Selection of Operating Fluids forHydrodynamic Power Transmitting Equipment

Dipl. Eng. Heinz Höller

2

For hydrodynamic power transmis-sion, the main task of the operatingfluid is to transport its kinetic energyin a closed circuit between the pri-mary part (pump) and the secondarypart (turbine). The three Föttingerunits converter, coupling and retarderwhich are based on this functionalprinciple can be used for a wide rangeof power transmission applications.The typical operating behaviour ofthese units, the variety of applicationsand the fact that other machine ele-ments are used more frequently,result in various specific requirementson the operating fluid, some of themcontradicting each other. When fluidsare selected, compromises are oftennecessary which then affect the over-all design of the units. It is virtuallyimpossible to have a uniform opera-ting fluid for all Föttinger units usednowadays.

1. Basics

Hermann Föttinger’s basic inventiondescribed the functional principle ofhydrodynamic power transmission forthe first time in 1905. The VDIDirective 2153 of April 1994 classifiesthe three Föttinger units developedfrom this basic invention in types andexplains their function in detail.

The three units are characterised bythe fact that the dual energy conver-sion is performed by rotating guidevanes, i.e. hydrodynamic machines.The guide vanes are located inside ahousing, and the energy-transportingfluid flows through them, one after the

other, in a closed circuit. The calcula-tion of the working circuit is basedprimarily on Euler’s and Reynolds’fluid equations and model specificati-ons, which were adopted for the usein practice. Fig. 2 shows this scheme,using a hydrodynamic coupling withconstant fill, sealed to the outside, asan example.

Euler’s turbine equation generallydescribes the energy conversion inrotating guide vane rows. The hydro-dynamic model equation can be de-rived from this equation via the Eulerfigure.

On account of the interaction ofseveral guide vane rows, two for thecoupling and retarder and at leastthree for the converter, Föttinger unitshave their own operating behaviour.Their interaction can be clearly recog-nised by the standard guide vanearrangement of hydrodynamic cou-plings. The mass between the pumpand turbine, which is necessary forenergy transport, can only flow in theevent of a speed difference (slip) bet-ween the two guide vanes, i.e. in therotating system.The circulating mass flow results in africtional connection of both guide

Selection of Operating Fluids forHydrodynamic Power Transmitting Equipment

L

1

2

3

P T

S T P

S G R

Fig. 1: Föttinger units - Basic elements

ConverterP = pumpT = turbineL = guide vane / reaction

member with housingCouplingP = pumpT = turbineS = shell (housing)Retarder/brakeR = rotorS = statorG = housing

(rotor-stator-housing

1

2

3

Characteristic relation(characteristic curve)

λ = f(νη, V) with νη = ϖT/ϖP

V = filling volume

During the establishment of thehydrodynamic model equation, theoptimum operating fluid is character-ised only by its density. The viscosityand the relevant flow losses causedby the real operating fluid flowingthrough the guide vane rows due tofriction, jerk and separation anglesare recorded implicitly only by thepower factor λ.

3

vanes. So the speed ratio (slip) auto-matically adapts as a function of theload (main characteristic curve).

The transmission behaviour of a cou-pling can clearly be described withthe help of two equations.

Hydrodyn. model equation

or

Though calculation technology hasmade much progress for many years,it is still difficult to develop a suffi-ciently precise geometry of theFöttinger units merely numerically.Thus, the power factors used in prac-tice are determined from the torquesand speeds measured on the teststand. By selecting the test fluid anddetermining the measuring tempera-ture ranges using Reynolds’ similaritylaws, the viscosity influence of theoperating fluid included in the powerfactor λ can be maintained withinacceptable limits. For power trans-mission, in addition to the high densi-ty, the low viscosity at a high viscosi-ty index can also be derived as furtherdesirable fluid property.

2. Operating behaviour

A specific characteristic of all threeFöttinger units is that the mechanicalenergy supplied to the input shaft(pump, rotor) can be convertedwearfree and infinitely.

The number of guide vanes and theirarrangement determine the operatingbehaviour. The Föttinger unit, called„converter“ with at least three guidevanes permits the conversion of thetwo power factors torque and speed,and thus it also represents an infinite-ly variable gearbox.

Couplings with two guide vanestransmit the introduced torque with-out changing it, with infinitely variableoutput speed. They belong to the fric-tional starting and variable speedcouplings.Hydrodynamic brakes or retarderswith two guide vanes completely

Fig. 2: Hydrodynamic principles

Eulersche Turbinengleichung

Geometrische Ähnlichkeit Hydraulische Ähnlichkeit

Hydrodynamische Modellgleichungen

CharakteristischeBeziehung

meistvernach-lässigbar

P T

P T

S

Euler’s turbine equation

Geometric similarity

Hydrodynamic modelequations

Characteristicrelation

Hydraulic similarity

[Reynolds fig.][Euler fig.]

can be neglec-ted in most ofthe cases

4

convert the introduced mechanicalenergy into heat. They are used assafety equipment in drive systemswith moving masses, or as drivenmachines for heat generation.Compared to mechanical converters,couplings or brakes, the inevitableconversion losses (heat) of Föttingerunits are produced in the operatingfluid directly, without mechanicalwear. The heat can only be dissipatedvia the machine surface for few appli-cations.As a rule, a partial current is takenfrom the working circuit, fed through aheat exchanger and returned again,cooled down, into the working circuit.This process is either possible bymaking use of the energy potential inthe closed loop of the Föttinger unit,or via an open loop by means ofpumps.

As a function of the unit type, designand operating behaviour, the externalpower flux to be cooled varies consi-derably. This external cooling flowshall be kept as small as possible fordifferent reasons. For the main func-tion of energy transmission, furtherdesirable properties are a high speci-fic heat, a high permissible operatingtemperature and a great effectivetemperature drop.

3. Type and design-specificrequirements

The variety of the requirements to bemet by the operating fluid can only besummarised generally in a specifica-tion (fig. 4) and shall be explained inmore detail with the help of four sel-ected examples.

3.1 Specification

The basic function of power transmis-sion in Föttinger units needs to besuitable for practical design, usingfluids suited for industrial uses andcovering a wide range of applications.

This paper can only indicate the qua-lity of the desirable properties, therelevant limits and standards need tobe determined as a function of therespective type and design. The addi-tional minimum requirements speci-fied for power transmission shall beexplained in the following.

Adequate protection against cor-rosion

Standard materials such as cast iron,steel, cast aluminium and copperbronze are used for Föttinger units. Inthe vehicle manufacturing industry,lightweight constructions are pre-ferred. For fluids on mineral oil basis,the protective effect may be affectedby ageing products getting in contactwith water.

No reaction of non-ferrous heavymetal

This specification is applicable to allfluids since reaction residues may causemalfunctions or even a unit failure.

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Drehmoment- undDrehzahlwandlung

nurDrehzahlwandlung

Wandlung derEnergieform, mech.Energie in Wärme

Fig. 3: Illustration, characteristics of Föttinger units

Converter Coupling Brake

Torque and speedconversion

Only speedconversion

Conversion ofenergy, mechanicalenergy in heat ch

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Fig. 4: Specified fluid properties

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Low cavitation trend

Fluids on the basis of mineral oil hard-ly tend to cavitate due to the wideevaporation spectrum. Nevertheless,cavitation may occur, but in mostcases it can be prevented by takingspecific hydrodynamic measures.

Resistance to ageing due to oxida-tion

The hydrodynamic mode of operationintensifies the contact with theambient air, in particular with partlyfilled couplings and brakes. However,the advantage of the high operatingtemperature of the fluids on hydrocar-bon basis should not disappear.

Non-toxic

This should be a standard require-ment by now.

Suitability for standard processes

Nowadays this is an important pro-duct requirement. Föttinger units withindividual volumes of 0.5 l to 10,000 lper unit are used world-wide. Prior totheir use, the units might have been insea transit for several weeks and thenstored on site for several months. Theoperating fluid should be compatiblewith various preservation agents.

It is always a problem when the struc-ture of a fluid procured locally underthe same designation is different fromthat in Europe. In particular for heavy-duty fluids, it should be possible thatthe unit manufacturer can rely on

identical quality requirements whenthe great fluid suppliers use the sameproduct designation.In addition, the expectations of manu-facturers of top-quality machines asto fluids being free from dirt and tothe fluid water content are often notfulfilled.

Nowadays, a safe disposal and afavourable price should go withoutsaying.

3.2 Constant fill coupling

Constant fill couplings are usedthroughout the power transmissionindustry, in the power flow betweenmotor and driven machine to ensure asmooth start-up, torque limitation andsystem separation in the event ofvibrations and jerks.The main components are supportedby relative bearings and are sealed tothe outside. The standard runnermaterial is aluminium cast alloy.Standard operating fluids are hydrau-lic oils HLP, viscosity class ISO VG 32.To limit the thermal inner pressure, thefilling opening is designed in such away that filling is limited to max. 85%.The heat losses due to slip duringstart-up and nominal operation aredissipated through the coupling sur-face. Excess temperatures are moni-tored via a thermal switching element.In case of any particular situations, afusible plug responds releasing adraining opening, and the couplingsprays off.

For the main fields of applicationunderground and for preservation ofground water, new fluids had to beapproved. The currently approvedfluids are shown in Fig. 6.

As a function of the type, minimumlubricating capability for antifrictionbearings and compatibility with thesealing material should be additionalfluid properties. Solutions for specialapplications at low temperatures arealso evaluated. The standard fluidmeets all product characteristicsduring normal operation.

Water or HFA fluids meet the require-ments of underground applications,but their use is often limited to thisapplication only, due to various dis-advantages. Considering their lowtemperature of use and their tenden-Fig. 5: Constant fill coupling

7

2

Fig. 6: Fluids for constant fill couplings

3.3 Variabl speed coupling,torque converter

For variable speed couplings, thetransmission behaviour can be ad-justed infinitely by varying the filling.In order to achieve this, these cou-plings have an external fluid circuitwhich allows the variation of the fillinglevels and also serves as coolingsystem. (Fig. 8). Variable speed cou-

plings for torque conversion are usedin drive systems where the operatingprocess should be varied via thespeed of the driven machine -econo-mically, smoothly and infinitely.Examples are pumps, fans or com-pressors.

The couplings are located in a hou-sing which serves as a fluid container(anti-friction bearings; for specialapplications also plain bearings).Aluminium cast alloys are preferablyused as runner material. In the exam-ple shown, a centrifugal pump, alter-natively a positive-displacementpump, delivers the cooling controlfluid which is then transportedthrough a heat exchanger to the wor-king circuit in an open collectingchannel. Filling is adjusted by a scooptube (discharge pump) which is par-tially filled for system reasons. On itsway from the scoop tube mouth tothe oil sump, the filling is mixedthoroughly with the air suppliedsimultaneously. To obtain compactunits, the distances between the fluid

1. Shaft sealing ring to protect thebearing space

2. Bearing greased with fluid grease3. Stainless steel4. Hard anodic oxidised AL cast

parts5. Low-temp. fusible plug

Year of construction 1992suited for operating fluids:water pH 4.5-8.5HFA (oil in water)HFC (watery polymer solution)

8

cy for cavitation, the coupling powerrange needs to be limited. The waterfilled coupling slightly differs from thestandard oil coupling.

The further development of flame-resistant HFD-U fluids for under-ground applications is more favour-able. No more additional constructivemeasures are necessary here. Theintroduction of an environmentalfriendly HEES fluid also contributes tothis. Both fluids were successfullytested on a test stand and are nowbeing tested by some customers inpractice.

Special applications at low tempera-tures which have so far been solvedby oils of the viscosity classes up toISO VG 5 have not yet been tested.Due to their unsafe hydrolytic beha-viour, fully saturated, synthetic esterscan only be used for sealed couplingtypes for the time being. Tests withopen coupling circuits are planned.

Fig. 7: Constant fill coupling 422 TW

Fig. 8: Variable speed coupling type SVNL, self-supporting

Schöpfrohr

Umlaufpumpe

scoop tube

scoop tube

circulation pump

9

surfaces and rotating runner parts arerelatively small. Circulation times forthe contents of the container can beless than one minute.

The fluid should foam only slightlyand have an excellent air release pro-perty to guarantee the pump function.With independent units, partial flowsfor lubrication and clutching are de-rived from the delivered main flow.The fluid flow should be resistant topressure and offer sufficient protec-tion against wear and tear of thepumps, switching elements and fit-tings. The units are connected, viavent filters and labyrinths, with theambient air and can therefore absorbair humidity. The minimum lubricatingcapability of the bearings should beguaranteed and should naturally becompatible with the sealing elements.Like variable speed couplings, indivi-dual converters are used as indepen-dent units in the power flow betweendrive motor (diesel engine or electricmotor) and driven machine. They arepreferred when excessive torque isrequired in the low speed range, inaddition to the infinite speed adjust-ment. Fields of application: rail cars,

building machinery, lifting gear, extru-ders etc.The design and function of theseindependent converters are similar tothose of the variable speed coupling.Consequently, the fluid propertyspecifications are identical.The specified standard operatingfluids are hydraulic oil HL or HLP ofthe viscosity class ISO - VG 32 for alltypes (see next page).The standard fluid does not fully meetthe specific requirements for anincreased range of applications.Limits for low temperatures:250 mm2/s (cSt) for units with centri-fugal pump or 1000 mm2/s (cSt) forunits with positive-displacementpump.For these individual units, the viscosi-ty class can be reduced up to ISO VG22; for lower ambient temperatures,sump heating should be provided.Fully synthetic oils up to ISO VG 150can be used for hot water cooling inthe vehicle or distance heating pumpdrives, considering the rated opera-ting viscosity. For these units, too, themanufacturers should consider more

and more the customers’ require-ments as to flame resistant and envi-ronmentally friendly fluids.In order to retain their market shareand not to lose out to drive units withother physical function principles, themanufacturers are also forced todevelop other suitable solutions forthe operating fluid water (fig. 10).This fill-controlled coupling type withexternal fluid circuit is supplied withwater from the mine water mainsunderground. The coupling can bedesigned with greased, sealed hou-sing bearings or non self-supportingbearings on the motor and gearboxshaft. There are considerable con-structive deviations compared to thedesign with standard operating fluids,leading to an own design series. Thetest results of the tested constant fillcouplings filled with HFD-U andHEES fluids are favourable to also usethese fluids in variable speed cou-plings and individual converters of thestandard oil series, with slight functio-nal limitations.

Fig. 9: Torque converter series ELIndividual converter with own oilsupply system for free installation inthe shafting

Fig. 10: Variable speed coupling for operating fluid „water“

motor gearbox

10

Föttinger unit Fluid properties

Extended for type/design Variable speed coupl. /EL converter

* only valid for the tested types, but not generally for the fluid group

-------------------------------------------Specifications at least

Energytransmission

High density Low viscosityHigh viscosity index High specific heat High operating temp.

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Fig. 11: Fluids for variable speed couplings, industrial converters

11

3.4 Hydrodynamic brakes,retarders

Hydrodynamic brakes as safety orload equipment convert the intro-duced mechanical energy, completelyand without wear, into heat.Frequently, the operating conditionsof brakes for industrial applicationsand those of variable speed couplingsare identical. Consequently, the sameoperating fluids can be used in mostof the cases. The retarders whichwere specifically developed for theuse in commercial vehicles requireother operating fluids. Fig. 12 showsan independent secondary retarderwhich can be mounted on the gear-box output or installed in the cardanshaft line.

With identical active rotor diameters,the retarder efficiency in vehicles isabout 100 times greater than that ofcomparable converters and cou-plings. Retarders with a power of800 kW weigh approx. 100 kg andfunction with a fluid volume ofapprox. 10 l.The braking signal from the driver isconverted in the prop valve into airpressure which then presses the ope-rating fluid from the oil sump into theworking circuit. The rotor connectedto the cardan shaft forms an internaland external braking circuit. The en-gine cooling water serves as coolingfluid, with a supply temperature rang-ing from 85°C to 95°C. The coolingflow returns into the working circuitdownstream of the heat exchanger.The bladed chamber filling and hencethe braking torque to be generatedadjust automatically, with a pressurebalance, on the intersection of thecooling circuit with the sump. For thecooling flow design, fluid tempera-tures of 180°C up to max. 200°Care assumed

The working circuit is only fully filled inthe low speed range, with vehiclespeeds of less than 30 to 50 km/h.For high vehicle speeds and thus forhigh braking forces, only partial fil-lings of 2-3 l, with a flow rate ofapprox. 10 l/s, contribute to heat dis-sipation. The braking fluid consider-ably gets in contact with air in thesump and the partly filled workingchamber. The air, which is partlyabsorbed in the fluid, should be ca-pable of escaping quickly enough andwithout foaming or formation of aero-sol when the retarder is switched off.To be compatible with the ABSsystem, switch-off times of less than0.6 s are expected.

The recommended standard opera-ting fluid for retarders in commercialvehicles is single-range engine oil HDof the viscosity classes SAE 10 W,SAE 20 W 20 and SAE 30. Thus,under normal operating conditions, oilchange intervals of 90,000 km intrucks and of 135,000 km in busescan be reached. In case of severeoperating conditions (braking fre-

quency and duration), these intervalsshould be halved. The world-wideavailability in vehicle service stationsis of utmost importance when selec-ting the fluids.The high heating-up rates of morethan 25 Kelvin/s may cause malfunc-tions if the operating fluid containswater. For operation at low tempera-tures, all 3 fluids are only suited to alimited extent. The drivers do not con-sider the possible reduction of thebraking torque to 30 % during winterin Europe to be unfavourable. Duringcold starts, the retarder is used asvehicle heating system and thereforereaches its operating temperaturevery quickly. The requirements as toenvironmental protection should onlybe considered in connection with thesystem „vehicle“. The two alternativefluids meet the requirements forvehicle retarders worse or better. Forthe use in buses, the change intervalsof fully synthetic oils could be increa-sed to 300,000 km.

Fig. 12: Hydrodynamic brake for vehicles

gearbox

rear axle

waterair

operating oil reservoir (sump)

Rated value

driver

12

HP

mul

ti-ra

nge

Fig. 13: Fluids for vehicle retarders

13

A special field are the further devel-oped integrated retarders.

The step-up ratio retarder is flangedto the gearbox and its pinion shaftengages in the gearbox output shaft.The pinions and cylindrical roller bea-rings are lubricated by the gearboxfluid, the retarder has its own oilreservoir filled with retarder fluid.

The retarder fluids described earliercan be used for this arrangement.

A competitive product is the fully inte-grated retarder with common oil cir-cuit of retarder and gearbox. For thissolution, the temperature of useshould be reduced to the temperaturelimit of the high-alloy gearbox oil.The oil circuit, cooling flows and heatexchangers are designed for theselower temperatures of use. A de-

crease in power has to be put up withhere.

Since both retarder systems are of-fered by one commercial vehiclemanufacturer, it is absolutely neces-sary to observe the specifications inthe instruction manuals.

3.5 Variable speed planetarygear

The variable speed planetary gear(fig. 15) is a large-scale integrated,hydrodynamic superimposing gearunit with high efficiency. It is used inpower engineering and power supplytechnology to feed great mass cur-rents, as required, to the process byvarying the driven machine speed.Driven machines are, preferably, boi-ler feed pumps, fans, coal mills, circu-

lation pumps, compressors anddistrict heating systems.Hydrodynamic superimposing gearunits are also known from the auto-mobile industry. The industrial seriesVORECON presented in this paperwas developed for a power rangefrom 1 to 50 MW in 3 variants.The series RW known as long versionis suited to operate driven machineswith parabolic load torque, virtuallyfrom standstill, infinitely withoutspeed drop, in two operating ranges.Speed control is performed in thebottom operating range by adjustingthe filling of coupling A. The superim-posing gear unit F acts as fixed pla-netary gear with supported ring gear.The retarder D serves as flexible sup-port. In the upper operating range,the variable speed coupling isconnected to a multi-disk clutch B.Speed control is performed via thevariable pitch control of the converterC, acting on the revolving planetarygear in the superimposing line.This large-scale integrated systemwith 3 Föttinger units and severalmechanical driving elementsdemands exacting, partly contradic-tory, operating fluid requirements. Thespecified standard fluid is a hydraulicoil free from zinc HLP of the viscosityclass ISO VG 46 with a load stage≥ 12.This fluid, which was mainly selectedas a function of the mechanical ele-ments, does not meet all require-ments of hydrodynamics.

The necessary compromises as toviscosity, temperature of use andfoaming were considered for enginee-ring, operation at low temperatures ispossible with the help of a sump hea-ting.

Fig. 14: Step-up ratio retarder for truck and bus

1 Rotor2 Stator3 Retarder

housing4 Oil reservoir5 Pinion shaft6 Drive wheel7 Heat

exchanger8 Cooling

waterconnections

14

∼∼

∼

∼

∼

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Fig. 16: Fluids for variable speed planetary gears

15

4. Summary

This paper can only give an approxi-mate survey on the function andtypes of the machines calledFöttinger units. The differences of therequired fluid properties, as a functionof the machine type and application,were explained with the help of someexamples. New tendencies to usespecific fluids were shown. Individualvalues and descriptions of relevanttest methods have been deliberatelyomitted and should be obtained fromthe unit manufacturers. The new VDIStandard 2153 „HydrodynamicPower Transmission“ offers a goodsurvey.

Literature

[1] VDI Directive 2153 -Hydrodynamic power trans-mission, publisher Beuth,Berlin 1994

[2] Hydrodynamics in power trans-missionVereinigte Fachverlage Krausskopf-IngenieurDigest, Mainz 1987.

[3] Rohne, E.: Fluids for hydro-dynamic power transmissionSpecial print J. M. Voith GmbH, Heidenheim G1221/1989

[4] Höller, H.: Hydrodynamic coupling with water asoperating fluid VDI-ReportsNo. 977, 1992

Fig. 15: Variable speed planetary gear

1 rotor2 stator3 retarder housing4 oil reservoir

5 pinion shaft6 drive wheel7 heat exchanger8 cooling water connections

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Voith Turbo GmbH & Co. KGCentral TechnologyVoithstr. 1D-74564 CrailsheimPhone (0 79 51) 32-0Fax (0 79 51) 32-605E-mail: info-voithturbo@voith.comInternet: http://www.voithturbo.com