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DYNAMIC ANALYSIS OF LARGE VIBRATING SCREEN

Wenying Li, Shibo XiongInstitute of Mechelectronic Engineering

Taiyuan University of TechnologyNo.53 Xikuang Street

Taiyuan, Shanxi Province, P.R. China

ABSTRACT

Large vibrating screen have been extensively used ingrading materials and expulsion of agents in the materialsindustry and coal preparation industry. Working life of thevibrating screen is important for decreasing the cost ofproduction and increasing productivity. A large model of thescreen was mounted in the laboratory for studying its modalperformance. The model is suspended with steel ropes.Modal test was carried out with artificially exciting by 500gimpacting hammer and by 100kg exciting force shakerrespectively. Synthesis and correction of the modalparameters are obtained from both testing methods. Designfaults of vibrating screen are determined based on theanalysis and dynamic correction of structure approachesabout the screen is put forward finally.

1. INTRODUCTION

Vibrating screen as principal equipment in fine coalpreparation industry is developing to large structure. Sinceexpensive price, lower working life and the maintenance costof the equipment failure, large vibrating screen productivecapacity is decreasing and resulted in the products costincreasing.

The kinds of failure in large vibrating screen consist of thefaults of cross members, the side plates and the dischargechute. In operating conditions, the vibrating screen isundertaken exciting force generated by exciters and materialfeeding and moving on the screening surface. Many naturalmodes are emerging and stronger alternating stresses are

formed on the components of the equipment. Cracks areemerged early and then developed the breakage of thestructure where stress concentration is emerged on thecomponents. These faults occurred generally on largevibrating screen.

According to the industrial criterion, a vibrating screenprototype was made based on theoretical analysis with FEM.Modal test is performed on this model and faults of designare found exactly.

2. MODAL TESTING

2.1 Model of vibrating screen and measurement points

Prototype of the screen is shown as figure 1. There are 159measured points described in figure 2.

2.2 Test with single exciting by impact hammer

Test system includes a Kistler 9726A20000 500g impacthammer, tri-axis accelerating sensor, data acquisitioning andanalyzing system, The test is carried out with single timeexcitation by impact hammer and response signals aresampled by tri-axis accelerating sensor sequentially frommeasurement points.

2.3 Test with random signal exciting by shaker

Test system consists of a HEV-100 shaker, a force sensorwith a charge amplifier, 15 single-axis accelerating sensor,16 channels signal analyzer and so on for signals processing.

Exciting point is placed on the drive module. 15 sensorsmeasure the response signals.

3. COMPARISON OF THE MODAL PARAMETERS FROM

TESTING DATA

3.1 determination of the frequency band

Operating frequency of vibrating screen is from 980rpm to1500rpm, I. e. 16.3Hz to 25Hz, generally is 16.3Hz a largeone. This prototype of screen is designed with drivingfrequency 980rpm. Considering run up and down, lowfrequency is from 0Hz. As exciting of material deeding andmoving on the screen surface, the exciting frequency band isvery width. Hence, It is reasonable that the band width is

determined from 0~200Hz according above facts.

3.2 Comparison of the modal parameters from two testmethods

3.2.1 Preferences of analytical parameters for modalextraction

z Analytical frequency bands: 0~200Hz;z Damping ratio: 0~10.0%;z Numbers of degree of freedom(NDOF): 1~50

3.2.2 Results of analysis of modal test data by impacthammer

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z 14 order modes, as shown as figure 3, were extractedfrom data that were acquired using hammer singleexciting. Stability of the modes is above 0.97.Damping ratios are shown in Table 1 between0.0115~0.0476.

Figure 1 Structure of Vibrating Screen Prototype

Figure 2. Layout of Response Pints andFirst Mode Shape by shaker

Figure 3. Mode Parameters for Hammer impacting Data

z Confirmed the modes by modal assurance criteria(MAC), the MAC values are accepted except for theMAC value of between mode 13 and mode 14 is 0.587.This indicated the independence of.modes

3.2.3 Results of analysis of modal test data by shaker

z 22 order modes, shown in Figure 4, were extractedfrom data that were acquired by shaker with randomtype of excitation. Stability of the modes is above 0.98.Damping ratios are .range from 0.0105 to 0.0534.

z Confirmed the modes by modal assurance criteria(MAC), the MAC values are accepted but for the MACvalue of mode 12 and mode 13 is 0.738, the MAC valueof mode 16 and mode 17 is 0.674, the MAC value ofmode 21 and mode 22 is 0.438,

3.2.4 Influence on the results of experimental modal analysisusing different testing methods

z Because of the insufficient energy generated by impacthammer, a number of modes are not excited on thisequipment. The prototype is larger for the hammer,which is another reason for lack modes. The number ofmodes, extracted from the test data obtained withrandom excitation by shaker, is more than by hammer.To reinforce the insufficient impact energy by hammerwe can use multiple impact exciting with hammer, which

is will be confirmed to do new test on later.zz Corresponding modal frequencies in both sets of mode

are good agreement, The correlative curve of bothgroups modes, shown in Table 2 and Figure 5, is oneline among direction 45

Table 1 Modal Parameters of Single Impacting Test

Figure 4 Modes Parameters of Estimation for RandomExciting by Shaker

Mode# Frequency(Hz) Damping (%) Stability

1 88.54 1.34 0.9922 118.95 1.65 0.9923 84.44 1.78 0.9924 129.09 1.42 0.990

5 14.70 3.58 0.9876 19.63 4.76 0.9877 161.38 1.73 0.9868 111.98 1.15 0.9869 176.73 1.48 0.986

10 61.04 2.08 0.98411 34.35 2.11 0.98412 146.87 1.33 0.98313 135.23 1.30 0.97914 171.31 2.43 0.974

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4. STRUCTRUAL DESIGN FAULTS OF THE PROTOTYPE

DETERMINED BY THE RESULTS OF THE ANALYSIS

4.1 Structural design faults of cross members and structuralcorrection design

Several kinds of stronger deformation of cross members are

at low frequency band, in which 1st

order mode shape (14Hz)

is vibrating up and down whole 8 members with greater

amplitude of vibration (see Figure 2 and Figure 6), 2nd

, 3rd

and 8th

order mode shapes (19Hz, 25Hz, and 62Hz) are

distortion of deformation of the whole members (see Figure 6,

Figure 9). 4th

order mode shape is expand and contract

deformation of screen body among the cross x-axis and the

front part of the side plate is of opposite phase with back part

(see Figure 6 and Figure 9). The others are tiny deformation.

Cross members structural correction design is to reinforce

the stiffness of the members, which can decrease the

amplitude of vibration, decline deformation of the structure,

reduce the alternating stress on the members and increase

fatigue life of the components.

4.2 Structural design faults of side plates and structural

correction design

The kinds of mode deformation of the side plates are among

the normal direction of its surface in addition to with vibration

of the cross members. The mode shapes that are projected

on the horizontal plane, are sine curves, cycles of the curves

are with range from 1 to 2.5. Deformation of side plates in the

middle part is greater than both ends even though the middle

part of the side plates consists of 3 layers steel plate by bolts

to joint together, which indicate a fact that is a stronger

exciting force on this part. This is a reason to result in fatiguefaults early. Structural correction design can use to increase

stiffness and amount of the brace members.

4.3 Structural design faults of brace members and structural

correction design

The deformation of brace members is great as shown in

figures, which indicate the structural behaviors are not good

agreement with the operating property of the screen.

Structural correction design can be also used to increase

stiffness of the brace members, which is agreement with side

plates to increase brace stiffness.

5. CONCLUSION

z Modal test method with impact hammer by single

exciting is not a good method for larger equipment

because using this method can not obtain complete

modes. The natural dynamic behaviors of the large

equipment can incompletely be excited for insufficient

exciting force. In order to obtain satisfactory results

from experimental mode analysis, using impact

hammer by means of multiple random exciting enhance

the insufficient impact energy or using proper shaker

with random or sine sweep excitation.

z

z Because the kinds of excitation include the operating

excitation by means of driver (exciter), feeding material

and moving the material on the screen surface, their

exciting frequency band is considerable width in

operating condition. Consequently, analytical frequency

band in the experimental mode analysis also should be

increase.

Figure 5 Correlation of modes from different test methods

Figure 6 First 4 orders mode shapes by hammer

Table 3 Compare Mode Frequency by Shaker with

by Hammer

Frequency (Hz) Frequency (Hz)Mode#

Shaker HammerMode

Shaker Hammer

1 14.17 14.70 12 109.422 19.18 19.63 13 110.37 111.98

3 25.62 14 118.15 118.954 34.15 34.35 15 128.69 129.095 46.31 16 135.51 135.236 48.73 17 146.62 146.877 60.60 18 159.23

8 62.58 61.04 19 162.13 161.389 82.80 84.44 20 172.18 171.3110 88.81 88.54 21 179.99 176.7311 102.48 22 191.61

0

50

100

150

200

250

Mode

Fre

uenc

b

ShakerHz

Mode Frequency by Hammer (Hz)

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Figure 7 5th, 6

th, 7

th, 8

thmode shapes by hammer

Figure 8 9th, 10

th, 11

th, 12

thmode shapes by hammer

Figure 9 2nd

, 3rd

, 4th, 5

th, mode shapes by shaker

Figure 10 Last 4 mode shapes by shaker

6. ACKNOWLEDGMENTS

The author wishes to thank his fellows Miss Yu Zheng, Mr.Ranfeng Wang, his teacher and the others. The researchwork is based on the financial support from Shanxi ProvinceScience & Technology Committee.

REFERENCE

[1] Zhenghao Wang, Study status of vibration screenstructure strength, Journal of Shinnying Arch. & CIF Eng.Inst. Vol. 15, No. 3, Jul. 1999

[2] Shanghais Wang, Gooey Wang, Present status anddeveloping tendency of vibration screen, Journal ofShinnying Arch. & CIF Eng. Inst. Vol. 15, No. 1, Pp.77~81, Pp. 278~281, Jan. 1999

[3] Up GAO, Wenham Cui, Strength Analyses on the SidePlate of Vibrating Screens, Journal of Anshan Institute ofI. & S. Technology, Vol.22 No.2 Pp.103~106, Apr. 1999