Received 12 March 2008; accepted 15 July 2008 Corresponding author. Tel: +86-13805207270; E-mail address: hyfcumt@sohu.com

Dynamic characteristics of conveyor belts

HOU You-fu, MENG Qing-rui School of Mechanical and Electrical Engineering, China University of Mining & Technology, Xuzhou, Jiangsu 221116, China

Abstract: The dynamic characteristics of a belt conveyor are determined to a large extent by the properties of the belt. This paper describes experiments designed to establish the dynamic properties of belting material. The dynamic elastic modulus, viscous damping and rheological constants of the belt were measured. Several properties were studied as a function of the tensile loading on the belt. These included longitudinal vibration, the natural vibration frequency in the transverse direction and the response to an impulse excitation. Vibration response was observed under several different excitation frequencies. Most of these properties have not been tested previously under conditions appropriate for the ISO/DP9856 standard. Two types of belt were tested, a steel rein-forced belt and a fabric reinforced belt. The test equipment was built to provide data appropriate for designing belt conveyors. It was observed that the stress wave propagation speed increased with tensile load and that tensile load was the main factor influenc-ing longitudinal vibrations.Key words: experimental investigation; dynamic characteristics; conveyor belt

1 Introduction

Belt conveyors are, in most cases, the most cost-effective solution for handling bulk material mass flows over short and medium conveying dis-tances. The belt is a key component of these convey-ors and its dynamic characteristics determine the working performance to a great extent. At present, experimental research on the dynamic characteristics of conveyor belts is mainly concentrated on testing dynamic elastic modulus and viscous damping fol-lowing the ISO/DP9856 standard. Little research on other dynamic parameters has been carried out.

F. Langebrake et al. tested the breaking and splice strength of steel cord belts by using a large magnetic flux leakage tester[1]. Bla ej et al. tested the tensile strength of the belt and the strength parameters of the rubber used for the adhesive-bond joint in splices by using a ZP40 testing machine[2]. Hou et al. reviewed the experimental research work on the dynamic char-acteristics of the belt published over the past two decades. They considered that the test piece used in the previous research work, based on the ISO/DP9856 standard, was too small (50 mm×300 mm) to acquire reliable test data and suggested that a larger one should be adopted[3].

This paper describes the design and construction of an apparatus to investigate the dynamic character-istics of conveyor belting. Two types of belt, a fabric

belt and a steel cord belt, commonly used in coal mines were examined.

2 Experimental

2.1 Parameters

The main parameters studied by the experiments are:

1) The dynamic performance parameters of the belt.

2) The relationship between the stress wave propagation speed and the tension force on the belt.

3) The dynamic response characteristics of the belt under different tension forces and exciting frequen-cies.

4) The natural frequency of transverse vibration of the belt.

2.2 Methods The first step is pretreatment of the test piece,

which includes the measurement of its size and peel-ing two ends off for gripping. Then the test piece is installed on the test apparatus and kept in tension for 24 hours under a given tension force. Each individual test is repeated ten times and the average value is re-ported as the final test data.

1) By using the shock response method, the stress wave propagation speed C was found and used to

J China Univ Mining & Technol 18 (2008) 0629–0633

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Journal of China University of Mining & Technology Vol.18 No.4630

calculate the dynamic elastic modulus dE . Accelera-tion sensors were fixed at certain points to record the response signal. The stress wave propagation time t can be obtained by comparison of the signals of an impact force and the response signals picked up by the acceleration sensors. The stress wave propagation speed C can be calculated and dE is then given by

the equation dECρ

= , where is the density of the

belt. 2) The rheological constant τ , of the belt is ob-

tained by analyzing the shock and vibration signals from displacement and acceleration sensors mounted at the same place. The viscous dampingη , of the belt can be calculated by the relationship between τ ,ηand dE .

3) The response signals of the belt were tested un-der different tensile loads and exciting frequencies by mounting two displacement and two acceleration sensors at specified places and then analyzing the interaction between the respective signals.

4) The natural frequency for transverse vibration of the belt was identified by using a swept sine-wave excitation.

2.3 Apparatus

The data from the experiments carried out in this paper are intended to assist in engineering applica-tions. To acquire reliable test data, the apparatus is built to simulate a real belt conveyor. The main fea-tures of the apparatus are as follows (see Fig. 1):

Fig. 1 Experimental apparatus

1) The test piece is supported on carrying idlers, just as real conveyor belts are.

2) The distance between two carrying idlers is ap-proximately the same as in actual belt conveyors.

3) The test piece is placed horizontally so that the sag is similar to that of actual belt conveyors.

4) The test piece is tensioned by a screw nut. 5) The longitudinal exciting force is applied by a

vibration exciter.The signals were recorded with a TEAC MR 30

tape recorder and were analyzed with an HP3562A dynamic signal analyzer. The test pieces included fabric belts and steel cord belts commonly used in coal mines. The cross sections of the belts are shown in Fig. 2. The design parameters of the belts are given in Table 1.

Fig. 2 Cross section of the belts

Table 1 Parameters of the belts

Test pieces Belt type Length (m)

Width (m) Tensile strength

Steel cord belts ST1000 3.2 0.25 1000 N/mm

Fabric belts 800*4 3.2 0.25 70 N/mm/layer

3 Results

3.1 Propagation speed of a stress wave along the belt

The stress wave propagation time can be obtained from the recorded impact force and response signals. The stress wave propagation speed can be calculated from the time. The results are shown in Table 2. The data in Table 2 show that the stress wave propagation speed varies with the belt type and with tensile load. The speed increases nonlinearly with an increase in tension. Under lower tension the stress wave propagation speed increases more quickly. As the tension force increased above a threshold the stress wave propagation speed changed only slightly. The stress wave propagation speed in the steel cord belts is greater than that of the fabric belts for a given tensile load.

Table 2 Stress wave propagation speed under different tensile loads (m/s) Tension force (kN) 6 8 10 12 14 16 18 20

Steel cord belts 1107.5 1304.5 1534.8 1525.2 1538.7 1554.1 1500.6 1582.8

Fabric belts 835.4 946.9 1008.1 1073.7 1085.4 1106.6 1187.2 1147.7

3.2 Dynamic parameters of the belts

The dynamic elastic modulus of the belts can be calculated from dEC

ρ= . The results are shown in

HOU You-fu et al Dynamic characteristics of conveyor belts 631

Table 3. The rheological constant of the belts can be found by analyzing the acceleration and displacement from an impact; the results are shown in Table 4.

Viscous damping coefficients can be calculated using the relationship between dynamic elastic modulus, the rheological constant and the viscous damping; the results are shown in Table 5.

Table 3 Dynamic elastic modulus under different tensile loads (N/mm) Tension force (kN) 6 8 10 12 14 16 18 20

Steel cord belts 25734.4 35708.7 49416.2 48838.1 49674.3 50713.2 47250.6 52557.2

Fabric belts 2788.9 3579.6 4064.2 4605.3 4708.9 4892.9 5135.8 5262.4

Table 4 Rheological constants under different tensile loads (s) Tension force (kN) 6 8 10 12 14 16 18 20

Steel cord belts 0.207 0.171 0.151 0.155 0.149 0.147 0.145 0.140

Fabric belts 0.255 0.245 0.250 0.253 0.248 0.261 0.249 0.246

Table 5 Viscous damping under different tensile loads (N·s/mm) Tension force (kN) 6 8 10 12 14 16 18 20

Steel cord belts 5326.9 6106.1 7461.8 7569.9 7401.4 7454.8 6851.2 7357.9

Fabric belts 710.9 876.8 1016.0 1165.1 1167.5 1276.8 1403.1 1294.4

From Tables 3, 4 and 5, it can be seen that the dy-namic performance parameters of the belts vary over a large range as the tensile load changes. This indi-cates that because of viscoelastic behavior the dy-namic characteristics of the belt vary under different boundary conditions.

3.3 Natural frequency of transverse vibration of the belt

The natural, transverse, vibration frequency is that frequency where the response of the belt to a swept

sine wave excitation is greatest. The results of swept sine wave tests are shown in Table 6. The data in Table 6 show that the natural frequency for transverse vibration in the two types of belt in-creases slightly with an increase in the tensile force in a nonlinear way. The natural frequency for transverse vibration of the steel cord belt is greater than that of the fabric belt. According to belt transverse vibration theory, the steel cord belts are suitable for high speed belt conveyors.

Table 6 Natural frequency of transverse vibration under different tensile loads (Hz) Tension force (kN) 6 8 10 12 14 16 18 20

Steel cord belts 11.5 12.3 14.9 14.7 15.3 15.7 15.4 15.8

Fabric belts 9.8 11.7 12.2 11.8 12.1 11.9 12.3 12.2

3.4 Response characteristics under different ex-citing frequencies

Harmonic excitation was applied to the belt at dif-ferent frequencies (5 Hz, 10 Hz, 15 Hz, 20 Hz, 25 Hz and 30 Hz) under various tensile loadings. The longi-tudinal vibration of steel cord belts was measured. The results are shown in Figs. 3 and 4.

From Figs. 3 and 4, it can be seen that: 1) The basic frequency of the longitudinal vibra-

tion of the belt is the same as the exciting frequency. 2) Waveform in the time domain varies with the

exciting frequency under the same tension force. Higher-frequency harmonics decrease gradually with an increase of the exciting frequency. At an exciting frequency of 30 Hz the longitudinal vibration is close to a first harmonic waveform.

3) By comparing the longitudinal vibration wave-

form shown in Fig. 3 with the one in Fig. 4 under the same exciting frequency, it can be seen that the higher-frequency harmonic components are more obvious when the tension is greater. Different vibra-tion waveforms have different effects on dynamic stress. Higher frequency harmonics intensify vibra-tion of the belt and lead to increased dynamic stress. This is also the harmful effect from excessive tension. 3.5 Response to shock excitation

The vibration response of steel cord belts to an impulse excitation is shown in Fig. 5. It can be seen that the response decays exponentially, similar to the response characteristics of an elastic body. This indi-cates that the vibration characteristics of the belts are mainly determined by the elastic properties of the framework material of the belts.

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Fig. 3 Longitudinal vibration of the belt in response to different excitation frequencies, 10 kN tensile load

Fig. 4 Longitudinal vibration of the belt in response to different excitation frequencies, 20 kN tensile load

HOU You-fu et al Dynamic characteristics of conveyor belts 633

(a) Shock response: 5 kN load (b) Shock response: 10 kN load

Fig. 5 Response of steel cord belts to a shock excitation under different tensile loads

6 Conclusions

The following conclusions can be drawn from the results of the experimental investigation:

1) Stress wave propagation speed increases nonlinearly with an increase in tension in the belts. For the same tension force, the stress wave propaga-tion speed of the steel cord belt is greater than that of the fabric belt.

2) The dynamic performance parameters of the belts, including the dynamic elastic modulus, the rheological constant and the viscous damping, vary with tension force.

3) The natural frequency of the transverse vibration in the belts slightly increases with the tensile load in a nonlinear way. The natural frequency of the trans-verse vibration in the steel cord belt is greater than that in the fabric belt.

4) Tension force on the belt is the main factor that influences longitudinal vibration: The effect of exci-tation frequency is smaller. This indicates that more attention should be paid to controlling tensile loading in belt conveyor design.

5) Steel cord belts have the same response charac-teristic to shock excitation as an elastic body. This indicates that the vibration characteristics of a belt are mainly determined by the elastic properties of its framework material.

Acknowledgements

The authors would like to gratefully acknowledge Prof. Zhang Yong-zhong for his valuable contribu-tions.

References

[1] Langebrake F, Klein J, Gronau O. Non-destructive test-ing of steel-cord conveyor belts. Bulk Solids Handling, 1998, 18(4): 565–569.

[2] Bla ej R, Hardyg ra M. Modeling of shear stresses in multiply belt splices. Bulk Solids Handling, 2003, 23(4): 234–241.

[3] Hou Y F, Huang M, Zhang Y Z. Dynamic Performance and Control Technology of Belt Conveyor. Beijing: Coal Industry Press, 2004. (In Chinese)