A preparation method for controlling magnet properties through molding process

Abstract

The present invention discloses a preparation method for controlling the performance of a magnet through a molding process, and belongs to the technical field of sintered NdFeB permanent magnet materials. S1. The upper punch assembly moves upward to a preset powder adding position on the mold, and then quantitatively adds sintered NdFeB powder to the mold; S2. The upper punch assembly moves downward to a preset micro-lowering position on the mold; S3. The lower punch assembly moves downward to a preset pre-pressing position on the mold to pre-press the sintered NdFeB powder in the mold; S4. After the pre-pressing of the sintered NdFeB powder is completed, the sintered NdFeB powder in the mold is magnetized and oriented; S5. After the magnetization and orientation operation is completed, the upper punch assembly and the lower punch assembly move synchronously toward the middle of the mold to perform bidirectional pressure on the sintered NdFeB powder; S6. After the pressing of the sintered NdFeB powder is completed, a demagnetization operation is performed. This technical solution is used to solve the problem of uneven distribution of powder in the mold cavity when pressing sintered NdFeB powder.

Description

Technical Field

[0001] The invention belongs to the technical field of sintered NdFeB permanent magnet materials, and particularly relates to a preparation method for controlling magnet performance through a molding process. Background Art

[0002] Sintered NdFeB permanent magnets are widely used in electronics, electromechanics, communications, instrumentation, medical, and military fields due to their excellent comprehensive magnetic properties. However, traditional preparation methods often suffer from poor magnet consistency and unstable performance during the molding process, which affects product quality and performance. Controlling the performance of sintered NdFeB through the molding process is a key step in the sintered NdFeB preparation process. The molding process determines the magnet's geometry, size, and orientation, which directly affect the magnet's ultimate performance.

[0003] At present, the manufacturing process of sintered NdFeB materials mainly includes the steps of permanent magnet powder preparation, pressing and sintering. The pressing step is mainly as follows: sintered NdFeB powder is added to the mold cavity, scraped flat, and an orientation current is applied. The upper and lower mold rods are used to press the powder in the orientation magnetic field to prepare rare earth permanent magnet materials. However, due to the magnetic field, the magnetic field at the two poles in the mold cavity is higher than the central magnetic field. This will result in more powder and less powder in the center, resulting in the final performance not being optimized. Summary of the Invention

[0004] In view of this, the purpose of the present invention is to provide a preparation method for controlling the performance of magnets through a molding process, so as to solve the problem in the prior art that when pressing and sintering NdFeB powder, there is more powder at the two poles and less powder in the center of the mold cavity, resulting in poor performance after pressing and molding.

[0005] In order to achieve the above object, the present invention provides the following technical solutions:

[0006] The present invention provides a preparation method for controlling the performance of a magnet through a molding process. A pre-pressing position, a powder adding position, and a slight reduction position are sequentially provided from the upper surface to the lower surface of the mold, and a lower pressing head assembly is provided above the upper surface of the mold, and an upper pressing head assembly is provided below the lower surface of the mold.

[0007] The preparation method comprises the following steps:

[0008] S1. The upper pressing head assembly moves upward to the preset powder adding position on the mold, and then quantitatively adds sintered NdFeB powder into the mold;

[0009] S2. After the sintered NdFeB powder is added, the upper pressure head assembly moves downward to the preset micro-lowering position on the mold;

[0010] S3, the lower pressure head assembly moves downward to the preset pre-pressing position on the mold to pre-press the sintered NdFeB powder in the mold;

[0011] S4. After the pre-pressing of the sintered NdFeB powder is completed, the sintered NdFeB powder in the mold is subjected to magnetization and orientation operation;

[0012] S5. After the magnetization and orientation operation is completed, the upper and lower pressure head assemblies move synchronously toward the middle of the mold to perform bidirectional pressure on the sintered NdFeB powder;

[0013] S6. After the sintered NdFeB powder is pressed, demagnetization operation is performed;

[0014] S7. After demagnetization is completed, the upper and lower pressing head assemblies move upward synchronously to push the sintered NdFeB powder out of the mold.

[0015] Furthermore, the pre-compression density of the sintered NdFeB powder is 2.5-2.8 g / cm 3 The compressive density of the sintered NdFeB powder is between 3.9-4.1 g / cm 3 between.

[0016] Furthermore, in step S1, after the sintered NdFeB powder is added, a triangular fork is used to evenly insert it into the mold cavity four times.

[0017] Furthermore, the upper pressure head assembly includes a lifting hydraulic rod and an upper pressure plate, and the lower pressure head assembly includes a telescopic hydraulic rod and a lower pressure plate. In step S2, when the upper pressure head assembly moves down to a slightly lowered position, a hammer assembly is provided on the upper pressure head assembly to knock and compact the sintered NdFeB powder in the mold.

[0018] Furthermore, four groups of hammer assemblies are evenly distributed on the outer circumference of the mold, and the hammer assembly includes a symmetrically arranged light rod, one end of the light rod is provided with an end plate, and the other end of the light rod is provided with a retaining ring, and the retaining ring is arranged close to the jacking hydraulic rod, the light rod is provided with a slider, a first spring is provided between the slider and the baffle arranged on the outside, the first spring is sleeved on the light rod, and the slider is provided with a vertical plate, one end of the vertical plate is fixed to the slider, the other end of the vertical plate is located on the outside of the middle and lower part of the mold, and the other end of the vertical plate is provided with a knocking block, and the knocking block is connected to the other end of the vertical plate, and at least one wedge block is provided on the part of the vertical plate located at the lower end of the mold, and a mounting block is provided on the output rod of the jacking hydraulic rod, and the middle part of the mounting block is threadedly connected to the output rod of the jacking hydraulic rod, and baffles are provided on the sides of the mounting block facing the wedge block, one end of the baffle is connected to the mounting block, and the other end of the baffle is arranged towards the wedge block, and the projections of the baffle and the wedge block in the vertical direction overlap.

[0019] Furthermore, a groove is provided on the side of the mounting block on which the baffle is set, and a gap is provided between the end of the groove and the upper surface of the mounting block. A rotating shaft is provided in one end of the groove facing the upper surface of the mounting block, and one end of the baffle is rotatably connected to the rotating shaft.

[0020] Furthermore, a fixing plate is provided on the upper surface of the mounting block, one end of the fixing plate is fixedly connected to the mounting block, and a second spring is provided on the other end of the fixing plate, and both ends of the second spring are respectively fixed to the fixing plate and the baffle.

[0021] Furthermore, the knocking block is detachably connected to the end of the vertical plate.

[0022] Furthermore, the knocking block is made of rubber.

[0023] Furthermore, the wedge blocks in the four groups of hammer assemblies are arranged at different heights.

[0024] The beneficial effects of the present invention are:

[0025] (1) The present invention optimizes the molding process to make it standardized and quantifiable, and to achieve stable performance. At the same time, the coercive force can be increased or the remanence can be slightly reduced to increase the coercive force through process control. The operation method is simple, low-cost, and easy to promote and apply.

[0026] (2) This technical solution improves the density of the powder through the pre-pressing process, that is, the powders at the center are in a tightly squeezed state, which effectively suppresses the problem of the center powder moving to the two poles due to the fact that the magnetic field at the two poles in the mold is higher than the central magnetic field strength under the subsequent magnetic field. Therefore, it effectively avoids the phenomenon that the powder in the center is less and the powder at the two poles is more during the final pressing, and effectively improves the unevenness of the powder density distribution. At the same time, the coercive force can be increased without affecting the residual magnetism through pre-pressing, and deformation can be reduced and dimensional consistency can be improved.

[0027] Other advantages, objectives and features of the present invention will be described in the following description and will be apparent to those skilled in the art to some extent, or those skilled in the art can be taught from the practice of the present invention. The objectives and other advantages of the present invention can be realized and obtained through the following description. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] In order to make the purpose, technical solutions and beneficial effects of the present invention more clear, the present invention provides the following drawings for illustration:

[0029] Figure 1 Schematic diagram of the pre-pressing position, powder adding position and slight reduction position set on the mold of the present invention;

[0030] Figure 2This is a three-dimensional schematic diagram of a hammer assembly provided on the outside of the mold in the present invention;

[0031] Figure 3 It is a schematic cross-sectional view of a hammer assembly provided on the outside of the mold in the present invention;

[0032] Figure 4 A schematic diagram of the glass body of the hammer assembly of the present invention;

[0033] Figure 5 For the present invention Figure 4 A local enlarged schematic diagram of point A in the middle.

[0034] The following are marked in the accompanying drawings:

[0035] a. Upper surface of the mold; b. Pre-pressing position; c. Powder adding position; d. Slightly lowering position; 1. Telescopic hydraulic rod; 2. Lower pressure plate; 3. Lifting hydraulic rod; 4. Upper pressure plate; 5. Mold; 6. Mounting plate; 7. End plate; 8. Polished rod; 9. Slider; 10. Retaining ring; 11. Vertical plate; 12. Wedge block; 13. Knocking block; 14. Mounting block; 15. Groove; 16. Baffle; 17. Rotating shaft; 18. Fixed plate; 19. Second spring; 20. First spring. DETAILED DESCRIPTION

[0036] like Figures 1 to 5 As shown, the present invention provides a preparation method for controlling the properties of a magnet through a molding process, wherein a pre-pressing position b, a powder adding position c, and a slight reduction position d are sequentially provided from the upper surface a to the lower surface of the mold, and a lower pressing head assembly is provided above the upper surface a of the mold, and an upper pressing head assembly is provided below the lower surface of the mold 5;

[0037] The preparation method comprises the following steps:

[0038] S1. The upper pressing head assembly moves upward to the preset powder adding position c on the mold 5, and then quantitatively adds sintered NdFeB powder to the mold 5;

[0039] S2. After the sintered NdFeB powder is added, the upper pressure head assembly moves downward to the preset micro-depression position d on the mold 5;

[0040] The advantages of setting the micro-lowering position d are: (1) lowering to the micro-lowering position d is conducive to positioning the mold 5 in the middle during orientation, controlling the molding position, and improving the service life of the mold 5. That is, when molding at the same location, if the mold 5 is damaged, the molding position can be changed by changing the micro-lowering position d, thereby avoiding the damaged part and improving the service life; (2) lowering to the micro-lowering position d is conducive to the self-flow of the powder. Following the micro-lowering process, the powder distribution will be more uniform.

[0041] It should be noted that in this solution, a represents the mold top surface, and the pre-press position b is calculated based on the set pre-press density. The distance between the upper and lower cylinders during pre-pressing can be calculated based on the powder weight, cavity size, and pre-press density, thus setting the pre-press position b. c is adjusted based on actual powder addition and is determined after all powder is added. d is set by changing the molding position based on the mold conditions. The pre-press density is calculated from the distance from the pre-press position b to the micro-dip position d, the cavity size, and powder weight. The powder addition height c is adjusted based on actual conditions. The abcd values for different molds are adjusted based on the pre-press density and the actual mold conditions.

[0042] S3, the lower punch assembly moves downward to the preset pre-pressing position b on the mold to pre-press the sintered NdFeB powder in the mold 5; it should be noted that when the lower punch assembly moves to the pre-pressing position b, the pre-pressing step has been completed.

[0043] S4. After the pre-pressing of the sintered NdFeB powder is completed, the sintered NdFeB powder in the mold 5 is magnetized and oriented. The magnetization and orientation operation is a prior art and will not be described in detail in this technical solution.

[0044] S5. After the magnetization and orientation operation is completed, the upper and lower punch assemblies move synchronously toward the middle of the mold 5 to perform bidirectional pressure on the sintered NdFeB powder. The upper and lower punch assemblies move at the same speed to extrude the sintered NdFeB powder, that is, the density of the sintered NdFeB powder after extrusion will be relatively uniform, and the pressing effect will be better.

[0045] Specifically, the magnetic field strength is 1.5-2T, the applied pressure is 10-15MPa, the pressing movement speed is 20-25mm / s, and the density after pressing is 3.9-4.1g / cm3.

[0046] S6. After the sintered NdFeB powder is pressed, demagnetization operation is performed;

[0047] S7. After demagnetization is completed, the upper pressing head assembly and the lower pressing head assembly move upward synchronously to push the sintered NdFeB powder after compaction out of the mold 5.

[0048] In one embodiment, the pre-compression density of the sintered NdFeB powder is 2.5-2.8 g / cm 3 The compression density of sintered NdFeB powder is between 3.9-4.1g / cm 3 between.

[0049] In one practicable manner, in step S1, after the sintered NdFeB powder is added, a triangular fork is evenly inserted into the mold cavity four times to preliminarily eliminate defects such as powder bridging and improve the subsequent pressing effect.

[0050] In order to further illustrate the influence of controlling the pre-compression density on the Br and Hcj of the magnet, N52 and N38MT are selected for specific implementation. During the manufacturing process, the smelting, hydrogen crushing and air flow grinding processes are the same as normal, and only the molding process is changed. Through the new molding process, the normal molding process generally corresponds to a pre-compression density of 2.2. The test is carried out on the same powder and the pre-compression densities of 2.5, 2.6, 2.7 and 2.8 are tested in the same time sequence. After molding, they are subjected to 200MPA isostatic pressing and sintered in the same environment to ensure consistency except for the pre-compression density. The experimental results are as follows:

[0051]

[0052]

[0053] From the above test results, it can be seen that in the magnet with the brand N52, the values of Br and Hcj increase with the increase of pre-pressing density within the pre-pressing density range; but in N38MT, when the pre-pressing density is 2.5 and 2.8, the values of Br and Hcj of the magnet increase more, and when the pre-pressing density is 2.6 and 2.7, the values of Br and Hcj of the magnet increase more. It can be seen from Examples 5 and 6 that when the pre-pressing density is too large (exceeding the range in this technical solution), the values of Br and Hcj will decrease instead.

[0054] In summary, in the preparation method provided by the present invention, by increasing the pre-pressing density, the coercive force can be increased by 0.5 without basically reducing the remanence, and at the same time, the coercive force can be increased more under the premise of slightly reducing the remanence, that is, the technical solution can fine-tune the performance of the blank according to the pre-pressing density. Specifically, the fine-tuning object is to only increase the coercive force or slightly reduce the remanence to increase the coercive force.

[0055] In one practicable embodiment, the upper ram assembly includes a lifting hydraulic rod 3 and an upper pressing plate 4, and the lower ram assembly includes a telescopic hydraulic rod 1 and a lower pressing plate 2. In step S2, when the upper ram assembly moves downward to the slightly lowered position d, a hammer assembly is provided on the upper ram assembly to tap and compact the sintered NdFeB powder in the mold 5. After the sintered NdFeB powder is added to the mold 5, it may have pores or be less dense, which may subsequently affect the performance of the pressed blank.

[0056] In one practicable manner, four groups of hammer assemblies are evenly distributed on the outer circumference of the mold 5, and the hammer assemblies include symmetrically arranged polished rods 8, one end of the polished rods 8 is provided with an end plate 7, the end plate 7 is fixed to the edge of the mounting plate 6, the other end of the polished rods 8 is provided with a retaining ring 10, the retaining ring 10 is arranged close to the jacking hydraulic rod 3, a slider 9 is provided on the polished rod 8, a first spring 20 is provided between the slider 9 and the retaining plate 16 arranged on the outer side, the first spring 20 is sleeved on the polished rod 8, a vertical plate 11 is provided on the slider 9, one end of the vertical plate 11 is fixed to the slider 9, and the other end of the vertical plate 11 is located on the outer side of the middle and lower part of the mold 5, and A knocking block 13 is provided on the other end of the vertical plate 11, and the knocking block 13 is connected to the other end of the vertical plate 11 and is arranged toward the mold 5. At least one wedge block 12 is provided on the portion of the vertical plate 11 located at the lower end of the mold 5, and a mounting block 14 is provided on the output rod of the jacking hydraulic rod 3. The middle part of the mounting block 14 is threadedly connected to the output rod of the jacking hydraulic rod 3, and a baffle 16 is provided on the side of the mounting block 14 facing the wedge block 12. One end of the baffle 16 is connected to the mounting block 14, and the other end of the baffle 16 is arranged toward the wedge block 12. The projections of the baffle 16 and the wedge block 12 in the vertical direction overlap.

[0057] It is not difficult to understand that the length of the baffle 16 is preferably able to enter the interior of the mold 5, which is convenient for the later pushing operation; of course, if the length of the baffle 16 is long, then the length of the output end of the lifting hydraulic rod 3 can be increased. The working principle of the above technical solution is: when the lifting hydraulic rod 3 moves downward, the baffle 16 contacts the inclined surface of the wedge block 12, thereby pushing the wedge block 12 to move outward, thereby compressing the first spring 20. When the baffle 16 and the wedge block 12 are disengaged, under the action of the first spring 20, the slide block will be quickly pushed back, thereby driving the knocking block 13 at the upper end of the vertical plate 11 to knock the outer surface of the mold 5, thereby causing the mold 5 to vibrate, and then drive the sintered NdFeB powder therein to vibrate, thereby achieving the purpose of vibration compaction. At the same time, the vibration of the mold 5 can also cause the residual sintered NdFeB powder on the inner side of the mold 5 to fall off, and then merge with the sintered NdFeB powder at the bottom, further improving the later pressing effect.

[0058] Of course, in this technical solution, when the jacking hydraulic rod 3 moves upward, it is necessary to rotate the mounting block 14 so that the baffle 16 avoids the wedge block 12 to avoid movement interference, that is, through the threaded connection, it is also possible to choose whether the hammer assembly is required as needed.

[0059] In one practicable embodiment, a groove 15 is provided on the side of the mounting block 14 on which the baffle 16 is set, and a gap is provided between the end of the groove 15 and the upper surface of the mounting block 14, that is, the groove 15 is not connected to the upper surface of the mounting block 14, but is connected to the small surface of the mounting block 14. A rotating shaft 17 is provided in one end of the groove 15 facing the upper surface of the mounting block 14, and one end of the baffle 16 is rotatably connected to the rotating shaft 17. It is not difficult to understand that there should be no contact between the upper surface of the baffle 16 and the end of the groove 15 to avoid interference between the end of the baffle 16 and the end face of the groove 15 when the baffle 16 rotates downward.

[0060] That is, the baffle 16 can rotate downward, but due to the obstruction of the inner wall of the upper end of the groove 15, the upward rotation angle is small. Therefore, with this arrangement, the baffle 16 can squeeze the wedge block 12 to move. At the same time, during the upward movement, after the two are separated, the baffle 16 automatically rotates upward, thereby avoiding motion interference during the upward movement.

[0061] In one feasible manner, a fixing plate 18 is provided on the upper surface of the mounting block 14, one end of the fixing plate 18 is fixedly connected to the mounting block 14, and a second spring 19 is provided on the other end of the fixing plate 18, and the two ends of the second spring 19 are respectively fixed to the fixing plate 18 and the baffle 16. The advantage of setting the second spring 19 is that the baffle 16 is always in a parallel state, that is, when the baffle 16 moves upward, it contacts the wedge block 12, overcomes the resistance of the spring and rotates, and then moves to the top of the wedge block 12. After moving up, the second spring 19 resets, driving the baffle 16 to reset, and then when moving downward, it can act on the wedge block 12. That is, this setting method can automatically avoid motion interference and push the wedge block 12 without human intervention.

[0062] In one practicable manner, the knocking block 13 is detachably connected to the end of the vertical plate 11 , and can be set to different heights as needed to knock on different heights of the side surface of the mold 5 .

[0063] In one practicable manner, the knocking block 13 is made of rubber to avoid the noise generated by metal collision and to protect the mold 5 .

[0064] In one feasible manner, the wedge blocks 12 in the four groups of hammer assemblies are arranged at different heights. The advantage of different heights is that the hammering timings of the four hammer assemblies are staggered, that is, the four hammer assemblies contact the mold 5 at different times, so that the mold 5 can generate multiple vibrations, thereby improving the compaction effect of the stripping box. At the same time, it can also avoid the problem of multiple hammer assemblies working simultaneously from different directions, resulting in the vibration being offset.

[0065] Finally, it should be noted that the above preferred embodiments are only used to illustrate the technical solutions of the present invention and are not limiting. Although the present invention has been described in detail through the above preferred embodiments, those skilled in the art should understand that various changes can be made in form and details without departing from the scope defined by the claims of the present invention.

Claims

1. A preparation method for controlling magnet properties through a molding process, characterized in that: A pre-pressing position (b), a powder adding position (c) and a slight lowering position (d) are sequentially provided between the upper surface (a) and the lower surface of the mold, and a lower pressing head assembly is provided above the upper surface (a) of the mold, and an upper pressing head assembly is provided below the lower surface of the mold (5); The preparation method comprises the following steps: S1, the upper pressing head assembly moves upward to the preset powder adding position (c) on the mold (5), and then quantitatively adds sintered NdFeB powder to the mold (5); S2. After the sintered NdFeB powder is added, the upper pressure head assembly moves downward to the preset micro-depression position (d) on the mold (5); S3, the lower pressure head assembly moves downward to the preset pre-pressing position (b) on the mold to pre-press the sintered NdFeB powder in the mold (5); S4, after the pre-pressing of the sintered NdFeB powder is completed, the sintered NdFeB powder in the mold (5) is subjected to magnetization and orientation operation; S5, after the magnetization and orientation operation is completed, the upper and lower pressure head assemblies move synchronously toward the middle of the mold (5) to perform bidirectional pressure on the sintered NdFeB powder; S6. After the sintered NdFeB powder is pressed, demagnetization operation is performed; S7, after demagnetization is completed, the upper and lower pressing head assemblies move upward synchronously to push the sintered NdFeB powder after compaction out of the mold (5); The upper pressure head assembly includes a lifting hydraulic rod (3) and an upper pressure plate (4), and the lower pressure head assembly includes a telescopic hydraulic rod (1) and a lower pressure plate (2). In step S2, when the upper pressure head assembly moves downward to the slightly lowered position (d), a hammer assembly is provided on the upper pressure head assembly to knock and compact the sintered NdFeB powder in the mold (5); The outer circumference of the mold (5) is evenly distributed with four groups of hammer assemblies, and the hammer assemblies include symmetrically arranged light rods (8), one end of the light rod (8) is provided with an end plate (7), the other end of the light rod (8) is provided with a retaining ring (10), and the retaining ring (10) is arranged close to the jacking hydraulic rod (3), the light rod (8) is provided with a slider (9), a first spring (20) is provided between the slider (9) and the retaining plate (16) arranged on the outer side, and the first spring (20) is sleeved on the light rod (8), the slider (9) is provided with a vertical plate (11), one end of the vertical plate (11) is fixed on the slider (9), and the other end of the vertical plate (11) is located on the outer side of the middle and lower part of the mold (5), and the vertical plate (11) is provided on the outer side of the mold (5). 1) is provided with a knocking block (13), the knocking block (13) is connected to the other end of the vertical plate (11), the vertical plate (11) is provided with at least one wedge block (12) at the lower end of the mold (5), the output rod of the jacking hydraulic rod (3) is provided with a mounting block (14), the middle part of the mounting block (14) is threadedly connected to the output rod of the jacking hydraulic rod (3), and a baffle (16) is provided on the side of the mounting block (14) facing the wedge block (12), one end of the baffle (16) is connected to the mounting block (14), and the other end of the baffle (16) is arranged toward the wedge block (12), and the projections of the baffle (16) and the wedge block (12) in the vertical direction partially overlap.

2. The method for controlling magnet properties through a molding process according to claim 1, characterized in that: The pre-pressed density of the sintered NdFeB powder is 2.5-2.8 g / cm 3 The compressive density of the sintered NdFeB powder is between 3.9-4.1 g / cm 3 between.

3. The method for controlling magnet properties through a molding process according to claim 1, characterized in that: In step S1, after the sintered NdFeB powder is added, a triangular fork is used to evenly insert the fork into the mold cavity four times.

4. The method for controlling magnet properties through a molding process according to claim 1, wherein: A groove (15) is provided on the side of the mounting block (14) on which the baffle (16) is arranged, and a distance is provided between the end of the groove (15) and the upper surface of the mounting block (14). A rotating shaft (17) is provided in one end of the groove (15) facing the upper surface of the mounting block (14), and one end of the baffle (16) is rotatably connected to the rotating shaft (17).

5. The method for controlling magnet properties through a molding process according to claim 4, characterized in that: A fixing plate (18) is provided on the upper surface of the mounting block (14), one end of the fixing plate (18) is fixedly connected to the mounting block (14), and a second spring (19) is provided on the other end of the fixing plate (18), and two ends of the second spring (19) are respectively fixed to the fixing plate (18) and the baffle (16).

6. The method for controlling magnet properties through a molding process according to claim 1, characterized in that: The knocking block (13) is detachably connected to the end of the vertical plate (11).

7. The method for controlling magnet properties through a molding process according to claim 1, characterized in that: The knocking block (13) is made of rubber.

8. The method for controlling magnet properties through a molding process according to claim 1, characterized in that: The wedge blocks (12) in the four groups of hammer assemblies are arranged at different heights.

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