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G Kum aresanInst itut e for En ergy Stu dies

ANNA UNIVERSITY

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Flu id Pow er - Def in i t ionFluid power is the technology that deals with the

generat ion, control, and transmission of power-using

pressurized fluids.

Fluid power is the use of a confined fluid flowing

under pressure to transmit power from one location to

another.

It is one of three commonly used methods of

transmitting power in an industrial sett ing; the others

are electrical and mechanical power transmission.


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Flu id Syst em Classif icat io nFluid system is divided into two areas:

1. Fluid transport

Fluid transport systems have as their sole objective the

delivery of a fluid from one location to another to accomplish some

useful purpose.

2. Fluid Power

Fluid power systems are designed specifically to perform

work. The work is accomplished by a pressurized fluid bearing directly

on an operat ing fluid cylinder or fluid motor.

Fluid power system broadly classified asHydraulics and

Pneumatics.


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Application Area

In general, hydraulic systems are usually preferred for applications

that require:

High power / large load capacity,

Precise positioning and

Smooth movement

Pneumatics is well suited for applications that require:

Low power / light to moderate load capacity

Low to moderate precision andQuick response


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Com parison o f Hydr aul ic and Pneum at ic syst em

Hydraulic System Pneumatic System

Incompressible mediumused

Used for Heavy loads

Very expensive tooperate and build

Medium is recirculatedwithin the system

Hydraulic systemprovide a very rigid

motion

Compressible medium used

Used for medium loadsLess expensive

Medium is exhausted into

the atmosphere

Pneumatic system behave in

a springy fashion


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Advan t ages o f Fluid Pow erThe secret of fluid powers success and widespread use is its

versatility and manageability. Fluid power is the muscle of

automation because of advantages in the following 4 major

categories.

1. Ease and accuracy of control

2. Multiplication of force

3. Constant force or torque

4. Simplicity, safety, economy

Additional benefits of fluid power systems include instantly

reversible motion, automatic protection against overloads and

infinitely variable speed cont rol and also highest power-per-weight

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Hydraulic fluid has 4 primary functions:

i. Power transmission or Energy transfer

ii. Lubrication

iii. Sealing

iv. Cooling


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Basic Pr incip le o f Hyd raul ics

Hydraulics is the science of transmitting force and/or motion throughthe medium of a confined liquid. In a hydraulic device, power istransmitted by pushing on a confined liquid. The transfer of energytakes place because a quantity of liquid is subject to pressure.

Pascal's Law(also called Pascal's Principle) says that

"changes in pressure at any point in an enclosed fluidat rest are transmitted undiminished to all points inthe fluid and act in all directions."


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Hydrau lic Jack


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M achine of M ul t ip ly ing For ce


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Fr ict ion losses


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Fr ict ion losses con t

Frictional Losses in Laminar Flow

Friction Factor f = 64/(Reynolds Number)


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v The max. recommended velocity for pressure lines is 6.1 m/s in order

to prevent turbulent flow and the corresponding losses


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Fast Moving Oil May Become

Turbulent


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Slow Moving Oil


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Fluid Pow er Sym bo ls

long chain thin Enclosure of two or more functions contained in

one unit.


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Reading Symbols For Pumps And Motors


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Fluid Pow er Sym bo ls Cont..

An arrow through a symbol at

approximately 45 degrees indicates that

the component can be adjust or varied

Variable displacement pump

Flow direction of

Hydraulic

Pneumatic

Fixed displacement pump


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Reading Symbols For Cylinders


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Ph ysical di f f eren ces bet w een Liqu ids and Gases


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Hydraulic mediums used in Industry

(i) Mineral oil

(ii) Synthetic oil

(iii)Water oil emulsion(iv)Glycol in water

(v) Chlorinated synthetic fluid

(vi)Phosphate ester fluid


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Proper t ies o f Fluids Afluid is defined as any

matter that flows when

force is applied.

Liquids like water or silver

are kinds of fluid.


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Fo rces in f lu idsForces in fluids are more complicated than forces in

solids because fluids can change shape.


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PressureA force applied to a fluid createspressure.

Pressure acts in all directions, not just the

direction of the applied force.


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If one variable increases along a streamline,at

least one of the other two m ust decrease.

For example, if speed goes up, pressure goes down.


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Strong

intermolecular

forces

High viscosity


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Surface tension

Surface tensionis the amount of energy required to stretch or increase

the surface of a liquid by a unit area.

Strong

intermolecularforces

High surface

tension


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Cohesionis the intermolecular attraction between like molecules

Adhesionis an attraction between unlike molecules

Adhesion

Cohesion


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Good lubricity:

Stable in viscosity:

Hydraul ic f lu ids sho uld h ave th e fo l low ing propert ies:


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System compatibility:


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Pour point:

Oxidation resistant:

Rust and Corrosion:The rust will have its effect on all the internal parts of the system. The

hydraulic fluid chosen should be a medium having a minimum of this

effect. Corrosion happens when acid reacts with the metal. Considering all

aspects, a suitable hydraulic oil is chosen


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High Bulk Modulus:

Good Heat Dissipation:


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Adequate low temperature properties:

Flash Point:


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Low foaming tendency:

Fire resistant:


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Low in volatility:

Good Demulsibility:


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Low coefficient of expansion:

Low specific gravity:

Non toxic, Easy to handle and available:


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Con clusion on Pro per t ies of hyd raul ic f lu id

1. Good lubricity

2. Ideal viscosity

3. Chemical and environmental stability

4. Compatibility with system materials

5. High degree of incompressibility6. Fire resistance

7. Good heat-t ransfer capability

8. Low density

9. Foam resistance

10. Non toxicity

11. Low volatility

12. Inexpensive and

13. Ready availability


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Com po nen t s of a Hydraul ic Pow er Syst em

Tank / Reservoir

PumpGear pump

Vane pum p

Piston pum p Electric motor

ActuatorCylinder (for linear mot ion)

Motor (for rotary mot ion)

Distribution line or Piping system

Valve

Direct ional Control Valve (DCV)

Pressure Control Valve (PCV)

Flow Control Valve (FCV)

Input segment

Output segment

Control segment


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W o r k & Po w erWork is done (or energy is transferred) when an object ismoved against a force, and is defined as:

Work = force (N) x distance moved (m) ; unit Nm=Joule

Power is the rate at which work is performed:Power = work (J)/time (s) ; unit J/s=Watt


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TorqueThe term torque is used to define a rotary force, and issimply the product of the force and the effective radius asshown in Figure

T= F (N) x d (m); unit Nm= Joule

where, d - distance


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Robert Boyle

(1627-1691)

Boyle was born into an

aristocratic Irish family

Became interested in

medicine and the new

science of Galileo and

studied chemistry.

A founder and an

influential fellow of the

Royal Society of London

Wrote extensively on

science, philosophy, and

theology.


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Boyles Law - 1662

Pressure x Volume = a constant

Equation: P1V1 = P2V2 (T= constant)

Gas pressure is inversely proportional to thevolume, when temperature is held constant.


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Graph of Boyles Law

Boyles Lawsays the

pressure is

inverse to the

volume.

Note that when

the volume

goes up, thepressure goes

down


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Charless Law

The volume of a fixed mass of gas isdirectly proportional to the Kelvin

temperature, when pressure is held

constant.This extrapolates to zero volumeat atemperature ofzero Kelvin.

V

T

V

TP

1

1

2

2

= =( constant)


J h L i G L (1778 1850)

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Joseph Louis Gay-Lussac (1778 1850)

v French chemist and

physicist

v Known for his studies onthe physical properties of

gases.v In 1804 he made balloonascensions to study

magnetic forces and to

observe the compositionand temperature of the air

at different altitudes.


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Gay-Lussacs Law - 1802The pressure and Kelvin temperature of

a gas are directly proportional, provided

that the volume remains constant.

2

2

1

1

T

P

T

P =

How does a pressure cooker affect the time

needed to cook food? (Note page 422)

Sample Problem 14.3, page 423


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The Combined Gas Law

The combined gas law expresses therelationship between pressure, volume

and temperature of a fixed amount of

gas.

2

22

1

11

T

VP

T

VP=


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The combined gas law contains

all the other gas laws!If the temperatureremains

constant...

P1 V1

T1

x=

P2 V2

T2

x

Boyles Law


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The combined gas law contains

all the other gas laws!If the pressureremains constant...

P1 V1

T1

x=

P2 V2

T2

x

Charless Law


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u The combined gas law contains

all the other gas laws!u If the volumeremains constant...

P1 V1

T1

x=

P2 V2

T2

x

Gay-Lussacs Law


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Hyd rau l ic Pum p Classif icat ion

Hydraulic pumps convert mechanical energy from a prime mover

(engine or electric motor) into hydraulic (pressure) energy. The pressure

energy is used then to operate an actuator. Pum ps push on a hydraulic

fluid and create flow.

1. Pum ps that discharge liquid in a cont inuous flow are nonpositive-

displacem en ttype.

2. Pumps that discharge volumes of liquid separated by periods of nodischarge areposit ive-displacem en t type.


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Pump

Piston pump

Radial pump

Positive displacementNon Positive displacement

Centrifugal Jet pump Axial type

Gear pump

External Gear

Internal Gear

Lobe pump

Screw pump

Vane pump

Unbalanced Balanced Axial pump

Bent axis

Swash plate

Rotary ram

Rotary cylinder


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Non posi t i ve d isp lacem ent pum p

This type is generally used for low-pressure, high-volume

flow applications. Normally their maximum pressure

capacity is limited to 250-300 psi.

Flow (Q)

Pressure (P)

Pmax

Pmin

Q0 Qmax


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Gear Pum p - External


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Gear Pum p Cont

Q

N

P

Q

QactualQtheoretical

( )

A

vol

T

3 3

T D

2 2

D o i

Q100

Q

Q (m / min) V (m / rev) N(rev / min)

V D D L4

=

=

=


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Gear Pum p Ext ern al (Helical Gear)


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Gear Pum p Cont

Advantages: Simplicity and compact

Low cost

Less sensitive to contamination

High operating speeds

External gear pumps may use spur, herringbone, or helical gear sets to

move the fluid.

Herringbone pump gives steadier flow than the spur gear pump

Helical gears can be designed with a small number of large teeth,

thus allowing an increase in capacity without sacrificing smooth flow


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Int ern al Gear Pu m p (Off-cen ter ed )


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Lobe Pu m p

Flow at deliver side is not smooth(Pulsation delivery)

Volumetric efficiency is higherthan gear pumps.


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Screw Pum p

Used in submarines

Suitable for high pressure applications

(up to 3500 psi (241 bar))

Advantage

Deliver non pulsating flow

Disadvantage

Expensive

Low in efficiency

The rolling action obtained with the threaddesign of the rotors is responsible for

the very quiet pump operation.

The symmetrical pressure loading aroundthe power rotor eliminates the need for

radial bearings because there are no radial

loads


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Gerot or Pum p (Cent ered Int ernal gear t ype)

The inner gear has one tooth less than the outer gear, and the

volumetric displacement is determined by the space formed by theextra tooth in the outer rotor.


G t P m C t

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Gerot or Pum p Con t ..

The tooth form of each gear is related to that of theother in such a way that each tooth of the inner gear is

always in sliding contact with the surface of the outer

gear.

Each tooth of the inner gear meshes with the outergear at just one point during each revolution. In the

illustration, this point is at the X.


Vane Pum p

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Vane Pum p

Unbalanced and Fixed displacement type

The rotor, which contains radial slots, is splined to the drive shaft and rotatesinside a cam ring.

Each slot contains a vane designed to mate with the surface of the camring as the rotor turns.Centrifugal force keeps the vanes out against the surface of the cam ring.


Analysis of Volum et r ic Displacem ent

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Analysis of Volum et r ic Displacem ent

max

C

R

max

D

D diameter of cam ring (m)

D diameter of rotor (m)

L - width of rotor (m)

N - rotor speed (rpm)

e - eccentricity (m)

e - max. possible eccentricity (m)

V - max. possible volumetric displacement

3(m / rev)


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Cont..

Some vane pumps have provisions for mechanically varying the

eccentricity. Such a design is called a var iab le d isp lacem ent pum p


V P U b l d

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Vane Pum p Unbalanced type

Variable displacement, pressure-compensated vane pump


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Vane Pum p Fixed d isplacem ent , balanced t ype

Complete hydraulic balance is achievedLow cost with respect to power outputLess noisyLong service lifeIt delivers pressure up to 170 kg/cm2


Pist on Pum p- bent ax is t ype

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st o u p be t a s t ype

A pist on pu m p work s on t he principle t hat a reciprocating pist on candraw in fluid when it retracts in a cylinder bore and discharge it when it

extends.

T he pistons are forced in and out of t heir bores as t he distance bet ween

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cont

D

T D

Stan( ) S Dtan( )

D

V YAS

Q V N NYADtan( )

= =

=

= =


Pist on Pum p Sw ash plate t ype

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Pist on Pum p Sw ash p la te t ype

q As the cylinder rotates, the pistonsreciprocate becausethe piston shoes follow the angled surface of the swash

plateq The inlet and outletports are located in the valve plate so that the pistons passthe inlet as they are

being pulled out andpass the outlet as they are being forcedback in.


Radial Pist on pum p

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Radial Pist on pum p

The pistons remain in constant contact with the reaction ring due to centrifugalforce and back pressure on the pistons.

As the cylinder barrel rotates, the pistons on one side travel outward Suction

When a piston passes the point of max.`e, its forced inward by the reaction

ring - Delivery


Cont

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Cont

Piston pump with stationary cam and rotating block


Com par ison of h ydrau l ic pu m p t ypes

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Com par ison o f h ydrau l ic pu m p t ypes


Com binat ion pum ps

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Com binat ion pum psFor a clamping cylinder, a

large flow, but low pressure, is needed during extension and retraction, but zero flowand high pressure are needed during clamping


Pum p p er fo rm ance

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Pum p p er fo rm anceThe performance of a pump is mainly a function of the precision of its

manufacture. Components must be made to close tolerances.

(i) Volumetric efficiency

Gear pump 80 to 90 %Vane pump 82 to 92 %

Piston pump 90 to 98 %

(ii) Mechanical efficiency (90 to 95 %)

P delivery pressure (Pa) ; QT Theoretical flow rate (m3/s)

TA Actual input torque (N-m) ; N Pump speed (rps)


Cont

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ContT

m

A

T100

T

=

(i) Overall efficiency


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Und erst anding At m osph er ic Pressur e

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