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APPLIED HYDRAULICS AND PNEUMATICS
UNIT-- I FLUID POWER SYSTEMS AND FUNDAMENTALS
Introduction to fluid power, Advantages of fluid power, Application
of fluid power system. Types of fluid power systems, Properties of
hydraulic fluidsGeneral types of fluidsFluid power symbols.
Basics of Hydraulics-Applications of Pascals Law- Laminar and
Turbulent flowReynolds numberDarcys equation Losses in
pipe, valves and fittings.
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Fluid power is the use of confined fluid flowing underpressure to transmit power from one location to another.
It is a controlled, flexible muscle that provides power
smoothly, efficiently, safely and precisely to accomplish useful
work.
Fluid power steers and brakes automobiles, launches
space craft, moves earth, harvests crop, coal mine, drive
machine tools, control airplanes, processes food and even
drills teeth.
Fluid power technology deals with the conversion,
generation, control and transmission of fluid energy (power)
using pressurized fluids to do some useful work.
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Advantages of Fluid power system
1.Fluid power system can readily start, stop, speed up or slow down
and position very high forces with close tolerances using simple
levers and push buttons.
2. Fluid power can simply performs work by achieving greater forces
through the multiplication of smaller forces efficiently.
3. Fluid power provides constant torque at infinitely variable speeds
in either direction with smooth reversals.
4. Fluid power systems are simple to maintain and operate. Hence
maximum safety, compactness and easy installation is assured.
continue
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5. High accuracy in controlling small or large forces with instant
reversible motion is possible
6. Since the medium is fluid, these systems are not subjected to any
breakage of parts as in mechanical systems.
7. Fluid power is efficient, economical, dependable, and readily
available and provides predictable performance.
8. Hydraulic fluid power systems have the provision of automatic
lubrication for less wear.
9. It easily provides infinite and step less variable speed control.
10. Automatic protection against overload is possible in Fluid power
systems.
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Disadvantages of Fluid power system
1. Hydraulic oils are messy.
2. It is impossible to eliminate the leakages completely.
3. Improper design of Hydraulic lines can burst and results in
injuries to the operator.
4. Prolonged Exposure to loud noise.
5. Leakage of most of the hydraulic oils in hot equipment area
causes fire, as they are highly flammable.
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Applications of Fluid power systems
Some of the areas applying Fluid power are given below.
1. Agriculture : Hydraulically driven farm equipment
2. Automobile : Fluid power steering and braking systems.
3.Automation : Hydraulically powered robotic dexterous arm,
Transfer machine, machine tools, Pneumatically
operated indexing, holding, gripping and feeding
devices.
4.Aviation : Hydraulic retractable landing wheels.
5. Domestic
Applications : Hydraulic powered brush drive to clean roads,
floors.
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6. Construction: Excavators, Earth movers, Concrete mixing
equipments.
7. Defence: Missile Launch Systems, navigation controls.
8. Fabrication: High speed pneumatic riveter, Injection moldings
machine, pneumatic hand tools, Hydraulic controlled
Goliath (forging press).9.. Material
Handling: Industrial Lift trucks, Rear crawler shovels, Hydraulic
jacks. hydraulic rams, onveyor systems, pneumatically
operated packing, wrapping and butting equipments.
10. Transportation: Hydraulically powered overhead sky tram.
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Types of Fluid Power Systems
Based on the type of fluid used, the fluid power systems can
be classified into two systems.
1. Hydraulic system [Fluid - Liquid]
2. Pneumatic system [Fluid - Compressed air]
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Hydraulic system
Hydraulic systems are power - transmitting assemblies employing
pressurized liquid as a fluid fortransmitting energy from energy -generating source to energyuse area to accomplish work. Figure
shows the simple circuit of a hydraulic system with basic components.
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Advantages of Hydraulic system
1. Large load capacity with almost high accuracy and precision.
2. Smooth movement.
3. Automatic lubricating provision to reduce wear.
4. Division and distribution of Hydraulic power is simpler and Easier
than other forms of Energy.
5. Limiting and balancing of Hydraulic forces are easily performed.
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Disadvantages of Hydraulic system
1. Hydraulic Elements needs to be machined to a high degree of
precision.
2. Leakage of Hydraulic oil poses a problems to hydraulic
operators.
3. Special treatment is needed to protect them from rust, corrosion,
dirt etc;
4. Hydraulic oil may pose problems ifit disintegrates due to aging
and chemical deterioration.
5. Hydraulic oils are messy and almost highly flammable.
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Pneumatic systemsPneumatic system carries power by employing compressed gas
generally air as a fluid fortransmitting the energy from an energy -
generating source to energy - use area to accomplish work. Figureshows the simple circuit of a pneumatic system with basic components.
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Functions of the components Hydraulic Actuatoris a device used to convert the fluid power into
mechanical power to do useful work. Actuators may be linear typeor rotary type to provide linear or rotary motion respectively.
Hydraulic pump is used to force the fluid to the rest of the hydraulic
circuit from the reservoir.
By converting the mechanical energy into hydraulic Energy.
Valves are used to control the direction, pressure and flow rate of afluid flowing through the circuit.
External power supply(Motor) is required to drive the pump.
Reservoiris used to hold the hydraulic liquid usually hydraulic oil.
Piping system carries the hydraulic oil from one place to another. Filters are used to clean the hydraulic oil used in that circuit.
Pressure regulatorregulates i.e., maintains the required level of
pressure in the hydraulic fluid
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Advantages of pneumatic system
1. Low inertia effect of pneumatic components due to lightdensity of air
2. System is light weight
3. Comparatively easy operations of valves
4. Power losses and leakages are less in pneumatic systems5. Low cost
Disadvantages of pneumatic system
1. Suitable only for light loads or small loads.
2. Availability of the assembly components is doubtful
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S.No. Hydraulic System Pneumatic System
1 . It employs a pressurized liquid It employs a compressed gas
usually air as a fluid. as a fluid.
2. Generally, Hydraulic systems Pneumatic systems are usually
designed are designed as closed system. as open system.
3. System get slow down if leakage Leakage does not affect the more.
system much occurs.
4. Valve operations are difficult. Easy to operate the valves.
5. Heavier in weight. Light in weight
6. Pumps are used to provide Compressors are used to provide
pressurized liquid. compressed gas.
7. System is unsafe to fire Hazards. System is free from fire hazards
8. Automatic lubrication is provided. Special arrangements are made for
lubricating the parts.
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Requirements / Characteristics of a hydraulic Fluid
A Good hydraulic fluid should possess the following characteristics
1. High corrosive resistant properties.
2. Good lubrication properties to reduce friction and wear.
3. Good sealing properties
4. Good Heat transfer capabilities.
5. Not effected by temperature changes.6. Free from acidity and should be non-toxic.
7. High Flash point and low pour point.
8. Chemically and environmentally stable.
9. Less volatile. 10. High degree of incompressibility.11. Ideal viscosity. 12. Good Fire and Foam resistant properties.
13. Low density. 14. Readily available.
15. Inexpensive.
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Functions of a Hydraulic Fluid
1. To transmit fluid power efficiently to perform useful work.
2. To lubricate the moving parts to minimize wear and friction.
3. To absorb, carry and dissipate the heat generated within the
system.
4. To seal the close clearances between mating parts against leakage.
5. To prevent the rusting or corrosion.
6. To rapidly settle and separate the insoluble contaminants and
abrasion.
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Properties of Hydraulic Fluid
1. Specific Gravity or relative density
Specific gravity of a fluid is the ratio of the specific
weight of a liquid to the specific weight of water
2. ViscosityViscosity is the most important property of a hydraulic
fluid, as it determines the ability of a fluid to be pumped
and transmitted through the system
3. Viscosity Index
Viscosity Index (V.I) is a relative measure of change in
viscosity for a given change in temperature
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4. Neutralization Number
The Neutralization number is a measure of the acidity or alkalinity of
a hydraulic fluid And referred as PH factor of the fluid.
Neutralization Number = (Total ml of titrating solutionX5.61) /Weight of sample used
5. Lubricity
A good hydraulic fluid must lubricate all the integral moving parts of
the system.
6. Demulsibility
Demulsibility is the ability of a fluid to resist emulsification by
separating it from moisture(water).
7. Oxidation
Another important property of hydraulic fluid is the oxidationstability. Oxidation is caused by the chemical reaction between the
oxygen of the dissolved air and the oil
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8. CorrosionRust is the chemical reaction between iron or steel and oxygen.
Corrosion is the chemical reaction between a metal and acid.
9. Defaming
Foaming is the result of entrainment of air in oil. Typically, an oil can
contain up to 10% by volume of dissolved air.
10. Pour point, Flash point and Fire point
Pour point is the lowest temperature at which a fluid will flow. Flash
point is the Temperature at which the oil gives sufficient vapour at
the surface to ignite when a test flame is passed over the surface. Fire
point is the temperature at which the oil will release sufficient vapour
to support combustion
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Types of Fluids
Generally, Fluids are broadly categorized into two major groups.
1. Gases
2. Liquids1. Gases
Gases are used as a fluid in pneumatic systems, preferably
air. In gases, the relative spacing between the molecules is too
large. Gases have definite mass, but no volume and definite shape.2. Liquids
Liquids are used as a fluid in hydraulic systems, preferably oils. In
liquid, the relative spacing between the molecules is much less.
Liquids have definite mass and volume but no definite shape
The liquids may fall on the following categories.
a) Water b) Petroleum oil c) Fire resistant fluids
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a) Water
Water was the first fluid used for the transmission of power. It is
treated with chemicals before use to remove undesirable
contaminants.
b) Petroleum - base Fluids
Petroleum - base fluids are refined from selected crude oils
and blended with additives to prevent wear, rust, oxidation and
Foamingc) Fire resistant fluids
In many fluid power applications, hazardous conditions and
safety requirements dictate the use of a fire resistant fluid.The
commonly used fire resistant fluids are
i)Water-in-oil Emulsion (invert Emulsion)
ii) Water Glycol Emulsion
ill) Synthetic fluids
iv) High water content fluids (HWCF).
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Fluid power symbols
Fluid power symbols are used to draw the hydraulic/
pneumatic circuit for easy understanding. They are used worldwide
in design. Operation and maintenance of fluid power systems.
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Basic of HydraulicsAccording to Pascal's law "The pressure generated on a
confined fluid at rest is transmitted equally undiminished in all
directions throughout the fluid and acts at right angles to thecontaining surfaces".
Here, a force is being applied to a piston which in turn exertsa pressure on the confined fluid which is equal everywhere and
at right angles to the containing surfacesAccording to Pascal's law
P1
= P2
(F1/A1) = (F2/A2)100/10 = F2/100
F2 = 1000N
A li ti f P l' l
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Applications of Pascal's law
Any fluid power system works on the basis of pascal's law.
Here, we discuss the applications of pascal's law employed in two
simple systems.
Hand operated Hydraulic Jack
This system uses a piston-type hand pump to power a
hydraulic load cylinder for lifting loads as shown in figure .It consists
of handle ABC, pivoted about point C and a piston rod is pinned to itatpoint B.
When handle is pulled up at A, the piston rises and valve
creates a vacuum in the space below it thus draws the oil from thetank through check value 1. This check valve allows flow to pass in
only one direction, as indicated by the arrow.
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When the handle is pushed down at A, the pump cylinder ejects the oil tothe load cylinder through check valve 2. Pressure builds up below the load pistonand thus lifts the load. Bleed valve is a hand-operated valve, which opened tolower the load by bleeding the oil from the load cylinder back to the oil tank.
i d li
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Air-to-Hydraulic pressure BoostersThis device uses shop air to increase hydraulic pressure
needed for operating hydraulic cylinders requiring small to
medium volumes of higher - pressure oil. It consists of a largeair piston cylinder driving a small hydraulic piston cylinder toclamp a work piece to a machine tool table by supplying high-pressure oil as shown in figure
Laminar and Turbulent flow
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Laminar and Turbulent flowConditions due to the construction or operation of
hydraulic machinery may effect the flow characteristics of the
fluid in the system. There are two type of fluid flow in a hydraulic
system.
1)Laminar or streamline flowLaminar or streamline flow is characterized by the smooth
flow of liquid particles in even layers with minimum frictional
losses.Laminar flow continues only in low velocity on the samedirection.This Fluid flow type is called stream line flow as all thefluid particles moves in a parallel path.
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Turbulent Flow
Turbulent flow is characterized by the irregular flow of liquid
particles due to the collision of fluid particles. Figure shows the
random or erratic pattern of a fluid flow, creating inefficiencies, highenergy losses due to friction, more resistance to flow and pressure
drop.Turbulent flow may lead to premature wear, cavitations and
pitting on valve surfaces
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Darcy's Equation
Darcy's Equation can be used to calculate the head loss due to
friction in pipes for both laminar and turbulent flow and expressed
asHL=fLv
2/2Gd--- DarcyWeisbach formula
where HL
= Head loss in pipe due to friction, m
f = Friction factor, dimensionless
L = Pipe length, m
V = Fluid velocity, m/sec
g = Acceleration due to gravity, m/sec2
D = Pipe inner diameter, m
ForLaminar flow f = 64 / Re
HL= (64Lv2/Re2gD) =(32Lv2/ Re g D) =(32Lv2 ) /(v D gD)= 32Lv2 / v D 2 g=32Lv/ D 2 g
=32vLv/ D 2 g
Pi h d d h i i l d h
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Pipe roughness depends on the pipe material and the
method of manufacture.
Re1ative Roughness = Surface roughness (E)/Diameter of pipe (D)
The friction factor (f) may also be taken from the Moody diagramfor both laminar as well as turbulent flow.
Losses in pipes, valves and FittingsWhen fluid is pumped through a fluid power system, a
portion of its energy is lost due to friction. Major losses occur as
the fluid flows through pipe, hoses and tubing. These losses can be
calculated using thumb rules for a given length of pipes. Minor
losses occur at valves, fittings, bends, enlargements, contractions
and orifices.
L i i
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Losses in pipes
Losses in pipes are due to the friction i.e., rubbing action
between the boundary surface and fluid. This friction builds up the
heat and results in energy loss. In addition to this, pressure drop mayoccur during the flow.
In hydraulic systems, pressure drop should be kept as low as
possible to obtain high transmission efficiency. This may be done by
increasing the design value of diameter of one pipe to some extent.
In other words, large diameter pipes reduces the pressure drop acrossthe flow.
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Losses in valves and Fittings
Minor losses occur as the fluid undergoes sudden expansions
or contractions, or as the fluid flows through the pipe fittings, valves
and bends.For many fluid power applications, majority of energy losses
are due to change in cross-section of the flow path and the change in
flow direction which are usual in these valves, fittings, tees, elbows
and bends.
Head losses are found to be proportional to the square the velocity
of the fluid. HL= kv2/2g
The head loss through the minor losses is equal to the loss
through some length of straight pipe.
Minor HL=Major HLkv2/2g=f L v2/2Gd
L=D(k/f)