423 e CA Ueberlastverhalten En

7/30/2019 423 e CA Ueberlastverhalten En

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Braunkohle, Surface Mining, 51 (1999) Nr. 5 September/Oktober Overload behaviour of coupling combinations

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Investigations on the overload behaviour

of coupling combinations on the basis

of a simulated bucket-wheel excavator drive

G. Kunze and S. Graul, DresdenR. Finzel and H. Hoffeld, Crailsheim

1. Problem

Design upgrades of bucket-wheel exca-vators are aimed at the extraction ofinhomogeneous and ever harder min-erals. This results in much higher stress

for the entire unit. In order to ensurestability and avoid damage to individualcomponents it is therefore all the moreimportant to prevent undue loads. Suchloads originate primarily from thedigging process (special events). There-fore, the drive system of the bucket-wheel excavator is usually protected bytwo independent systems. For electricalsystems this means limiting the motorcurrent intake, mechanical or hydro-dynamic systems use a torque-con-trolled coupling. Although this approachhas proven itself in the past, it is still

only of limited use when it comes toreacting to extremely short or hightorque jolts, because of the existingsaturation effect in electrical machineryand, most of all, because of the overall

inertias in the drive line. These require-ments, described on the basis of abucket-wheel drive, apply to the majorityof electrically powered driven machines.The bucket-wheel excavator is merelyused as an example.

The drive of a bucket-wheel is deter-mined by the required digging and liftingperformance. The resulting torque Terf ismultiplied with the application factor ka,resulting in the actual nominal torqueTn. There are no standard definitionsregarding the scale of the applicationfactor. DIN 3990 is based upon ka= 1.75, and [1] to [3] refers to simulatedor measured values of ka = 1.33 to 3.3.The application factor takes all dynamicadditional loads into consideration anddepends on the driving and the drivenmachine. In practice, dimensioningoccurs in such a way that the drivecomponents with an operating factor (ka 1.6) are specified to have high fatiguelimits and simultaneously able to toler-

ate a finite number ( 105) of shocks(shock factor ks = 2.2 related to Tn). Theservice life of the drives is usuallyquoted with 50.000 operating hours.The application and the shock factor aredetermined by processing and engi-

neering aspects and by relevant experi-ences of the machine manufacturers.Modern dimensioning methods use loadcollectives, as far as these are availablefrom previous measurements.

A limitation of the motor current isalways assumed when selecting a suit-able overload protection device. Thelimitation is usually set in such a waythat the electric motor is switched offwith a time delay of 2 to 10 seconds inthe event of the nominal torque beingexceeded 1.5 times. The mechanicalsafety devices are expected to respond(TSchalt) in dependence on the plantdesign at 1.3 to 1.7 times the nominaltorque. Additionally, they are required tofulfil the following requirements:

Fig. 1Characteristic curves of torque-controlled couplings (overload couplings); a) Torque T depending on slip s; b) Torque T depending on duration tof a operating process:1 Friction coupling (load-holding); 2 Magnetic powder coupling (limited load-holding properties); 3 Fluid coupling (load-holding); 4 Shear pincoupling (load-separating); 4.1 Coupling with delayed gripper elements (load-separating and automatically re-setting).


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Interruption of the driveline, as clo-sely as possible to the place ofoverload (bladed wheel), in order todecouple mass inertias of all high-speed drive components.

Reaction with high separating pre-cision (operating precision), in orderto avoid unwanted switch-off.

Re-start of operation after switch-offwithout time delay, in order to avoiddowntimes.

Adjustability of release torque; suita-bility for heavy-duty operation, nomeasuring technology or electroniccontrol. Avoidance of wear, thermaloverload or maintenance.

Overload protection with torque-con-

trolled couplings rely on friction or form-locking devices in the driveline.Couplings that have been developed forthis purpose distinguish themselves bytheir design, but also regarding theirprecision and reaction speed. Apartfrom selecting the coupling, the defini-tion of a suitable place of installation inthe driveline is also decisive for theeffect of a certain overload protectionsystem. A problem is presented by theunpredictable systems behaviour of anoverload-protected drive on the onehand because of insufficient knowledge

of the precise coupling characteristics(e. g. operating speed), and on theother hand because of the impact onthe operating properties as a result ofdrive dynamics. Any reciprocal effect onthe reaction behaviour resulting fromthis needs to be taken into considera-tion. Drive simulation is therefore im-perative for the safe design of an over-load device. This simulation is equally

important for the identification of meas-uring collectives. Unfortunately, thisresearch directive is not adequatelysupported.

The main difference in the operatingbehaviour of overload couplings ischaracterised by operating principlessuch as auto re-set, load-separatingand load-holding". Generally, and thisalso applies to the drive system re-viewed in this paper, friction, safety,shear-pin, fluid and magnetic powdercouplings are used for such applications[4]. This also applies to friction brakescombined with a triple-shaft planetarygear [5]. A comparison of principlecoupling curves (Fig. 1) shows that slip-ping (power loss) fluid couplings trans-mit high, load-holding torques. Con-siderable losses also occur duringoperation with friction and magneticpowder couplings. Owing to their un-stable friction characteristics, frictioncouplings provide insufficient operationprecision. Major disadvantages are fric-tion wear and fatigue of shear pins orother reaction elements. Magneticpowder couplings are thermally sensi-

tive. If the powder is overheated, it willweld together, resulting in downtimes.With a view to quick operation re-starts,shear-pin couplings are becoming lessand less attractive.

The above evaluation suggests thatnone of the popular coupling typesmeets the complex requirements foreffective overload protection. Therefore,a combination of fluid and Safeset-safety couplings would present an inno-vative solution. To prove this, the rele-vant drive characteristics were ex-amined through simulation and are out-lined in the following.

Depending on its operating characteris-tics and its design, the Safeset-safetycoupling (Fig. 2) can be allocated to thegroup of load-separating or load-holdingoverload couplings. It uses a friction-locked oil-pressure sleeve for its oper-ating mechanism. Its very low massinertia and compact design also presentan advantage. The hollow pressuresleeve between shaft and hub can bepumped up to nearly 1000 bar and istightly closed via the shear valve. In the

event of overload there is a slight rela-tive movement between shaft and hub,during which the shear-off valve opensand instantly takes pressure off thepressure sleeve (load separation).Since the hub is fixed to the shaftthrough roller or sleeve bearings, thecoupling will not suffer any damagefrom unrestricted spinning. The releasetorque depends on the excess pressure

in the pressure sleeve and can be ad-justed with relative precision. The valvehead is replaced after each release,and the pressure sleeve needs to bepumped up again. A Safeset-safetycoupling without shear-off valve oper-ates over a limited time-span and with alimited slide path and is hence load-holding.

Against this background, a special drivesystem for armoured-face conveyorswas developed. The principle is basedon the combination of a fluid couplingwith a special face curve and a load-holding Safeset-safety coupling. Thecomponents are to be combined in thesequence "electric motor fluid cou-pling gearbox Safeset-safety cou-pling". The fluid coupling allows thestart-up (break-away) of the plant undervarious conditions. If partly filled, creepspeed operation is possible. In theevent of an overload (e. g. throughblockage or increased operating resis-tance) the fluid coupling limits themaximum torque in accordance with itscharacteristic curve, and the chain canbe stopped whilst maintaining its torque

at 100% slip. Other components are notendangered, because the reaction andslip torque only correspond to themaximum systems torque provided bythe motor (asynchronous motor pull-outtorque). If the chain is subjected to asudden increase in tension (e. g. by achain blockage close to the drive), thehigh amount of delayed masses wouldresult in peak torques in the driveline.The short length of chain is unable toabsorb this. In this case, a slip move-ment is created, while the drive line isnot separated via the Safeset-Safety

coupling. During the compensating slidemovement, the slip is increased andhence also the torque transmitted fromthe fluid coupling to the Safeset. Thisoccurs until the kinetic energy of themoved masses has been exhausted.Since the pull-out torque of the fluidcoupling is always lower than the slidingtorque, the Safeset is re-set. The slid-ing movement between the shaft andthe hub "grips" again and the fluid cou-pling continues to limit the transmittabletorque.

Fig 2Operating principle of Safeset safetycoupling: 1 Shear-off valve; 2 Hub;3 Pressure sleeve; 4 Shaft

Fig. 3Bucket wheel drive scheme: 1 Gearbox;2 Coupling; 3 Electric motor; 4 Main gear;5 Bucket wheel; 6 Main shaft; 7 Rotatingplanetary gear (power-split)


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With a drive design like the one de-scribed it is possible to combine thespecial reaction and overload charac-teristics of two torque-controlled cou-plings. New solutions are expectedespecially for such machines, wherestarting under load and protectionagainst static und dynamic overload areidentical. This applies e. g. to compres-sor, pump, shredder, belt conveyor,

bucket wheel and bucket chain con-veyor drives. Using the example of abucket wheel excavator, this effect willbe explored in the following, taking anysystems influences into account.

2 Simulation tool and modelformation

The software used for the simulation isITI-SIM3.0. It has a visual, interactivedevelopment environment with standardWindows surface. Drive models areconstructed with model objects fromvarious technical genres and placed onthe surface, combined and parameter-

ised by the mouse. The model objectsdo not restrict the flexibility, since freelydefinable objects and program inter-faces are available in form of user-DLLs. The advantage of this software isthat it allows the easy creation of oneand two-dimensional, multiple, trans-latory and rotary models. Additionally,comprehensive analogue and digitalcontrol processes can be illustrated.Individual model objects or groups canbe combined, shown and administratedas macros.

A bucket wheel drive rated at 3 x

1680 kW (Fig. 3) is used as a research

object. Each of the motors has anoverload coupling and a gearbox, actingon a power split planetary gearbox. Thisdriveline exists in threefold. The plane-tary gears transmit the torque via twogear spindles and a total of 6 pinions ofthe main gearbox to the main wheelwhich is connected with the bucketwheel via the main shaft. The describeddrive structure and all drive characteris-

tics (stiffness, damping, inertia, etc.) areincorporated in an SIM drive model.This is a model with concentratedparameters. It is necessary to simplifycertain aspects of the model gearbox, e.g. criteria like tooth backlash are disre-garded. The gearbox inertia is dis-tributed to adjacent knots in relation tothe transmission ratio. The model takesefficiencies into consideration. the self-operating overload coupling is availableas a model object in the ITI-SIM. Forthe modelling of the fluid coupling andthe Safeset-safety coupling, relevant

measuring data from test bed trials areused. The torque requirement at themain shaft is approximated to theparameter of the bucket wheel bymeans of a displaced sinus function,depending on the torsional angle (seehigh load frequency in Fig. 5). This kindof excavating power for bucket wheelsis known from own measurements.

The simulation of various overload con-ditions during the excavation processoccurs by modifying the increase oftorque at the bucket wheel. In order tosimplify the operation, the model objects

required for torque (extraction and liftingresistance), fluid coupling und Safesetare combined in macros. The electricgrid for the power supply of the motorsis regarded as ideal and hence repre-sents a model limit. Following relevantexperiences, the boom of the bucketwheel excavator is incorporated in themodel formation as dampened single-mass model. This appears necessary,since its dynamic behaviour greatly in-fluences the torque requirements andhence the systems behaviour of thedrive (see low load frequency Fig. 5).

Fig. 4 represents the described simu-lation model with software environment.It is constructed in such a way, that thefollowing 10 variations can be calcu-lated:

Fig 4SIM-model for bucket wheel drive (case study 7)

Fig. 5Torque at the bucket wheel across the duration of a slew path (representative measuringexample)


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Modelvariation

Drive concept

1 Fluid coupling and Safesetbefore planetary gearbox

2 Fluid coupling and Safeset

before main gearbox (simplifiedmodel)

3 Fluid coupling and Safesetbefore bucket wheel

4 Safeset before planetarygearbox

5 Safeset before main gearbox

6 Safeset before bucket wheel

7 Fluid coupling and Safeset

before main gearbox (detailedmodel)

8 Fluid coupling

9 Multiple disc friction couplingon motor shaft

10 Sun wheel with friction brake,triple shaft planetary gearboxand torque scales

The case studies are allocated severalparameter variations, resulting in a highnumber of simulation calculations. Eachsimulation comprises the following op-erating conditions: start-up, remaining inidle, start-up of section, remaining insection, special situations (e. g. blockedsection).

3 Simulation results

First of all, a place is determined whichwould present the best installation con-ditions for the Safeset safety coupling.For this, case studies 1 to 6 are used.

The parameters are adjusted to thespecial characteristics of Safeset, i. e.the selected release torque is so highthat the slipping angle of the Safesetremains low owing to the very low slip-ping capacity. Case study 1 forms thestarting point, where the release torqueof the Safeset is set to 2.5 times thepull-out torque TK of the fluid couplingunder consideration of the transmissionratio. For the remaining case studies 2

to 6, a release torque is set whichcorresponds with the transmission ratio,in order to be able to compare thesimulation results. The pre-set torqueincrease at the bucket-wheel for theload situation blockage is identical withall simulations.

The most important results are summa-rised in Figs. 6 to 8. Fig. 6 shows the

flow of torque in the main shaft, whilethe bucket-wheel is blocking. Thetorque increases demonstrate that theload with all case studies with Safesetand fluid coupling is lower than that forthe variations without fluid coupling. Fig.7 shows the relevant maximum andminimum torque increase of casestudies 1 to 6. Here it becomes clearthat the installation point of the Safesetsafety coupling is decisive for thesystems behaviour. The closer theSafeset is positioned to the bucket-wheel, the lower the increase. Thisbehaviour is explained by the fact thatthe torque increase needs a certain timein order to reach the overload coupling,due to spring stiffness and inertiafactors. Until slipping occurs, the torquein the main shaft can therefore risefurther. In case studies 3 and 6 (Safe-set at the main shaft), the main shafttorque equals that of Safeset safetycoupling. A time delay until the overloadcoupling reacts is hardly noticeablehere.Energy losses and slip angles for thedifferent case studies are shown in Fig.8. Here it becomes clear that the energy

to be absorbed by the overloadcouplings increases the closer it gets tothe bucket wheel. This helps to recog-nise the transmission efficiency takeninto account in the model. The longerthe distance to be covered by an over-load in the driveline, the lower theenergy that has to be taken up by theoverload coupling. The sliding pathsand slip angles decrease with theproximity of the overload coupling to thebucket-wheel, since the release torqueis increasing while the energy losses tobe absorbed remain virtually identical.

As a result of the statements made sofar, the most favourable installation pintfor the Safeset safety coupling is themain shaft of the bucket-wheel. Thisfinding is not new, because technicalsolutions for this issue were alreadysought some 60 years ago [5]. Today,the market is able to offer overloadcouplings (Safeset) with very high re-lease torque (up to 36000 kNm).

Fig. 6Torque at main shaft simulation of load situation "blocked section

Fig. 7Minimum and maximum torque increase at main shaft simulation of load situation blocked

section


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The relocation of the installation point tothe gear spindles of the main gearbox(case study 2) represents a reasonablecompromise (technology, costs). Sincethe planetary gears operate on a power-split principle, only one Safeset safetycoupling is required. The installationspace needed for the Safeset is verysmall, it can therefore be integrated intoone of the gear spindles.

Further investigations are carried outwith case study 2. For this, an extendedcase study 7 is created in which thepower-split is reproduced. The torqueincreases established so far (hightorque increases, low slip angle) are toohigh even for optimum installation pointsof the Safeset safety coupling. Therelease torque in case study 7 is there-fore set to 1.7 times the pull-out torqueof the fluid coupling. The torque in-

crease achieved by this can be seen inFig. 9. This reaction behaviour issufficient for bucket-wheel drives. Thechronological sequence of certainquantities prior to and during a blockageof the bucket-wheel is shown. Therelease, slip and re-setting can be seenfrom the flow of torque in the Safeset.The torque in the main shaft increasesfurther even after the Safeset has re-

leased, because the inertias to be de-layed are very high. After the kineticenergy has been absorbed by theslipping Safeset, the drive line closesagain. This can also be seen from thespeed curve of the primary and secon-dary side of the Safeset.

The behaviour of the fluid coupling ist ofgreatest importance for the operation ofthe drive system. Owing to the blockagein the bucket-wheel, the speed in thedriveline (see turbine speed) is de-creased. Since the motors are notswitched off in oder to maintain thestarting torque, the slip increases andhence also the torque transmitted by thefluid coupling. Such an increase intorque is often sufficient for clearing theblockage of the bucket-wheel. Shouldthis not be the case, the torque of thefluid coupling increases until pull-outtorque has been reached. At this pointin time, the torque already tends to fall.This demonstrates that the overload in

Fig. 9Simulation results for case study 7 load situation blocked section

Fig. 8Energy loss and slip angle in the overload coupling simulation of load situation blockedsection


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the main shaft is not caused by the in-creasing torque of the fluid coupling, butexclusively by the sudden delay of highinertias. If the bucket-wheel cannot befreed with the pull-out torque of the fluidcoupling, its operating point shifts to thebreakdown area of the motor curve.

With the help of simulation it was alsopossible to establish that a change ofthe pre-set torque increase (excavationprocess) results in different reactionbehaviour. This is why the torque in-crease here treated as an increase of

circumferential force at the bucket-wheel will be varied for the followingsimulations. During the excavationprocess, the circumferential force canexperience big (sudden blockage ofthe bucket-wheel) or small (smoothblockage of bucket-wheel) changes. Inthis paper, its increase varies between550 kN/ and 2300 kN/. The measuringunit describes the increase of thecircumferential force per degree of

bucket-wheel rotation. The results of thesimulation are shown in Fig. 10. In the500 550 kN/ range, the increase offorce is so low that the Safeset is notreleased. This can be seen from the factthat the slip angle shows the valuezero. The maximum torque increase inthis area is 1.7 times the pull-out torqueof the fluid coupling. The Safeset isreleased from 550 kN/. The maximumtorque increase increases 1.8-fold andremains initially in this range. From950 kN/ onwards, the maximum torque

increase in the main shaft rises further.This indicates that the Safeset is nolonger able to limit the torque on itsown. The fluid coupling is increasinglyinvolved. It reaches the point of pull-outtorque excess earlier and earlier. Thetime span between der start of theblockage (t = 0) until pull-out torque isreached can be read. In the last simu-lation variation, the torque increasereaches a value of 2.2 times the pull-out

torque at 2300 kN/. This equals ablockage of the bucket-wheel after arotation of 2. Since such a situationhardly ever occurs in practice, thesimulation was discontinued at thispoint.

In order to be able to evaluate the re-viewed also in relation to other overloadcouplings, three further models (case

study 8 10) were created. These areclassic overload situations which arelargely identical with the parameters ofcase study 7 stiffness, damping, in-ertia, increase of circumferential forceand release torque. As a comparativecriteria, the torque increase in the mainshaft prior to and during the blockage isshown in Fig. 11. Case study 9 (multipledisc coupling) has the highest torqueincrease, since the coupling only takesup the inertia of the motor from thedriveline. Case study 8 (fluid coupling)on the other hand shows a lower torqueincrease, because the fluid couplinglimits the torque provided by the motorto its pull-out torque. Case studies 7(Safeset safety coupling and fluidcoupling) and 10 (brake in triple-shaftplanetary gear) are equal from a torquelimitation point of view. This can betraced back to the virtually identical in-stallation point of the limiting elements.Consequently, the torque increase de-pends largely on the inertia which, afterrelease of the relevant limiting element,is still connected with the main shaft.Once the brake in the triple-shaftplanetary gear has released the drive-

line in model 10, the torque in the mainshaft decreases rapidly. This results inload alternations with low amplitude. Inmodel 7, the driveline is not separatedand the kinetic energy can reduce in theSafeset safety coupling. The frictiontorque of the Safeset has a dampeningeffect in the driveline.

4 Summary

The simulation of multiple drive varia-tions represents the investigation ofindividual situations at a real bucket-

wheel drive. The results may howeveralso be transferred to similar drives. Thereviewed combination of two load-hold-ing overload couplings (fluid and Safe-set safety coupling) represents a tech-nically interesting solution. With thiscoupling combination, the drive underreview shows a much more favourablebehaviour from an availability and com-ponent-wear point of view than with anyother overload coupling. Equal resultsare achieved with an active brake in atriple-shaft planetary gear. The brakesolution is however likely to cause muchhigher expense with a view to design

and control systems. Such a drive re-quires an extensive and failure-insensi-tive measuring system, in order to de-termine the release torque. Such

Fig. 11Torque increase in the main shaft for case studies 7 10.

Fig. 10Torque increase in the main shaft, maximum slip angle and energy loss in the Safeset-safetycoupling, moment when pull-out torque of fluid coupling is reached, in dependence onincrease of circumferential force at the bucket-wheel for case study 7.


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equipment is not required if fluid andSafeset safety couplings.

Deriving from own experience (e.g. witharmoured-face conveyor) and the in-vestigations described earlier, it can beestablished that the suggested couplingcombination is particularly suited formachines with process-inherent block-ages. This characteristic can also be

allocated to many other mining andmaterials handling machinery. It wasalso possible to demonstrate that alter-nating situations in the systems be-haviour of the driveline have a stronginfluence on the operation of overloadcouplings. Adequate selection ofcouplings and calculations in order toavoid overloads can only be done bymeans of simulation. Modern drivetechnology can no longer do without it.

Literature

[1] Wnsch, D.; Christianhemmers, A.;Schlecht, B.: Simulation zur Auslegungvon Schaufelradantrieben. In:

Antriebstechnik. - Mainz 37 (1998) 5. -S. 142-145

[2] Van den Heuvel, B.: Metechnischeund rechnerische Untersuchungen zurBestimmung der Betriebs- und

Sicherheitsfaktoren von Verzahnungenin Schaufelradantriebssystemen. In:Braunkohle. - Clausthal 46 (1994) 9. -S. 4-14

[3] Pajer, G.; Kurth, F.; Pfeifer, M.:Tagebaugrogerte undUniversalbagger. - 2. Aufl. - Berlin:Verlag Technik, 1971. - 296 S.

[4] Schlecht, B.: Planetengetriebe alsberlastsicherung in Sonderantrieben.In: Antriebstechnik. - Mainz 36 (1997)10. - S. 28-31

[5] Schlecht, B.: Effektiveberlastsicherung inSchaufelradantrieben. In: Braunkohle. -Clausthal 50 (1998) 1. - S. 9-14

Authors

Dr.-Ing. R. Finzel undDipl.-Ing. H. HoffeldVoith Turbo GmbH, Crailsheim

Prof. Dr.-Ing.habil. G. Kunze andDipl.-Ing. S. GraulTechnische Universitt DresdenInstitut fr Frdertechnik,

Baumaschinen und Logistik

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