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Since the design of China’s first fine-crushing, complexly swinging jaw crusher in 1979, within just 15 years, this type of crusher has been widely adopted and used in various industries such as cement and building materials across the country, with hundreds of manufacturers producing it. At present, this product has developed into a complete series. In addition, to meet various requirements, several new specifications have been developed. Our company has now become one of the domestic manufacturers that offers the largest range of such crushers as well as the highest quality products. Over the past few years, many scholars in China have conducted extensive research and discussions on determining the operational and structural parameters of jaw crushers. However, there has been little exploration into what differences should be considered in design for fine crushing (also known as two-stage or three-stage crushing) compared to rough crushing. This article aims to provide a preliminary exploration of this topic. I. Traditional design methods: In the past, the design of jaw crushers was based on traditional rock mechanics, which posits that the material within the crusher’s crushing chamber is subjected to forces such as compression, splitting, bending, and grinding caused by the reciprocating movement of moving jaws relative to fixed jaws. Theoretically, the material is crushed when the swing of the moving plate generates a compression deformation greater than that required for the material to be broken. In design, taking into account factors such as the bending of the plate, as well as the gaps between components like the working mechanism and the transmission mechanism, the amplitude of the moving plate’s swing chosen in practice is much larger than the theoretical value; this value is usually determined based on experience. This value is the main basis for determining the magnitude of the eccentricity of the eccentric shaft. Considering that the crushing chamber gradually narrows from top to bottom, while the filling degree of the material within the chamber increases in the same direction, reaching its maximum at the discharge outlet, a certain limit is imposed on the stroke of the moving parts at the discharge outlet in order to prevent excessive compaction of the material there, which could lead to damage to the machinery due to overload. Furthermore, given that the horizontal travel of the moving jaw gradually decreases from the feed port and discharge port toward the crushing chamber, it is appropriate to lower the positions of the elbow plates and the support points of the moving jaw slightly, in order to ensure that the upper part of the moving jaw has enough travel to crush large pieces of material, while still maintaining the required relationship between this travel distance and the position of the minimum discharge port. According to relevant test data, there is a mutually restrictive relationship between the rotational speed of the eccentric shaft and productivity as well as energy consumption; that is, there is an optimal rotational speed or a limit value for such speed. This value is recommended to be between 300–400 revolutions per minute (as it is also influenced by factors such as the characteristics of the moving parts, inertial forces, the physical properties of the material, and the working environment). In summary, in traditional design, the determination of operational parameters and structural parameters still relies primarily on the conventional crushing principles from rock mechanics, treating the material crushing process as one based on the mechanism of single-grain crushing. It focuses on the relationship between external forces and the crushing of individual materials, while ignoring the mutual crushing effect among the materials. II. Advances in the principles of crushing The material crushing process is a highly complex one in which the size of the material changes continuously, and it is closely related to numerous factors. Among those related to the material itself are its physical and mechanical properties, shape, size, and inherent defects; as for external factors, these include the particle size distribution of the material, the nature and magnitude of the applied force, as well as the environment and interactions surrounding the material at the moment of crushing. Rock mechanical mechanics takes these factors into account and explores them in two aspects: one is the physical properties of rocks, namely the relationship between their mechanical properties and the difficulty of crushing them; the other aspect concerns the laws governing the crushing process of rocks under external forces, as well as the external conditions involved. With the rapid development of science and technology and further in-depth research on crushing mechanisms, a new theory has gradually emerged on the basis of traditional rock mechanical mechanics – crushing physics. It **significantly expanded the scope of research in this field, bringing such theory gradually closer to the actual crushing process of materials. This theory mainly includes the following aspects: single-grain crushing and laminated crushing, selective crushing, and the crushing limit. This article provides a brief overview of single-particle crushing and layered crushing related to the topic. The representative theory for single-particle crushing—the fracture theory—was proposed by A. A. Griffith. Its main idea is that, under ideal conditions, if the applied force does not exceed the strain limit of the material to be crushed, the material will compress and undergo elastic deformation; when the load is removed, the material will return to its original state without being crushed. Given the presence of numerous fine cracks within solid materials, stress concentration occurs, which further expands the existing microcracks. It mainly describes several processes: the expansion of existing cracks due to stress concentration, the formation of numerous new crack sources, the development of surface cracks, and the creation of new surfaces as a result of particle fragmentation. Over the years, many scholars have used the aforementioned theories, based on material mechanics, to study and analyze the crushing process of materials. They have proposed various hypotheses regarding the relationship between the crushing process, the input energy, changes in the material’s potential energy, and changes in particle size – these are what are commonly referred to as crushing theories. As time passed and research progressed, it was found that the actual strength of materials during crushing is lower than the theoretical value, yet the energy required for crushing is higher than what is predicted theoretically. In single-particle crushing experiments, American scholar S. H. Bergström used special colloids to capture the fragments that were ejected as a result of crushing. By performing calculations, he determined the remaining crushing energy of those fragments and found that 45% of the crushing energy of the ejected fragments was not utilized. After conducting momentum-based experiments on single-grain crushing, West German scholar K. Schunert concluded that if the utilization rate of energy consumed in single-grain crushing is set at 100%, then no type of crusher exceeds 4000, whereas roller crushers can achieve a rate of over 70%. The above situations reflect, from different perspectives, the same trend: namely, the use of laminated crushing can improve energy efficiency. Laminated crushing is different from single-grain crushing. It can be understood as the difference between the crushing of a mass of material within the crusher’s crushing chamber and the crushing of individual particles; layered crushing takes into account, on top of the crushing of individual particles, the interactions between those particles or between different layers of material during the crushing process. According to relevant information, there is a quantitative threshold between laminar crushing and single-particle crushing: when the solid volume in the container accounts for 10%, it exhibits single-particle crushing behavior; whereas when it exceeds 45%, it shows laminar crushing behavior. Here, some sources suggest that a layer count of more than 6 is required to meet the conditions for laminated crushing. At present, the research on the mechanism of laminar crushing still needs further development and refinement. The following aspects generally form a consensus: (1) There is a limit to the probability of crushing in laminar crushing, which is also what distinguishes it from the crushing of single grains. Unlike single-particle crushing, where a particle is crushed as soon as the stress generated by the applied energy exceeds the particle’s limit stress, in the case of crushing a group of materials, the previously crushed fine particles can act as a buffer or protective layer for the larger particles, preventing them from being crushed and thus reducing the probability of crushing to a certain limit value. (2) To achieve the best crushing effect, the material must have a certain degree of looseness within the crushing chamber. There is a data alias with a value of 40%, and it is further suggested that a loose density of 16% after compaction is optimal. This means that the material undergoes a compression process during crushing, and this process is closely related to the porosity of the material mass. (3) The stress-strain of layer crushing is related to the shape of the applicator (or working mechanism). According to available information, during laminated crushing, when the stress reaches a certain limit value, the strain induced by carriers of different geometric shapes varies. Among them, plate-to-plate is the smallest, while sphere-to-sphere is the largest. (4) During laminated crushing, there is an appropriate specific energy consumption. According to available information, during the laminated crushing process, too large or too small a feeding rate can result in high energy consumption per unit mass; when there are 6–10 layers of material, the specific energy consumption is at its lowest. Applying load slowly increases the probability of crushing, thereby reducing specific energy consumption. Under the same input of work, energy utilization is higher when materials are in contact with each other in a plate-to-plate or pillar-to-pillar configuration. It is effective to unload the material pile at the right time during compression, and to reapply load promptly after the pile becomes loose, thus improving energy utilization. In summary, during laminated crushing, the stress conditions on w-grains differ from those during the crushing of individual particles. The stress conditions within the material layer are complex and depend on various factors, such as the characteristics of the applied load (the static or dynamic nature of the load, its intensity, and the duration of application), the particle size distribution of the layer, its packing structure, and its degree of looseness. Moreover, the efficiency of crushing the material layer is also related to the properties of the material itself (including its mechanical, physical, and geometric properties). To this end, in the design of crushers, corresponding adjustments must be made when determining the relevant operating parameters, structural parameters, and model type, based on the aforementioned characteristics. III. Laminated crushing and grinding – Type I crushers. As far as we know, there are many crushing devices that operate based on the principle of laminated crushing; these include fine-grinding jaw crushers, rotary cone crushers, inertial cone crushers and their industrial applications, as well as roller mills. The topic of this article is the further improvement of fine embedded crushers; therefore, only discussions related to this topic are presented. In the past, when designing crushers, consideration was given to the single-particle crushing mechanism. Additionally, for the purpose of simplifying calculations, the operating conditions of the crusher were set as shown in Figure 2. It is assumed that the input materials are in the form of spheres, arranged in a sequence of gradually decreasing size. Under these conditions, the point where the maximum crushing force acts is naturally located at 2/3 of the height of the crushing chamber. Through numerous practical tests related to TL, it was found that the actual operating conditions differ significantly from the theoretically established ELI values; this difference is even greater in the case of crushers. Based on the measurements of PI, -400, and the strength of multi-PEN crushers, the maximum crushing force actually acts between 1/2 and 1/3H of the crushing chamber (depending on the particle size distribution of the material within the chamber). Regarding the fine material (the material returned from the second-stage crushing or closed-circuit crushing screens), the material to be crushed is usually that which has undergone seven stages of primary crushing. It often contains a large amount of fine-grained material, while the real one. The proportion of large particles at 0.85B is relatively small; for example, when calculating the feeding screening characteristic curve of the PEX-150X 750 crusher using the arithmetic mean method, its average particle size is around SO mm, which is quite different from the maximum feeding particle size of 120 mm. As a result, its actual operating conditions differ significantly from those assumed theoretically (as shown in Figure 2), and the position of the maximum crushing force inevitably shifts downward. The aforementioned phenomenon confirms, from another perspective, that J represents a complex crushing process in which, during fragmentation, the material within the crushing chamber exists in a clustered geometric configuration, leading to interactions between particles – this is what is known as laminated crushing. Once it is clear that the fine-grinding crusher operates based on a layer-by-layer crushing mechanism, it becomes possible to have a better understanding of the differences between its operating parameters, structural parameters, and the selection of model versions compared to conventional designs. At least the following aspects require attention: (1) Determining the size of the moving particle’s stroke. Except at the feed end where the mechanism of crushing M particles per order must be considered (to ensure that the maximum feed particle size is broken down), the rest of the area should be treated according to the lamination crushing mechanism. (2) Therefore, the discharge end of the crushing chamber should also be considered as a laminated crushing process; the size of the discharge opening is not solely determined by the size of the product particles, as previously specified as S} ≤ 0.3–0.4d-. The constraint relationship requires re-evaluation. (3) When designing the crushing chamber, the first thing to consider is how to ensure that the material remains in a layered crushing state throughout the crushing process inside the chamber; that is, the looseness of the material should be maintained at around 40% to ensure optimal operating conditions. Secondly, to ensure the desired particle size of the product, the crushing chamber must be capable of supporting a sufficiently long crushing process. (4) Given the renewed understanding of the minimum discharge port size, in order to ensure the desired product particle size, it is necessary to reconsider the appropriate selection of the equipment’s rotation speed accordingly. IV. Selection of new models As mentioned above, the fine crushing 3A-type crusher, which is based on the laminated crushing mechanism, differs from traditional design methods in terms of the evaluation of its dynamic characteristics (rq). The lower travel distance of the moving component in the new model should not be directly determined by the particle size of the product or the size of the discharge opening; rather, it must meet the requirements for laminated crushing, that is, it is necessary to ensure that the material mass undergoes sufficient compression. Secondly, the stroke required for crushing due to the interaction between layers of material must also be taken into account; finally, it is necessary to ensure that the material mass is not over-compacted. Recently, some scholars have proposed a new indicator for evaluating the characteristics of jaw movement—the dynamic nip angle. They believe that improper handling of this dynamic nip angle is the main reason for the low processing capacity and severe wear of the tooth plates in existing reciprocating jaw crushers, and have therefore proposed new models with an optimal dynamic nip angle. As is well known, one of the structural parameters is the meshing angle. The determination is based on ensuring the normal operating conditions of the Type 30 crusher. The appropriate size of the bite angle should ensure that even with the largest pieces of material [D2]. At 8^0.85, the material to be crushed will not either be forced out of the crushing chamber. The current method for calculating the nip angle ensures a normal crushing process under various operating conditions, which is why it remains widely used to this day. It is true that in the lower part of the crushing chamber (within the 1/3H region), the intermeshing angle deviates significantly from the theoretical value. However, this does not have much practical significance. As mentioned earlier, the actual condition of the material inside the crusher’s crushing chamber is a complex crushing process characterized by interactions between particles within layers or groups of particles (i.e., layered crushing). The loading force is no longer applied to individual particles; therefore, the meaning of the intermeshing angle essentially disappears. What is shown in Figure 3 is strong evidence, and this goes back as far as 3. Before the New Year, we conducted comparative tests on various types of swinging motion, including horizontal movement, on an experimental pellet crusher. The moving jaw of a horizontally moving jaw crusher has a relatively fixed amplitude; it can perform approximately horizontal reciprocating movements, and there is not much difference in the engagement angles at the upper and lower parts of the crushing chamber. However, its actual processing capacity is much lower than that of the pendulum type, which from another perspective indicates that there is no substantial inherent relationship between the dynamic meshing angle and the crusher’s processing capacity. Therefore, when evaluating the advantages and disadvantages of the dynamic 3f motion characteristics, we still use the horizontal component Sx and the vertical component Sy of the particle’s movement path. In fact, over the long term, using these values to measure various types of swinging motions has proven to be fairly accurate in reflecting the actual performance. To make the selection of new models more reasonable, we consider several swing types currently used for crushing, or that can be employed for such purposes: (1) For various swing-type crushers, the stroke length of the moving parts at the feed end is still based on the assumption that the maximum feed particle size D is 0.8~0.85B. (2) Since the energy consumption of the crusher is proportional to the magnitude of the eccentricity, the eccentricity for various types is specified as a constant (r = 10 mm in the table). However, when its feeding stroke differs significantly from the requirements specified above, the eccentricity is adjusted to meet those requirements in sequence. (3) When various swing types are compared, their structural parameters [such as the shape and size of the crushing surface] are consistent and reasonable (such as the size of the drive angle and the position of the elbow plate support points). In traditional designs, since the stroke of the moving parts at the inlet and outlet points is actually limited, the s-average value and the characteristic value S/S are introduced in order to better evaluate the motion characteristics of these moving parts. In the 1950s, the All-Union Institute for Building and Road Construction Machinery conducted comparative tests on an experimental impact crusher, examining simple pendulum motion, compound pendulum motion (with positive and negative supports), and combined pendulum motion. In the early 1980s, the Luoyang Mining Machinery Research Institute carried out similar comparative tests with horizontal movement added as a factor. In all these performance comparisons, the s-average value and S/S were used to assess the motion characteristics of the moving parts, achieving very good results. Today, we follow this approach to evaluate the processed values (as shown in Table 2). To make the evaluation clearer, based on existing domestic and international experimental data as well as relevant sample records, the processing capabilities of each swinging pattern are arranged in a qualitative order and listed in Table 2 for comparative analysis. As mentioned above, the negative-support reciprocating jaw crusher exhibits superior motion characteristics of the moving jaw as well as a slightly higher processing capacity (some sources report the opposite, such as the test results from the All-Union Institute for Construction, Road Building, and Manufacturing). However, there are several issues that cannot be ignored: (1) With the extension line of the elbow plate and the rubber contact point on the moving jaw plate serving as the boundary, the movement trajectories of the upper and lower parts of the moving jaw are in opposite directions. This leads to obstruction during the crushing of materials, resulting in increased wear in the middle section of the jaw plate, which in turn affects the metal utilization efficiency of the jaw plate and the processing capacity of the equipment. (2) Due to the change in the force direction of the eccentric shaft, the split design of the frame bearing cover must be modified accordingly, which increases the difficulty of machining the frame. (3) With the same structural strength, its external dimensions and weight are not smaller than those of a simply supported compound pendulum. The negative support gyroscope was popular in many industries around the world during the 1950s and 1960s, but soon after its manufacturers decreased significantly in number. The occurrence of this phenomenon may be related to the fact that the benefits it brings are far outweighed by the existing problems. However, at present, some manufacturers have already begun producing larger-sized products in the 600 X 900 format. V. Design of the crushing chamber and other considerations The design of the crushing chamber plays an important role in the structure of a crusher. Its rationality will directly affect technical and economic indicators such as the equipment’s processing capacity, power consumption per unit, product quality, and metal wear of the tooth plates. The shape of the tooth plate (including the tooth profile), the size of the mesh angle, and the height of the cavity shape are the main factors that determine the crushing chamber. The designs regarding the shape of the tooth plates vary widely [usually, most are used in jaw crushers] and each has its own characteristics. In the 1960s, the All-Union Institute for the Manufacturing of Construction and Roadbuilding Machinery conducted comparative tests on more than a dozen different shapes of toothed plates, confirming that each shape had its own advantages and disadvantages as well as specific applications. However, for over 30 years, these results have not been widely adopted in production. The reasons for this are mainly as follows: (1) Certain tooth plates with special shapes can achieve relatively ideal results when first put into use, but as wear increases and the shape of the tooth plate changes, its performance declines significantly. (2) Some specially designed tooth plate shapes can achieve lower specific wear and better product quality, but they cannot be used in reverse. This results in a low metal utilization rate for the tooth plates, making the actual usage uneconomical. (3) Gears with special shapes can sometimes cause many difficulties in manufacturing. As mentioned earlier, the operating principle of the fine OH-type crusher is based on the mechanism of layered crushing, and its crushing chamber design must first meet the requirements of this mechanism. At the same time, requirements such as low energy consumption, high utilization rate, and ease of production for the street panels should also be considered. Here, it is recommended that the crushing chamber of the new model be composed of teeth plates with a small tooth angle, a deep chamber shape, and symmetrical curved shapes, which appears to be more reasonable. The discharge II dimension of the new model is no longer equivalent to that of a single product particle; rather, it should be considered as a condition resulting from the interaction between layers. Its size is clearly larger than the values assumed in traditional concepts; moreover, the large Sx stroke will cause the product particles to become coarser. To avoid this phenomenon, it is appropriate to increase the rotation speed of the eccentric shaft, thereby increasing the probability of particle back-and-forth movement (that is, reducing the discharge interval). Foreign sources report that the rotation speed of this type of crusher can reach 500–650 rpm. As the speed increases, the inertial forces acting during motion inevitably rise, which in turn requires adjustments to the stroke of the plate support or linkage system, so as to achieve an appropriate balance of forces. Due to the low hanging position of the inverted structure, the center of rotation drops, resulting in better stability of the entire machine. Therefore, this type is more suitable for use in mobile crushers. The inverted design presents new challenges related to material discharge adjustment and overload protection of the machine. How to address these issues without increasing the cost or complexity of the machine still needs to be explored and improved through practical experience. Additionally, the inverted design brings about some unique difficulties, such as the disassembly and assembly of moving parts, as well as the sealing and lubrication of bearings in the moving parts and the frame. All of these issues need to be properly resolved one by one in future operations.