Introduction to Hydraulics

Introduction to Filter Technology

Introduction to Accumulator Technology

Introduction to Hydraulics

Inndex

SSERVICEENTRE

SSERVICEENTRE

SSERVICEENTRE

Introduction to Hydraulics

Introduction to Filter Technology

Introduction to Accumulator Technology

Introduction to Hydraulics

11

PrefaceHYDACoperatesworldwide,offeringanextensiveproductrangetocoverallareasoffluidtechnology.The products range from components and sub-systems, through to complex controlled and regulated drive units for mobile and industrial machines and systems.

In addition we offer our customers a comprehensive package of technical services within the framework of HYDAC Fluid Engineering,formediasuchashydraulicoils,lubricationoils,cooling/cuttingfluidsandwater.Our objectives are exclusively to increase machine and system availability and to reduce our customers operational costs.

HYDAC has at its disposal a worldwide network of expertise, high quality standards and customer knowledge and is thereforebestplacedtofulfiltheexactingdemandsoftheinternationalmarket. The continuous expansion of our global presence with strong local focus enables us to respond to the needs of our customers in almost every part of the world.With10salesofficesinGermany,over40overseascompanies,somewiththeirownproductionorassemblyfacilities,more than 500 sales and service partners and over 5000 employees, HYDAC is always close to its customers.

To bring our staff, service partners and costumers in the position to reach the continuously growing needs in there business environment, we offer trainings, seminars and practical trainings in our Training Center.

Doing so, the concept of lifetime learning gets more and more important. Learning after school, apprenticeship or university never stops, because learning is the most important tool to achieve education and so for the creation of individual life- and working chances.

Lifetime learning brakes through the borders of the conventional education structures and the classification in strict arranged parts of the education way, which ends often with school or university degree. It also includes education as a way to more self dependence in life to identify and use. The ability to lifetime learning will be a fundamental key for personal, economic and social success in the future.

For these innovative forms of learning, we have decided on a learning concept, which combines the multimedia possibilities of E-Learning with the advantages of the in-house training and practical training.

For this concept we are developing a book collection, which includes the basic topics of hydraulics and continuative hydraulic systems.

This book can be used attendant to our seminars and trainings and also as a reference book for your business experience.

Jrgen Ringle Manager HYDAC Training Center

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I Introduction to hydraulics

1 IntroductionHydraulics is not a recent invention. Already Archimedes (285 - 212 BC) experimented with water power. That is when he discovered buoyancy forces and some hydrostatic laws.

Heron of Alexandria (approx. 100 BC) used air and water pressure for sundry technical gimmicks and shenanigans, which at the time were believed to be witchcraft and wizardry. Among other things he developed an opening mechanism for temple gates.

Function:

Whenafirewaslitonthealtarinfrontofthetemple,theairinasubterraineanvessel,whichalsocontainedwater,washeated up. The air expanded and displaced part of the water via an ascending pipe into a second vessel, which was suspended and lowered with increasing weight. Since it was connected to a mechanical gear system, the temple gates opened,whenthevesseldescended.Oncethefirewasextinguished,thewholeprocessstartedinreverse.Tothepeopleat the time it seemed as if the gods themselves opened and closed the temple gates.

As of the 16th. century Bernoulli, Pascal and Torricelli successfully occupied themselves with hydraulics and formulated the essential and fundamental laws of hydraulics.

Inthelate18th.century,duetotheinventionofthesteamengine,theveryfirstmethodsweredeveloped,whicharestillbeing used today.

Fig. Door mechanism of Heron of Alexandria

temple

temple gates

altar

gear water supply tank

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1.1 Important notions

Hydor Greek word for waterHydrology Science of the element water and its various shapes and propertiesHydraulic Formerly: Thescienceoffluidflowthroughpipes,channelsandbasins.

Moderndefinition: Transmissionandcontrolofforcesandmovementinducedbyfluids.

Fluids Fluids, gas and steamLiquids Substances for the transfer of energy, like water, emulsions (water and oils), mineral oils, bio-oilsandsyntheticfluids.

Fluid Technology Technologyofmechanicalpropertiesoffluids(hydromechanics,aero-mechanics)Hydromechanics Technologyofmechanicalandphysicalpropertiesandreactionsoffluidsinbothstatic (hydraulicstatic) or in motion (hydraulickinetic).Hydrodynamics Generic term for hydraulicstatics and hydraulickineticsHydrostatics Mechanicsofstaticfluids(equilibriuminfluids),also:scienceofgenerationandtransmission of forces and performance through static pressure of a given liquid.Hydrokinetics Scienceofmechanicalandphysicallawsoffluidsinmotionandtheresultingforces and performance.

1.2 Typical applications of hydraulics

Industry Machine tools

Injectionmouldingmachines

Presses

Iron-andsteelfactories,rollingmills

Nuclearandotherpowerplants

Miningindustry

Mobile Technology Excavators and cranes

Constructionworks,agriculture,forestindustry

Cars,trucks,railway

Ship Building Industry Rudder blade adjustments

On-boardcranes

Bow-gates

Bulkheadslides

Off-Shore Technology Hydraulic rams

Seafloormills

Wavecompensators

Public Water Ways Locks and weirs

Bridges

Shipliftingsystems

Custom Made Machines Custom made machines

Pilotoperatedaerials

Roboticsandhandlingtechnology

Testingmachines

Aeronautics,astronautics

Aerospace Industry Special requirements due to highly specialized technology

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1.3 Types of energy transformation (comparison)

Hydraulics Pneumatics Electricity Mechanics

Source of energy E-Motor E-Motor Mains E-Motor (Drive/Motor) Combustion engine Combustion engine Battery Combustion engine Hydraulic accumulators Pressure container Load Spring tension

Elements for Pipes, hoses Pipes, hoses Electric wires or cables, Mechanical energy transmission Magneticfields parts, levers or cranks, shafts

Source of energy Fluids Air Electrones Solid and elastic bodies

Performance High pressures Low pressures Small forces, (E-motor to Large pressure, great forces Small forces hydraulic motor tall design (quite small design Medium design relation 1:10) tall design often more advantageous compared to hydraulic solutions)Dynamic Excellent Moderate Good Good variable adjustments by means of pressure by means of pressure electric control system (acceleration, and volumeflow and volumeflow deceleration)

Power output Linear and rotational Linear and rotational Mainly rotational move- Linear and movements via movements via ments, linear movements rotational hydraulic cylinders pneumatic cylinders through magnets: movements and hydraulic motors and motors Small forces, short lifts, achievable prob. linear motors

1.4 Advantages and disadvantages of hydraulicsLikewithotherkindsofdriveunits,hydraulicsystemshavetheiradvantagesanddisadvantages:

Advantages:

tremendousforces(torque)canbeeasilytransferredevenwithrelativelysmallhydraulicunits.fullloadmovementispossiblerightfromthebeginning(startingpoint).steplesscontrolofvelocity,torqueandliftingpower.equallysuitableforquickandrapidmovementsandextremelyslowprecisionmovements.simpleoverloadprotectionandrelativelyeasyenergystoragebymeansofaccumulatortechnology.higheconomicefficiencyduetosimplecentralizeddrivesystemsincombinationwithdecentralizedtransformationof hydraulic energy into mechanical energy.

Disadvantages:

duetocompressibilityoffluidscausedbyaircontaminationinthehydrauliccircuit,pressureshocksanduneven movements in the system may occur.temperaturechangeshaveaninfluenceontheviscosity.Thiscancauseamongotherthingsanincreaseoflosses duetoleakageandorificeblockage.lossofefficiencyduetofrictioninfluids.highprecisioninproductionofhydraulicunitsisessential.

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1.5 SI-Basic units

Physical description Unit

Name SymbolLength Metre mMass Kilogram kgTime Seconds sElectric current Ampere ATemperature* Celvin KSubstance Mol molBrigthness Candela CdPressure Pascal Pa

* The iron and steel industry maintains the Celcius-temperature scale.

Prefix Abbreviation Power

Pico p 10-12

Nano n 10-9

Micro m 10-6

Milli m 10-3

Centi c 10-2

Deci d 10-1

Deca da 10Hecto h 102

Kilo k 103

Mega M 106

Giga G 109

Tera T 1012

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2 Physical basicsHydromechanics is the basic principle of hydraulics.

Hydromechanicsisthestudyofmechanicalandphysicalpropertiesplusthebehaviourofstaticanddynamicfluids.

Hydrostatics:

Studyofstaticfluidsandtheirequilibriuminahydrauliccylinderorapress.

Significantproperty:

Pressure [p] Pa

Hydrodynamics:

Studyof dynamic fluids, like the conversion and translation of energy flows in turbines of hydropower plants or thephysicalbehaviourofpressurefluidsinvalvesandpipes.

Significantproperty:

Volumetricflow/volumeflow[Q]l/min

Fig. Overview: hydrodynamics

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2.1 Hydrostatics (physical properties of pressure)

Asmentionedbefore,hydrostaticsdealswithstaticfluidsandgases.Othersourcesrefertohydrostaticsasthestudyofthe state of equilibrium.

Imagine a static cuboid, which is exposed to a certain pressure force Fn. A state of equilibrium is obtained due to the counter pressure (back pressure) p of the level surface on which the cuboid is resting.

Hydrostaticsisanimportantpartofthevastfieldofhydraulicssinceitalsodealswithrequiredforcesorpressures.Oneexample is a hydraulic cylinder. A certain force is generated inside the cylinder acting on the piston surface, which has to override an external resisting force. As a result an operation is carried out, like the pressing (moulding) of an component.

According to DIN 24312 pressure p is the quotient of standard force Fn, which acts vertically on a given surface, and surface A.

Inhydraulics,apressuredesignationinPascal[Pa]isnotcommon:

Pressure

[ ] [ ][ ]

p Newton FMeter A

Pascal= =2

1 1 0 00001

1 10

2

5

Pa Nm

bar

bar Pa

= =

=

.

Fig. Hydrostatics

pressure force F

cuboid

back pressure p

level surface

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2.1.1 Hydrostatic pressure

Pressure is not only created by external forces. The mass of a body can generate a weight force, which in turn generates a gravitational pressure.

Insideacolumnoffluidspressureisgeneratedbytheweightofthefluidmassaboveagivensurface.

Thepressuredependson:

heightofcolumn: [h]=mdensityoffluid: [r]=kg/m3gravitationalacceleration: [g]=m/s2

Thisistheformulaforhydrostaticpressure:

Example:

Whichpressureisgeneratedbyafluidcolumnofhydraulicoilwithaheightof10m(r=0.85kg/dm3)?

Solution:

Thehydrostaticpressureisafunctionofheight,notoftheshapeofthevessel.Ifyoufillvesselsofdifferentshapebutwithequalfloorsizeandthesamefluidandfillingheight,theresultingforcesonthefloorareequal.

This is also called hydrostatic paradox.

Thatis:

p h g Pa= r [ ]

Fig. Hydrostatic paradoxon

if A A Athen F F F

1 2 3

1 2 3

= == =

p = 10 0.85 1000 9.81 = 83385 = 83385 Pa

1 bar =

Nm2

1100 000 Pa83385 Pa = 0.83385 bar

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2.1.2 Further properties of pressure

In addition to pressure other terms are used in physics and will be explained in the following.

Absolutepressure:

The absolute pressure indicates the pressure compared to a vacuum. Pressures are examined and added, like: absolute pressure=relativepressure+atmoshericpressure.Inavaccuumtheabsolutepressure=0.

Relativepressure:

Therelativepressuresignifiesarelativepressurerelationship.Itdescribesthepressuredifferencebetweentwodifferentactual states.

Atmosphericpressure:

Atmospheric pressure exists all over the world. It is generated by the masses of air above us and differs from place to place. This is usually due to the geographical altitude of a location in relation to the surface of the oceans. The higher you are, the less air mass is above you and consequently the atmospheric pressure decreases with increasing height.

Thus the atmospheric pressure is equal to the gravitational pressure, generated by earths atmosphere.

This effect is used in altimeters by measuring the air pressure. Subsequently the altitude can be calculated.

Fig. Overview: pressure terms

atmosphere

upperfluidlevel

abso

lute

pre

ssur

e

rela

tive

pres

sure

(gra

vita

tiona

l pre

ssur

e)at

mos

pher

ic p

ress

ure

(gra

vita

tiona

l pre

ssur

e)

lowerfluidlevel

21

2.1.3 The law of Pascal

Due to the comparatively high pressures, which are used in modern hydraulic units, the gravitational pressure can be neglected. Thus it follows that the pressure must be equally high at all places.

The system pressure can be calculated with Pascals law.

Pascalslawstates:

Thataconfinedfluidtransmitsexternallyappliedpressureuniformlyinalldirections.Moreexactly,inastaticfluid,forceistransmittedatthevelocityofsoundthroughoutthefluid.Theforceactsnormalandverticallyonallsurfaces.

Twomoreprinciplescanbederivedfromthat:

theprincipleofthetranslationofforcestheprincipleofthetranslationofpressure

Example:

WhatpressurepiscreatedinacontainerifaforceF=1toactsonapistonwithasurfaceA=20cm2?

F: Force in daNA: Surface in cm2

p: Pressure in bar

Formula:

p= barF= daNA= cm2

p=1.000daN/20cm2

p=50bar

p FA

=

Fig. The law of pascal

force F

rodcylinder road area

pressure p

Forces act vertically on the internal walls of a vessel

pressure tank

A

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2.1.3.1 Power conversion

Let us have a look at two pistons with different active areas, which are connected with each other. A force F1 acting upon area A1 causesa uniformandevendispersionof pressurewithin the fluid chamber (Pascals law).Consequently a resulting force F2 acts upon the active area of the piston A2.

Generically:

Inourcase:

Consequently:

The ratio of the forces is equal to the ratio of the active areas. This means that the larger force acts upon the larger surface.

The path length of the piston is reciprocal to the active piston areas. With a piston ratio of 2:1, the smaller piston moves twice the distance s1 than the larger piston (s2).

Inpracticewecanfindtheconversionofpowerin:

manuallyoperatedliftingplatformshydraulicliftingplatformsforcarspresses

Example:

Please determine which piston generates a greater force? Surface A1 with a diameter of 30 mm, surface A2 with a diameter of 50 mm and a constant pressure of 50 bar.

Formula:

Fig. Power gear ratio

p FA

and p FA

p p

11

12

2

2

1 2

= =

=

whereas according to Pascalss law

FA

FA

or F F AA

1

1

2

22 1

2

1

= = : ( )

F p A F p A

F F

1 1 2 2

1

= =

=

22 =

p FA

=

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2.1.3.2 Pressure conversion

In analogy to power conversion there is also the possibility of pressure conversion.Letshavealookatthefigurefurtherdown.Twopistonswithdifferentactiveareasareconnectedsolidlytoeachotherby a rod.

IfpressurepactsuponactivepistonareaAwegetforceF:

the force F.

The force F generates with the ring surface A2 the pressure p2:

Thus we have a pressure conversion where the pressure ratios are reciprocal to the surface (area) ratios. This means that the higher pressure can be found on the side of the smaller area (surface).

Inpracticewecanfindpressureconversionin:

pneumatic-hydraulicpressureintensifierssequentialdifferentialcylindersbrakepowerassistunit(brakeforcebooster)

p FA

F p A= => =

F p A p A p p AA

= = => =1 1 2 2 2 1 12

Fig. Pressure conversion

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2.2 Volumetricflow/volumeflow(hydrokinetics)

Inhydraulicsnotonlypowerprovidedbyacylinderisofgreatimportancebutalsothevelocityandtheefficiencyofthecontrolofthefluidflow.

Thereforeweshouldalsolookatthelawsofhydrodynamics,alsoknownasthescienceofflowingandmovingfluids.

FlowinahydraulicunitiscalledvolumeflowQ(technicalterm).Itindicatesthevolumeoffluids,whichflowsthroughthesystem in a certain time unit (sec).

AFrenchmanJosephMichelMontgolfiergainedfirstresultsintheareaofhydrodynamicsalreadyin1796.Basedonhisfindingshedevelopedahydraulicwaterram,apump,withwhichitwaspossibletotransportwatertoahigheraltitude.

Waterfromareservoirflowsthroughapipeintothehydraulicram.Initiallythebuffervalveisopenandtheshuttervalveclosed.Theflowvelocityincreases,whenthewaterpassesthroughthebuffervalve.Onceacertainvelocityhasbeenreached the valve shuts suddenly. Thus the pressure before the valve is increased, which causes the shutter valve to open.

The pressure surge pushes water into a boiler (pressure vessel). The air inside the vessel is compressed and now can push the water via a standpipe in a reservoir on a higher level.

After the water has accumulated in the vessel, the pressure inside the pipe leading to the ram decreases again. The buffer valve opens and the shutter valve closes. Since all valves are back in the starting position, the whole process can start from the beginning.

Thenameofthepump(ingerman:widder=ram)isderivedfromitscharacteristicnoisesmadebytheopeningandclosing of valves. People at the time were reminded of the noises made by rams, hence the name of that pump.

Fig. Principle of a hydraulic water ram

flattervalve/shuttervalve

hydraulic water ram

plug/stopper

buffer valveboiler

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2.2.1 Thelawofvolumetricflow

VolumeflowQisgivenbythevolumeoffluidVdividedbytimet.

Liquid volume V is itself given by area A times length s.

IfAsissubstitutedforV,Qisthengivenby:

Distance s divided by time t is velocity v.

Flow Q hence equals the cross-sectional area of the pipe A multiplied by the velocity of the liquid v.

2.2.2 Flowphenomenaandflowpatterns

Insidepipestwodifferenttypesoffluidflowsoccur:

laminarflowturbulentflows

Theflowtypedependon:

thecross-sectionofthepipethevelocityofthefluidflow(volumeflow)theviscosityofthefluid

Thechangefromlaminartoturbulentflowoccursattheso-calledReynoldsNumber:

Formula:

This number is only valid for round and smoothe pipes with even surface.

Q Vt

V A s

Q A st

v st

Q A v A v A v

=

=

=

=

= = =

1 1 2 1

Fig.Thelawofvolumetricflow

Re /= v d mm s u

2

Re = 2320

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2.2.2.1 Laminarflow

Iftheflowvelocityislow,thefluidparticlesflowinlines(hencelaminarflow).Thefluidsgenerateflowlevels,whichmove with different velocities.

If the velocity is at its peak in the center, the outer level sticks to the interrnal surface of the pipe. There is a certan friction, which causes loss of pressure, which in turn can be calculated by means of the Bernoulli-equation.

2.2.2.2 Turbulentflow

If thevelocity increasesbeyondacertain level, theflowgets turbulent.Turbulencesoccuraswellasfluctuationsof velocities.

TheBritOsboneReynoldswasthefirsttopointoutthatthischangefromlaminartoturbulentflowcouldbedescribedbyaparameter(ReynoldsNumber).Thereexistsacertainvalue,whichisresponsibleforaparticularbehaviouroffluidflow.

Fig.Turbulentflow

Fig.Laminarflow

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2.2.3 Fluid friction and pressure loss

Hydraulicenergycannotbetransferredwithoutlossthroughpipesandothercomponetslikevalvesandfilters.Ontheinsidesofpipes, valvesandfilters friction isgenerated,which in turncreatesheat,whichof course isa lossof pressure (pressure difference between inlet and outlet of a component).

Pressure differnces are usually indicated by Dp.

Thequantityofpressurelossusuallydependson:

theviscosityofthefluidthelengthofthepipesthecoss-sectionofpipestheroughnessoftheinsideofthepipesnumberanddesignofpipebendsthevelocityofflowanddesignandnumberofvalvesandfilters

Fig. Pressure loss due to friction inside pipes

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2.3 Hydraulic energyLike a combustion engine in a vehicle a hydraulic aggregate is usually called a drive unit.

The aggregate provides the energy, which is used by the actuator (like a cylinder) to do its job.

The operations are controlled by valves or the energy is led to individual actuators. Together with control valves and the actuators we can call this assembly a hydraulic system.

Thedesignofahydraulicaggregatedependsonthesupposedefficiency,whichinturnisaresultofpressureandvolumeflow.Theenergylossesoftheaggregatearedescribedbyges(efficiency).

ThuswegetanoverallefficiencyPanofanaggregateof:

P p Q kWanges

= [ ]

600 h

Fig. HYDAC aggregate solution

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2.3.1 The law of conservation of energy

The law of conservation of energy is one of the core issues in physics. It states, that the total energy in a closed system always remains constant. A closed system is a system without any interdependancy to the outside world. There is no exchange of energy, matter and information.

Within a closed system one form of energy can be transformed into another, like electric energy to warmth. Wheras it is impossible to generate or destroy energy within a closed system.

Thustheequation:

For example in a hydraulic unit on one side you have the electric energy of a motor, which drives the unit. On the other side you use the energy to move loads. Inside the unit electric energy is transformed into hydraulic energy, which is transformed into mechanical energy. During these transformations part of the energy is transformed to warmth.

However during these transformations the law of conservation of energy is always valid.

Fig. Closed system

E before E afterges ges =

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2.3.2 The Bernoulli Principle

Thetotalenergyofaflowofliquiddoesnotchange,aslongasenergyisnotsuppliedfromtheoutsideordrainedtotheoutside.Neglecting the typesofenergy,whichdonotchangeduringflow, the totalamountofenergy ismadeupof potential energy, kinetic energy and pressure energy. The potential energy depends on the height of head of liquid and onstaticpressureandkineticpressure.Thekineticpressuredependsontheflowvelocityandbackpressure.

ConsideringboththecontinuityequationandtheBernouilliequationthefollowingmaybededuced:

If the velocity increases as the cross-section decreases, movement energy increases.

Thefollowingmighthelpasanexplanation:

Sincethetotalenergyremainsconstant,thepotentialenergyand/orthepressureenergymustbecomesmaller,ifthecross-section area is reduced. There is no measurable change in potential energy. The static pressure, however, changes, dependentuponthedynamicpressure,i.e.dependentonthevelocityofflow.

Thehightofthefluidcolumnsisameasureforthepressureexistingatpreciselythispoint.

Fig. The Bernoulli Principle

g h pp

v p p g hst ges st constant + + = = +2

2r

+ =

=

v g p static pressure

g h essure c

st2

2r

r Pr aaused by hight offluid column v 2 r2

= ( )back pressure dynamic

55

5 Basics of hydraulic symbols

5.1 General remarksFor hydraulic circuit diagrams graphic symbols are required. They are standardized in DIN ISO 1219. In order to read and understand circuit diagrams it is necessary to learn the symbols and their function. The symbols dont tell you anything about the design and construction of the components, they only illustrate their function.AnoverviewofthesymbolsofDIN1219youfindinthe appendix.

5.2 SymbolsWith the help of the following basic symbols you can draw a major part of circuit diagrams.

5.2.1 Basic symbols (excerpt)

Lines, main line, electrical line

Internal or external control line, drain line, leakage line

To group two or more components in a sub-assembly

Mechanical connection (shaft, lever, piston rod)

Pump, motor (circle ), energy transfer unit

Measuring device (circle )

Check valves, rotary connection, mechanical pivots, rollers (circle )

Control elements, drive unit

Preparationdevices(filters,separators,lubricationdevices,heatexchangers)

Cylinders, valves

56

5.2.2 Function symbols (excerpt)

Showsdirectionofflowandoperatingmedium(filled=hydraulic,open=pneumatic)

Arrows,(straight),linearmovement,pathanddirectionofflow throughavalve,directionofheatflow

Arrows (curved), rotational movement, direction of rotation viewed on shaft end

Adjustability in pumps, motors, springs, selenoids

Closed path or connection

Linear electrical positioning elements acting in opposition

Spring

Throttle

57

5.2.3 Operation symbols (excerpt)

Push button

Push button, pull-out knob

Spring repositioning

Roller shaft (two directions)

Electrical, 1 winding

Internal control channel

External control channel

Hydraulic operation (1-stage, 2-stage)

Pneumatic operation (1-stage)

58

5.3 Symbols for hydraulic motors and hydraulic pumpsHydraulic motors and pumps are represented in a circuit diagram by a circle. The triangles inside the circle tell you whe-theritisapumporamotor,howmanyconnectorsandthedirectionofflow.For inlet and outlet you need two connectors. For the transfer of the energy a shaft is drawn to the motor or pump.

5.3.1 Hydraulic motors

Hydraulic motors transform hydraulic into mechanical energy. The turning movements are illustrated by triangles (they point to the inside g Motor)

5.3.2 Hydraulic pumps

Hydraulic pumps transform mechanical into hydraulic energy (triangles point to the outside g pump). An arrow < 45 indicatesthatthevolumeflowcanbeadjusted.

5.3.3 Hydraulic pump and hydraulic motor

Hydraulic pump and hydraulic motor can function as a unit and work both as a pump or motor (triangles point in the same direction gHydraulicpump/-motor).

5.3.4 Direction of rotation

The direction of rotation of a hydraulic pump and motor is indicated with a curved arrow on the shaft.

(1 direction) (2 directions)

Fig.Hydraulicpumpwithasingleflowdirection andadjustablevolumeflow

Fig.Hydraulicpumpwithtwoflowdirections

Fig.Hydraulicpump/-motorwithonedirectionofflow Fig.Hydraulicpump/-motorwithtwodirectionsofflow

Fig.Hydraulicpumpwithonedirectionofflow constant displacement volume and one

direction of rotation

Fig.Hydraulicmotorwithonedirectionofflow,constantdisplacementvolume/sweptvolume

two directions of rotation

Fig.Hydraulicmotorwithasingleflowdirection Fig.Hydraulicmotorwithtwoflowdirections

59

5.3.5 Hydraulic pumps and hydraulic motor classes

An additional motor is required to drive a pump, which is connected with shaft to the hydraulic pump or the hydraulic motor.

5.4 Symbols for cylindersCylinders transform by means of a linear movement hydraulic into mechanical energy.

5.5 Symbolsandnaming(example:directionalvalve)

We take a directional valve to explain the basic symbols and names of hydraulic valves. Directional valves open and shut the hydraulic pipes and facilitate the exchange between pipe connectors. This way volumeflowsandconsumers(cylinder,motors)canbecontrolled.Becauseofthisfunctiondirectionalvalveshaveatleast two switching modes and at least two connectors.

The symbols are always drawn in a non-operational state with connectors and designation.

5.5.1 Design of a directional valve

Switching Modes (Number of squares)0=neutralpositiona,b=functionalposition

2 switch positions 3 switch positions

Numberofconnectorsandconnectionswithinasitchingmode,i.e.:

2 connectors 3 connectors 4 connectors

Thearrowsinsidethesquaresshowthepossibledirectionofflow.Thesesigns indicatethattheflowisshutoff.

Since there exist manifold demands on the valves there are numerous connections between the various connectors.

Fig.Hydraulicpumpwithonedirectionofflow,electro motor and one direction of rotation

Fig.Hydraulicpumpwithonedirectionofflow, combustion engine and one direction of rotation

Fig. Simple acting hydraulic cylinder with return spring

Fig. Double acting differential cylinder

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Example:

Flow shut off Flow in two directions possible Flow from P to A, T is shut off.

Whichflowdirectionsareopenedorshutoffdependsonthetasksofthesystemtobedesigned.

5.5.2 Characteristics and naming of a directional valve

Namingofconnectors:

P = PumpT = Tank,returnflowA,B = ConsumersX,Y,Z = Controlconnectors

The naming is not laid down in DIN Standard. So other symbols are possible g follow manufactors data.Symbols indicate the component always in the neutral position.

Example:

Connector P is connected with the pump, connector T with the tank and connectors A and B with the cylinder.

Pronunciation: 4 stroke 3 directional valve

61

Furtherexamplesofdirectionalvalves(nooperationalmodes):

2/2directional valve

3/2directional valve

4/2directional valve

4/3directional valve

6/3directional valve

Directional valve continually variable (any number of switch positions and intermediate positions)

The operational consequences of the different switch positions becomes obvious when you slide the whole sign against thefixedpositionsoftheconnectors.

5.5.3 Different centre positions of directional valves

A4/3directionalvalvewithrotating central position is used to control double actingcylinders.Iffixeddisplacementpumpsareappliednoheatingoffluids occurs. If differential cylinders are used, the connection must not be exposed to pressure. Otherwise a creeping of the piston due to leakage oil transfer cannot be excluded.

A4/3directionalvalvewithblocked central position (all ports blocked) is used to control double acting cylinders or hydraulic motors. The central position ascertains a stop of the piston in any position (emergency stop, hard stop). Preferably these cylinders are used with cylinders of equal sized surfaces, since in differential cylinders leakage oil transfer from P to cylinder pipes might cause creeping of the piston.

A4/3directionalvalvewithfloatingmid-position (both cylinder lines are connectedtotankbyfullflowdiameterofthepistonvalve,Pblocked)makesa soft halting possible. However the piston will move a little bit longer. Pressure relief via A, B and T prevents a creeping of piston in differential cylinders. This type of directional valve is also used in vertically built cylinders, which are safeguarded by pilot controlled check valves.

A4/3directionalvalvewiththrottledfloatingmid-position (both cylinder lines are connected to spool by small notches to tank, P blocked) are used to control doubleactingcylindersorhydraulicmotors.Athrottledfloatingmid-position results in a softer halt than could be obtained with valves with blocked mid- position. A pressure relief due to the notches prevents a creeping of the piston in differential cylinders.

A4/3directionalvalvewithcontinuousflowinmid-position(allportsare connected to each other) results in a soft halting, but the piston needs more time toStoppcompletely.Thefluidflowinfixeddisplacementpumpsoccurs pressureless in order to avoid heating. This type of valve is usually only used in cylinderswithequalsurfaces,sincepressureheadsandflowresistanceinthe pipes can cause a creeping of the piston in differential cylinders.

62

5.5.4 Operational methods

So farwehavenamedanddiscusseda4/3directional valve. Inorder to slide thesymbols into thedifferent switch positions (a, 0, b) the appropriate operational mode has to be added.Operational modes are drawn on the left or right side of the symbol of a directional valve. The selection depends on the demands on the unit. You have to be careful here, since some operational modes require a return spring to reposition the valve into neutral.

In circuit diagrams directional valves are drawn in the neutral position (0), in other words not when they are being operated but when they have been repositioned by a return spring.

Fig.4/3directionalvalvewithtwo-sidedelectro-magneticswitchandspringcentering

In order to keep the directional valve in the neutral position a return spring is attached left and right (spring centring). The directional valve can be moved into position a or b with the help of an electric element. The valve is repositioned with the return springs.

Example:

Fig.3/2directionalvalvewithpushbutton/pull-outknob

Noreturnspringisrequiredfortthis3/2directionalvalvesincethisvalveismovedbymechanicalforceintotherequiredposition(pushbutton/pull-outknob).

Fig.4/2directionalvalvewithanelectricelementandreturnspring

Here we need a return spring. The valve is switched with the help of an electric element into position b and returned to position a with a return spring.

63

5.6 Further symbols of hydraulic valves5.6.1 Pressure valve

Pressure valves control the pressure within a hydraulic unit. They are symbolized by a single square with an arrow. The position of the arrow tells you, whether the connectors are connected with each other or not. The throttle cross section can be adjusted variably.

Two main groups of pressure valves are called pressure limiting valves (to limit the pressure inside a unit) and pressure reducing valves (to reduce the operational pressure to a certain predetermined level).

Pressurelimitingvalve:

The control line sits before the valve. Therefore the valve can be controlled when the opening pressure increases by opening outlet (2) to the tank against an opposing force (adjustable spring force).

Fig. Pressure limiting valve (Control line before valve, throttle cross section shut-off)

Pressurereducingvalve:

Pressure reducing valves feature the control line after the valve. With an increasing opening pressure the valve can keep the pressure constant no matter how high the load. Outlet (2) needs to be shut off against an opposing force. Therefore pressure reducing valves are also called pressure control valves.

Fig. Pressure reducing valve (Control line after valve, throttle cross section open)

5.6.2 Check valves

Checkvalvesarevalveswhichpermitonlyonedirectionofflow.

They where also called now return valves.

Flowisshutoffbyalockingelement(i.e.ball,cone).FlowfromAtoBispreventedbyalockingelement.Volumeflowispossible from B to A if the pressure before the locking element is higher than the spring pressure.

Fig. Spring check valve

Fig. Throttle cross section open Fig. Throttle cross section shut

64

5.6.3 Flow control valve

Flowcontrolvalvesinfluencethevolumeflowbychangingthecrosssectionofflowcarryingdevice.

Inaflow(circuit)diagramitisdrawnlikethis:

Fig. Adjustable throttle valve

Byreducingthecrosssectionofapipeyouthrottlethevolumeflow.

Sinceflowcontrolvalvesandcheckvalvesareusuallyonlyusedforoneflowdirectionanothercheckvalveisaddedtothe system.

Fig. Adjustable throttle check valve

5.7 Further general hydraulic symbols5.7.1 Symbolsofstorageandprocessingofpressurefluids

Symbolsforstoragecomponentsareusuallyovals:

Examples:

Symbolsforprocessingthepressuremediumaredrawnlikethis:

Examples:

Fig. Separators Fig. Cooling devices Fig. Filters Fig. Heating devices

Fig. Hydraulic accumulator (only for upright positions) Fig. Closed accumulator (with three lines)

65

5.7.2 Symbols for checking and measuring instruments

Additional components for hydraulic units, like control and measuring instruments are represented by circles ():

Examples:

Fig. Pressure control Fig. Temperature control Fig.Volumeflowindicator

67

6 Basics of hydraulic circuit diagrams6.1 The circuit diagram (general)A hydraulic circuit diagram is a graphic representation of all components and their connections of a hydraulic unit for whichstandardizedsymbolsarebeingused.MandatorystandardisDINISO1219.Acircuitdiagramisalwaysdrawnandalsohastobereadthatwayinthedirectionofthevolumeflowstarting,forexample,withahydraulicpumpandfinishingwiththeoutletportuser,cylinder,motor.Thehydraulicsymbolsshouldbedrawnhorizontaly,thelinespreferablywithout crossings and directly. Components are always represented in neutral position, neither exposed to pressure nor volumeflow.Thebasicsymbolscanberotatedandmirrored.Theyshoulddisplayallnecessarycharactersfor listingrelevantdatalikeconnectors,pressure,volumeflow,electricconnectionsandcomponentsettings

YouwillfindmoreinformationfordrawinghydrauliccircuitdiagramsinDIN ISO 1219-1.

6.2 Assembly of a basic power unitThe power unit is necessary for the power supply of the whole system.

It consists of a tank(3)forthehydraulicfluid.Thepump (1) is connected over a suction hose with the tank an is activated with and electric motor(2).Thatiswherethevolumeflowforthesystemisgenerated.Thecheck valve (4) is needed tostoptherefluentoilwhenthemotorisstopped,sothatthemotorcantrunbackwardsandwillbedestroyed.Tosecurethe system from over pressure, we have the pressure control valve(5).Thisvalveleadsthevolumeflowdirectlytothetank, if the load doesnt need it. The manometer (7) shows the system pressure.In the return line of the system (T) is a returnfilter(6).Itcleansthefluidfromcausedcontamination.

Fig. The circuit diagram

Fig. Circuit diagram: assembly of basic power unit

68

6.3 Actuating users with directional valvesToactuate the cylinders,weusea4/3directional valve in this circuit.The volumeflow from thepower unit canberedirectedwiththisvalve.Inposition0thedirectionalvalve,thecylinderstaysinhispositionandthevolumeflowisleadover the pressure control valve to the tank. If we supply a voltage to the solenoid y2, the directional valve switches in positionb.ThevolumeflowrunsfromPtoAandthecylinderextends.ThefluidfromthecylinderflowsovertheconnectorsBtoTthroughthereturnfilterintothetank.Switchingthedirectionalvalvewithsolenoidy1,thefluidflowsfromPtoBandin the return line of the cylinder from A to T. The cylinder retracts.

Fig.Circuitdiagram:4/3directionalvalve

69

6.4 Application of pilot controlled check valves in load holding circuitsOne of the objectives of check valves is that they have to hold a load over a certain period of time in a given position, like with working platforms. If pulling loads act upon cylinders and hydraulic motors, for example with cranes, a tight sealing of the line exposed to the load pressure is required in order to prevent a slow but continuous sliding down due to leakage. Thereforecheckvalvesarebeingused,whichcanblockvolumeflowinbothdirectionseffectivelyandwhichcanbe released easily under certain circumstances.

Fig. Circuit diagram: twin-check valve (RPDR06)

70

6.5 Speed control of actuators with throttle valvesFor speed control of actuators we use different throttle valves.

example1withaconstantcross-section(orifice)example2withathrottlevalve(volumeflowcanbeadjusted)example3withaflowcontrolvalve,withaconstantvolumeflowtotheuser(the necessary directional valves are missing due to better overview)

Fig. Circuit diagram: speed control of actuators with throttle valves

71

6.5.1 Feed line control (primary control)

Applied if counterforces occur at the actuator

Advantage:

Pressure is only exerted directly at the actuator, this means the actuator is only exposed to this pressure.

Disadvantage:

Apressurereliefvalve,whichdivertsanexcessivequantityoffluidof thepumpbackto thetank,mustbeset to the highest available actuator pressure in the system. Heat generation is fairly high and the pump has a higher power consumption.Theheatduetofrictiongeneratedbythevolumeflowpassingthroughtheflowcontrolvalveisdirectlytransferred to the actuator.

Fig. Circuit diagram: feed line control (primary control)

72

6.5.2 Discharge control (secondary control)

Appliedwithpullingloadsattheactuator,sothattheactuatorcannotrunfasterthanthepumpgeneratesfluidflow.

Advantage:

Nobackpressurevalveisnecessary.Thefrictionheatgeneratedbythefluidpassingthroughtheflowcontrolvalve,istransferred to the tank.

Disadvantage:

The pressure relief valve has to be set to the highest possible level for the pressure of the actuator (generation of heat). The actuator is exposed to the pressure due to the hydraulic context.

Attention:pressureconversionpossible!

Fig. Circuit diagram: discharge control (secondary control)

73

6.5.3 Bypass control

Advantage:

Sincetheflowcontrolvalvelimitstheflowtotheactuatorbydivertingacertainpartofthetotalvolumeflowbacktothetank,onlythepressureasrequiredbytheloadisbuiltupattheactuator.Theheatgeneratedbythepassageofthefluidthrough the valve is transferred to the tank.

Disadvantage:

The actuator is not hydraulically involved gtheactuatorcouldmovefasterthanitissuppliedwithfluid;abackpressurevalve might have to be implemented.

Fig. Circuit diagram: bypass control

74

6.6 Various rapid traverse-controls in differential circuit designsRapidtraverse-controls(rapidmovingofthecylinder)canbeachivedthroughaddinganotherpowerunit(withoutfigure)orbyreturningthefluidfromtheoutletcylinderchambertotheinletofthecylinder.

Attention:pressureconversionpossible!Examples:

extend

Fig. Rapid feed forward back without intermediate stop Fig. Rapid feed forward back with intermediate stop

Fig. Rapid feed forward back with intermediate stop

Fig. Rapid feed forward and working feed forward without intermediate stop

Fig. Rapid feed forward and working feed forward with intermediate stop

extend

extend

extend

retract

retract

retract

retract

back

rapi

d fe

ed

forw

ard

wor

king

feed

75

II Introduction to equipment technology

1 Hydraulic pumps and hydraulic motors1.1 General remarksHydraulic pumps and motors are instruments in which mechanical energy is transferred into hydraulic energy and vice versa.

In most cases hydraulic motors have the same principal technical design as hydraulic pumps.However leakage oil return in motors goes to the outside as opposed to pumps, where the leakage oil is returned to the suction chamber. Some pumps like constant axial piston pumps in bent axis design can be directly used as motors.

Depending on the displacement principle we distinguish several types of pumps and motors, which differ in the designs.

Fig. Overview hydraulic pumps and hydraulic motors

76

1.1.1 Hydraulic pumps

Hydraulicpumpsarehydrauliccomponents inwhichfluidsaredisplacedandenergyprovidedbyelectricalmotorsor combustion engines is being transformed into hydraulic energy.Pumpstakefluidsoutofatankorcontaineranddisplacethesefluidsthroughpipesandcontrolanddistributingelementsto the different drive units, which provide work by transformation of hydraulic energy into mechanical energy.Dependingonthecomponents,whichtransportthefluidsthepumpsarecalledrotatingcirculationpumpsandoscillatingpiston pumps.

Circulationpumps:

Gear pumps

External gear pumps Constant displacement volume Up to 250 barInternal gear pumps Constant displacement volume Up to 315 barGear ring pumps Constant displacement volume Up to 100 barScrew pumps Constant displacement volume Up to 175 bar

Vane pumps

Fixed vane pumps Constant displacement volume Up to 175 barRotary vane pumps Constant displacement volume Up to 175 barVane pumps Displacement volume (constant and variable), Up to 125 bar pressure adjustable in the NELL stroke pumpScrew pumps Constant displacement volume Up to 175 bar

Thrustpumps:

Piston pumps

Hand pumps Constant displacement volume Up to 700 barIn-line piston pumps Constant displacement volume For highest pressuresRadial piston pumps Constant displacement vol. (also adjustable) Pressure control up to approx. 630 barSwash plate pumps Constant displacement vol. (also adjustable) Pressure/outputcontroluptoapprox.400barBent axis pumps Constant displacement vol. (also adjustable) Pressure/outputcontroluptoapprox.400bar

77

1.1.2 Hydraulic motors

The job of hydraulic motors is simply to transform hydraulic energy as provided by the pump into mechanical energy. This transformation will produce a certain torque.Quiteoftenthetypeclassificationofhydraulicpumpsisalsovalidforhydraulicmotors.

Thedemandsontheperformanceofhydraulicmotors,like:

maximumoperationalpressuretorquelife-timecircledirtresistancemaintenancepulsationnoiselevelsparepartsweightsizebuild-inpossibilitiescost

arethesame,whichareplacedontohydraulicpumps.Inthiscaseonedoesnotspeakofvolumeflowperrotationbutofcapacity per rotation.

The most important characteristics of hydraulic motors are the torque delivered to the shaft and the drive speed range.

Classificationofhydraulicmotors:

The drive speed range plays a major role in the discussion about the right hydraulic motor. Especially the lowest possib-le drive speed with which a hydraulic motor can deliver the torque to the shaft in a uniform steadiness is important.

Thereforehydraulicmotorshavecertainadvantagesoverelectricalmotorsorcombustionengines:

advantageousweightoperformancerelationshipcompactdesignreasonablecoststeplessdrivespeedadjustment

Motorsaredistinguishedasfollows:

1 - 150 min-1 Slow Piston motors50 - 750 min-1 Medium Vane motors300 - 3.000 min-1 Fast Gear and screw motors

When designing a hydraulic unit with a hydraulic motor it is advisable to be able to use the motor as a pump by reversing thedirectionofflowonashortorlongtermbasis.

78

1.2 External gear pump and motorDescription:

Volume is created between the gears and housing.

m = Modulez = Number of gearsb = Width of gearsh = Height of gearsp = Pressure

Function:

By turning the upper cog wheel in the direction of the arrow the lower cog wheel is turned in the opposite direction. Germanengineerstalkaboutthecombingofthetwowheels.Thefluidistakenoutofthesuctionchamberandpropelledthrough the gears and displaced on the pressure side. Since on the pressure side the cavities between the cogs of the onewheelaresealedbythecogsoftheotherbeforetheyareemptied,thepressurefluidenclosedhastobetransportedto theoutsidebymeansofsomedrilledholes in thehousing.Dependingon themanufacturer thefluidused for the lubrication of the bearings and let out at the upstream side. At the same time a pressure balance is achieved at the bearingbracketsandtheefficiencythusimproved.

The pumps are manufactured from different materials. Special designs according to customers' requirements are also available. Certain permissible operational parameters, like pressure upstream, time, peak pressure (downstream), range ofviscosity,contaminationandfluidsareverydifferentfrommanufacturertomanufacturer.Therearedifferentdesignsinrelation to operational parameters, like time, reverse operation and interruptions.

The pumps are either single pumps, multiple pums and combinations thereof. Until the last century a version with three wheels was built. They were used in lubrication units of water plants and combustion engines.

Repairs in these pumps are economically not feasible. Therefore no spare parts are available.

Application:

The external gear pump is a simple, robust and inexpensive construction with a high degree of reliability. However they feature a high degree of irregularity and a high noise level.

TechnicalData:

constantdisplacementvolumeupto250barcanbeusedasa hydraulic motor

V m z b h p=

79

Externalgearmotor:

The external gear motor is not suitable for low speed drives, since it features a rather bad total torque for low speeds. A speed reduction gear unit can be built in and is in comparison to piston motors (slow running) rather inexpensive. In order to secure a smooth start of the engine the required load torque has to be limited.

Fig. External gear pump

shaft

gears

housing

80

1.3 Internal gear pump and motorDescription:

Volumeiscreatedbetweenthegears,housingandspacing/sealingelement.

m = Moduleb = Numberofgearsoftheinnermostcogwheel

z = Width of gearsh = Hight of gears

Function:

By turning the innermost cog wheel in the direction of the arrow the outside cog wheel is turned in the same direction. The rotationalmovementcausesthecogwheelstodivert,sothatthespacesbetweenthegearsaresetfreeandcanbefilledwiththefluidfromthesuctionchamber.Thepumpisavailableindifferentdesignsandwithdifferentpressureranges.Pressurecompensationandincreasedefficiencyissolvedindifferentwaysbythevariousmanufacturers.Multiplepumpsand pump combinations are available, also special designs for particular requirements and locations (mobile sector, offshore etc.).

Repair kits are also available. Whether a repair is economically feasible or not, cannot be discussed here.

Application:

Noise reduced pump with low degree of irregularity

TechnicalData:

constantdisplacementvolumeupto315barcanbeusedasa hydraulic motor

Fig. Internal gear pump

V m z b h=

housing

shaft internal gearoutside gear

sickle

81

1.4 Ring gear pump and motorDescription:

The rotor has one gear less than the internally geared stator. Planetary movement of the rotor.

z = Number of gears of the rotorb = Width of gearsA = Area

Description:The ring gear pump works like an internal gear pump. The out side cog wheel has one more gear than the innermost cog wheel.

Thedisplacementofthefluidiscreatedbythefactthatthegearsoftheinnermostcogwheelwhenturningalwaystouchtheoutsidewheelsothathermeticallysealedchambersareformed,inwhichthefluidaretransportedfromthesuctionsidetothepressureside.Forlubricationandotherlowpressureunits,likefilters,coolingcircuitsetc.asimplepumpwithfixedsidepanelsisavailable.Forhigherpressurerangestherearemodelswithoneortwoaxialflexiblesidepanels.Alot of effort was put into a pump design with limited fuel consumption.

Pumps in standard versions cannot be repaired.

Application:

pumpswithlowerdisturbancewhilstrunningandacompact,spacesavingdesign

TechnicalData:

constantdisplacementvolumeupto100bardifferentpressurestagescanbeusedasa hydraulic motor

Ringgearmotor:

This type of motor has the highest performance in relation to its dimensions. The motor can be applied where low drive speeds are required, if a high degree of irregularity is not important.

V z A A b= ( - )max min

internal gear

shaft

outside gear

housing

Fig. Ring gear pump

82

1.5 Planetary srew pump and motorDescription:

Ring gear pumps as discussed on the previous page can be used as a motor provided some changes in the design are made. It is then called planetary screw motor.

Function:

By using a commutator and a control plate with controls slots you can achieve 56 displacement events per stroke. Since a planetary screw motor features such a high capacity or swept volume it belongs to the slow running motor types. You will findveryfewdesignsandmanufacturersofthispumponthemarket.Aplanetaryscrewpumpisusuallyemployedinpolymer production as a dosage pump. In hydraulics it is mainly used as a motor unit, because it is a typical slow running aggregate with slow starting features when exposed to heavy load. Compared to their size these motors offer very high torques.

rotor

control plate

outside rollers

shaft

housinghollow wheel

inside rollerscommutator

rings

Fig. Planetary screw motor

83

1.6 Screw spindle pump and motorDescription:

Volume is created between the spindles and the housing.

Function:

Two or more spindles one is driving and the other been driven are placed in a housing. With the rotational movement chambersarecreated,whicharelimitedbythehousingandthespindleshaft.Thefluidfilledchamberscontinuallymovefrom the suction side to the pressure side when the pump is running.

Application:

thisreducedpumphasahighdegreeofsynchronismaccuracyandcontinuouspulsationfreevolumeflow.therotationalpartsarecounterbalancedtoahighdegree.Furthermoreduetothedesignnoextremelypressurized fluidscanbefoundintheunit.Thereforehighdrivespeedscanbeobtainedandlargevolumescanbetransported despite the small dimensions of the pump.

Screwspindlemotor:

Screwspindlemotorsareusuallynotusedasadriveunit.Howevertheyareappliedassensorsforthevolumeflow.

Fig. Driven spindles

driving spindle

driven spindlerpressure side

suction side

84

1.7 Single chamber vane pump and motor (single stroke and pressure controlled)Description:

Volume is created between the circular stator, rotor and vanes.

b = Width of vanese = Eccentricityd = Internal gear stator

Function:

Asinglechambervanepumpfeaturesavariablerotor,whichrunsinsideacircularstatorandsingleordoublevanesfixedinto slots. The stroke movement of the vanes is limited by a ring with a circular internal form. The displacement chamber consists of the rotor, two vanes, the inside of the ring and the control plates. The volume inside the displacement chamber is proportional to the distance between stator and rotator. Caused by the rotation of the rotator the volume inside the displacementchamberschangescontinually.Whenthevolumedecreasesthefluidinsidethedisplacementchambersiscompressed and the impact direction reversed.

With this pump design the displacement volume can be adjusted by changing the eccentricity of the stator to the rotor. The maximum pressure can also be adjusted (zero-stroke). The increased pressure caused by the system presses on the inside of the stator, thus causing a force in the direction of the spring. Once the predetermined spring force is reached, which equalsacertainpressure,theeccentricityofthestatorisdiminished.Justtherequiredamountoffluidisproduced.Iftheconsumerdoesnotrequiremorefluid,thepredeterminedpressurepointisreachedandnomoreflowoccurs.

Application:

stationary pumpadjustable in pressure, volume and outputzero-stroke effectbypassfilterinstallation

Technicaldata:

displacement volume constant and adjustable pressure adjustable as zero-stroke pumpup to 125 barcan be used as a hydraulic motor

Fig. Single chamber vane pump

stator

rotor

vane

adjustment unit

V b e D= 2

85

1.8 Double chamber vane pump and motorDescription:

Single chamber vane pumps have only one displacement event per rotation. Since the internal curvature of the stator has a double eccentric cam design double chamber vane pumps obtain two displacement events per rotation.

b = Width of vaneK = Vanestrokesperrotation(=2)

Function:

The only difference between single and double chamber vane pumps is that the stator of the double chamber vane pumps has a double cam form internal surface. The effect is that each vane carries out two strokes per rotation.

Fig. Double chamber vane pump

86

1.9 Radial piston pump and motor with eccentric shaftDescription:

The rotating eccentric shaft causes radial oscillating piston movements to be produced.

z = Number of pistonse = Eccentricitydk = Diameter piston

Function:

This is how a valve controlled radial piston pump with eccentric shaft works. The drive shaft is eccentric to the pump elements. The pump elements consist of piston, cylinder sleeve, pivot, compression spring, suction valve and pressure control valve. The pivot is screwed into the housing. The piston is positioned with a slipper pad on the excenter. The compression spring causes the slipper pad to always lie on the excenter, when the eccentric shaft rotates and the cylinder sleeve is to be supported by the pivot.

There are different designs and pressure stages depending on the manufacturer, but always with an odd number of pistons(3/5/7/9etc.).Theconnectionoftheindividualpistonsonthepressuresideisregulatedbymeansofacheckvalveindifferentdesigns.Forexamplepipesarefittedintothepumporholesaredrilledintothehousing.Onthesuctionside, thepumphas tobefittedalwaysbelow theoil level.Breathing ismandatorybeforestarting theunit.Thus the mounting position is predetermined by the manufacturer, because they determine the position of the breather valve.

Thispumpisverypronetoconataminationduetoitsvalvecontrolmechanismanditsverynarroworifices.Somepumpsof some manufacturers are not suitable for large load changes with high pressure alterations. Problems on the suction side often lead to an immediate destruction of the pump.

Most manufacturers provide repair kits.

Application:

Radial piston pumps are used for high pressure units (operating pressure above 400 bar). In presses, machines for processing plastic, in clamping hydraulics for machine tools and in many other applications, operating pressures of up to 700 bar are required. Only radial piston pumps can satisfactorily operate at such high pressures even under permanent use.

Radialpistonmotor:

A radial piston pump is a typical slow running unit without swept volume control. Sequential motors generate extremely high torques.

Fig. Radial piston pump with eccentric shaft

housing

dK

e

spring

eccenter

slipper pad

piston

V d zk e= 2 42

87

1.10 Radial piston pump and motor with eccentric cylinder blockDescription:

The pistons rotate within the rigid external ring. Eccentricity e determines the stroke of the pistons.

e = Eccentricityz = Number of pistonsdk = Diameter piston

Function:

Aradialpistonpumpwitheccentriccylinderblockoperatesasfollows:

Thepistonsfixedtoslipperpadsrotateinastaticoutsideringorcylinder.Eccentricityedeterminesthestrokeofthepistons.The volume in the cylinders diminishes (gpressure built-up) or increases (gsuction) due to the stroke movements.Therearedifferentdesigns,butalwayswithanoddnumberofpistons(3/5/7/9etc.).Theconnectionoftheindividualpistonsonthepressuresideisregulatedbymeansofacheckvalveindifferentdesigns.Forexamplepipesarefittedintothepumporholesaredrilledintothehousing.Onthesuctionside,thepumphastobefittedalwaysbelowtheoillevel.Breathing is mandatory before starting the unit. Thus the mounting position is predetermined by the manufacturer, because they determine the position of the breather valve. Swept volume control is possible.

Thispumpisverypronetocontaminationduetoitsvalvecontrolunitandextremelysmallorificesbetweenpistonandbush (5 to 8 m). Some pumps of somemanufacturers are not suitable for large load changeswith high pressure alterations. Problems on the suction side often lead to an immediate destruction of the pump.

By repositioning the stator ring the performance of the pump can be adjusted to the requirements of the unit.

Most manufacturers supply repair kits.

Technicaldata:

displacementvolumeconstantoradjustablepressureadjustableuptoapprox.630bardifferentmanufacturersg different pressure stagescanbeusedasa hydraulic motor

Fig. Radial piston pump with eccentric cylinder block

housing

dK

e

slipper pad

piston

shaft

V d zk e=

2

42

88

1.11 Axial piston pump and motor in swash plate designDescription:

The rotating displacement pistons are supported by a swash plate. The angle of the swash plate determines the piston stroke.

z = Number of pistonsdk = Diameter piston

Function:

The cylinder, tightly connected to the shaft and the pistons, is in a parallel position to the drive shaft. The ends of the pistons are designed as ball- and socket joint, are positioned on slipper pads held in place by discs at an angle. When the shaft starts to rotate, the cylinder, pistons and slipper pads start to rotate as well. Since the pistons with the slipper padsareattachedtotheswashplatepistonstrokesoccurinsidethecylinder.Thefluidiscontrolledbykidneyshapedslots in the control plate. With the exception of the housing all components of the pump are manoeuvrable.

Dependingonthemanufacturerpumpsmustbepre-chargedwithfluidviatheleakageoilportbeforetheyarebroughtintoservice.Thepumpsmustbefittedbelowtankfluidlevel.Thepositionisdeterminedanywaybythemanufacturers,since they decide on the position of the leakage oil port. With revolutions larger than 1500 per minute most pumps must befedfromtheentryside.Occasionallythesepumpsareeitherfittedwithfeedpumpsorthetankmustbepre-charged.Problems on the suction side can cause immediate destruction of the pump. These pumps are very prone to contamination due to the revolving movement of the drum on the control plate and the stroke of the piston with the pressure compensation nozzles.

Thepumpsaresuppliedwithahugevarietyofcontrolunitsforpressureandvolumeflow.Agreatadvantageofthesepumps is the fact that a compensation of suction and pressure side occurs during operation with a concurrent drive turn of the pump due to the swivelling of the tilted axle. There are other designs with connection possibilities for more pumps.

Repairs are possible with knowlegeable mechanics.

Application:

mobiletechnologystationaryhydraulicsinjectionmouldingmachinespresses

Technicaldata:

displacementvolumeconstantoradjustablepressureandoutputadjustablealwaysanoddnumberofpistonshighrestpulsationdifferentmanufacturersg different pressure stagesuptoapprox.400barcanbeusedas hydraulic motor

V d r zk h= 2 42 ( tan )

89

Axialpistonmotorinswashplatedesign:

Therearethreedifferentdesigntypes:bentaxisdesignswashplatedesigndisplacementdesign

With the exception of the displacement design all designs permit a swept volume regulation.

A motor in swash plate design permits high revolutions per minute since it is perfectly counter balanced. The motor in swash plate design in cylindrical form increases the possibilities for applications in comparison to the bent axis design motors.

With axial piston motors in swash plate design a complete mass balancing is not possible, which is the reason why high revolutions per minute cannot be obtained.

dK

swashplate

displaement pistoncylinder

control plate

shaft

rh

Fig. Axial piston pump in swash plate design

90

1.12 Axial piston pump and motor in bent axis designDescription:

Depending on the swivel angle, the pistons move within the cylinder bores when the shaft rotates.

z = Number of pistonsdk = Diameter piston

Function:

The stroke plate in which the middle pivot and the axial pistons are positioned, stands vertical to the drive shaft. The cylinder with the piston is positioned at an angle of usually 25 to the drive shaft. When the drive shaft rotates the cylinder rotates as well thus causing the pistons to perform a stroke. A control plate, also called pilot plate, with kidney shaped slotsdealswiththeinputandoutputofpressurefluid.

Overtheyearsthekinkanglehasbeenchangedrepeatedly.Itstartedwithanangleof25to32.Todaywefindanglesof 40. It also depends on the manufacturer with which angle the pump is equipped. Therefore it is very important to check on the angle, when faulty pumps have to be replaced.

Dependingonthemanufacturerpumpsmustbepre-chargedwithfluidviatheleakageoilportbeforetheyarebroughtintoservice.Thepumpsmustbefittedbelowtankfluidlevel.Thepositionisdeterminedanywaybythemanufacturers,since they decide on the position of the leakage oil port. With revolutions larger than 1500 per minute most pumps must befedfromtheentryside.Occasionallythesepumpsareeitherfittedwithfeedpumpsorthetankmustbepre-charged.Problems on the suction side can cause immediate destruction of the pump. These pumps are very prone to contamination due to the revolving movement of the drum on the control plate and the stroke of the piston with the pressure compensation nozzles.

Thepumpsaresuppliedwithahugevarietyofcontrolunitsforpressureandvolumeflow.

Repairs are possible with knowlegeable mechanics.

Application:

mobilehydraulicsstationaryhydraulicsinjectionmouldingmachinespresses

Technicaldata:

displacementvolumeconstantandadjustablepressureandoutputcontrollabledifferentmanufacturersg different pressure stageshighrestpulsationuptoca.400bartwodesigntypes(constantandvariable)canbealsousedas hydraulic motor

V d r zk h= 2 42 ( sin )

91

Axialpistonmotorinbentaxisdesign:

Axial piston motors in bent axis design are variable displacement motors in bent axis design with hydraulic adjustment.

The adjustment is done through a regulating piston and pivot attached to the backside of the control plate. The regulating piston is controlled by the pilot piston which is activated either by applied pressure or a solenoid. A separate pilot oil pump is not necessary since the respective highest operational pressure is taken from connectors A or B as adjusting oil. In order to guarantee a proper functioning of the adjustment the pressure has to be at least 15 bar.

dK

drive shaft

driveflange

cylinder

pilot plate

piston

rh

Fig. Axial piston pump in bent axis design

92

Fig. Selection criteria in planning a hydraulic unit

1.13 Rotary vane motorWithrotaryvanemotorsthesweptvolumecanbeincreasedbymultiplefillingsperrotation.Theresultisahigherspecifictorqueandaconsiderablysmallerloadbearing.Thisisachievedbyahigherpressurefluidintakethanwithvanemotors.A disadvantage is the reduced sealing and the constant swept volume.

1.14 Roll vane motorThis type of motor is mostly used in the tooling industry as a feed drive. It is a full-TLA-constant machine with low mass-moment of inertia, which shows a good response sensibility, low reversing time and a high torque.

In combination with servo valves this motor is being used as positioning drive for control circuits.

Function:

The rotor features two vanes on opposite sides. Therefore a radial load balancing is secured.

The main drive cog wheel is attached to the rotor, which drives the roll vanes in a way that rotor movement and roll vane movement are synchronized timewise. The cog wheel gear ratio is chosen in such a way that the peripheral speed of the rotating elements is equal. Therefore there is no sliding against the sealing surfaces but a rolling movement. In order to obtainahighvolumetrictorque,highestdegreesoftolerances(5m)arenecessary.Averyfinefiltrationofthesystemisnecessary.

1.15 Selection criteriaWhen planning a hydraulic unit several criteria for choosing the appropriate pump or motor have to be considered.

The criteria for the individual design principles are shown in the table at the bottom of this page.

1 =verygood/verylarge2 =good/large3 =medium4 =low

Usable rpm range 2 1 2 2 3 3 2 2 2 2

Usable pressure range 2 2 3 3 3 3 1 1 1 1

Viscosity range 1 2 3 1 3 3 1 1 1 1

Maximum noise level 4 1 2 1 2 2 3 3 3 3

Life time circle 3 2 2 1 1 1 2 2 2 2

Cost 1 2 2 3 2 2 3 3 3 3

Exter

nal g

ear

Intern

el ge

ar

Gear

ring

Scrow

spind

le

Single

cham

ber v

ane

Doub

le ch

ambe

r van

e

Radia

l pist

on ec

centr

ic sh

aft

Radia

l pist

on ec

centr

ic cy

linde

r bloc

k

Axial

pisto

n ben

t axis

Axial

pisto

n swa

shpla

te

97

3 Cylinders3.1 General remarks3.1.1 Drive units

Drive units like hydraulic cylinders, swivel drives and hydraulic motors with linear, swivel motion and rotary movements are components which transform hydraulic into mechanical energy.

When planning a hydraulic unit you usually start with the drive unit, because the necessary forces, paths and times of the machines to be built are predetermined. Having said this it is clear that the most important parameters for pumps, motors, swivel drives and cylinders like volume flow, pressure, torque and payload are obvious, since the volume flow determines the speed and the pressure determines the torque and thus the payload of the drive units.

Fig. Overview: hydraulic cylinders

98

3.1.2 Hydraulic cylinder

Hydraulic cylinder perform linear movements and thus transfer forces. The maximum force of the cylinder (traction and force generated by pressure) depends on the area acted upon (piston and ring area) and the maximum permissible operation pressure.

Hencefollowstheequation:

The force of the cylinder is constant throughout the entire stroke area.There are countless types of hydraulic cylinders, but the basic design is always the same. Piston rod, piston with washer, cylinder casing, two lids and ports. As of a certain stroke speed shock absorbers at stroke end are built in. Furthermore breathing valves at both ends of the cylinder are necessary. We distinguish between single and double acting cylinders.

Thecorecharacteristicsofthehydrauliccylinderare:

theforcewillbegenerateddirectlywithoutaconnectinglink.theforcecanbeusedoneverypointofthemovementinanydimensiontothenominalforce.theusabledeviationcanbemodifiedintheconstructionallimits.thespeedofthemovementcanbecontrolledbythevolumeflow.withthechoiceoftheforceyoucanadjustthedimensionsofthecylinder.

Cylinders are used in single and double acting design in hydraulics and pneumatics. In principle they differ only in the use of the medium and the force.

3.2 Design3.2.1 Single acting cylinder

Single acting cylinder transfer force only in one direction. They either exert traction and force generated by pressure. To return the piston into the start position a return spring is used or the weight of the piston and the load does the job. Basically simple acting cylinders have only one effective piston area on which the forces can act. Depending on the design we distinguish between plunger piston cylinders with or without an internal stroke limiter.

F p A=

piston with washercylinder casing

lidslidsports

piston rod

Fig. Hydraulic cylinder

Fig. Plunger cylinder with internal stroke limiter (left) and plunger cylinder without internal stroke limiter (right)

Fig. Single acting cylinder with return spring

99

3.2.2 Double acting cylinder

Doubleactingcylinderhavetwoopposingeffectiveareaswhichareofthesameordifferentsize.Theyarefittedwithtwopipeports,whichareisolatedfromeachother.ByfeedingfluidviaportsAorB,thepistonmaytransferpushingorpulling forces in both stroke directions. We distinguish between single and double rod cylinders.

3.2.2.1 Single rod cylinder

A single rod cylinder or differentiate cylinder have a piston rod only on one side of the piston. The name differentiate came into use, because with this design you have to differentiate effective areas. The area ratio of piston area to annulusareaisindicatedbythefactor.Sincewehavetwodifferenteffectiveareasizeswehavealsotwodifferentspeeds for extension and retraction. The stroke velocities are inversely proportional to the areas. (large area g low velocity, small area g large velocity).The larger the effective area the bigger the force which can be transferred. Hence a bigger force is available for the extension of the piston.

Theforceactingonthepistoniscalculatedasfollows:

force[F] = Npressure[p] = Paeffectivearea[A] = mtorque [h]

Someadvice:

Thetorquedependsontheusedsealkitandrangesfrom0.8to0.98.Averycommonratio=0.5.

Fig. Single rod cylinder

Fig. Double rod cylinder

Fig. Single rod cylinder

piston area annulus area

F p A= h

100

3.2.2.2 Double rod cylinder

Double rod cylinder have a piston, which is rigidly connected to two piston rods, which have diameters smaller than that of the piston. As a rule of thumb you can say that the effective areas of both pistons are equal in size. Thus the force transferred as well as the velocities on extension and retraction is equal in size.

3.2.2.3 Single acting rod cylinder with different piston rod diameter

In some cases single acting rod cylinder with two rods are needed. With this design force transference and velocity r elate to each other according to the area ratio j.

3.2.3 Tandem cylinder

In double acting cylinder operating in tandem the effective areas of both piston are added. By using this arrangement large forces may be transferred for relatively small external diameters without increasing the operating pressure. However the longer length of this type can be a disadvantage. Usually this model is used in large presses.

Fig. Double rod cylinder

Fig. Single acting rod cylinder with different piston rod diameters

Fig. Tandem cylinder

101

3.2.4 Rapid traverse cylinder

Rapid traverse cylinder are used primarily in presses. In this cylinder, as long as the complete working force is not required, only part of the effective piston area, the so-called rapid traverse piston is placed under pressure. The complete effective piston area is only later connected to the hydraulic pump via a control system by means of pressure control valves or limit switches. The high rapid traverse velocity due to small volume and the high pressing force due to large effective piston areas are very advantageous.

Legend:

A1: rapid travers processA2: pressing forceS: suction

Returnflow(A1) and (A2).

3.2.5 Telescopic cylinder

Telescopic cylinder consist of several rods sliding into each other. In general telescopic cylinders are manufactured in a simple design form and used wherever little space is available but relatively large stroke forces are required. Within the individual stroke units different velocities and stroke forces may occur.

Due to new developments in the material sciences nowadays more and more lighter materials are used in mobile technologies. Therefore double acting telescopic cylinders have to be used, since the own weight of the much lighter containers cannot move them back to the start position.

Fig. Single acting and double acting telescopic cylinder

Fig. Telescopic cylinder

Fig. Single acting rapid traverse cylinder

Fig. Double acting rapid traverse cylinder

102

3.2.6 Cam system

The cam system works as a force diffuser, witch is used in the moulding technology.Withthissystem,valvescanbesavedandthehydraulicsystemcanbedesignedeasierandmoreefficiently.

Design:

Thesystemconsistsofaninputcylinder(driveunit),whichisfittedveryaccessiblyintheunit.Thereceptivecylinder(working unit) is connected with the input cylinder by means of a pipe system.

Function:

Due to theextensionand retractionof the inputcylinder thedisplacedhydraulicfluid issqueezed into the receptive cylinder. When the displaced volume of cylinder Z1 equals the received volume in cylinder Z2 a synchronization control can be achieved. By changing the volume of the cylinders (length, diameter), also a reduction of forces or an elongation of paths can be achieved.

Pleasenote:When several cylinders are used on the receiving end in order to obtain a synchronization control, they should be connectedmechanically.Synchronizationisdifficulttoachieveifyouuseapipesystemwithdifferentpipelengths.Thesame is true for hoses, since they expand if pressurized.

Fig. Cam system

103

3.3 Design principlesThe design of a cylinder is determined by its purpose, demands placed upon it and its application.

3.3.1 Tie rod cylinder

In tie rod cylinder cylinder head, cylinder pipes and cylinder bottom are tightly attached to each other with tie rods. They have a very compact design and are mainly used in machine tool industry, manufacturing devices and automotive industry.

3.3.2 Mill type cylinder

In mill type cylinder, the top and the base of the cylinder and cylinder tube are connected together via threads or retaining rings. Due to the robust design, hydraulic cylinders with screwed or welded constructions are also suitable for use in applications with extreme operating conditions.

Fig. Tie rod cylinder

Fig. Mill type cylinder

104

3.4 Mounting3.4.1 Joint mounting

Thejointmountinggivesthecylindermovingpossibilitiesinoneortwoways.Thismountingcanbefixedonthebottomor on the rod.

a. swivel bearing on the bottom and rod with swivel bearing misalignment only in one directionb. swivel bearing on the bottom and rod with joint bearing equalisation of inaccuracies in the parallelism of the axle bolts will be equalizedc. joint bearing on the bottom and rod with joint bearing misalignment across the normal rotatable direction

a. b.

tiltingangle

c.

Fig.Possiblefixturesforjointmountings

Fig. Joint mounting on the bottom of the cylinder

105

3.4.2 Trunnion mounting

Thecylinderisfittedbymeansofaswivelmechanismattachedtothecylinder.Anypositiononthecylinderispossible.The most favoured position, however, is the the centre of gravity of the cylinder.

3.4.3 Flange mounting

Thecylinderisfittedbymeansofaflangeeitherattheheadorthebottomofacylinder.Thescrewsareputunderstressduring the pulling sequence of the cylinder.

Pleasenoteforbothdesigns:

mainlyverticalpositioningthescrewsoftheflangeshouldberelievedduringmainfunctionalstress

Fig. Cylinder with swivel mechanism

Fig. Flange at cylinder head

Fig.Mountinghintsforflangeattachments

106

3.4.4 Foot mountingFitting or mounting of cylinder by means of brackets attached to the cylinder. The screws of the brackets are mainly put under stress by shearing strain. Depending on the position of the brackets, you have to watch out for an additional tilting momentum.

Fig. Cylinder with brackets

107

3.5 Buckling/bendingofpistonrod

Wherever cylinders are built in either horizontally or in a strongly tilted position, you have to take into consideration, that the cylinders might buckle or bent due to their dead weight. This is especially true for very large cylinders with a considerable dead weight and stroke length.

Aloadcalculatedwiththisformulawillactuallymakethepistonrodbuckle:

Maximumoperationalload:

K = Buckling load N

Sk = Length of buckling in mm, check the table on this page

E = Flexibility=2.1105N/mm2 for steel

S = Safety (ca. 2.5 - 3.5)

I = Torque of inertia for circle diameter mm4

oneendfree,oneendfixed twoendsguidedwithjoints

oneendguidedwithjoint, twoendsfixed oneendfixd

load has to be carefully guided not suitable, since tensions since tensions may accur otherwise are to be expected

K E ISk

= 2 2

F KS

=

Fig.Buckling/bendingofpistonrod

108

3.6 Shock absorber at stroke end (end cushioning)The stroke end cushioning system causes and secures a deceleration of the piston velocity at one or both stroke ends, in order to generate mass forces. If stroke end and the end position of the piston are identical, a stroke end cushioning system is used. That of course requires constructive measures at the cylinders. A deceleration is brought about by measures at the control unit by means of appropriate valves. The end cushioning serves as a protection for the cylinder and the entire unit. Pressurization at starting point is initiated by check valves and acts immediately in the piston surface (or ring surface) in such a way that there are no losses in performance or delays in the start-up process.

Function:

Attached to the piston is a conical damping bush. When the piston moves with the damping bush into the drilling in the cylinderbottom,thecross-sectionthroughwhichthefluidcanescape,iscontinuouslydecreaseduntil it iscompletelyclosed.Nowthefluidfromthepistonchamberhastoflowthroughthedrillingtothethrottleandtheadjustablethrottlevalve. The cushioning effect can be controlled by means of the throttle valve. A small cross-section of the throttle valve results in a high cushioning effect. As an extension support for the piston to move out of its end position a check valve is fittedaswell.Thishastheeffectthatduringtheextensionthethrottleisbypassed.

Wedistinguishbetweenthreedifferentkindsofendcushioning:

constantshockabsorberslotprogressiveshockabsorberslotringholeshockabsorbe

Fig. Shock absorber at stroke end

piston rodthrottle valve

piston chamber

bore throttle

damping bush

check valve

109

3.6.1 Constant cushioning

A constant cushioning causes a sudden breaking and a slow sliding into the end position.

3.6.2 Progressive cushioning slot

Progressive shock absorbing has the advantage that the velocity at end point is low and therefore a soft sliding into the end position is possible.

3.6.3 Ring hole shock absorber

Ringholeshockabsorbingcausesadecelerationofvelocityintothefinalposition.

Fig. Constant cushioning

Fig. Progressive cushioning slot

Fig. Ring hole shock absorber

111

4 Hydraulic valves4.1 Check valvesCheckvalvesblockthevolumeflowofahydraulicunitinacertaindirection.Thepressurefluidcanflowunhinderedintheopposite direction. It is possible to compare this valve with a diode in electronics. Since the valve is designed as a seat valve no leakage will occur.

Thefollowingclosingelementscanbeusedinthesevalves:

A ball is by far the cheapest closing element, but because of its mass it is only meaningful to use it in small valves. A cone is the most widely used closing element and turned hollow due to its weight. The production of a cone however is more tedious than that of a ball or disc.

4.1.1 Check valve

Description:

Theyarevalveswhichpermitflowinonedirectionandblockflowintheoppositedirection.

Design:

The check valve basically consists of a housing with integrated valve seat, a cut and tempered cone and a pressure spring. The valve is also being produced with a ball as a closing element.

Function:

Asandwhenfluidsflowthroughthevalveaforcegeneratedbythesystempressureactsontheclosingelement,whichworks against the spring force. When this force exceeds the spring force, the closing element, in this case a cone, and releasestheflowfromBtoA.IfthereisavolumeflowfromAtoB,theconeispushedintothesealinglipandsealsthevalve without loss of leakage oil.

Fig. Check valve (RV)

Caution!Inadesignwithoutspringitisimportantthatthevalveisfittedvertically.Thiswaythedeadweightoftheconehelpstoletit sit in the neutral position.

housing seat valve

coneseat valve

Fig. Cone-seat Fig. Ball-seat Fig. Disc-seat

112

Design:

HYDACofferscheckvalvesinthefollowingdesigns:

pipefitting(rv)sandwichdesign(rvp)valvesinathreadedplug(RVandRVE)

Pleasenote:The opening pressure of the valve is increased by the pressure at port A.

Modelcode:

Themodelcodeiscomposedofthefollowingsequences:

113

4.1.2 Pilot operated check valves

Design:

Hydraulicpilotoperatedcheckvalvescanbeopenedviaanadditionalpilotoilconnectorforflowintheoppositedirection.They consist of a housing, control spool, a valve seat, a ball and a closing spring.

Function:

Thevalvepermitsfreeflowfrom2to1inthedirectionofflow.Intheoppositedirectiontheballispushedonthevalveseatbytheclosingspringandthepressureatport1andthusblockstheflowdirectionfrom1to2withoutlossofleakageoil. The pressure at port 1 acts on the control spool and counteracts the pilot pressure at port 3. Therefore during the hydraulicopeningport1hastobewithoutpressure.Ifthepilotpressureatport3issufficientlyhighthepistonismovedandtheballispushedawayfromthevalveseat.Nowthevalveisunblockedandfluidcanflowfrom1to2.Thereturnspringfittedunderneaththepistonpermitsanundelayedbackswitchoncethepressureislowered.

Fig. Hydraulically pilot operated check valve (ERVE)

Pleasenote:The existing pressure countervails the opening pressure at port 3.

housing

piston

return spring

ball

closing

1

2

3

114

Application:

With hydraulically pilot operated check valves, creeping movements in cylinders, which are operated by spool valves and which are loaded, can be prevented.

The valve is located in the return line of the cylinder and prevents the cylinder from extending even more under loading. Only as and when pressure is built up in the feed line, the pilot control unit unblocks the check valve in the return line of the cylinder.

Fig. Circut diagram: hydraulically pilot operated check valve (ERVE)

115

4.1.3 Check valves with pre-decompression

Description:

By opening a check valve a sudden opening of the entire cross section might happen. Decompression shocks might occurwithhighvolumeflow,whichnotonlyproducealotofnoise,butalsomightbeharmfultotheentireunit.Damagetoscrewedfittingsandvalvescouldbetheresult.

Function:

In order to prevent this, you could choose valves with preopening features. If pressure acts on the piston, it pushes down on the preopening ball. Only a small part of the cross section is opened. Only then the main cone is pushed off its seat. Thiswayasmoothreleaseofthepressurizedfluidispossible.

Fig. Pilot operated check valve with predecompression (ERVE-R1)

piston

check valve

closing spring

housing

ball

piston

116

Modelcode:

Themodelcodeiscomposedofthefollowingsequences:

Pre-decompression:

forlargevolumeflowsforveryhighpressures(otherwisetheunitwillbecometoolarge)

Pleasenote:The existing pressure countervails the opening pressure at port 3.

117

4.1.4 Lowering speed controlled valves

Description:

Lowering speed controlled valves belong to the group of check valves. These are valves whose block position is canceled by a hydraulic operation or if the pre-set pressure has been reached.

Theyfulfilthefollowingtasks:

actuatorvelocitycontroldependingonthevolumeflowintakeoverreactionoftheactuatorwithpullingloadsisavoidedclosedflowpassagesinblockpositions(theactuatorscanmaintaintheirrespectiveposition)limitationofactuatorpressure(maximumload)tothepredeterminedpressurefreevolumeflowintakebyintegratedcheckvalveburstcontrolatpipesleadingtotheactuatororatcontrolpipes

Fig. Lowering speed cont