stability
4. Compatibility with system materials
5. High degree of incompressibility
6. Fire resistance
7. Good heat-transfer capability
8. Low density
9. Foam resistance
10. Non-toxicity
11. Low volatilityy
In addition a hydraulic fluid must be inexpensive and readily
available.
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Properties of hydraulic fluids
No single hydraulic fluid possesses all of the desirable
characteristics. The fluid power designer must select the fluid that
comes the
h closest
l
to being
b i ideal
id l overall
ll for
f a particular
i l application.
li i
Hydraulic fluids must also be changed periodically, the frequency
d
depending
di not only
l on the
h fluid
fl id but
b also
l on the
h operating
i conditions.
di i
Laboratory analysis is the best method for determining when a fluid
should be changed.
Generally a fluid should be changed when its viscosity and acidity
increase due to fluid breakdown or contamination. Preferably, the
fluid should be changed while the system is at operating
temperature. In this way, most of the impurities are in suspension
and will be drained off.
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Properties of hydraulic fluids
Much hydraulic fluid has been discarded in the past as it was more
expensive to test the fluid than to replace it. This situation has
changed
h
d as the
h needd to conserve hydraulic
h d li fluids
fl id has
h developed.
d l d
Nowadays, hydraulic fluid test kit is available that provides a quick,
easy method to test hydraulic system contamination. Even small
hydraulic systems may be checked. The test kit may be used on the
spot to determine whether fluid quality permits continued use.
Three key quality indicators can be evaluated: viscosity, water
content, and foreign particle contamination level.
Chemical properties dealing with maintenance of the quality of the
hydraulic fluids will discussed afterwards. These properties include
rate of oxidation, fire-resistance, foam-resistance, acidity, etc.
17
Fluids: Liquids and Gases
Liquids
Li
id are considered
id d to
t be
b incompressible
i
ibl so that
th t their
th i volume
l
does not change with pressure changes. This is not exactly true, but
the change in volume due to pressure changes is so small that it is
i
ignored
d for
f most engineering
i
i applications.
li i
Gases, on the other hand, are fluids that are readily compressible. In
addition, their volume will vary to fill the vessel containing them.
Gases are greatly influenced by the pressure to which they are
subjected. An increase in pressure causes the volume of the gas to
decrease, and vice versa.
Air is the only gas commonly used in fluid power systems because it
is inexpensive and readily available. Air also has the following
desirable features as a power fluid:
1. It is fire resistant.
2. It is not messy.
3. It can be exhausted back into the atmosphere.
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Fluids: Liquids and Gases
The disadvantages of using air versus using hydraulic oil are:
1 D
1.
Due to its
i compressibility,
ibili air
i cannot be
b usedd in
i an application
li i
where accurate positioning or rigid holding is required.
2. Because air is compressible, it tends to be sluggish.
3. Air
i can be
b corrosive,
i since
i
i contains
it
i oxygen andd water.
4. A lubricant must be added to air to lubricate valves and
actuators.
5. Air pressures of greater than 250 psi are typically not used due
to the explosion dangers involved if components such as air
tanks should rupture. This is because air (due to its
compressibility) can store a large amount of energy as it is
compressed in a manner similar to that of a mechanical spring.
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