The Concept of Functional Virtual Prototype in the Design of ...

M. POPOVIĆ, Z. JUGOVIĆ, R. SLAVKOVIĆThe Concept of Functional VirtualPrototype in the Design of ExcavatorCutting TeethRESEARCHCutting teeth for continuous excavator is an element of bucket assembly which has a dualfunction in the process of excavation, that is destruction of soil (media) and its excavation.Since most excavator processes depend on the processes in the cutting zone, it is necessary topay particular attention to the cutting teeth design and testing of completed prototypes inworking conditions. Prototype testing might be a problem, regarding distinctive excavatordesign,working conditions and work technology. This paper presents an alternative method forresolving these problems by utilization of simulations and simulation models throughdevelopment of functional virtual prototype. Further more, it presents an example of thedevelopment of the basic model of functional dependence of cutting teeth wear and appropriateload. Development objective of such and similar models is their implementation into the virtualprototype.Keywords: cutting teeth, design, virtual prototype, rotor excavator1. INTRODUCTIONIn order to present a high-quality, low-cost productto the market in a very short time, it is necessary toshorten the time for development and innovation ofthe product, reduce design and production costs,and provide high quality of the product as well.Considering the fact that much has been alreadydone in the field of the production optimization, itis necessary to search after new reserves forattaining stated goals in the first phases of thedevelopment, particularly during the designprocess of the product, but also during theproduction and examination of the prototype.Modern approaches in the product design as wellas methods used to this effect implyinterdisciplinary bases of knowledge and constantcommunication among them.All previously mentioned refer to severalsignificant and unique requests that are imposed bythe design process[5]. Design solutions also haveto satisfy complex production requirements as wellas the requirements concerning the assembly work.Design engineers must take into consideration anumber of aspects reffering to fixture,maintenance, storage, lifetime, reliability, safety,recycling etc. The product has to be designed insuch way so as to make a profit and satisfy therequirements both of the users and the market, inother words, design engineers have to be aware ofthe user’s expectations about the product. Finally,it is necessary to take into account the fact thatboth processes within which the product and itsparts are being produced and the conditions underwhich they are used are changeable. Consideringthe changeable nature of production andexploitation conditions, the requirement refferingto the product robustness (strength, resistance etc.)is an essential design requirement.Production design, from simple to the mostcomplex forms, with fulfilment of previouslystated requirements, is a very difficult, demandingand complex task.It is, therefore, of great importance to find creativeand efficient solutions to the problems that mightoccur during the processes of production andexploitation. It is particularly important in earlyphase of design when ideas are generated anddecisions which determine the nature andcharacteristics of the product are made.Marko Popović, Zvonimir Jugović,Radomir Slavković,Technical Faculty, Svetog Save 65,32000 Čačak, Serbia, marko@tfc.kg.ac.rsTribology in industry, Volume 31, No. 3&4, 2009. 37

2. INTEGRATED DESIGN APPROACHThe aim of integrated approach is to shorten thedevelopment time and additionaly improve theproduct in this phase [6]. Realization of integratedapproach can be observed through simultaneousand virtual concept. Simultaneous concept is basedon need for close communication between designand technology. The point is that the product andtechnology development are simultaneous.Decisions concerning the design process are madeon the basis of technology development and viceversa. In such a way the time for development isshortened and possible misunderstandingsconcerning the realization of proposed solutionsare avoided. Virtual concept is based on theunification of the product development in spaceand time. It is models, modeling and simulationthat represents the basics of the concept. Themodels are used to simulate the product, itsproperties, the production and the process ofexploitation as well. The aim is unification of thesimulation and its simultaneous realization fromseveral different aspects depending on the currentsubject of consideration.Figure 1. Classical V approach to design with the physical prototypes [1]It is physical prototype production and testing thatis a bottleneck in almost every design process of anew constituent element of an continuousexcavator or in reconstruction of the existing ones.The question concerning this issue has been raised.Continuous excavators are recognized as onesegment of the continuous excavation process andthe process of coal transport. Since the process iscontinuous, if any element of the system does notwork, the entire line is out of order. Consideringthe fact that such systems are expected to workcontinually, disregard for planned stoppages andoverhauls, any other stoppage would generateserious problems in terms of the satisfaction of theset requirements and it would also reflect oneconomic costs [9]. In addition, workingconditions are quite unfavourable and there is adanger concerning large dimensions of rotorexcavator components, a danger regarding workperformance and technology as well as currentcondition of the machine and her impact onprototype behaviour (e.g. prototype can be valid,however, due to relatively bad condition of theassembly or subassembly, it is not possible to getvalid results). We have to take into account thequality and assembly work of the completedprototype as well as the impact that the prototype38breakdown has on another machine elements (e.g.concurrent fracture of several cutting teeth cancause bucket damage). On the other hand,prototypes are principal instances as they have akey role in the final confirmation of the defaultsolution.Concerning the above mentioned issues, it is of animmense importance to find an alternativeapproach to assembly work which would implyassembling of the physical prototype only when allpossible previous adjustments are made andafterwards tested and when all other possibilitiesare exhausted. A possible alternative lies in thedevelopment and application of the virtualprototype. Nowadays, virtual prototypes are usedfor better perception of future product appearanceeven at the early stage of development as well asfor simulation and testing of the productcharacteristics before the production of physicalprototype. Thus, it is possible to achieve majoreconomies in, or even to eliminate, costs and timeneeded for production and modification of physicalprototypes. Apart from visual representation of theproduct, virtual prototype has the task of descriptionand simulation of the product physical performance([4], [5]). As distinguished from the physicalTribology in industry, Volume 31, No. 3&4, 2009.

prototypes, virtual prototypes make use of moderncomputer science, so the focus of design processshifts from physical to virtual environment, whichrepresents a significant advantage in resolvingproblems concerning strip mining excavators.3. FUNCTIONAL VIRTUALPROTOTYPEThe design process, known as process V (Figure 1.),consists of two clearly defined phases. The firstphase includes the top-down analysis that leads toa gradual and hierarchical definition of the systemfunctions and its components to the lowest level,i.e. the level of the basic physical examination. Thesecond phase includes systematically repeatedserial of corrected prototype tests up to a pointwhen the complete validation of the system hasbeen realized.Classical CA-x approach covers computersupported technologies aimed at the generation ofproduct model, the analysis of its condition anddefinition of necessary technological processingparameters. Such approach is also known as “art topart”. Solutions attained in such a way satisfyrequirements for solving the problem in the casesof less complex systems (products and processes)design. The notion of complexity is not related tothe number and complexity of the components, butto the possibility and appropriate account of innerand outer interactions in the system. By generatingthe complex systems, whereat the elements areconnected through basic metaphysicalmanifestations on the lowest level, early defectidentification becomes practically impossible,therefore the success of the “first” design is hard toachieve. On the other hand, the later we perceivethe problems during testing, the higher costs ofcorrecting them are required (for example,perceiving errors in linking components of thecutting tooth is more convenient than perceivingproblems in versification of the adopted concept ofthe cutting tooth). Furthermore, it should bepointed out that the problem of building optimalsystem on the whole cannot be solved by optimaldesign of certain components of the system.Therefore it is necessary to define new horizons inthe field of the existing technologies and steptowards new ones. It is feasible to be done bydevelopment of new devices as well as integrationand modification of the existing technologies, suchas CAD/CAM/CAE, so that the subject of theirexamination might be extended to a complexsystem.Figure 2. The diagram of the Functional Virtual Prototype concept [2]The concept Functional Virtual Prototype – FVP(Figure 2.) enables perceiving not only the functionsof some of its components, but also the function ofthe entire system and its operative performances.The approach based on FVP is a range of wellorganizedtasks used in the design process or theprocess of improving the existing system throughconstructing separate models that describe thesystem. By the analysis of the system functionsand categorization performed in accordance withthe specification of the realization, a complexmodel that provides better comprehension of itsTribology in industry, Volume 31, No. 3&4, 2009. 39

essential meaning can be built. Basically, thisconcept is founded on the descriptive andpredicative models of the objects, parts of thesystems and their environment. Thus, computersimulation and testing are placed in the foreground,whereas physical production and testing are usedas utmost tools for design versification andimprovement of the simulation models.4. BASIC ELEMENTS NECESSARY FORGENERATON OF THE VIRTUALPROTOTYPEThe most important task of the virtual prototype isa complete conveyance of design, production,development and testing phases on a computer,which would considerably reduce the time andcosts of product development. Without distinctionof what type of functional virtual prototype it isabout, i.e. no matter what process or system ismodelled, the following phases in construction ofsuch models can be defined [5]:1. Production2. Testing (testing simulations)3. Estimation4. Development5. AutomationDesign of the system virtual prototype (of theproduct, process etc.) is completed in the firstphase. In this phase structure defining anddisintegration into integral subsystems andcomponents are performed. Clear correlationsestablished between the integral components at thisstage enable the development of true and complexmodel with possible variations. For example,geometry and mass features of separatecomponents of the cutting elements can be attainedfrom solid models, whereas mechanicalcharacteristics can be achieved from componentmodels of the definite elements or fromexperimental testing.One of the significant phases includes formation ofthe virtual equivalent for laboratory testing andtesting in the field. Within laboratory testing, thevirtual prototype requires installation of the virtualtest equipment for procedure reproduction andreproduction of conditions realized by theutilization of real appliances and testing machines.Thereupon, test constitution reflecting actual orneeded exploitation conditions are performed.Figure 3. Synoptic diagram of the FVP for bucket wheel excavators40Tribology in industry, Volume 31, No. 3&4, 2009.

At the next stage model estimation is performed,i.e. virtual and physical model are tested in theidentical manner, and afterwards comparison anddeduction are performed. By additional adjustmentof the virtual test we achieve a reliable model forfurther testing.In the process of the virtual prototype utilization,its development and improvement are carried out,within which simpler initial virtual prototypes arecomposed into complex functional virtualprototypes. Thus, modular solutions that cansignificantly expand activity field of the basicvirtual prototype are obtained, whereat furtheradjustments refer only to some newly-addedsegments (e.g. in the process of bucket testingwhereat not only cutting teeth but special blades ina form of a knife are added as well. Concerningthis, VM teeth buckets can be used with anaddition of new modules that are later adjusted).Improvement is always carried out in twodirections, in the direction of improvement of themodel accuracy and constancy, and in the directionof improvement of the product design.The final phase of the design is process automationof the virtual prototype construction. It is onlywhen enough “knowledge” and “experience” inexisting models are generated, that we canapproach this phase. Current generation of newsimilar constructions is achieved by the automationin a short time. Thus the structure and mostcomponents are “lent”, whereas adjustments referonly to those segments that have difference fornew and existing design. The utilization of FVPcan be automated only when one completed,through many cycles estimated model of the virtualprototype exists and when it is tuned through thechange of its characteristic parameters.5. WEAR MODEL OF CUTTING TEETHThe following text presents an example of a modeldefining that connects cutting elements load withthe extent of their wear. When we talk about a partof the cutting tooth wear and their influence on theprocess of cutting soil, we talk about the enormousconsumption of materials in two characteristicplaces. The first one is the tooth-face wear, whichis a consequence of excavated material influence.The second is the dorsal surface wear, which is aconsequence of the interaction between back partof the tooth and the forehead of the excavatedmaterial block. In general, the worn wedge-shapedtool with finished width, as shown in the Figure 4.,can be approximately assumed.Figure 4. Wear model of the cutting teethThe Figure 4., presents the following variables: α -cutting angle, β - wedge angle, γ - relief angle, δ -angle of tooth wear, δ st =β+γ, F trL - frictional force(resistance) on the dorsal surface of the tooth, F nL -a normal component of the frictional force on thedorsal surface of the tooth, F trG – frictional force(resistance) on the tooth-face, F nG - a normalcomponent of the frictional force on the tooth-face,a – wear length, s – thrust length. The forcesdefined by this model are valid only with thefollowing hypothesis:a) The force function F nG is a feature of theexcavated material and it is proportional to thetooth-face (A G =b⋅h): FnG= b ⋅ h ⋅ kB,whereat: b – is the width of the cutting blade,k B – is the specific resistance to chipproductions [kN/m 2 ], h – is the thickness ofthe chip .b) On the basis of the wedge effect and previouslygenerated crack that is typical for brittlebreaking with a small deformation of the soil(the initial destruction phase has beencompleted), the direction of the force effect F nGlies close to the perpendicular to the motionpath (Figure 4.).c) Either elastic or plastic deformation of theexcavated material occurring on the backsideof the wear surface, brings about the force F nL ,a characteristic of the material itself,represented as the function of the penetrativedepth s: FnL= b ⋅ a ⋅ s ⋅ kv, whereat b – is thewidth of the cutting blade, k v – is the specificresistance of the wear surface. [kN/m 3 ]d) Direction of force F nL lies perpendicularly to thewear surface or it is inclined at an angle in regardto the perpendicular (in this case the force on thetop of the tooth is taken into consideration).e) The wear specific resistances are values thatshould be determined and they present thefunction of the excavated materialcharacteristic.Tribology in industry, Volume 31, No. 3&4, 2009. 41

f) The presented tooth model should beobserved in the plane that has been inclined atsome angle (ε) in relation to the movementdirection. Owing to the angle, the frictionalforces would not lie in the plane of the wedge.Consequently, this angle is a result of theteeth arrangement in the bucket and currentposition of the examined tooth in themovement trajectory (sequence rotation of theworking wheel and swerving the arrowexcavator).By observing the previous figure, it is possible toprovide an analysis of the forces. Examination ofthe model in the plane is enabled by thedecomposition of the acting forces into adequatecomponents (Figure 5.).When we look at Figure 5., and results obtainedfrom the research [7], it is evident that the normalforce F N must represent sum of two forces, andthese are a force exerted from chip generates abovetooth-face and the other exerted from wear surfaceeffect on relief tooth surface, which acts even in thecase of „sharp“ teeth due to blade radius andexcavated material structure. This „additional“ forceand force of friction caused by it are notproportional to the dimensions of chips but to wearsurface. Wear angle δ can be considered constantfor one cycle of the teeth wear. The size of thisangle depends on: design parameters of the teeth,type of excavated material, excavator worktechnology, bucket characteristics regardingassembly work etc.Figure 5. Example of decomposition of the actingforcesThe represented model takes into considerationfunctions of the average values. Basically, thefunction differs from the model of soil failure,nevertheless it is applicable in this place due toexamination of the conditions describing theprocess in the “middle”, not to the examination ofthe current conditions.Observing the plane model, determined bydirections of the tangential and perpendicularforce, in relation to the forces defining within rotorexcavator, according to the Figure 5., the followingexpressions can be defined:FFtrLtrG= μ ⋅FLGnL= μ ⋅ FnG→F→FrLrG→= F→= FtrLtrG→+ F→+ FnLnG→= F→= FTLTG→+ F→+ FNLNGFigure 6. Measured force load for a single tooth(laboratory model)42Tribology in industry, Volume 31, No. 3&4, 2009.

In accordance with previously shown equations,we can notice the mutual dependence of the size ofthe wear surface and a component which increasesthe total excavation resistance. Diagrams (Figure6.) show the results of realized measurings thattestify previous statement. All three components ofexcavation resistance are measured withinlaboratory testing. With regard to the subject ofthis paper, it is more important to examine trendsand relative ratio of the obtained results thanapsolute value of the obtained resistances. Theconclusion that may be drawn is that the increaseof wear surface leads to the increase of totalrequired cutting force, for it is necessary toincrease the cutting force whereas the soil stressfailure remains unaltered.Table 1. Measured values for previous diagramsL 1 δ st =35° δ=0° a=0 mmL 2 δ st =35° δ=5° a=7 mmL 3 δ st =55° δ=0° a=0 mmL 4 δ st =55° δ=5° a=8.5 mmThe model shown on Figures 4.,5., represent a basefor connecting cutting teeth load with parametersof its wear. In such a way, variations and possibleappearances of wear surface and its dimensionswould represent the input, and the output would bethe load of cutting elements and other elements ofthe rotor excavator system. The aim of thedevelopment of such and similar models is theirimplementation in functional virtual prototype. Thenext figure shows a model which offers aparametric definition of the cutting teeth wear [8].Figure 7. Example of parametric values forcutting teeth wear6. CONCLUSIONThe paper presents the application of the conceptof functional virtual prototype in the developmentof cutting teeth for the continuous excavator. Initialmodel is singled out and described as an example.It is this model that establishes connection betweenthe load and the degree and form of cutting toothwear. It represents only one segment that is to beused for complex virtual prototype generating.Such approach in the development of a newgeneration of teeth used for continuous excavatorswould bring about the following positive effects:• Risk minimization concerning the failure of theteeth conception and its final design• Time required for the prototype developmentand testing would be shortened• The number of repetitive steps within andbetter organization of design and testingprocesses• Improved reliability of design process• Testing of similar structures would also bepossible to carry out repeatedly and therewould be a possibility for other excavatorcomponents to be testedREFERENCES[1] Herve, Y., Desgreys, P., Functional virtualprototyping design flow and VHDL-AMS,Forum on specification & Design Languages(FDL'06), Darmstadt (Germany), September19-22, pp. 69-76, 2006[2] Legendre, A., Herve, Y., Functional VirtualPrototyping Applied to Medical DevicesDevelopment, Systems VIP, Strasbourg, 2008.[3] Ryan, R., Digital testing in the context ofdigital engineering - Functional VirtualPrototyping, Mechanical Dynamics, Inc.,Michigan, USA, 1999.[4] Mandić, V., Virtuelni inženjering, Mašinskifakultet, Kragujevac, 2007[5] Kolarević, M., Brzi razvoj proizvoda,Beograd, Zadužbina Andrejević, 2004.[6] Ognjanović, M., Konstruisanje mašina,Mašinski fakultet, Beograd, 2003.[7] Hitzschke, K., Jacob, K., ExperimentelleAnalyse der Belastung des Schaufelrades durchden Grabvorgang, Teil 1,2, Hebezeuge undFördermittel 24, Berlin, 1984.[8] Popović, M., Jugović, Z., Slavković, R.,Integrisani pristup konstruisanja habajućihreznih elemenata kod rotornih bagera,Serbiatrib’07, Kragujevac, 2007[9] Popović, M., Istraživanje i razvoj dvodelnekonstrukcije zuba kod rotornih bagera,Magistarski rad, Tehnički fakultet, Čačak, 2007.[10] Franek, F., Vorlaufer, G., Edelbauer, W.,Bukovnik, S., Simulation of Tribo systems andTribometrology, Tribology in industry,Vol.29, No.1&2,2007.[11] Ivković, B., Tribološke površine u tribomehaničkimsistemima, Tribology in industry,Vol.21, No.2, 1999.Tribology in industry, Volume 31, No. 3&4, 2009. 43

44Tribology in industry, Volume 31, No. 3&4, 2009.