ANALYSIS OF WORKS ON THE THEORY AND PRACTICE OF IMPACT GRINDING PROCESSES

​АНАЛИЗ РАБОТ ПО ТЕОРИИ И ПРАКТИКЕ ПРОЦЕССОВ УДАРНОГО ИЗМЕЛЬЧЕНИЯ

Жақыпбеков Бауыржан Серикулы

магистрант, Южно-Казахстанский университет им.М.Ауэзова,

Казахстан, г. Шымкент

Жумадуллаев Даулет Кошкарович

доктор PhD, старший преподаватель, Южно-Казахстанский университет им. М.Ауэзова,

Казахстан, г. Шымкент

Хаиров Ануар Нуриевич

директор ЮФ АО «НГСК КазСтройСервис»,

Казахстан, г. Шымкент

АBSTRACT

This paper presents an analysis of the theory and practice of impact grinding in order to determine the main factors that determine the process of impact grinding.  The analysis of works on the theory and practice of impact grinding allowed us to establish the main factors that determine the process of impact grinding and determine the directions for further experimental research of the developed structures of impact-centrifugal mills. The object of further research is the critical speed of impact destruction, as well as the impact loading of the material, which will depend on its mechanical and physical properties, and to a large extent, on the speed of impact loading and design features of the shredder. Therefore, taking into account theoretical research, more specific dependencies for determining the critical speed (), as well as for the dispersed composition of grinding products, should be found experimentally.

АННОТАЦИЯ

В данной работе представлен анализ теории и практики ударного шлифования с целью определения основных факторов, определяющих процесс ударного шлифования. Анализ работ по теории и практике ударного измельчения позволил установить основные факторы, определяющие процесс ударного измельчения, и определить направления дальнейших экспериментальных исследований разработанных конструкций ударно-центробежных мельниц. Объектом дальнейших исследований является критическая скорость ударного разрушения, а также ударное нагружение материала, которое будет зависеть от его механических и физических свойств и в значительной степени от скорости ударного нагружения и конструктивных особенностей материала. измельчитель. Поэтому с учетом теоретических исследований следует экспериментально установить более конкретные зависимости для определения критической скорости (), а также для дисперсного состава продуктов измельчения.

Keywords: impact grinding, factors, process, theoretical research, mechanical and physical properties.

Ключевые слова: ударное измельчение, факторы, процесс, теоретические исследования, механические и физические свойства.

In the process of impact in impact grinders, the particles of the crushed material and the working body interact. In the contact zone, high stresses develop and complex phenomena occur due to the fact that materials react to impact loads differently than under static loading. The different behavior of materials is a consequence of the manifestation of its dynamic properties of inertia and elasticity. Initially, the research was based on Newton's classical mechanics, according to which it is assumed that the bodies involved in the impact are absolutely rigid. Consequently, any force, tension, impulse applied to one part of the body, spread instantly throughout the body. This allows us to operate with point masses in calculations, that is, conditionally transfer the entire body mass to the center of gravity. According to the classical theory, with an elastic central impact of two bodies of the same material and equal masses, an exchange of velocities will occur. Consequently, with an elastic impact, the recovery coefficient will be equal  to unity. In works [1-2], the issue of impact is considered from the main provisions of the theory of elasticity and is based on the theory of Saint-Venant, which states that in any body, including a solid one, any load propagates with a finite speed, and therefore the impact is not an instantaneous phenomenon. , but proceeds in time. In this case, when the bodies collide, a complete or partial transition of the kinetic energy of the bodies into the energy of their deformation occurs.

According to the theory of elasticity [3, 4], stresses and strains propagate from the contact area in colliding bodies not instantly, but with finite velocities, and the propagation velocity of individual types of stresses is different. The magnitude, duration and distribution of the resulting forces are largely determined by the reaction of the material and the shape of the body to which the shock load is applied. The stress distribution caused by this load is localized and at the same time mobile. Stresses and strains move in the form of longitudinal and transverse waves. Longitudinal waves in the material propagate by compression and tension, and transverse waves in the form of shear movements.

The propagation velocity of a compression (tension) wave is determined by the formula:

where – modulus of elasticity of the material;

– material density.

The speed of propagation of shear waves is determined by the following relationship:

where – material shear module.

The speed of the primary longitudinal wave usually exceeds the propagation speed of the transverse wave by about 2 times [5].

 As a result of an impact, a very complex field of stresses arises in the body, changing not only from point to point, as with a static load, but at a given point with time. The stress field becomes even more complicated as a result of wave reflection from the boundaries of the body. Therefore, stresses and strains at a point must be considered as the sum of successive shock waves, such as longitudinal, transverse, surface (Rayleigh wave), etc., and waves reflected from the boundaries of the body. For these reasons, the general mathematical description of the impact process turns out to be so complicated that it goes beyond the scope of the modern theory of elasticity. To solve particular applied problems of impact theory, one has to apply simplifications and assumptions, but this can lead to unacceptable quantitative and qualitative errors. Therefore, the theoretical developments available in the literature [6-8] relate to materials of a strictly defined geometric shape (ball, rod), homogeneous in structure, constant in their properties and particle size distribution, which practically does not occur in the practice of impact grinding. Considering the foregoing, we can conclude that impact grinding is an extremely complex process and is less fully studied than the destruction of a body under static loading. The absence of a sufficiently fully developed theory of impact and the almost complete absence of the theory of impact grinding leads to the fact that the methodology for calculating impact grinding machines has not yet been created. In experimental studies of impact fracture, there are also great difficulties associated with the short duration of the process. As a result, it is difficult to determine the magnitude of the loads, the magnitude of the deformation, the time of interaction of the particle with the impact surface.

Experimental studies [9] show that the transformation of the impact kinetic energy into deformation work depends to a large extent on the duration of the impact. Many researchers tried to determine the impact time, and some of them obtained mathematical dependencies. For the most part, the dependences were obtained to determine the impact time of two balls or cylinders [6-10]. However, subsequently, the experimental studies carried out do not confirm the obtained dependencies even for these bodies of the simplest shape (ball, cylinder). However, most researchers note that the impact time should be as short as possible, and the impact intensity as high as possible. With an increase in the intensity of loading, the stresses in the deformable body increase significantly, and at the same time, its plasticity decreases [11]. The decrease in the plasticity of the material upon impact due to high stress concentrations reduces the work of deformation, which means that the energy intensity of destruction decreases [11].

A very important characteristic of the impact grinding process is the critical fracture rate (), that is, the rate of impact loading of a material particle, at which its guaranteed destruction will begin. This indicator () will determine the minimum rotation speed of the rotor with hammers or blades in almost all designs of centrifugal impact grinders. Practically all researchers working in the field of impact grinding have been engaged in theoretical and experimental studies to determine the critical rate of fracture onset (). Today, a large number of mathematical dependencies are known to determine , for example, the formula of Academician V.P. Goryachkin and Professor G.I. Pokrovsky [12].

Using a different theoretical approach, a similar dependence was obtained jointly with colleagues, academician of the Academy of Sciences of Ukraine V. N. Poturaev [12]. the above dependences can be represented in a general form:

However, the experimental results by definition given in [12] show that the real critical velocity is several times higher than that obtained from the theoretical dependences given in the above works. In the works of V. A. Bauman, B. V. Klushantsev, L. A. Glebov, F. G. Plokhov [12], in the formulas for determining (), the size of the initial material supplied for grinding is additionally taken into account. At the same time, P. M. Sidenko proves that the speed of the destructive impact does not depend on the size of the initial piece of material. There are also other works [12], where dependencies are given for determining the critical impact velocity (). In these formulas, additional parameters are also introduced, which cannot be found in the reference literature for a wide range of crushed materials.

Summing up the results of literature studies on determining the critical impact fracture rate, it should be noted that all researchers agree that the mechanical properties and density of the material are the determining factors, however, the design parameters of the grinder will also have a significant impact, which naturally should be taken into account on the basis of experimental studies. The dispersed composition of the material crushed by shock loading will also depend on its mechanical and physical properties, to a large extent on the speed of shock loading and the design features of the grinder. Therefore, taking into account theoretical studies, more specific dependencies for determining the critical speed (), as well as for the dispersed composition of grinding products, should be found experimentally.

An analysis of the works on the theory and practice of impact grinding made it possible to establish the main factors that determine the process of impact grinding and determine the directions for further experimental studies of the developed designs of impact centrifugal mills. As an object of further research, the critical rate of impact destruction, as well as the impact loading of the material, which will depend on its mechanical and physical properties, and to a large extent, on the impact loading rate and design features of the grinder, was chosen.

References:

1. Duda V. Cement / Pod.gen. ed. B. E. Yudovich. – M.: Stroyizdat, 1981. 464p.
2. Chirkun D.I., Levdansky A.E. Theoretical studies of a passing centripetal classifier. Abstracts of reports int. sci.-tech. Conf., Minsk, 27 – 29 Sept. 2005 / NPO Center. - Minsk, 2005. P. 65 - 68.
3. Chirkun D.I., Levdansky A.E. Classification of polydisperse powders in a centripetal classifier / Mater. int. sci.-tech. Conf., Minsk, 16 – 18 Nov. 2005 / BSTU. - Minsk, 2005. - P. 116 - 117.
4. Levdansky A.E. Sorting of bulk materials in gas-centrifugal and inertial-reflective classifiers: Diss. cand. tech. Sciences: 05.17.08. - Minsk, 1993. - 158 p.
5. Vasilenko P.M. The theory of particle motion on rough surfaces of agricultural machines. - Kyiv: Naukova Dumka, 1960. - 284 p.
6. Avdeev N.E. Centrifugal separators for grain. - M.: Kolos, 1975. -152 p.
7. Bashuk V.A. Study of the dust collection process using a new type of louvered inertial dust collector: Diss. cand. tech. Sciences: 05.17.08. - Lvov, 1973. - 144 p.
8. Pirumov A.I. Aerodynamic fundamentals of inertial separation. Moscow: Gosstroyizdat, 1961. 425 p.
9. Targ S.M. Course of Theoretical Mechanics. - M .: Higher School, 1998. - 416 p.
10. Yanovsky L.P. Influence of fluid pulsations on the average motion of a suspended particle // IFZh. - 1970. - v. 19. - No. 5.
11. Deich M.E., Zaryankin A.E. Hydrogasdynamics / M. E. Deych. – M.: Energoatomizdat, 1984. – 384 p.
12. Levdansky A.E., Levdansky E.I. Highly efficient flow processes and apparatuses. - Minsk: BSTU, 2001. - 236 p.