the principles of hydraulics

8/13/2019 The Principles of Hydraulics

1/13

Principles of Hydraulics

Hydraulics is a branch of engineering that deals with the practical application ofwater or other liquids at rest or in motion. The two major divisions of hydraulicsare hydrostatics and hydrodynamics.

HydrostaticsHydrostatics is the study of liquids at rest and the forces exerted on them or bythem. Equilibrium is the condition when all forces and torques are balanced byequal and opposite forces and torques. Most hydraulic systems applyhydrostatic principles. For example the fluid in a automotive bra!ing system is atrest and the pressure throughout the system is in equilibrium. The bra!e systemis activated by applying pressure to the foot pedal. The fluid in the systemtransmits the applied force from the foot pedal to the slave cylinder piston. Theslave cylinder piston transmits the force to bra!e pad which applies pressure tothe bra!e drum "rear wheel on newer vehicles#. The pressure is equal in all parts

of the system but higher than the pressure of the fluid when the system is atrest.

Simplified auto brake system

HydrodynamicsHydrodynamics is the study of force exerted on a solid body by the motion orpressure of a fluid. For example fluids are transferred through a non$positivedisplacement pump by angular velocity. % non$positive displacement pump is apump that is not sealed between its inlet and outlet. %ngular velocity forces areproduced by a rotating object. The fluid is force to the discharge "outlet# port byrotating impeller blades. The output of the pump may be reduced or completely

&E' ()*+ MET ,(* Hyd$-ne (


2/13

bloc!ed if the pressure of the discharge circuit is increased because there is nopositive displacement of fluid.

Hydrodynamic pump, non-positive displacement pump

Hydraulic History (abbreviated)Early hydraulic systems consisted of diverting streams for village irrigation andwater supply and digging wells. There exists evidence of hydraulic principle use

in early Mesopotamia "currently geography encompassed by raq#. This recordpredates the existence of 'hristianity by over a thousand years. /ne of the firstrecorded machines for hydrostatic use is the %rchimedes water$screw.0eveloped by the 1ree! %rchimedes it believed that earlier Egyptian technologyinfluenced the development of this invention.

Archimedes screw, a spiral screw turned insidea cylinder, was once commonly used to lift waterfrom canals. he screw is still used to lift water inthe !ile delta in "#ypt, and is often used toshift #rain in mills and powders in factories.

&E' ()*+ MET ,(* Hyd$-ne 2


3/13

$i%uid &haracteristicsn hydraulics the term fluidrefers to gases as well as liquids. % fluidis asubstance that tends to conform to the outline or shape of its container "such asa liquid or gas#. Fluids yield easily to pressure. % liquidis a fluid that can flowreadily and assume the shape of its container. Fluids have no independent

shape but do have a definite volume. 3iquids do not expand indefinitely and areonly slightly compressible. % gas is a fluid that has neither independent shapenor volume and tends to expand indefinitely. /xygen hydrogen nitrogen etc.are gases.

3iquids ma!e convenient fluids for transmitting force because they are not highlycompressible li!e gases. The term fluid is used in reference to a liquid becauseliquids are specifically used in hydraulic systems. &or! produced in a hydraulicsystem is dependent on the pressure and flow of the fluid in the system.

ransmission of Hydraulic 'orce

To understand the principals of hydraulics it is necessary to understand 4laise-ascal5s theorem upon which all hydraulics are based. -ascal reali6ed thatenclosed fluids under pressure follow a definite law. -ascal5s theorem is nowstated as a law of physics7 pressuri6ed fluid within a closed container8such as acylinder or pipe8exerts equal force on all surfaces of the container and is thesame in every direction. Force is the energy that produces movement. %lthoughthis law and its potential for technology were reali6ed in the (9 thcentury it wasnot until the 2*thcentury that fluid power became a means of energytransmission.

Pascals law of e%ual pressureof enclosed fluids on all surfacesin all directions

Pressure-ressure is the force per unit area. -ressure is expressed as atmosphericgaugeand absolute. %tmospheric pressure is the force exerted by the weight ofthe atmosphere "air# on the Earth5s surface. The weight of the atmosphereacting over a height of several hundred thousand feet above the Earth5s surfacevaries slightly with weather conditions and variation in gravity. For practicalpurposes and to establish a standard for weight of the atmosphere at sea level is

&E' ()*+ MET ,(* Hyd$-ne ,


4/13

determined to be (:.9 pounds per square inch "psi#. %tmospheric pressure isexpressed in psi and is measured with a mercury barometer. % mercurybarometer is an instrument that measures atmospheric pressure using a columnof mercury.

orricellis ercury *arometer

% mercury barometerconsists of a glass tube that is closed on one end andcompletely filled with mercury. The tube is inverted and the open end issubmerged in a dish of mercury. % vacuum "low pressure# is created at the topof the tube as the mercury tries to run out of the tube. ;acuum is a pressurelower than atmospheric pressure. The pressure of the atmosphere on themercury in the open dish prevents the mercury in the tube from running out of thetube. The height of the mercury in the tube corresponds to the pressure of theatmosphere on the mercury in the open dish.

% mercury barometer is commonly calibrated in inches of mercury "in. Hg#. %tsea level the atmosphere can support 2


5/13

-ressure outside a closed system "such as normal air pressure# is expressed inpounds per square inch absolute. The difference between gauge pressure andabsolute pressure is the pressure of the atmosphere at standard conditions"(:.9psi#. % pressure gauge reads * psi at normal atmospheric pressure. Tofind absolute pressure when gauge pressure is !nown the atmospheric pressure

of (:.9 psi is added to the gauge pressure. %bsolute pressure is found byapplying the formula?

psia @ psig A (:.9

&here

psia @ pounds per square inch absolute

-sig @ pounds per square inch gauge

(:.9 @ constant "atmospheric pressure at standard conditions#

-ressure other than atmospheric pressure is considered to be artificial and is

produced to transfer or amplify force in hydraulic systems. This transferred oramplified force is used to do wor! such as lifting a car with a hydraulic jac!running a conveyor with a hydraulic motor or stamping steel in automotivecomponents.

%rea force and pressure are the basis of all hydraulic systems. The forceexerted by a liquid is based on the si6e of the area on which the liquid pressure isapplied. n hydraulic systems this area usually refers to the face of the pistonwhich is circular in shape. %rea is always expressed in square units such as sq.in. or sq. mm.

% circle with a diameter the same as a square has less area. The area of a circleis exactly 9B.+:C of the area of a square with the same measurements.

+./ of s%uare

The area of a piston can be found if the force and pressure applied to a cylinderis !nown. The applied pressure on a piston can be found if the amount of forceand the piston area are !nown. %lso the force produced by a piston can be

&E' ()*+ MET ,(* Hyd$-ne +


6/13

'

P APressure 0

'orce

Area

found if the area and pressure applied to a piston are !nown. Two of the valuesmust be !nown to find the un!nown value.

The relationship between force pressure and area can be recalled using theforce pressure and area formula pyramid from the standard calculations

handout sheet.

4y covering the letter of the un!nown value the formula for finding the solution isshown.

Head pressureHead is the difference in the level liquid "fluid# between two points. Head isexpressed in feet. Head pressure is the pressure at any point below the surfaceof the fluid.

n an open cylinder the pressure of the fluid at any depth in the cylinder isproportional to the height of the column of fluid. The pressure in a column of fluidis determined by the columns height and the fluid5s weight not the shape of thevessel. The pressure at the same level in each vessel is identical if the pressuresurrounding the different surrounding the different$shaped vessels is the same

and the fluid in each vessel is the same. The pressure of the fluid at any level ina vessel is based on the height of the fluid above that level and is the same atthat level regardless of the shape of the vessel.

n a hydraulic system head pressure is the energy or pressure that supplies ahydraulic pump. %tmospheric pressure and head pressure combine to feed theinta!e "suction# line connecting a hydraulic pump to a reservoir.

Head is classified as static or dynamic. Dtatic head is the height of a fluid abovea given point in column at rest. Dtatic head pressure is a force over and area

created by the weight of the fluid itself. Dtatic head pressure is potential energy.The pressure of water per foot of static head is calculated by using .*,>( lb)cu in.or 2.,(5 head of water for each psi.

0ynamic head is the head of a fluid in motion. 0ynamic head represents thepressure necessary to force a fluid from a given point to a given height. 0ynamichead pressure is the pressure and velocity of a fluid produced by a liquid inmotion. 0ynamic head pressure results when a valve is opened and fluid is

&E' ()*+ MET ,(* Hyd$-ne >


7/13

allowed and open flow. 0ynamic head pressure may be used to direct an openflow of fluid. For example dynamic head pressure was used in early prospectingdays to wash away the sides of mountains to retrieve gold. This wasaccomplished by piping water from higher la!es and using dynamic headpressure to produce a high pressure and high velocity.

Hydrostatic head. he fluid hei#ht incolumns A and * is identical, but thepressure readin# on the #au#esdiffers because of the different fluiddensities. Hydrostatic head refers tothe vertical column hei#ht1hydrostatic pressure refers to the

force e2erted by the fluid.

'lowFluid flow is the movement of fluid caused by a difference in pressure betweentwo points. n a hydraulic system fluid flow is produced by the action of a pumpand expressed as a measurement of gallons per minute "gpm# or liters perminute "lpm#. Fluid flow in a hydraulic system is affected by friction and theviscosity of the fluid. Fluid flow is based on the volume and capacity of thesystem and the velocity of the fluid in the system. Fluid flow also affects thespeed of a hydraulic system. n a system with flowing fluid pressure is causedby total resistance to fluid flow from a pump. -ressure results only when there isresistance to flow. esistance to flow is comprised of friction throughout thesystem and actuator loads. % pressure change occurs to a fluid due to its flow is

generally expressed in psi.

'rictionFriction is generated throughout a hydraulic system between the piping wall andthe fluid and within the fluid as fluid layers slide by one another. The faster thefluid flows the greater the friction. %ny friction generated becomes a resistanceto fluid flow. -ressure must be increased to overcome the friction. Each

&E' ()*+ MET ,(* Hyd$-ne 9


8/13

component in a hydraulic system offers resistance and reduction of availablewor!ing pressure.

% fluid flows because of a difference in pressure. The pressure of a moving fluidis always higher upstream. -ressure drop is the pressure differential between

upstream and downstream fluid flow cause by resistance. The pressuredeveloped in a hydraulic system is designed to be used as hydraulic leverage.-ressure and fluid flow are independent of each other but both assist in theoutput. -ressure provides the force and flow rate is used to provide speed. Flowrate is expressed in gpm and is typically determined by the capacity of the pump.

Fluids follow the path of least resistance. For example a hydraulic systemconsisting of a telescoping cylinder for lifting purposes will extend the largestportion first. Force @ pressure times area determines which section of thecylinder will extend first. The larger area of the telescope5s base cylinder willprovide greater lifting power for a given pressure and will extend initially. The

larger force from the base cylinder area begins to move the weight as a lighterresistance compared to the other cylinders smaller area.

'luid flow in a telescopin# cylinder

3olume

;olume is the three$dimensional si6e of an object measured in cubic units.egardless of the shape of the figure volume is expressed in cubic units "cu in.cu ft mm, m, etc.#. The volume of a figure is found by calculating the area ofthe figure and multiplying by the length. The volume of a cylinder is found byapplying the procedure?

&E' ()*+ MET ,(* Hyd$-ne B


9/13

Find the area of cylinder.% @ .9B+: x 02

&here% @ area "in sq units#.9B+: @ constant

02

@ diameter squared

Find volume of cylinder.; @ % x 3

&here; @ volume

% @ area "in square units#3 @ length "in units#

&apacity

'apacity is the ability to hold or contain something. 'apacity is expressed incubic units and is calculated from a containers volume. Fluids are measured inounces pints quarts gallons liters etc.

Fluid measurements can also be expressed in cubic units "cu in. cu ft etc.#because fluids occupy three dimensions. For example one gallon of fluid equals2,( cu in.

The quantity of fluid required to fill a specific volume is determined by calculatingthe volume and dividing by 2,(.

3ess hydraulic fluid is required to retract a piston than is required to extend apiston. This is due to the rod ta!ing up part of the cylinder volume "reducedcapacity#. The volume that the piston rod occupies must be subtracted from thetotal volume of the cylinder when determining the volume of fluid that a cylinderdisplaces when retracting.

Sin#le actin# cylinder. 4educt thevolume of the rod from the totalcylinder volume to determine fluid

capacity of rod end.

&E' ()*+ MET ,(* Hyd$-ne


10/13

3elocity;elocity is the distance a fluid travels in a specified time. ;elocity generallymeans the change in position of a fluid particle during a certain time interval.

This may be represented as a distance in feet per second "ft)sec#.

;elocity is measured as a vector. % vector is a quantity that has a magnitudeand direction. % vector is commonly represented by a line segment whose lengthrepresents its magnitude and whose orientation represents its direction.

The velocity of a fluid particle is determined by subtracting it5s initial position fromits final position and dividing by the value of value of the initial time subtractedfrom the final time. ;elocity is found by applying the equation?

; @ x2 x(

t2 t(

&here; @ velocity "in ft)sec#

x2 @ final position "in ft#

x(@ initial position "in ft#

t2@ final time "in sec#

t(@ initial time "in sec#

The velocity of the hydraulic fluid in a system should not exceed recommended

values because turbulent conditions result with loss of pressure and excessiveheating. The concept for predicting turbulence "non laminar flow# is based uponthe eynolds number. More about eynolds numbers and their calculation willbe covered in later lessons.

'low 5ateFlow is the movement of fluid. Flow rate is the volume of fluid flow. % fluid inmotion is always flowing but its rate of flow may change. Fluid velocity dependson the rate of flow in gallons per minute "gpm# and the cross$sectional area of apipe or component.

The velocity of a fluid increases at any restriction in a pipe or component if theflow rate remains the same in the system. 'ommon restrictions include valveselbows pipes reducers etc. %lso the velocity of a fluid decreases as the cross$sectional area of a pipe or component increases.

The law of conservation of matter states that the mass or volumetric flow rate ofan incompressible fluid through a pipe is constant at every point in the pipe. Thevelocity must increase at any restriction if there are no lea!s in the system and

&E' ()*+ MET ,(* Hyd$-ne (*


11/13

the flow rate remains constant. The velocity increases four times to maintain aconstant rate of flow if the original pipe diameter is changed to one$half of itsoriginal si6e.

Speed

The speed of a cylinder rod is determined by volume capacity and fluid flowvelocity. To determine the speed at which a cylinder rod moves the flow rate atwhich hydraulic fluid is directed into the cylinder must be !nown.

The speed of a cylinder rod is independent of pressure "yup it5s true#. The speedof rod extension is usually expressed in inches per minute "in. )min#. The speedof rod extension is directly proportional to the flow rate.

Two methods of increasing speed at which a load "or cylinder rod# in a hydraulicsystem moves are by using a smaller diameter cylinder or by increasing the rateof fluid flow to the cylinder. % small diameter cylinder produces an increase in

speed and a decrease in applied force as compared to a larger cylinder. Twocylinders of different diameters having the same length have different fluidcapacities and if both receive the same rate of fluid flow the rate of travel andpressure output are different.

'low rate must be kept belowthe rate at which turbulenceoccurs in the system, derived bythe calculation of a 5eynolds

number

echanical Advanta#eMechanical advantage is the ratio of the output force of a device to the inputforce. Mechanical advantage is achieved when an applied force is multipliedresulting in a larger output force. 0evices that produce mechanical advantageinclude levers bloc! and tac!les gears etc.

&E' ()*+ MET ,(* Hyd$-ne ((


12/13

Mechanical advantage results from a force applied a certain distance from afulcrum. % fulcrum is a support on which a lever turns or pivots and is locatedsomewhere between the effort force and the resistance force. n determining theforce needed to balance a lever)fulcrum mechanism the effort force must be

farther from the fulcrum than the resistance force or must have and effort forceequal to or greater than the resistance force.

-ascal5s law states that pressure exerted on enclosed fluid is transmittedundiminished in every direction. This is demonstrated by a fluid filled bottle. %sa cor! is pressed further into the bottle the pressure throughout the bottle isincreases until the incompressible fluid bursts the bottle. The bottle burstsbecause the force applied to one area "the cor!# is equal to the pressuremultiplied by the larger area "the body of the bottle#. The resulting force within avessel is a product of the input force and the input pressure area divided by theoutput pressure area.

Fluids are well suited for being transmitted through pipes hoses and passagesbecause of these force characteristics. This force is energy which can producemovement wor! or leverage when applied to a hydraulic application. Forexample interconnected hydraulic cylinders of different diameters producehydraulic leverage in a typical hydraulic car jac!.

&E' ()*+ MET ,(* Hyd$-ne (2


13/13

'orcemultiplication withinterconnectedcylinders ofdifferent si6es.

%t times system pressure must be determined before calculating either inputforce or the output force. This may be required when determining the input forcerequired to produce a given output force with given si6e cylinders. The requiredinput force is determined by calculating the area of the output cylindercalculating the pressure in the system and determining the input force based onthe system pressure and area of the input cylinder.

Hydraulics is the branch of engineering that deals with the practical application ofwater or other liquids at rest or in motion. Hydrostatics is the study of fluids atrest and the forces exerted on them or by them. Hydrodynamics is the study ofthe forces exerted on a solid body by the motion or pressure of a fluid. % liquid isa fluid that can flow readily and assume the shape of its container. Fluid flow isthe movement of fluid caused by a difference in pressure between two points. na hydraulic system fluid flow is produce by the action of a pump and isexpressed as a measurement of gallons per minute or liters per minute.

&E' ()*+ MET ,(* Hyd$-ne (,

the principles of hydraulics

Documents