INCREASING THE OPERATIONAL LIFE OF BELT CONVEYOR PARTS USED IN THE MINING ORES TRANSPORTATION

ABSTRACT

A reasonable choice of functional materials to ensure optimal operating conditions for belt conveyors should be carried out based on the results of a system analysis, including structural, deformation-strength, corrosion-mechanical, materials science and technological aspects of creation and operation. The operation of conveyors in the mining industry of Uzbekistan is carried out under the influence of a chemically active environment, which accelerates the processes of metal corrosion and rubber aging on the spots of actual contact, the components of the roller – tape interface, which are maximally heated by friction to high temperatures. Along with fatigue phenomena occurring in the surface layers during volumetric deformation of the mating bodies, these processes have a significant impact on the wear intensity and operational life of the roller–tape friction pair. The obtained research results confirm the prospects of replacing metal in the manufacture of conveyor rollers with composite materials based on polymer and oligomeric matrices, since higher durability of the rubber-fabric belt is ensured under normal operating conditions and especially when the roller bearing is braked or wedged, when rolling with slippage or sliding friction is realized.

АННОТАЦИЯ

Обоснованный выбор функциональных материалов для обеспечения оптимальных условий эксплуатации ленточных конвейеров должен быть осуществлен, исходя из результатов системного анализа, включающего конструкционные, деформационно-прочностные, коррозионно-механические, материаловедческие и технологические аспекты создания и эксплуатации. Эксплуатация конвейеров в горнодобывающей промышленности Узбекистана осуществляется в условиях воздействия химически активной среды, которая ускоряет процессы коррозии металла и старения резины на пятнах фактического контакта, компонентов сопряжения «ролик – лента» максимально нагреваемых при трении до высоких температур. Наряду с усталостными явлениями, протекающими в поверхностных слоях при объемном деформировании сопрягаемых тел, эти процессы оказывают существенное влияние на интенсивность изнашивания и эксплуатационный ресурс пары трения «ролик – лента». Полученные результаты исследований подтверждают перспективность замены металла при изготовлении роликов конвейера композитными материалами на основе полимерных и олигомерных матриц, поскольку обеспечивается более высокая долговечность резинотканевой ленты при нормальных условиях эксплуатации и особенно при затормаживании или заклинивании подшипника роликоопоры, когда реализуется качение с проскальзыванием либо трение скольжения.

Keywords: conveyor roller, rock transportation, roller-belt pair, roller shells, conveyor belt, tribotechnical characteristics, polymer composite material, wood-polymer composite (WPC).

Ключевые слова: ролик конвейера, транспортирование горной породы, пара «ролик-лента», обечайки ролика, ленточный конвейер, триботехнические характеристики, полимерный композиционный материал, древесно-полимерный композит (ДПК).

Introduction. One of the directions of complex mechanization and automation of the process of mining ores, coal and non-metallic minerals is the creation and implementation of progressive in-line modes of transport that increase throughput, labor productivity and reduce production costs, as well as contribute to improving operational safety and working conditions. Currently, belt conveyors are the most efficient and high-performance type of conveyor transport. Their use in production technology makes it possible to optimize mining operations and increase the load on the face. For open-pit mining, belt conveyors in most cases most fully correspond to the technical and economic parameters of mining machines and allow the use of in-line and cyclic-in-line technology. In this regard, the problem of improving belt conveyors, which transport more than 50% of bulk products at different stages of the production cycle, is of great importance [6, 3-4].

The designs of belt conveyor elements used (roller supports, rollers, drums, belts) are largely determined by the operating conditions of the conveyor and significantly depend on the parameters of deformation and strength characteristics and granulometric composition of the transported mass. So, when moving a mass with a high specific gravity consisting of large fragments, tapes with a significant number of gaskets and a larger thickness of the working lining are used, rollers with increased outer diameter and axis diameter are used, etc. However, the problem of ensuring a given resource of these conveyor elements during the transportation of coarse rocks is far from an optimal solution, since in practice their premature failure occurs. There are cases when the service life of conveyor rollers transporting coarse-grained rock did not exceed 40-50 hours. The low durability of the rollers is associated with the use of roller supports that are not adapted to these conditions, as well as ignoring the significant factors acting on the support, which leads to the choice of rollers with a service life that does not correspond to the optimal operating conditions of the conveyor [7, p. 5-6].

The rollers are the most massive element of the conveyor belt. To select a roller when designing a conveyor, it is necessary to take into account the magnitude and nature of the loads, the width of the belt and the speed of its movement, the mode of operation of the conveyor, the properties of the transported cargo, the required durability of bearings and a set of wearing factors. However, taking into account all factors is difficult; therefore, the standard size of the rollers is usually chosen according to the parameter of the permissible load, which is determined based on the effective load on the roller, taking into account the specified bearing life, the characteristics of the conveyor operating conditions and the size of the pieces of rock. If the effective load is not adequately determined, the selected roller may have either insufficient or excessive durability, which is economically impractical. Therefore, an adequate assessment of the magnitude of the acting loads during the movement of coarse rock is an urgent task [16, p. 202-203].

Determining the effective loads on the rollers of the supports of belt conveyors transporting bulk cargoes presents certain difficulties. Firstly, because the loads largely depend on the speed and tension of the belt, the distance between the roller supports, that is, on the design parameters of the conveyor itself. Secondly, due to manufacturing, assembly and installation errors, the rollers of the supports and drums have pronounced eccentricities, which, under certain operating modes of the conveyor, cause significant fluctuations of the belt with the transported mass in the vertical direction and additional dynamic load on the conveyor elements. Thirdly, the flow of lumpy material entering the conveyor belt is characterized by unevenness and represents a statistical process. Therefore, the dynamic loads perceived by roller supports as a result of the action of a moving belt with an arbitrarily positioned mass must be described using random functions. It should be noted that while the task of determining the effective loads on the rollers of the supports has been considered in many studies, the other factors noted above have not yet been sufficiently studied and require additional research.

Methods. The analysis of studies devoted to the problem of increasing the operational life of mining equipment conveyor belts indicates the predominance of mainly computational, analytical and constructive approaches, without taking into account the operating characteristics that cause corrosion damage, abrasive and fatigue wear of structural elements that determine the total resource of the system.

It is obvious that a reasonable choice of functional materials to ensure optimal operating conditions for belt conveyors should be carried out based on the results of a system analysis, including structural, deformation-strength, corrosion-mechanical, materials science and technological aspects of creation and operation.

Research aimed at increasing the operational life of the tape–roller friction pair is of both scientific and practical interest. Due to the fact that it is difficult to improve the operational characteristics of the belt in the conditions of operation of mining enterprises, the main directions of solving this problem are: optimization of the design and parameters of the service characteristics of conveyor elements in contact with the surface layer of the belt; the use of shock-absorbing devices that reduce shock loads; development of methods and designs that ensure the centering of the tape; removal of adhesive particles of the transported cargo from its surface, etc.

The key place in the set of directions is occupied by the establishment of patterns of friction and wear of both mating elements and the development on this basis of methods to increase their wear resistance, as well as the study of the stress–strain state of the roller shell, the choice of its optimal geometric parameters and material.

Along with fatigue phenomena occurring in the surface layers during volumetric deformation of the mating bodies, these processes have a significant impact on the wear intensity and operational life of the roller-tape friction pair. Therefore, it is important to study more closely the features of the wear process of the roller and tape in conditions close to operational ones, and to assess the effect of the air-salt environment on the tribotechnical characteristics of this pair [1-15].

The simulation of the operating conditions of the friction unit "rubber-cloth tape - steel roller" was carried out on a friction machine SMT-1, which implements the "shaft - partial liner" contact schemes for sliding friction and "shaft - shaft" for rolling friction. Rolling slippage was determined by the difference in the diameters of the mating bodies rotating at the same frequency. The speed of relative movement of the rubbing bodies varied in the range of 0.25 - 3 m/s, which corresponds to the high-speed mode of operation of lifting and transport mining equipment. The nominal pressure P in the zone of contact of the belt with the roller (rolling friction) under normal conditions of rock transportation corresponds to 0.1 - 0.4 MPa, and in contact of the belt with the bottom and guides of the grate or with the table of plowshares (sliding friction) does not exceed 0.1 MPa. When transporting large-sized loads of rock, the short-term effective value of P can reach 4 MPa. In laboratory tests, the range of load changes is chosen to be wider, since when loading the conveyor and beating the rollers, the impact of a shock load is possible. Air and air-salt containing a mixture of sodium chloride and potassium chloride in a ratio of 1:1 were chosen as the working medium.

Results and Discussions. Studies have shown [1, p. 15-23; 2, p. 32-48] that in the load range corresponding to the normal operating conditions of the conveyor, the pressure value does not significantly affect the linear wear rate of the rubber sample (Fig. 1).

It should be noted that the data on the intensity of wear give an idea of the comparative wear resistance of a rubber sample during frictional interaction with rollers made of various materials, including polymer and composite. However, they cannot be used to calculate the durability of a rubber band, because unlike laboratory tests, a lighter thermal loading mode is implemented in operational conditions. The average surface temperature in the friction zone of the belt along the conveyor roller practically does not exceed the ambient temperature, since the time of heat generation (finding a fixed section of rubber in contact) is small compared to the time of its dispersion.

1 – in an air environment; 2 – in an air-salt environment; V = 1 m/s

Figure 1. Effect of pressure P on the linear wear rate of a rubber sample during rolling friction on a steel counterbody

The obtained research results confirm the prospects of replacing metal in the manufacture of conveyor rollers with composite materials based on polymer and oligomeric matrices, since higher durability of the rubber-fabric belt is ensured under normal operating conditions and especially when the roller bearing is braked or wedged, when rolling with slippage or sliding friction is realized. Moreover, intensive wear of the steel roller shell during jamming can lead to its complete wear (the thickness of the wear layer is equal to the thickness of the shell) and the formation of sharp edges that can destroy the conveying belt and exclude the possibility of its further operation (Fig. 2). Note that this type of destruction does not take place in the manufacture of shells made of polymer-based composites.

Modeling of the operating conditions of the tape-roller rolling friction pair showed that at a pressure of P = 1 MPa and a speed of movement V = 2 m/s, the average value of the linear wear intensity of the steel shell of the roller in the air corresponds to (0.45 ÷ 0.63)·10^-11. Under long-term operation in a corrosive environment (sodium and potassium chloride salts), this indicator takes on higher values (from 1.4·10^-11 to 2.3·10^-11, depending on the humidity of the environment). Thus, as a result of fatigue and corrosion-mechanical wear over 3 years of operation of the roller, the thickness of the worn layer of the metal counterbody will correspond to an average of 2 mm, which is half the thickness of the shell wall. With sliding friction, the I_h value of the shell increases by 10-20 times and averages 3 · 10^-10.

Figure 2. Characteristic damage to the steel shell as a result of wear in a corrosive environment during rolling friction (a), wedging of bearings and sliding friction (b) and damage to the conveyor belt (c)

The wear rate of the shell made of wood-polymer composite (WPC) during rolling friction in a corrosive environment is slightly higher than that of metal: I_h = (2.1 ÷ 2.3)·10^-11, despite the fact that WPC samples are not subjected to corrosion and mechanical wear. Due to sliding friction, due to a lower heat sink from the contact zone compared to steel, the wood-polymer composite wears out 1.3 – 1.4 times more intensively than metal (I_h = (3.8 ÷ 4.2)·10^-10).

Conclusion. Thus, replacing the steel shell of the roller with a wood-polymer one is advisable, since this significantly increases the durability of the conveying belt. However, the shell made of WPC should have a large thickness, or the composition of the composite and the technology of its processing should be optimal from the point of view of tribotechnical characteristics. It should be noted that laboratory model tests of the investigated friction pairs were carried out with a uniform load distribution along the length of the contact pad (along the shell formation), and the rollers made of steel and WPC had the shape of a solid cylinder.

Consequently, the durability and wear resistance of the roller were determined only by the processes occurring in the friction contact zone. In the actual operating conditions of the conveyor, the roller shell, which is a thin-walled hollow cylinder, experiences not only local contact deformations, but also deformations due to its stress-strain state as a loaded cylindrical shell. Multiple alternating deformations of the material during rotation under load of the shell lead to the development of fatigue processes in its volume.

Acknowledgments. The authors of this work express sincere thanks to Prof. Struk V.A. and his team and also Prof. Riskulov A.A. for their practical supporting in conducting experimental research and obtaining results.

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