Analysis of asymmetrical effect of tension forces in conveyor belt on the idler roll contact forces in the idler housing

Measurement 52 (2014) 22–32

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Analysis of asymmetrical effect of tension forces in conveyor belt on the idler roll contact forces in the idler housing Vieroslav Molnár a,⇑, Gabriel Fedorko a, Beáta Stehlíková a, Marianna Tomašková b, Zdenka Hulínová c a

Faculty of Mining, Ecology, Process Control and Geotechnology, Technical University of Kosice, Park Komenskeho 14, 042 00 Kosice, Slovak Republic Faculty of Mechanical Engineering, Technical University of Kosice, Letna 9, 042 00 Kosice, Slovak Republic c Faculty of Civil Engineering, Slovak University of Technology in Bratislava, Radlinskeho 11, 813 68 Bratislava, Slovak Republic b

a r t i c l e

i n f o

Article history: Received 13 January 2014 Received in revised form 15 February 2014 Accepted 24 February 2014 Available online 4 March 2014 Keywords: Tensioning of conveyor belt Description of rheological properties of the conveyor belt Influence of asymmetry on the belt position and on the belt damaging

a b s t r a c t This paper analyses an impact of asymmetrical tension forces, which are occurring in the conveyor belt, on the contact forces in the guiding idlers. It describes causes of an increased wear-out of the conveyor belts due to an asymmetric tensioning of the transport belt. There were investigated five measuring points loaded with a symmetrical and asymmetrical tensioning of a specimen section of the pipe conveyor transport belt, using the Friedman test for the factor of asymmetry. The tension force determines also changes of the contact force layouts in the individually investigated idler rolls. A newly developed method was applied in order to identify states of the particular experimental phases for the process of the transport belt tensioning and relaxation. This method is based on differences of the contact force time behaviours. It is possible to present a hypothesis, taking into consideration the obtained results that a side slipping of the running conveyor belt is caused by the tension force asymmetry also on the real operational conditions. Ó 2014 Elsevier Ltd. All rights reserved.

1. Introduction A belt conveyor is one of the most often applied transport equipment, which is used for transportation of the bulk solids [1]. The belt conveyor is such kind of the transport and handling machinery, which is specified to carry a loose material and it is employed in the various industrial branches, such as the mining industry, building industry, metallurgical industry, as well as in the seaports, thanks to its numerous advantages [2]. The tubular belt conveyor system (which is called also the ‘‘pipe conveyor’’) is a specific type of the belt conveyor with respect to its tubular shape and it is able to transport

⇑ Corresponding author. Tel.: +421 556023147; fax: +421 556028023. E-mail address: [email protected] (V. Molnár). http://dx.doi.org/10.1016/j.measurement.2014.02.035 0263-2241/Ó 2014 Elsevier Ltd. All rights reserved.

a large variety of the bulk materials. The pipe conveyor is a relatively important continuously working transport equipment applied in the modern intensive production, for example in the industry of metallurgy, industry of coal, building industry and in the other different industrial branches [3]. A safe and reliable operation of the pipe conveyors requires an application of the modern control system. The main purpose of design and development of the belt conveyor control system is to minimize a physical labour necessity, to reduce energy consumption, together with reduction of the required information and in this way to increase also the global efficiency of the conveyor operation, as well as to reduce an occurrence of accidents [4]. A correct functioning of the control system requires obtaining of the indicators that are able to predict a possible emergence of the crisis situation on time. These

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indicators should be able to evaluate also an optimal solution, which can lead even to a preventive shut down of the whole transport system. Petrikova et al. [5] presents an experimental and numerical investigation of the conveyor belt behaviour. The investigated belt is made from the carbon black filled rubber, reinforced by the plain weave textiles. The mechanical properties where investigated by means of the tensile tests with the different loading rates, together with the relaxation tests, whereas the friction coefficient was measured, too. Makowski and Zimroz [6] analysed new techniques of the local damage detection in a machinery based on a stochastic modelling, using the adaptive Schur filter. Hu and Li [7] studied a failure diagnostic strategy of the belt conveyor. They applied a special failure diagnostic method, which is based on the multi-sensor fuzzy information fusion. Prenner and Kessler [8] are dealing with the development of an energy recovery system for the belt conveyors. Czuba and Furmanik [9] performed an analysis of a grain motion in the transfer area of the belt conveyor. They presented a study of grain motion in a parallel chute and a methodology for calculation of the impact angle and tangential speed of the grain at the point of contact with the receiving conveyor belt. Yu et al. [10] published an investigation and comparison of a soft-start systems applied for the belt conveyors. It is possible to utilize various research and analytical methods during investigation process of the conveyor belt properties. Fedorko et al. [11] studied a failure analysis of the textile rubber conveyor belt, which is damaged by the dynamic wear. Zimroz and Król [12] examined the most frequent failures, types and the location of the failures together with their importance in the context of maintenance of a conveyor belt transportation system. Zhao and Lin [13] analysed two typical failure forms of the rollers and transport belt of the belt conveyor and they described the suitable maintenance methods for a prevention and for failure elimination in order to ensure the normal belt conveyor operation. Liu and Wang used the extenics theory to establish the matter-element model for the quality evaluation of the belt conveyor [14]. Pang and Lodewijks [15] applied the Finite Element Method (FEM) for a research of the transport belt characteristics in the case of pipe conveyors. This study presents a design of the pipe conveyor test rig and the testing results. The FEM was used in order to simulate the static behaviour of the belt, which is embedded on one idler set of the pipe conveyor. Fedorko and Ivancˇo [16] analysed the force conditions in a belt of the classic belt conveyor by means of the Finite Element Analysis method (FEA). Hu and Guo [17] utilized a new method of the virtual prototyping technology for the belt conveyor dynamic design in a connection with investigation of the conveyor belt properties. Rao et al. [18] investigated an influence of the constant cyclic strain loading on the fatigue and fracture behaviour of the steel cord/rubber composites. Kozhusko and Kopnov [19] studied the fatigue behaviour of three types of the fabric conveyor belt subjected to a shear loading. Hardygora and Golosinska [20] defined the relationships between the impact resistance of the belt and the design the measuring and analysing of the acoustic emission from a belt

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on different conditions inside an enclosure. Fazenda et al. [21] dealt with acoustic diagnosis of driving belt physical condition in enclosures. Bolden et al. [22] studied a structural impact damage under varying laboratory conditions. Luo et al. [23] researched and analysed the belt-conveyors using the purely mechanical belt-broken seizing machine. Zhang and Steven [24] dealt with a dynamical analysis. They monitored the conveyor’s starting and stopping behaviour in order to improve a design and reliability of the long overland pipe conveyor systems. Harrison [25] calculated a safety factor for the high-tensioned inclined belts using the NDT signal analysis. The important factors of conveyor belts are electronic expert systems described by various authors. Li et al. [26] dealt with an intelligent detection system for mine belt tearing based on machine vision. Wang et al. [27] researched on belt conveyor monitoring and control system. The fault detection and control of belt conveyor were done through the on-site sensors information signal collection by remote monitoring of belt conveyor and the motor protection. Li et al. [28] suggested automatic defect detection method for the steel cord conveyor belt based on its X-ray images. Xu et al. [29] designed the intelligent protection system of coal conveyor belt, which can improve intelligent degree of the whole burning coal transport and reduce the manual greatly and save a lot of funds for enterprise. Li et al. [30] tested online monitoring and fault diagnosis system for belt conveyors which can locate the position of the faulty idlers with a limited number of sensors, which is important for operating belt conveyors in practices. Górniak-Zimroz et al. [31] applied GISs to support belt conveyor maintenance management. They selected the GIS environment and the database standard, based on MS access, was dictated by the widespread use of the environments in the mines. Aldrich et al. [32] dealt with online analysis of coal on a conveyor belt by use of machine vision and kernel methods.

2. Material and methods of experiment 2.1. Problem formulation The transport belt of the pipe conveyor is a very costdemanding component involved in the framework of the whole transport system. This statement is confirmed by a fact that the high financial expenses are related not only with the purchase of a new transport belt, but also every process of the belt replacement requires an undesirable down-time of the given technology and in this way it causes the additional financial loses. Therefore the producers are developing new constructions of the transport belts with a sufficient resistance and durability in order to ensure the belt operational time as long as it is possible. This intention is also in the interest of the projection agencies and belt conveyor users. A more detailed analysis of the damaging processes and transport belt wearing during operation is an important question, which is relevant for each of the above-mentioned concerned parts.

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One of the most often occurring occasions that are causing an extensive wear-out of the transport belts of the pipe conveyors, is an asymmetric layout of the tension forces, which causes also changes of the contact forces on the guiding idler rolls in the idler housing. This process cannot be observed during the initial phase of the pipe conveyor operation. However, there is available an effective tool, namely it is application of a special testing equipment, which enables to perform a detailed monitoring of the process and recording of the individual data together with an experimental simulation of the various possible situations. The transport belt has to be tensioned during the pipe conveyor operation equally in order to transfer the circumferential driving force from the driving pulley to the transport belt and to ensure in this way a continuous material movement and transport. The equal arrangement of the tension force is a necessary condition for a correct transformation of the transport belt from the flat shape to the piped shape. The pipe conveyor transport belt, which is tensioned unequally, is exposed to the various kinds of the wearing process that are arising simultaneously. The first kind of the wear is a gradual change of the initially equal distribution of the belt mechanical characteristics in the belt cross section. A result of this wear-out process is an excessive stretching of the belt, which can lead to the dynamical wearing of the transport belt finally. The second kind of the wear, which is caused by the unequal tensioning, is a mechanical damage of the upper and bottom covering layer as well as mechanical damage of the belt side edge due to a cyclic repeating friction between the belt and the conveyor supporting steel structure installed along the transport way. If the initial signals advising this wearing process are neglected, this situation can result in a damage of the inner belt construction (Fig. 1). Such kind of the transport belt damage is a serious negative occurrence, which is able to cause a total destruction of the transport belt. Another consequence of the unequal tensioning of the conveyor belt is a side slipping of the belt from the driving and tensioning pulley during a current operation. Such phenomenon is able to cause a withdrawal of one or more idler rolls from the idler housing (Fig. 2) and if the belt is running further despite of this critical situation, the empty holder of the idler roller is able to cut up the belt totally (Fig. 3).

Fig. 1. Side edge wearing of the transport belt due to a friction between the belt and the steel construction as a result of the unequal tension force.

Fig. 2. The missing idler roller in the idler housing as a result of the unequal tension force.

Fig. 3. Detail of damaged rubber-covering layer of the pipe conveyor transport belt.

A damage of the pipe conveyor transport belt due to an unequal tensioning can be eliminated by means of a simple mechanical device. This mechanical device serves for returning of the slipping belt back to the correct position. However, this system is not reliable fully. From this reason it is suitable to apply also another system. It should be an automatic electronic expert system applied in the framework of the pipe conveyor current operation. The main task of this system is monitoring and adjustment of the transport belt position. It is based on a continuous monitoring of the selected idler housing characteristics. Change of the contact force values in the guiding rollers is one of the possible identification factors that are able to predict occurrence of an undesirable shifting of the transport belt. The contact forces are changing due to a mutual interaction among the transport belt and guiding rollers. An occurrence of asymmetric tensioning is a phenomenon, which is typical for all kinds of the belt conveyors operating with the rubber-textile or steel-cord conveyor belts. The primary consequence of the asymmetric tensioning is a gradual damage of the conveyor belt edge in all these cases. If this phenomenon is neglected, so the

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damaged zone is extending towards the middle of belt and the internal structure of the belt (textile structure or steelcord structure) is damaged. At the same time there is also a risk of a side slipping of the belt from the idler rolls or pulleys, which causes a stopping of operation, at least or there is necessary a replacement of the whole belt, at worst.

Table 1 Parameters of measurements. Marking of measurement

Description

Kind of asymmetry

Working notations

M1

Symmetrical tensioning Asymmetry, difference 1000 N Asymmetry, difference 1000 N Asymmetry, difference 2000 N Asymmetry, difference 2000 N

TF23 = TF24

TF23 = TF24

TF23 > TF24

TF23 > TF24

TF23 < TF24

TF23 < TF24

TF23 > TF24

TF23  TF24

TF23 < TF24

TF23  TF24

M2

2.2. Test equipment

M4

The 8 m long specimen of the transport belt was fixed into the test equipment, which was described closely in Molnár et al. [33] according to Figs. 1 and 2 with a designation of the idler housings and idler rolls.

M5

3. Theory/calculation The experimental analysis was divided into the five individual steps, taking into consideration a knowledge base created during a study of the damage process of the pipe conveyor transport belts. The individual steps are interconnected mutually in a sequence. Therefore it is necessary to perform these steps precisely in order to obtain the correct and accurate partial results. The procedure (Fig. 4) is based on a determination of the experiment final task and it corresponds to the rules valid for a design of

M6

experiments [34]. Some of the partial tasks had to be solved due to the test equipment characteristics. 3.1. Task of the experiment The main purpose of the experiment is to verify, whether a change of the transport belt tensioning causes also a change of the contact forces with regard to the symmetry or asymmetry of the tension forces. 3.2. Regimes of the experiment There were realised measurements using the parameters according to Table 1 in order to investigate an influence of the asymmetry during tensioning of the transport belt. (a) The set-up of the factor asymmetry is realised using tensioning in the test equipment positions marked ID23 and ID24 (This procedure is described closely according to Fig. 1 in Molnár et al. [33]). The abbreviation TF means the Tension Force – so, it describes character of the operation. (b) The determined values of the tension force were selected according to the experiments using the results published in Molnár et al. [35,36,33]. The tension force TFTotal, which is a sum of two forces in the given experimental equipment TFTotal = TF23 + TF24, was defined by the values 6000 N, 12,000 N, 18,000 N, 24,000 N and 30,000 N. (c) The data were recorded by means of the sampling frequency 10 Hz, thus every 0.1 s. (d) Two educated persons performed a change of the tension force independently. Each of the persons adjusted one tension force TF23 and TF24 on the values according to the items (a) and (b). The testing equipment remained 60 s without an intervention after each set-up process and afterwards the tension force was changed. A growing change of the tension force was realised in this way. 3.3. Data obtaining for the measured values analysis

Fig. 4. Procedure for analysis of tension force asymmetry in conveyor belt.

The facts known about the measuring equipment and the resulting consequences were taken into consideration for determination of a methodology specified for

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Fig. 5. Time behaviour of the contact forces and tension forces in the idler housing No. 2 for the tension force TFTotal 12,000 N, 18,000 N and 24,000 N.

evaluation of the factor asymmetry influence. The belt tensioning was performed manually by two service persons simultaneously. From this reason it was not possible to keep a fully synchronisation of the time behaviour during the process of the tension force increasing. The model – test equipment is a static one and after achieving of the required tension force the system is stabilised. Jump movements of the transport belt on the idler rolls are typical for this stable state. A ‘‘relaxation creep’’ of the transport belt material is another important attribute. The both aspects are influencing the contact forces. Fig. 5 presents the time behaviour of the contact forces and tension forces in the idler housing No. 2 for the tension force TFTotal 12,000 N, 18,000 N and 24,000 N. The horizontal axis detects the absolute time of the experiment. There are two vertical axes: the axis of the contact forces in the idler rolls ID7–ID12 is on the left and the axis of the tension forces TF23 a TF24 is on the right. Fig. 5 represents end of endurance with the tension force defined at the value TFTotal = 12,000 N for the kind of asymmetry TF23  TF24. The next phase is tensioning. The vertical line ‘‘Line 1’’ defines a time, which was necessary for achieving of the tension force TF23 – it was reached the maximum value before the endurance. Tensioning at the TF24 was not finished in this time. The following time interval in the graph illustrates the endurance phase with the adjusted tension force value 18,000 N. It is possible to see a decrease of the TF23 and TF24 during this phase. The time behaviour of the contact force in the ID11 is decreasing during the endurance phase (blue colour) and the time behaviour in the ID12 is increasing in a polynomial form. The measuring process continues during the next tensioning phase. The beginning of this phase is marked with the vertical line ‘‘Line2’’, which enables recording of a delay in the position TF23 in comparison to the position TF24, as well as monitoring of reactions from the tensioning, i.e. the change of the contact forces in the positions ID7–ID12.

It was necessary to determine a methodology for a selection of data obtained from the measured contact forces M1–M6 for every tension force in order to evaluate the factor asymmetry: (a) In the same phase of the experiment. (b) In the same time from beginning of the relaxation phase. (c) For the same tension force. 3.3.1. The same phase of the experiment The data used for analysis are obtained from the relaxation phase with regard to the defined task of the experiment. 3.3.2. The same time from beginning of the relaxation phase This time was determined up to 30 s. The first vertical line in the graph (Line 1) – i.e. the end of the tensioning phase is the time point 248.1 s in the absolute time of the experiment. The second vertical line (Line 2) – i.e. the end of the relaxation phase and beginning of a new tensioning phase is the time point 310.2 s in the absolute time of the experiment. So, the difference between the ‘‘Line 1’’ and ‘‘Line 2’’ is approx. 60 s. The analysed data are coming from the middle part of the relaxation phase. This brief analysis confirms a complying with the experimental methodology by the service persons. It is not possible to determine exactly a start point of the relaxation phase according to the records obtained from the measurements and determination of the start point using the graph is time demanding and subjective. From this reason it was proposed a method of the phase analysis based on the differences between successive values. This method is founded on a logical assumption that during the tensioning phase a difference between two successive values of the tension forces is a positive number. The evaluated tension force value is the TFTotal. A calculation process, which used the given sampling frequency, was not able to determine the boundary of

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the tensioning phase uniquely. From this reason the difference between the successive values was calculated using the step 1–25 in the time interval from 233.9 s to 248.9 s in the absolute time of the experiment. This modification simulated a reduction of the sampling frequency. There was also specified a number of negative differences for each step during the given time interval – i.e. a number of wrong signals concerning finishing of the tensioning phase. The shortest possible step D = 13 was determined from the calculations and the wrong signals about finishing of the tensioning phase were eliminated in this way. Fig. 6 illustrates a reduction of negative differences in the tensioning phase (transition of the curves below the x-axis) if the difference step is increasing. The step D = 15 (1.5 s) and the number 10 of successive negative signs of the signals were selected in order to determine the end of tensioning phase During the time between two tensioning phase, i.e. approx. in the middle of interval, the tension force differences are oscillating about the zero level for all steps and the system is stable sufficiently. This fact confirms a suitability of the adopted decision to use the data obtained in the time 30 s after the relaxation phase beginning. 3.3.3. The same tension force The largest problem was to meet a requirement of the same tension force during the measuring process. It was impossible to keep the same tension force on the given

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conditions. From this reason the factor of asymmetry was evaluated using only a relative method taking into consideration results from the previous calculations Molnár et al. [36,33], that documented a dependence of the contact force values on the tension forces values, as well as mutual relations among the contact forces. There were applied 100 successive values of the contact forces in each of the positions ID1–ID18 for the every defined tension force in the given time of the relaxation phase. The arithmetic mean for each kind of asymmetry and for each position is a basis for the next calculations. The idler housing No. 1 with the positions ID1– ID6, the idler housing No. 2 with the positions ID7–ID12 and the idler housing No. 3 with the positions ID13–ID18 were evaluated individually, whereas the sum of contact forces in the every idler housing = 1. A ratio of the contact force in each of the idler housing positions was defined as a new variable, which is evaluated for the every kind of asymmetry. Calculation of a new variable is presented in Table 2 and in Fig. 7, where the contact force [N] distributions are summarized and the contact force ratios in the individual positions ID1–ID18 are arranged, as well. 3.4. Selection of statistical method for influence evaluation of the factor ‘‘tension force asymmetry’’ and its application 3.4.1. Determination of the factor evaluation criteria An impact of the factor is considered to be relevant, if the change of tensioning symmetry causes a change of

Fig. 6. Time behaviour of negative difference reductions in the tensioning phase with an increasing step of difference.

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Table 2 Contact force [N] distributions and contact force ratios in the individual positions ID1–ID18. TFTotal determined at 24,000 N, Measurements M1 TF23 = TF24 Position ID1 ID2 Average contact force [N] 354 169 Ratio 0.167 0.080

ID3 375 0.177

ID4 223 0.105

ID5 370 0.175

ID6 624 0.295

Sum No. 1 2116 1.00

Position Average contact force [N] Ratio

ID7 73 0.096

ID8 35 0.045

ID9 119 0.155

ID10 192 0.250

ID11 171 0.223

ID12 176 0.230

Sum No. 2 767 1.00

Position Average contact force [N] Ratio

ID13 158 0.739

ID14 107 0.501

ID15 80 0.372

ID16 71 0.331

ID17 27 0.128

ID18 33 0.154

Sum No. 3 214 1.00

Fig. 7. Contact force ratios in the individual positions ID1–ID18.

the contact force distributions in the positions of the idler rolls in the idler housing. The ratios for each of the measurements M1–M6 were determined and the matrixes of the measured results were created consequently for each of the positions ID1–ID18, as it is presented in Table 3 for the position ID7. The corresponding values between Tables 2 and 3 are marked with italics. 3.4.2. Statistical test and its evaluation The matrix of results has an internal structure of a double classification with one result for each combination of the factor levels. It is possible to say, taking into consideration method of the matrix obtaining that application of a

non-parametric test was more suitable. The configuration meets a requirement concerning the data arrangement for the Friedman test. The groups were created by means of the defined factor asymmetry of the TF23 in relation to the TF24. The block factor was the tension force. In the case that the null hypothesis about a conformity of the group distribution functions will be neglected, it will be realized a multiply comparison according to the Nemenyi method [37]. The calculations were realised using the same input data matrixes, together with the block factor asymmetry and with the group factor tension force. 4. Results

Table 3 Matrix of the measured results for the position ID7.

Table 4 presents results of the statistical hypothesis testing.

Measurements at ID7 Tension force [N]

M1

M2

M4

M5

M6

TFTotal = 12,000 TFTotal = 18,000 TFTotal = 24,000 TFTotal = 30,000

0.080 0.089 0.096 0.104

0.092 0.094 0.103 0.108

0.092 0.087 0.097 0.108

0.104 0.097 0.104 0.114

0.103 0.090 0.099 0.108

4.1. Evaluation of the statistical test results 4.1.1. Factor of asymmetry The obtained results proved a fact that if the tension forces are not symmetrical, so the contact force

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V. Molnár et al. / Measurement 52 (2014) 22–32 Table 4 Results of the statistical hypothesis testing. Position

Test criterion Friedman test for factor asymmetry M1, M2, M4, M5, M6

Set-up of asymmetry for statistically relevant difference

Test criterion Friedman test for factor tension force TFTotal = 12,000 N, TFTotal = 18,000 N, TFTotal = 24,000 N, TFTotal = 30,000 N

Set-up of tension force for statistically relevant difference

ID1

13.4

M5–M6

14.04

ID2 ID3 ID4 ID5 ID6

3 12.6 8.8 7.8 13

ID7

12.4

ID8 ID9

7.4 7.4

15 11.88

ID10 ID11 ID12 ID13 ID14 ID15 ID16 ID17 ID18

7.6 4.6 5.4 7 6.2 12.2 8.2 11 8.6

11.16 8.28 6.12 15 15 15 15 15 15

12,000 N–30,000 N 12,000 N– 24,000 N 12,000 N–30,000 N 12,000 N–30,000 N 12,000 N–30,000 N 12,000 N–30,000 N 12,000 N–30,000 N 12,000 N– 24,000 N 12,000 N–30,000 N 18,000 N– 30,000 N 12,000 N–30,000 N 12,000 N–30,000 N 18,000 N– 30,000 N 18,000 N–30,000 N 12,000 N–30,000 N

Critical value

8.80

M5–M6 M4–M5

15 15 15 15 11.88

M1–M5

12.12

M5–M6

M1–M6 M1–M6

12,000 N–30,000 N 12,000 N– 30,000 N 12,000 N–30,000 N 12,000 N–30,000 N 12,000 N–30,000 N 12,000 N–30,000 N

7.8

distributions are changing only in several positions from the group ID1–ID18. In the case of the idler housing No. 1 influenced are the positions ID1, ID3 and ID6. A statistically relevant difference in the distribution function, concerning a population of selections, was recorded for levels of the factor M5–M6, i.e. for the opposite asymmetry of the type TF23  TF24 and TF23  TF24. A similar situation is in the position ID6 also between the levels M4 and M5, thus TF23 < TF24 and TF23  TF24. Fig. 8 illustrates a change in distribution of the contact forces in the position ID7 due to the factor of asymmetry. In the case of the idler housing No. 2 the factor of asymmetry was relevant statistically only for the position ID7 and between the levels M1 and M5, i.e. TF23 = TF24 and TF23  TF24.

In the case of the idler housing No. 3 the factor of asymmetry was relevant statistically for the positions ID7 and ID17, between the levels M1 and M6, thus TF23 = TF24 a TF23  TF24. 4.1.2. Factor tension force The results confirmed a finding that a change of the tension force causes a change of the contact force distributions in the positions ID1–ID18, excepting the position ID12. The limit values TFTotal = 12,000 N and TFTotal = 30,000 N are situated among the results of the statistically important differences between the individual levels. Fig. 9 illustrates a change of the contact force distributions in the position ID7 due to the tension force factor. Figs. 10 and 11 describe differences caused by the change of tensioning symmetry for the TFTotal = 24,000 N.

Fig. 8. Change in distribution of the contact forces due to the factor asymmetry.

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Fig. 9. Change in distribution of the contact forces due to the factor tension force.

Fig. 10. Differences caused by the change of tensioning symmetry for the asymmetry 1000 N.

Fig. 11. Differences caused by the change of tensioning symmetry for the asymmetry 2000 N.

A difference of the contact force ratios between the asymmetric and symmetric set-up of the tension forces was applied in order to present the change. The horizontal axis

represents a state for the symmetric distribution M1 TF23 = TF24. The changes caused by the change of symmetry are depictured in the graph. The changes for the

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asymmetry 1000 N are in Fig. 10 and the changes for the asymmetry 2000 N are in Fig. 11.

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of research project VEGA 1/0258/14 Study of input parameter relations for interoperable transport efficiency based on mathematical model application.

4.2. Summarization of results The results of statistical testing confirm a truth that a change of the tension force asymmetry causes also a change of the contact force distribution. Another factor, which influences the change of the contact force distributions, is a tension force value. Concurrence of both these factors cannot be confirmed or rejected according to the realized analysis. 5. Conclusions The transport belt installed in the pipe conveyor, which is tensioned unevenly, is subjected to the various kinds of wearing simultaneously. The symmetrical distribution of the tension force is an unavoidable condition of a correct transformation of the belt from the flat shape into the cylindrical shape. The main task of the described experiment was to investigate, whether a change of the belt tensioning, the symmetry or asymmetry of the tension forces, causes a change of the contact forces. The achieved results emphasize a reality that the asymmetrical loading due to asymmetry of the tension forces influences distribution of the contact forces. Layout of the contact forces is affected also by the tension force values. It is possible to postulate a hypothesis that a deviation from the direct belt running, i.e. the belt slipping, is caused by the asymmetry of the tension forces also on the real operational conditions. The experimental results pointed out a difference in the contact force distributions in the case of limit values of the tension forces 12,000 N and 30,000 N. A new method was developed for identification of the experiment phase (tensioning and relaxation), which is based on differences of the contact force time behaviours. The obtained results create a new information base, which can be used for online monitoring of the operational parameters of the pipe conveyor. One of the online monitoring purposes is prediction of a damage possibility for the transport belt or prediction of a possible operational condition worsening. It is necessary to develop a new electronic measuring system in order to perform the online monitoring of the pipe conveyor current operation. This system can be developed using also the presented experimental results and it will mean an important contribution to increasing of the operational safety, as well as reliability of the pipe conveyor. The next research should be focused on investigation of the asymmetry effect during tensioning of the transport belt, which is conveying a material. Acknowledgments This work is a part of research project VEGA 1/0922/12 Research of effect of material characteristics and technological parameters of conveyor belts on size of contact forces and resistance to motion in pipe conveyors with experimental and simulation methods. This work is a part

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