World’s largest movable mining machine vibration testing – numerical and experimental approach

D. Pietrusiak1, P. Moczko

1, E. Rusiński

1

1 Wroclaw University of Technology, Faculty of Mechanical Engineering,

Department of Machine Design and Research

Wyspianskiego 27, 50-370 Wroclaw, POLAND

e-mail: damian.pietrusiak@pwr.edu.pl

Abstract Bucket wheel excavators operating in open pit mines are the largest movable machinery in the world. Due

to that fact, preparation and realization of numerical end experimental testing is a complex task. The paper

will give brief description of the experimental testing SChRs 4600.30. bucket wheel excavator testing.

The presented machine sticks to the class of the world’s biggest machines. The structure is over 70m long,

36m high and the total mass is around 3300 tons (without the discharge bridge). The experimental testing

sets covered excavation and machine travel. In the travel measurement set, the machine was adjusted in

direction parallel (1st case) and perpendicular (2nd case) to the crawler sets (travelling direction). The

investigation was done with numerical and experimental method.

1 Introduction

Bulk handling and earth moving machinery belong to the class of mechanical objects which are very often

subjected to the unfavorable operational conditions. Those machines are used from the area of small

construction field up to large mining fields. The biggest one are surface mining machines used in the

continuous surface mining, common in lignite mines. Object, which investigations are presented in the

paper, is bucket wheel excavator SChRs 4600.30. The structure is over 70m long, 36m high and the total

mass is around 3300 tons (without the discharge bridge). Two machines of that type (Figure 1) are located

in the largest Polish surface mine – “Bełchatów Lignite Mine”. To relies the size of the machine, in figure

2a it was compared with the biggest Boeing aircraft (Boeing 747-8). However, bucket wheel excavators

working in lignite mines can reach twice as much dimensions. The profile of one of the biggest machine

working in Poland (SchRs 4600.50) is presented in figure 2b. Information about investigation of dynamics

of this object can be found in the papers[1,2]. Large size bucket wheel excavators structure such

as SChRs 4600.30, consist of bucket wheel boom, central pylon supported on the slew platform,

counterweight boom, crawler undercarriage and discharge bridge (Figure 2a). In case of the biggest

machines, such as SChRs 4600.50, the pylon is replaced by two masts: one for bucket wheel boom and

the other for counterweight boom suspension (Figure 2b).

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Figure 1: Bucket wheel excavator SchRs 4600.30

Figure 2: Comparison of profiles of Boeing 747-8 to the biggest surface mining machines operating in

Polish mines: a) bucket wheel excavator SchRs 4600.30, b) bucket wheel excavator SchRs 4600.50

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Excavator SchRs 4600.30 was designed in Germany in 80’[3]. The assumptions made on the design stage

were adjusted to the geological conditions present in that time in German’s mines. At present, the machine

is used in very hard geological formations located in Poland. This leads to the high exposition to shock

impulses [4] and high value of harmonic excavation forces. The excitation generated by excavation forces

(11 buckets located on the cutting diameter) on bucket wheel equals around 0,57Hz, and amplitude of

excitation force can reach roughly 100kN [5]. Moreover, the technological movements of rotation, boom

hoisting and especially travel, excites structure vibrations. Example of vertical acceleration measured on

the bucket wheel boom for different operational conditions are presented in Figure 3. Another vibration

sources are belt conveyors, located along the structure, which transport the material. The vibrations are

generated by material hitting and moving over idlers and by transfer points (chutes) dynamic forces

presence (Figure 4).

a)

b)

Figure 3: Acceleration trace in vertical direction measured in vicinity of the bucket wheel:

a)excavation, b) travel

Figure 4: View on material transfer (chute) point

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During standard operation, the excitation sources can act simultaneously. Another, very dangerous to the

structure, excitation source are shocks which can be generated while the bucket hit a rock. The energy

[6,7] which is released in that event is big enough, that can completely destroy the bucket (Figure 5). The

load carrying structure is in that case also heavily overloaded.

Figure 5: Destroyed bucket

Mentioned above are the main excitation sources. It is worth noting that on the structure a lot of different

heavy mechanical equipment is installed, which can influence vibration parameters. As a result a need

arose to precisely determine the dynamic characteristics of the machine. This will help to establish its

resonance ranges, which are used then to plan its modernization in order to prevent resonance vibration

[8-10] of existing machine (if resonance is present) or to assure safety in case of technical parameters

change such as number of buckets, bucket wheel rotational velocity, etc.[11,12]. All such actions tend to

increase durability of such large and very expensive excavators in terms of alternating loads. Due to the

machine size and complexity, both numerical and experimental testing is a challenge.

2 Methods of bucket wheel excavators dynamic investigations

Technical requirements and recommendations for design of the earth moving or bulk handling machinery

like bucket wheel excavators, spreaders or reclaimers can be found in three standards DIN (German

Standard)[13] , AS (Australian Standard)[14] and ISO (International Organization

for Standardization)[15]. In each of them, one can find information about the fatigue design and limited

information about the dynamic loads in calculation process. Unfortunately, neither analytical/numerical

nor experimental requirements for dynamics phenomenon investigations are precisely defined. Eventually,

researchers around the world have been developing on both field of investigations. Great development in

mathematical modeling present achievements of Serbian researchers from University of Belgrade

[16-20]. Also Romanian and Czech researchers published papers presenting investigations on dynamics of

heavy machine dynamics [21-25]. Recently, Polish researchers presented first fully validated finite

element model of bucket wheel excavator [26]. The Polish team from Wroclaw University of Technology

presented also many papers on experimental testing of heavy machines dynamics [26-30].

As the mass of machines of that class can reach up to around 5000 tons, the experimental modal analysis

requires special equipment. In figure 6 and 8 there is presented method of excitation impulse generation.

In the first procedure (Figure 6) specially designed triggering device is used. Its essential part are pins

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which are rapidly sheared/destroyed under know force. Recently, to enable online measurement of the

applied force, special design force sensor was developed (Figure 7). Its measurement range is up

to 300kN. Via this device a mass is hanged upon bucket wheel boom and lifted. Due to the triggering

device it is possible to excite the investigated object and collect force trace via force sensor. Generated

impulse characterizes with long duration of force increase and its sudden drop to zero.

Figure 6: Excitation procedure with use of the triggering device

Figure 7: Force sensor

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Assumptions of the procedure presented in figure 8 is similar, however the mass of known weight

is hanged upon the bucket wheel boom. After lifting it up, release is executed with use of explosives.

Figure 8: Mass release with use of explosives [23]

As the complexity of the structure and its mass makes the modal analysis in classic approach difficult

to perform, operational deflection shape or the operational modal analysis finds application in recent years

[28]. Because surface mining machines are dedicated to work 24h, 7 days a week, the operational

procedures are highly advantageous.

3 Bucket wheel excavator SchRs 4600.30 – measurements set up

If analyze structures of bucket wheel excavator, spreaders or stacker, all can notice that in most cases the

design is based on the same principles. The truss structure (superstructure/upperbody) is placed on the

slewing platform embedded on the undercarriage portal frame. Undercarriage porta frame, together with

truck sets is the undercarriage support for the whole structure. While it is relatively stiff, there are no

significant vibrations in this element. However, the superstructure is free supported on the undercarriage

portal frame (most common via slewing bearing) [31]. Due to that fact, the rigid body modes of the

superstructure are significant while the swinging mass [28] of the first modes generates alternating loads

in the undercarriage. Moreover, in case of the most extended ends of the boom, the vibrations amplitude

can reach up to over 1m. This causes additional forces overloading the mechanical components and the

precise operation impossible.

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There are global mode shapes that are common for all machines of that type [28] however, local modes

can occur very complex. Due to that fact, it is recommended to prepare the numerical model first, before

experimental testing. Exemplary mode shapes determined with numerical model are presented in Figure 9

and 10. Mode shape 1 (Figure 9) presents most common global mode shape of the superstructure which is

common for all types of the machines. It is simply swinging of the structure in the main vertical plane.

The node of this mode is located close to the slewing bearing. More complex, fourth mode shape

(Figure 10) characterizes with dominating bending of the pylon simultaneously with bucket wheel boom

twisting.

Figure 9: First mode shape of the excavator SchRs 4600.30, f=0,40 – numerical model

Figure 10: Fourth mode shape of the excavator SchRs 4600.30, f=1,59 – numerical model

As a result of numerical modal analysis of testing SchRs4600.30 bucket wheel excavator, 5 significant

modes were distinguished (Table 1). As all can see, the frequency band do not exceed 2Hz, what

is common in this type of machines. Only for smaller machines the frequency band can shift higher

[28,32].

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Mode Frequency [Hz]

1 0,40

2 0,54

3 1,36

4 1,59

5 1,89

Table 1: Results of the numerical modal analysis

On the basis of conducted analysis, measurement points and directions were defined (Figure 11). Twelve

points, with one, two or three direction of measurements were selected. The total number of measurement

channels equals 15. Selection of measurement points can be limited by the accessibility to some areas.

In general, the structure is covered with the walkways, however those are planned in the manner to give

access to points important from the operation point of view. When need to access to the off-walkway

location, machine stop and use of aerial platform may be required. Unfortunately this procedure

depreciates on of the most important advantage of operational modal analysis, which is testing without

machine stop.

Figure 11: Location of measurement points

4 Measurements and results

The measurements were taken during excavation and travel (without excavation). The two sets differ

significantly while during excavation, the machine is in interaction with the slope and loaded with the

mass of overburden/coal. The machine design allows also for travel during excavation but in very short

range. In purpose of the modal testing the travel is done without load and slope contact. This allows

for relatively long travel and dedicated superstructure position (perpendicular and parallel to the truck

sets). Figures 12 presents the autoMAC matrices for both experimental cases. In figure 13 the crossMAC

matrix are presented. The numeration of the modes is related in global manner according to the table 2.

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Figure 12: autoMAC matrices for travel (a) and excavation (b)

Figure 13: crossMAC matrix

As visible in figure 12b the two modes are almost of the same shape and frequency. It was marked as the

same pole mode. The identified modes are compared in the table 2 where it is clearly visible that some of

the modes are not present when excitation method was changed. The analysis of MAC matrices reveals

that common poles are present. Additionally, the change in modes or it complete decay was observed.

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It is visible, that travel gives the matrix which is more clear, however in comparison to the excavation the

excitation is not enough to excite higher modes. The crossMAC matrix (Figure 13) presents, that the shape

of first mode remains the same but the frequency is shifted. The rest of modes almost don’t correlate.

Travel Excavation

Mode Frequency [Hz] Mode Frequency [Hz]

1 0,38 1 0,44

2 0,55 --- ---

--- --- 3(3) 1,32(1,47)

4 1,52 4 1,57

5 1,97 --- ---

Table 2: Results of the experimental modal analysis

Figure 14 presents first mode shape of the excavator on the experimental model. It remains unchanged

in numerical analysis (Figure 9) and both experimental cases. The shape is close to the rigid body mode

of the superstructure. This mode is especially crucial due to the generation of alternating loads

in undercarriage of the machine.

Figure 14: First mode – experimental method

Figure 15: Second mode – experimental method

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From the operational point of view the second mode (Figure 15), which is present, both in numerical and

experimental testing, is crucial. The frequency is matching the excitation frequency which equals 0,57Hz

and is generated by the material discharge from the buckets. Fortunately, experimental analysis during

excavation proved, that due to the changes in support in the system (contact of the buckets with the slope)

that mode is no longer present.

5 Summary and conclusions

The paper presents investigations on specific object what makes the modal analysis testing interesting and

bringing new knowledge, which makes it useful. The methods developed to excite bucket wheel

excavators are the example of the special approach of modal testing realization. Challenging also are the

environmental conditions in which the testing is performed. The hundreds of meters of cabling must be

installed in dust (very often electricity conducting carbon dust – dangerous to the electronic devices), mud

or in heavy weather conditions, while testing is always outdoor. The vicinity of many rotating and moving

elements installed on the machine, makes the task even more complex [33,34].

For the bucket wheel excavator SchRs 4600.30 in the numerical and experimental testing 5 modes

significant in the investigated frequency band were identified. The comparison between excitation

techniques, pointed out that excavation delivers more energy to the system. However, some modes

(second mode – 0,57Hz) can completely decay in excavation, due to additional support. The modes

unchanged in shape are 1st and 4

th one. In that case the frequency shift was observed (about 0,06Hz-

1st mode, about 0,05Hz – 4

th mode). For non-experienced user the shift may looks negligible, but when

compared to the frequency band which is investigated (0-2Hz) it is visible that the frequency change can

be meaningful.

Presented measurements are unique in world scale. Up to now, the complex measurements of bucket

wheel excavators dynamics was not performed. As a result, resonances and high vibration amplitudes are

not rare thing. This leads to the fact that the durability of many machines of that type is strongly

decreased. Years of operation in such state lead to the situation where fatigue cracks of load carrying

structures is common. With the use of presented approach, big modernization of excavation unit

of SchRs4600 bucket wheel excavator was made to improve operational condition and extend life time

of machines [1,12]. It also influences on the safety of the mining machines [35].

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