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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.

7/11/2013 12:51 AMR.GANESA MOORTHY,ASSO.PROF/MECH

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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.

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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.

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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)

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