Fluid Power Application - 002b Physical Properties of Hydraulic Fluids

8/13/2019 Fluid Power Application - 002b Physical Properties of Hydraulic Fluids

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Fluid Power ApplicationFluid Power Application(Physical Properties of Hydraulic Fluids)(Physical Properties of Hydraulic Fluids)

Hydraulics & PneumaticsHydraulics & PneumaticsKMD 3133KMD 3133

By,By,MohdMohd DarnalisDarnalis A.Rahman A.Rahman

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Learning ObjectivesLearning ObjectivesUpon completing this chapter, students should be able to:Upon completing this chapter, students should be able to:

1.1. ExplainExplain the primary function of hydraulic fluid.the primary function of hydraulic fluid.2.2. DefineDefine the termthe term fluid fluid ..3.3. DistinguishDistinguish between a liquid and a gas.between a liquid and a gas.

4.4. Appreciate Appreciate the properties desired of a hydraulic fluid.the properties desired of a hydraulic fluid.5.5. DefineDefine the termsthe terms weight weight ,, density density andand specific gravity specific gravity ..6.6. UnderstandUnderstand the termsthe terms pressure pressure ,, head head , and, and force force ..7.7. DifferentiateDifferentiate between gage pressure & absolute pressures.between gage pressure & absolute pressures.8.8. CalculateCalculate the force created by a pressure.the force created by a pressure.9.9. Apply Apply PascalPascal s law to a hydraulic system.s law to a hydraulic system.10.10. UnderstandUnderstand the termsthe terms kinematic viscosity kinematic viscosity andand absolute viscosity absolute viscosity ..11.11. ConvertConvert viscosity from one set of units to another set of units.viscosity from one set of units to another set of units.12.12. ExplainExplain the difference between viscosity and viscosity index.the difference between viscosity and viscosity index.


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Weight, Density and SpecificWeight, Density and SpecificGravityGravity

WeightWeight All objects whether solids or fluids, are pulled towards the All objects whether solids or fluids, are pulled towards thecenter of the earth by a force of attraction.center of the earth by a force of attraction.This force is called weight.This force is called weight.

F = force (Newton)F = force (Newton)W = weight (Newton)W = weight (Newton)m = mass (kg)m = mass (kg)G = gravity acceleration (m/sG = gravity acceleration (m/s 22))

F = W = mg

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Weight Density (Specific Weight)Weight Density (Specific Weight)Defined as weight per unit volume.Defined as weight per unit volume.

= weight density (N/m= weight density (N/m 33))W = weight (N)W = weight (N)

V = Volume (m V = Volume (m 33))

VW

volumeweight

densityweight

=

=


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Specific Gravity (Specific Gravity ( SgSg))Defined as the weight density of the fluid divided by theDefined as the weight density of the fluid divided by theweight density of water.weight density of water.For example, the specific of oil can be found usingFor example, the specific of oil can be found using

Specific gravity is a dimensionless parameter.Specific gravity is a dimensionless parameter.

water

oiloilg S

=

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Mass DensityMass Density Also known as density which defined as mass per unit Also known as density which defined as mass per unitvolumevolume

= mass density (kg/m= mass density (kg/m 33))W = weight (kg)W = weight (kg)

V = Volume (m V = Volume (m 33))

V

m =


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Mass DensityMass DensitySince weight is proportional to mass (W=mg), it follows thatSince weight is proportional to mass (W=mg), it follows thatspecific gravity can also be defined as the mass density of thespecific gravity can also be defined as the mass density of thegiven fluid divided by the mass density of water.given fluid divided by the mass density of water.

Solving for the mass densitySolving for the mass density

= N/m= N/m 33g = m/sg = m/s 22 = kg/m= kg/m 33

g

Vg V

mgW

=

==

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Force, Pressure and HeadForce, Pressure and Head

Force and PressureForce and PressureIn fluid power systems it is pressure rather than forceIn fluid power systems it is pressure rather than forcethat is transmitted equally in all directions.that is transmitted equally in all directions.

Pressure defined as force per unit area.Pressure defined as force per unit area.

PP = pressure (N/m= pressure (N/m 22))F = force (N)F = force (N)

A A = area (m= area (m 22))

AF

P =


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HeadHeadWe can conclude that a 1We can conclude that a 1 --ft column of water develops atft column of water develops atits base a pressure of 0.433 psi. The 1its base a pressure of 0.433 psi. The 1 --ft height of waterft height of wateris commonly called a pressure head.is commonly called a pressure head.

Figure 2.5 page 43Figure 2.5 page 43

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HeadHeadThe equation below allows calculation of the pressureThe equation below allows calculation of the pressureproduced at the bottom of any column of any liquid.produced at the bottom of any column of any liquid.Observe that, pressure does not depend on the area ofObserve that, pressure does not depend on the area ofthe bottom of the column but only on the column heightthe bottom of the column but only on the column heightand the weight density of the fluid.and the weight density of the fluid.

HP =


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Atmospheric Pressure Atmospheric PressureIs the pressure developed on the surface of the earthIs the pressure developed on the surface of the earthdue to the force of attraction between the atmospheredue to the force of attraction between the atmosphereand the earth.and the earth.Neither the equation discussed earlier can be used to findNeither the equation discussed earlier can be used to findthis pressure because of the compressibility of air.this pressure because of the compressibility of air.Which means that density of air is not a constant.Which means that density of air is not a constant.

The density is greatest at the earthThe density is greatest at the earth s surface ands surface anddiminishes as the distance from earth increases.diminishes as the distance from earth increases.



Gage & Atmospheric PressureGage & Atmospheric Pressure Are measured relative to the atmosphere. Are measured relative to the atmosphere. Absolute pressures are Absolute pressures are measuredmeasured relative to arelative to aperfect vacuumperfect vacuum such as that existing in outersuch as that existing in outerspace.space.Gage pressureGage pressure are labeledare labeled psigpsig oror psipsi

Absolute pressure Absolute pressure are labeledare labeled psiapsia ..Where atmospheric pressure equals to 14.7Where atmospheric pressure equals to 14.7psiapsia or 0 psig.or 0 psig.


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PascalPascal s Laws Law

IntroductionIntroductionReveals the principle of howReveals the principle of how liquids transmitliquids transmitpowerpower ..DefineDefine pressured applied to a confined fluid ispressured applied to a confined fluid istransmitted undiminished in all directionstransmitted undiminished in all directions ..The liquid transmits the pressure, created by theThe liquid transmits the pressure, created by theforce of the stopper, throughout the container asforce of the stopper, throughout the container as

illustrated in figure belowillustrated in figure below



Figure 2-11page 53


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PascalPascal s law can be applied to produce a usefuls law can be applied to produce a usefulamplified output forceamplified output force such as in a hydraulicsuch as in a hydraulicpress.press.

An An input forceinput force of 10 lb is applied to a 1 inof 10 lb is applied to a 1 in 22piston. This develops a 10 psi pressurepiston. This develops a 10 psi pressurethroughout the container.throughout the container.This 10 psi pressure acts on 10 inThis 10 psi pressure acts on 10 in 22 piston,piston,producing a 100 lbproducing a 100 lb output forceoutput force ..



Figure 2-12page 54


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Analysis of Hydraulic Analysis of HydraulicPressPress

Does a hydraulic pressDoes a hydraulic pressproduceproduce more energymore energy than itthan itreceivesreceives ??

A fluid power system like any A fluid power system like anyother power system,other power system, cannotcannotcreate energycreate energy . (. ( ConservationConservationof energyof energy ))

According to Pascal According to Pascal s Law,s Law,1

2

1

2

2

2

1

1

21

AA

FF

AF

AF

PP

=

=

=

(a)



Figure 2Figure 2 --1313Page 55Page 55


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SolutionSolutiona. From Eqn. (a)a. From Eqn. (a)

b. From Eqn. (b)b. From Eqn. (b)

1000N10030

300F

AA

F 21

21 ===

3mm3030030

SAA

S 12

12 ===



SolutionSolutionc. Energy input = Fc. Energy input = F 11 x Sx S 11

d. Energy output = Fd. Energy output = F 22 x Sx S 11

m N m N == 303.0100

m N m N == 3003.01000


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Bulk ModulusBulk Modulus

TheThe stiffnessstiffness of a hydraulic system isof a hydraulic system isdirectly related to thedirectly related to the incompressibilityincompressibility ofofthe oil.the oil.It is theIt is the measuremeasure ofof incompressibilityincompressibilityTheThe bulk modulus, thebulk modulus, the compressibilitycompressibility oror stiffer fluidstiffer fluid ..


Viscosity Viscosity

Most important property of a hydraulic fluid.Most important property of a hydraulic fluid. A A measure of concentrationmeasure of concentration whichwhich affectaffect fluidfluid

movement (flow).movement (flow). Viscosity Viscosity , fluid flow easily (thin appearance), fluid flow easily (thin appearance) Viscosity Viscosity , fluid flows with difficulty (thick, fluid flows with difficulty (thickappearance)appearance)In reality,In reality, ideal viscosityideal viscosity of hydraulic fluid is aof hydraulic fluid is acompromisecompromise ..


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Viscosity Viscosity Too highToo high

Resulting toResulting to ;;1.1. High resistanceHigh resistance to flow, which causesto flow, which causes

sluggishsluggish operation.operation.2.2. Increased powerIncreased power consumptionconsumption due todue to

frictionalfrictional losseslosses ..3.3. Increased pressure dropIncreased pressure drop through valves andthrough valves and

lines.lines.4.4. High temperaturesHigh temperatures caused bycaused by frictionfriction ..


Viscosity Viscosity Too LowToo Low

Resulting toResulting to ;;1.1. Increased leakageIncreased leakage losseslosses past seals.past seals.

2.2. Excessive wearExcessive wear due to breakdown of the oildue to breakdown of the oilfilm between matingfilm between mating movingmoving parts.parts. MovingMovingpartsparts , such as internal components of a, such as internal components of apump (reciprocating piston in cylinder).pump (reciprocating piston in cylinder).

Fluid Power Application - 002b Physical Properties of Hydraulic Fluids

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