holistic simulation of a bucket wheel excavator

CUSTOMER APPLICATIONS | Berthold Schlecht, Carsten Schulz, Institute of Machine Elements and Machine Design, Chair of Machine Elements, Dresden University of Technology

2 | SIMPACK News | July 2013

Holistic Simulation of a

“A single simulation of the drivetrain that neglects the bucket

wheel boom is not sufficient for a complete system analysis”

Holistic multi-body simulations can be used to improve our understanding of complex systems, determine design loads, and find operational modes to extend the fatigue life of machine elements. This requires a complete mod-eling and verification of all mechanical and electrical parts, including the con-trol loop of the motor.

INTRODUCTIONLarge-scale equipment for lignite mining, like the largest bucket wheel excavator in the world, shown in Fig. 1, exhibit high stability and insensi-tivity to mechanical vibrations because of their exception-ally high mass. The experiences of recent years have nevertheless shown that even established mining technologies can be enhanced. Measurement evaluations at the drivetrain of the bucket wheel or of the superstructure clearly show that the whole system is sensi-tive to mechanical vibrations. Increasing requirements during operation and excava-tion in hard clay-iron layers are forcing op-erators to adapt their operation and control strategy.As part of a research project, the dynamic behavior of the drivetrain of the bucket wheel was analyzed and optimized with the help of SIMPACK.

SYSTEM ANALYSISFig. 2a shows the measured values of the torque at the bucket wheel during coal mining. The transformation of the signal into the frequency domain illustrates the high dynamics connected to the mining process (Fig. 2b). Due to the engagement of the bucket wheel at 1.1 Hz, the system responds with its first natural torsional frequency at 1.8 Hz. Furthermore, one can also see a reaction of the bucket wheel boom at 0.35 Hz, which is the first natural frequency in the vertical direction. This

establishes that the response function of the drivetrain depends on the response of the bucket wheel boom.Because of this dependency between sub-systems, it is necessary to consider elastic structures like gear housings, the super-structure or the torque support of the drive-train. A single simulation of the drivetrain that neglects the bucket wheel boom is not sufficient for a complete system analysis.

DRIVETRAIN MODELINGThe Chair of Machine Elements at Dresden University of Technology uses full elastic models with six degrees of freedom for the mechanical components of the sys-tem. These models are based on origi-nal manufacturer drawings and have been verified with measurements on the real system. This ensures the highest possible modeling standard, a basic requirement

for a complex system analysis.The gear-box of the bucket wheel

excavator, whose gear pairs can easily be mod-eled by the SIMPACK element Gear Pair (Force Element 225), has a total transmission ratio of 243. The nominal torque of the three motors of 16 200 Nm adds up to 11 700 kNm at the bucket wheel. For considering higher orders of bending vibrations, gear shafts are modeled as beam elements. The mathematical ap-proaches of beam elements, which are integrated within the SIMBEAM elements of SIMPACK, ease the modeling of all shafts. All shafts are elastically mounted. The bearings are repre-sented by spring-damper elements with a characteristic curve. A simplified representation of bearings via a fixed op-erating stiffness is also allowable for normal load cases, because significant changes in the bearing stiffness only occur in low speed ranges.

Berthold Schlecht, Carsten Schulz, Institute of Machine Elements and Machine Design, | CUSTOMER APPLICATIONS Chair of Machine Elements, Dresden University of Technology

SIMPACK News | July 2013 | 3

ELASTIC STRUCTURESIn addition to the bearing stiffness, the stiff-ness of the housings affects the behavior of the drive train. Because of this, all gear housings (especially the torque arm) must be integrated as elastic bodies. These are, after their construction with the help of CAD, modeled by finite element models and integrated into the multi-body system. The full elastic multi-body system can now be simulated without difficulty despite the large number of degrees of freedom.In addition to the detailed drivetrain model, it is necessary to integrate the superstruc-ture of the bucket wheel excavator into the multi-body system. After modeling the complete geometry, all supporting

structures of the super-structure subsystems

are meshed via 1D or 2D elements (Fig. 3). As the

number of nodes is too large for a multi-

body system, the finite element model has to be

modally reduced.When integrating the superstructure, one also has to consider the kinematics and kinetics of the rope system because, it con-nects the superstructure subsystems which are necessary for stability. Now the multi-body system of the bucket wheel excavator is able to lift and lower the bucket wheel boom. This can be seen as an addition to the complete model but, one should take into account that the operating position of the excavator influences the natural frequencies of the bucket wheel boom. Depending on the position and the length of free rope, the natural frequencies of the bucket wheel boom change.The subsequent comparison between nu-merically calculated and measured natural frequencies shows very good compliance.

LOADS AND EXCITATIONSIn addition to modeling mechanical parts, the loads and excitations acting on the bucket wheel and motors must be consid-ered. Fig. 4 shows the outer and inner ex-citations. The characteristic force function, being associated with the mining process, has a broad excitation spectrum and high dynamics. In contrast, the control loop of the motor reacts slowly, which can increase the dynamics.

Bucket Wheel Excavator


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© Jürgen Leuffen


The tooth mesh frequency is an example of an inner excitation which can also be defined with SIMPACK Force Element 225. This effect is associated with a permanent amplitude modulation which is an excitation for torsional vibrations. As the tooth mesh frequency is, however, much higher than the first natural torsional frequency, vibra-tions caused by the tooth mesh frequency have a much lower amplitude.After verifying the excitation function and the reaction of the control loop of the motor based on measurements, one can assume that the holistic multi-body system will give reliable results. To combine the ele-ments of the holistic simulation — excitation function, multi-body model, motor control loop — the Chair of Machine Elements gen-erally uses Co-Simulation with MATLAB® and Simulink®. This allows for a comfortable calculation of complex excitation functions, a comfortable routing of signals into and out of SIMPACK and the storage and evalu-ation of results.

LOAD CASE SIMULATIONCalculations in the time domain provide information about the time histories of loads and displacements of the system. The transformation of those time signals in the frequency domain shows the main influ-ential variables concerning the amplitudes. Based on the system reaction (frequency and amplitude), engineers can evaluate the complete behavior of the drivetrain. When specifying improvements, this makes the quality of simulation results critical.

Fig. 2a: Measured torque: time domain

Fig. 2b: Measured torque: frequency domain

Fig. 1: Bucket wheel excavator 293 (model and real excavator)

Berthold Schlecht, Carsten Schulz, Institute of Machine Elements and Machine Design, | CUSTOMER APPLICATIONS Chair of Machine Elements, Dresden University of Technology

Motor reaction

Tooth stiffness variation

Load function

SIMPACK News | July 2013 | 5

Fig. 3: Finite element model of the bucket wheel boom

Fig. 4: Main influential variables

These optimizations are especially impor-tant for the bucket wheel gearbox. Before defining new operation strategies or con-structional changes, one has to compare the current behavior of the simulation with real measurements. Fig. 5 shows the compari-

son between simulation and measurement for a standard load case. Initially, the bucket wheel excavator slews into the surface. Consequently, the torque and dynamics increase quickly. A quick visual comparison of the torques in the

simulation and actual measurement shows a good correlation. The signals in the fre-quency domain confirm this.The dynamic factor of all drivetrain ele-ments is interesting in regard to the general directional behavior of the drivetrain which


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Fig. 6: Normalized torques

Fig. 5b: Comparison of simulation and measurement: frequency domain

Fig. 5a: Comparison of simulation and measurement: time domain

can be seen indirectly in Fig. 6. There is a visible gap between the dynamic factor of the coupling and the rest of the gearbox. This illustrates the elastic function of the coupling which reduces the vibration on the motor shaft. This function prevents the mo-tor shaft from overloading. In regard to the control of the drivetrain, this indicates that the speed-controlled motor controls are too soft and do not react quickly enough. It also means that, even though the control loop has a load limiter, the loads in the gearbox may be much higher.The results prove the high quality of the holistic simulation and show that SIMPACK is suitable for large models with complex excitations. Furthermore, the results show that the damping of the first torsional natu-ral frequency at 1.8 Hz is too low. This can be explained by the supporting effect of the surrounding ground which could easily be taken into account by an additional damper element.

CONCLUSIONThe results of the holistic simulation of bucket wheel excavator 293 show that the current system behavior can be calculated correctly. Further detailed work is justified by the accurate conclusions of the system behavior. This allows the definition of an improved motor control that also takes gearbox dynamics into account.Besides system analysis, one can also calculate load spectrums, after simulating normal and special load cases. These can accordingly be used for new construction or revisions. This also includes the calculation of bearing lifetime. The complete system analysis of the bucket wheel excavators with the help of SIMPACK not only increases system knowledge, but also delivers ways to improve operation. This lies within the interest of the operating company, as they are currently working to implement an improved motor control.

holistic simulation of a bucket wheel excavator

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