machine design, Vol.6(2014) No.4, ISSN pp

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1 machine design, Vol.6(2014) No.4, ISSN pp CONVEYOR IDLER S TURNING RESISTANCE TESTING METHODOLOGY Radivoje MITROVIĆ 1, * - Žarko MIŠKOVIĆ 1 - Milan TASIĆ 2 - Zoran STAMENIĆ 1 1 University of Belgrade, Faculty of Mechanical Engineering, Belgrade, Serbia 2 Tehnikum Taurunum, College of applied sciences,,belgrade, Serbia Original scientific paper Received ( ); Revised ( ); Accepted ( ) Absact: This paper describes the conveyor idler s turning resistance testing methodology developed at the University of Belgrade - Faculty of Mechanical Engineering, with examples of existing testing methodologies, relevant international standard and key features of newly developed testing machine and methodology based on analytical determination of conveyor idler s moment of inertia (using standard calculation procedures or available commercial 3D modelling software) and measurements of tested conveyor idler s rotational speed deceleration during the rotation stopping (without active forces acting on tested conveyor idler). Sample results of conducted tests are also presented and used as a basis for qualitative comparison of tested conveyor idlers Key words: conveyor idlers, idlers testing, turning resistance, qualitative comparison 1. INTRODUCTION Due to the lack of natural resources, one of the main priorities of 21 st century science is improvement of energy efficiency and reduction of overall power losses in all phases of elecic power production and ansport. The best place to cut energy losses in case of thermal power plants is at coal digging sites where necessary resources are being excavated. High percent of energy losses in elecical power production process is caused by ansportation systems - mostly belt conveyors and their key elements: conveyor idlers (rollers). The power consumption disibution in long belt conveyors (laid horizontally), given in percentage of power used to overcome particular resistances to the motion (of about 1.5km in length), is presented in Fig.1 [1]. Conveyor idlers, Fig.2, are assemblies commonly consisted of shaft, shell, pair of sealing groups and pair of rolling bearings, with main purpose to provide support for conveyor belt and ansported material (usually - coal), Fig.3. Very important characteristic of conveyor idlers is their turning (rotational) resistance, because it directly depends on level of damage of built-in rolling bearings Turning resistance of a single idler is defined as a tangential force applied to the roller shell in order to overcome the frictional resistance in the bearings and the seals. This component can be only experimentally determined and its boundary values are recommended by national and international standards. Taking into account that there are over conveyor idlers per standard open pit mine, it is clear that economic and energy savings due to optimal choice of the best conveyor idler for specific exploatational conditions are Fig.1. Conveyor power consumption for long horizontal profiles Fig.2. Typical conveyor idler assembly *Correspondence Author s Address: University of Belgrade, Faculty of Mechanical Engineering, Kraljice Marije 16, Belgrade, Serbia, rmiovic@mas.bg.ac.rs

2 Conveyor section Table 1. Samples of allowed values of measured turning resistance according to the DIN , 2, 3 Conveyor belt Conveyor idler diameter [mm] Max turning resistance [N] 133 2, ,6 Fig.3. Conveyor idlers working principle Conveyor garland However, many conveyor idler s turning resistance testing machines are developed with different working principles, other ways for load generation and various testing parameters than those in DIN Examples of those testing machines are shown in Fig.5 [2]. very significant and that s the main reason for development of new conveyor idler s turning resistance testing machine and methodology. Also, one of the purposes of the new methodology is to provide basis for qualitative comparison between conveyor idlers made by different manufacturers, new and repaired conveyor idlers, conveyor idlers with different s applied etc. However, many conveyor idler s turning resistance testing machines are developed with different working principles, other ways for load generation and various testing parameters than those in DIN Examples of those testing machines are shown in Fig.5 [2]. 2. REVIEW OF EXISTING CONVEYOR IDLER S TURNING RESISTANCE TESTING METHODOLOGIES AND MACHINES There are many different standards dealing with the conveyor idler s turning resistance (such as PN-91 M , SABS 1313 etc.) but the most relevant one for the area of Western Balkan is DIN , 2, 3 (used as a basis for relevant national standards). DIN defines conveyor idler s turning resistance testing machine (Fig.4), parameters of the testing and allowed values of measured turning resistances. According to the mentioned standard, at the beginning of the testing, conveyor idler should be worked out for at least 1hr by turning with rotational speed of 650RPM, under the affect of radial load of 250N and after that actual measurement should be conducted (using load cells or weight measuring systems). Allowed values of measured turning resistances in described experimental conditions are shown in Table Fig.4. Conveyor idler s turning resistance testing machine according to the DIN , 2, 3 Fig.5. Examples of conveyor idler s turning resistance testing machines

3 3. CONVEYOR IDLER S TESTING MACHINE DEVELOPED AT THE UNIVERSITY OF BELGRADE - FACULTY OF MECHANICAL ENGINEERING In order to build an installation for conveyor idler s testing, which would to be enhanced comparing to the existing test frames, a new testing machine was developed at the University of Belgrade - Faculty of Mechanical Engineering. A general view of developed testing machine is shown at Fig.6, along with the data acquisition system, specially designed to fulfill requirements necessary for appropriate measurements. Vibrations of idler s rolling bearings [mm/s]; Noise [db]; Conveyor idler s rotational speed [RPM]. It s important to mention that wide specum of working conditions could be simulated on developed testing machine, including load from 2-9kN, rotational speed between RPM etc. [3]. Also, by varying pressure in car wheel tires its possible to simulate uneven radial load disibution - similar to the exploitational loads acting on side idlers in conveyor garlands (Fig.7). F N F N /2 F N /2 CPU Vibration sensors Holder Magnets Fig.7. Conveyor idler s exploitational load disibution and conveyor idlers testing machine working principle 4. PRINCIPLE OF CONVEYOR IDLER S TURNING RESISTANCE TESTING METHODOLOGY Microphone Pressure sensor Temperature sensor Fig.6. Conveyor idler testing machine and data acquisition system developed at the University of Belgrade Faculty of Mechanical Engineering During the typical conveyor idler testing performed on newly developed installation, it s possible to measure, record and monitor following working parameters: Temperatures in idler s rolling bearings [ o C]; 4.1. Analytical approach During the regular work in exploitational conditions there are two moments acting on the conveyor idler: active moment M and reactive resistance moment M (Fig.8). Active moment is caused by friction between conveyor idler and conveyor belt with ansported material (radial load: F N ). If conveyor idler is separated from conveyor belt during the regular work, active moment equals zero (act no more) and reactive moment cause deceleration and 109

4 eventual stopping of conveyor idler s rotation. This situation is described analytically by following equations: partial moments of inertia are calculated and summarized; 110 Fig.8. Active and reactive moments acting on the conveyor idler in exploitational conditions Working state: I M (1) M Rotation deceleration: 0 I M M 0 M M (2) Where: I conveyor idler s rotational parts moment of inertia [kgm 2 ] ε angular acceleration [rad/s 2 ] Conveyor idler s rotational parts moment of inertia could be easily determined analytically or by commonly used 3D modelling software. Angular acceleration is determined by measurement of conveyor idler s rotational speed change during the deceleration time and by following equations: 2 d( n) d 60 2 dn ( ) dt dt 60 dt (3) Where: ω angular velocity [rad/s] n conveyor idler s rotational speed [RPM] Because developed conveyor idlers testing machine allows direct measurement of conveyor idler s rotational speed (~8 measurements per minute) its possible to indirectly determine reactive moments change in time, which has to be divided by conveyor idler s outer diameter in order to gain conveyor idler s turning resistance Determination of tested conveyor idler s moment of inertia Conveyor idlers rotational parts moment of inertia is commonly determined by: Analytical method conveyor idler s rotational parts are being approximated by cylinders (Fig.9) and Fig.9. Analytical method for determination of conveyor idler s rotational parts moment of inertia Numerical method - conveyor idler s rotational parts moment of inertia is being determined by available software for 3D modelling (Fig.10). Idler s shell Sealing group Fig.10. Numerical method for determination of conveyor idler s rotational parts moment of inertia It s important to mention that results of both methods were practically the same difference was less than 1% Testing procedure Bearing outer ring Material density Model mass Model volume Surface area Center of mass Moment of inertia 0,086723kgm 2 Step 1: tested conveyor idler should be mounted on the testing machine and worked out for at least 1hr, under the act of minimal radial load which can provide safe turning (without sliding). Rotational speed should be larger than 650RPM; Step 2: tested conveyor idler and car wheel tires should be separated as quick as possible by pressure regulation in pneumatic cylinders (Fig.11);

5 2π/60 and by predetermined value of conveyor idler s rotational parts moment of inertia, and after that divided by conveyor idler s outer diameter like in the following equations: 2 dn 2 df ( t) g( t) 60 dt 60 dt 2 2 d(0,4985 t - 48,507t +1165,5) 60 dt 0,1044t - 5,0796 Pneumatic cylinders M (0,1044t 5,0796) 0, ,0091 t - 0,4405 (5) Fig.11. Car wheel tires, tested conveyor idler and pneumatic cylinders Step 3: it must be secured that the data acquisition system is turned on in order to record rotational speed values, during the conveyor idler rotation stopping; Step 4: recorded rotational speed values should be presented graphically, for example in form of diagram shown at Fig.12; F 0,0091 t - 0,4405 0,1145 t - 5,5409 0,159 / 2 (6) Step 7: last equation represents conveyor idlers turning resistance change in time and should be graphically presented as shown on Fig.14 (saight line). Fig.14. Sample of conveyor idler s turning resistance change in time Fig.12. Sample of the recorded rotational speed values during the conveyor idler s rotation stopping Step 5: rotational speed values should be interpolated by polynomial line of second order, like in Fig.13: Fig.13. Sample of the measured rotational speed values interpolation with polynomial line of second order Step 6: generated polynomial equation should be differentiated with respect of time, multiplied by Step 8: from the last generated diagram value of turning resistance at 650RPM should be acquired and compared with the allowed values presented in the Table RESULTS AND DISCUSSION In total, 7 samples (conveyor idlers) were tested. Among them, two were brand new and five were repaired (using usual repairing procedure). Also, bearings from tested conveyor idlers were d by different types common indusial and modified by advanced additives. Experimental conditions were: At the beginning of testing, all samples were worked out for at least 2hrs; Testing radial load: 9kN Testing rotational speed: ~1120RPM Taking into account described experimental parameters, one hour of testing equals 20,2hrs of work in exploitational conditions. Results of performed testings are shown on Fig.15 and Fig

6 Table 2. Review of tested conveyor idlers Code Type Grease NVGM NVPM RVGM Modified indusial Common indusial Modified indusial New/ Repaired N N R Biult in bearings N o of samples Turning resistances of repaired conveyor idlers with improved bearing is up to 30% smaller than turning resistance of repaired conveyor idlers with common indusial bearing. Table 3. Measured idler s rotation stopping times (from 1120RPM to zero RPM) and turning resistances at 650RPM Sample Turning resistance [N] Conveyor idler s rotation stopping time [s] NVGM1 2,5 87 RVPM Common indusial R 3 NVPM2 3 N/A RVGM1 4,7 36 RVGM2 5,5 38 RVPM1 5,7 32 RVPM2 6,1 28 ACKNOWLEDGEMENTS Fig.15. Turning resistances of new conveyor idlers Results presented in this paper are realized within the Projects TR14033 and TR35029 so authors would like to express their sincere gratitude to the Minisy of Education, Science and Technological Development of the Republic of Serbia, as well as to the company Termoelekane i Kopovi Kostolac d.o.o. (Serbia). Fig.16. Turning resistances of repaired conveyor idlers Measured idler s rotation stopping times (during deceleration from 1120RPM to zero RPM) and turning resistances at 650RPM (for comparison with allowed values according to DIN 22112) are shown in Table 3. Based on the presented results, following conclusions were made: Repaired conveyor idlers have significantly worse characteristics than new conveyor idlers; Repaired conveyor idlers turning resistances are up to 2,3 times larger than turning resistances of new conveyor idlers (regardless the applied bearing ); Turning resistance of new conveyor idler with improved bearing is 22% smaller than turning resistance of new conveyor idler with common indusial bearing ; REFERENCES [1] Antoniak J. Resistances to the motion in mining belt conveyors, Acta Montanistica Slovaca, Vol. 6, No. 2 (2001), pp , ISSN [2] Gładysiewicz L.; Król R.; Bukowski J. Tests of belt conveyor resistance to motion, Maintenance and Reliability, No. 3 (2011), pp , ISSN [3] Miović R.; Stamenić Z.; Mišković Ž.; Tasić M.; Jovanović D. Installation for carrier roller idlers of belt conveyors testing on the open pit mining, Proceedings of the 7th International Scientific Conference IRMES 2011, Zlatibor, ISBN , pp , Mechanical Engineering Faculty - University of Niš, Serbia 112