JP2020101115A - Impeller, pump including the same and method for manufacturing impeller - Google Patents

Abstract

To reduce the number of processes for balance adjustment of an impeller, to reduce a manufacturing cost.SOLUTION: This slurry pump 100 comprises an impeller body 20 having a cavity 22 inside a boss opening rearward, and a hub 30 engaged so as to cover an opening part of the cavity 22 of the impeller body 20. In the impeller body 20, a balancer 21 is integrally formed on the inner surface of the cavity 22, and a counterweight 40 for balance adjustment is mounted on a mounting part 33 formed on the inner surface of the hub 30 on the cavity side.SELECTED DRAWING: Figure 1

Description

The present invention relates to a pump, and more particularly to a technique for manufacturing an impeller.

Among various pumps, for example, a slurry pump (sludge pump) used for sewage treatment or human waste treatment includes an impeller having a single blade in view of non-blocking property, as described in Patent Document 1, for example. ing. In general, an impeller of this type is made of cast iron, and counterweights are arranged at appropriate positions in order to cancel an imbalance due to an asymmetrical single blade.

JP, 2005-337130, A

However, the amount of imbalance and the direction of the center of gravity of the impeller differ depending on the cast impeller. Therefore, when manufacturing the impeller, after measuring the balance of the impeller material, a counterweight is added and the appropriate position of the impeller itself is cut, and the balance of the impeller is checked again. Is needed.
Here, for example, in an automobile, a balance adjustment work is generally performed in which a balance weight is attached to the outer peripheral portion of the wheel. On the other hand, in this type of slurry pump, the outer peripheral portion of the impeller is a liquid contact surface that gradually wears. Therefore, since the balance weight cannot be attached to the liquid contact surface, there is a problem that the balance must be adjusted by cutting the outer peripheral portion of the impeller material.

Therefore, the present invention has been made in view of such a problem, and provides an impeller, a pump including the same, and a method for manufacturing the impeller, which can reduce the number of balance adjustment steps and reduce the manufacturing cost. This is an issue.

In order to solve the above problems, an impeller according to an aspect of the present invention includes an impeller main body having a cavity inside a boss that opens rearward, and a hub engaged so as to cover an opening of the cavity of the impeller main body. A balancer is integrally formed on the inner surface of the cavity of the impeller body, and the hub has a counterweight for balance adjustment mounted on a mounting portion formed on the inner surface of the cavity side. It is characterized by being

In order to solve the above-mentioned subject, a pump concerning one mode of the present invention is provided with an impeller concerning one mode of the present invention.
According to the impeller and the pump including the same according to an aspect of the present invention, a cavity is formed inside the boss of the impeller body, the balancer is integrally formed on the inner surface of the cavity, and the mounting portion formed on the inner surface of the hub is The balance of the impeller is adjusted by installing the counter weight.
Therefore, the counterweight can be attached to a place other than the liquid contact surface. As a result, the number of man-hours for balance adjustment is significantly reduced, and the manufacturing cost can be reduced, as compared with the conventional balance adjustment by cutting.

Further, in order to solve the above problems, a method of manufacturing an impeller according to an aspect of the present invention, an impeller body in which a cavity is provided inside a boss that opens rearward and a balancer is integrally formed on the inner surface of the cavity, A hub is engaged so as to cover the opening of the cavity of the impeller body, and a counterweight is mounted on a mounting portion formed on the inner surface of the hub to adjust the balance.
According to the method for manufacturing an impeller according to one aspect of the present invention, a cavity is formed inside the boss of the impeller, a balancer is integrally formed on the inner surface of the cavity, and a counterweight is attached to the mounting portion formed on the inner surface of the hub. Attach it and adjust the balance. Therefore, the weight can be attached to a place other than the liquid contact surface. As a result, the number of man-hours for balance adjustment is significantly reduced, and the manufacturing cost can be reduced, as compared with the conventional balance adjustment by cutting.

Here, in the method for manufacturing an impeller according to one aspect of the present invention, the counterweight is composed of a plurality of types of shims having different thicknesses, and the balance is adjusted by adjusting the type or number of the plurality of types of shims. Is preferred.
Further, in the method for manufacturing an impeller according to one aspect of the present invention, the balancer is mounted when the impeller body and the hub are combined in a state where the counterweight is absent to achieve a static balance around a rotation axis. It is preferable that the portion is provided so as to be located at the upper portion. As a result, it becomes easy to measure the necessary amount of weight when measuring the balance in the balance adjusting step.

As described above, according to the present invention, the number of man-hours for adjusting the balance of the impeller can be reduced, and the manufacturing cost can be reduced.

It is a schematic diagram explaining one Embodiment of the impeller which concerns on 1 aspect of this invention, and the pump provided with this, The figure shows the cross section along an axis line. It is explanatory drawing of the impeller main body which comprises the impeller of FIG. 1, the same figure (a) is a front view, (b) is the cross section (meridional surface cross section) along an axis, (c) is a rear view. It is explanatory drawing of the hub which comprises the impeller of FIG. 1, the same figure (a) is a front view, (b) is the cross section (meridian plane cross section) along an axis, (c) is a rear view. It is explanatory drawing of the counterweight for balance adjustment which comprises the impeller of FIG. 1, and the figure is the figure seen from the front. It is a figure explaining the balance adjustment process in the impeller of FIG. 1, the same figure (a) is the schematic diagram seen from the front side, (b) has shown the schematic cross section along an axis line. It is a figure explaining the balance adjustment process in the impeller of FIG. 1, the figure (a) is the schematic diagram seen from the front side, and the figure (b) has shown the schematic cross section along an axis line. It is a figure explaining the balance adjustment process in the impeller of FIG. 1, the same figure (a) is the schematic diagram seen from the front side, (b) has shown the schematic cross section along an axis line. It is a figure explaining the comparative example of the impeller which concerns on 1 aspect of this invention, and the figure shows the cross section along an axis line. It is a figure explaining the comparative example of the impeller which concerns on 1 aspect of this invention, the figure (a) is a front view of the impeller main body which comprises the impeller of a comparative example, (b) is a meridional surface sectional view, (c). Is a rear view. It is a figure explaining the comparative example of the impeller which concerns on 1 aspect of this invention, the figure (a) is a front view of the hub which comprises the impeller of a comparative example, (b) is a meridional sectional view, (c) is. It is a rear view.

An embodiment of the present invention will be described below with reference to the drawings. The drawings are schematic. Therefore, it should be noted that the relationship between the thickness and the plane size, the ratio, and the like are different from the actual ones, and the drawings include portions where the dimensional relationship and ratio are different from each other.
Further, the embodiments described below exemplify a device and a method for embodying the technical idea of the present invention, and the technical idea of the present invention is that the material, shape, structure, and arrangement of components are Etc. are not specified in the following embodiments.

The present embodiment is an example of a slurry pump (sludge pump) used for sewage treatment and night soil treatment. As shown in FIG. 1, the slurry pump 100 of the present embodiment includes a frame 1 and a casing 2 provided in front of the frame 1. A single-blade impeller 10 is housed inside the casing 2.
The casing 2 has a casing body 4 at the center in the axial direction, a front cover 3 at the front of the casing body, and a back cover 5 at the rear of the casing body. A discharge port 4t is formed in the upper part of the casing body 4, and a suction port 3n is formed in the center of the front cover 3 so as to project forward along the axial direction.

The casing body 4 and the back cover 5 are fixed to each other with bolts and nuts, and the casing body 4 and the front cover 3 are fixed to each other with bolts and nuts, and the housing space of the impeller 2 is provided inside the casing 2. Well defined. A portion where the impeller 10 and the inner surface of the front cover 3 face each other serves as a clearance adjusting portion, and an adjusting plate 6 also serving as a plate ring packing having a sealing property is interposed in this portion.
An impeller 10 is supported in a cantilever manner at the tip of a shaft (rotating shaft) 7 that projects in the casing 2. Further, a shaft sealing portion 8 is provided in the center of the back surface of the back cover 5, and the tip side of the shaft 7 is shaft-sealed.
The shaft 7 has a base end side that penetrates the rear side of the casing 2 and extends into the frame 1, and a shaft support portion 9 in the frame 1 allows the base end side of the shaft 7 to rotate at a position rearward of the shaft sealing portion 8. Supported by. Further, an output shaft of a motor (not shown) is connected to the base end of the shaft 7 so as to be able to transmit driving force.

Next, the impeller 10 will be described in detail.
As shown in FIG. 1, the impeller 10 of the present embodiment includes an impeller body 20 having a cavity 22 inside a boss that opens rearward, and a hub engaged so as to cover an opening of the cavity 22 of the impeller body 20. 30 is provided.
In the impeller body 20 of the present embodiment, the balancer 21 is integrally formed on the inner surface of the cavity 22. On the other hand, in the hub 30 of the present embodiment, the counterweight 40 for balance adjustment is detachably attached to the attachment portion 33 formed on the surface facing the cavity 22 side.

Specifically, as shown in FIG. 2, the impeller body 20 has a single blade 23 formed in a spiral shape on the outer peripheral surface of the substantially conical body portion. The tip portion of the single blade 23 has an asymmetrical shape that projects outward in the axial direction from the tip portion of the outer peripheral surface of the substantially conical shape. A bolt insertion hole 24 for mounting is coaxially formed at the tip of the outer surface of the substantially conical shape. The inner surface of the substantially conical main body portion is a substantially conical cavity 22. The substantially crescent-shaped balancer 21 is integrally formed on the inner surface of the cavity 22.

The balancer 21 according to the present embodiment combines the impeller body 20 and the hub 30 in a state where the counterweight 40 is absent (when the balance weight is adjusted before the counterweight 40 is mounted) to rotate the shaft 7 around the rotary shaft. When the static balance is obtained, the mounting portion 33 of the counterweight 40 formed on the hub 30 is provided so as to be positioned at the upper portion (see the description of the balance adjusting step described later).
The impeller body 20 has a hub mounting portion 25, which is a spigot recess, into which the outer peripheral surface of the hub 30 is fitted, at the base end portion of the substantially conical body portion. A plurality of hub mounting pin holes 26 are formed on the surface of the hub mounting portion 25 that faces the axial direction and are spaced apart from each other in the circumferential direction. Of the plurality of hub mounting pin holes 26, the fixing pin 60 is inserted into one of the hub mounting pin holes 26 at a position facing the pin hole 39 formed on the hub 30 side at the time of the above balance adjustment, and the relative position in the circumferential direction. Is fixed (determined).

As shown in FIG. 3, the hub 30 has a disc portion 37 and a cylindrical shaft portion 36 that projects toward the rear at the center of the disc portion 37. At the center of the shaft portion 36, a mounting screw portion 38 is formed along the axis. The disc portion 37 is formed with a concave step portion on the outer peripheral surface thereof so that the spigot fitting to the hub mounting portion 25 on the impeller body 20 side is possible. On the back surface of the disk portion 37, a plurality of back blades 35 are radially formed so as to be spaced apart from each other in the circumferential direction.
On the front surface of the disk portion 37, a mounting surface 34 having a circular hub-side mounting hole 31 in the substantially central portion is provided at a higher position. The hub side mounting hole 31 is formed on the mounting surface 34 so as to face the weight side mounting hole 41 of the counterweight 40.
Further, on the front surface of the disk portion 37, the fixing pin 60 is provided at a position opposite to the mounting portion 33 on which the counterweight 40 is mounted, and at a position facing one hub mounting pin hole 26 during balance adjustment. A pin hole 39 to be inserted is formed.

As shown in FIG. 4, the counterweight 40 is formed of a plate member, and has a circular weight-side mounting hole 41 penetrating the surface 43 of the plate and left and right formed as a peripheral surface shape in plan view of the plate. And a pair of hook portions 42. As shown in FIG. 1, the counter weight 40 has the spring pin 50 inserted into the hub-side mounting hole 31 from the weight-side mounting hole 41, whereby the counter-weight 40 is mounted on the mounting portion 33 of the hub 30.
As shown in FIG. 3, the hub 30 of the present embodiment is provided with a boss portion 30a that projects in the axial direction at the center of the surface of the impeller body 20 facing the cavity 22 side. Engagement portions 32 formed of a pair of left and right recesses are formed on the left and right side surfaces of the outer periphery of the boss portion 30a. The pair of left and right engaging portions 32 are provided so that the pair of left and right hook portions 42 of the counterweight 40 engage with each other when the plate surface of the counterweight 40 faces the mounting portion 33 in the axial direction. There is.

As a result, the mounting portion 33 on which the counterweight 40 is mounted engages with the spring pin 50 inserted into the hub-side mounting hole 31, the pair of left and right hook portions 42 of the counterweight 40, and the pair of hook portions 42. The counterweight 40 is securely held by the mounting portion 33 in cooperation with the engaging portion 32 formed of a pair of left and right concave portions.

Next, a method of manufacturing the impeller 10 in which the counterweight 40 is mounted on the mounting portion 33 of the hub 30 to adjust the balance will be described with reference to FIGS. 5 to 7.
As described above, the method for manufacturing the impeller 10 according to the present embodiment includes the impeller body 20 in which the cavity 22 is provided inside the boss opening rearward, and the balancer 21 is integrally formed on the inner surface of the cavity 22, and the impeller body. A hub 30 that is engaged so as to cover the opening of the cavity 22 of 20 and a mounting portion 33 formed on the inner surface of the hub 30 on the cavity side are mounted with a required amount of counterweight 40 to balance. To adjust.

In particular, in the method for manufacturing the impeller 10 according to the present embodiment, when the impeller body 20 and the hub 30 are combined to balance the static weight around the shaft 7 without the counterweight 40 as the impeller body 20, the counterweight 40 The balancer 21 is provided so that the mounting portion 33 of FIG.
In the step of adjusting the balance, as shown in FIG. 5, the impeller body 20 and the hub 30 are combined on the rotation axis of the work shaft 7K corresponding to the shaft 7 without the counterweight 40. Since the balancer 21 is provided such that the mounting portion 33 of the counterweight 40 is located at the upper portion, when the static balance around the work shaft 7K is taken in a state where the impeller body 20 and the hub 30 are combined. , The center of mass G is located lower than the axis. Therefore, the mounting portion 33 of the counterweight 40 can be located at the upper part. The operator uses, for example, an oil-based pen M to put a marker P at the upper position so as to serve as a mark.

As shown in FIG. 6, the balance is adjusted in a state where the impeller body 20 and the hub 30 are combined on the rotation axis of the work shaft 7K corresponding to the shaft 7, and the mounting portion 33 of the counterweight 40 is positioned at the upper portion. While maintaining the static balance in this posture, the required amount of the counterweight 40 to be attached is determined using the temporary balance adjusting material m made of clay or the like.
In the example of the figure, clay is used as the temporary balance adjusting material m, and the first adjusting material m1, the second adjusting material m2, and the third adjusting material m3 are sequentially attached to determine the required total attached amount. It shows the image. Further, the wavy line indicated by the symbol F indicates an image in which a proper balance is applied along the circumferential direction while maintaining a static balance.

Here, as shown in Table 1, the counterweight 40 of the present embodiment includes a plurality of types of shims P1 to P5 having different thicknesses, and based on the total attachment amount determined by the temporary balance adjusting material m, The type or number of the plurality of types of shims P1 to P5 is adjusted to adjust the balance so as to correspond to the required mass.
When the required mass of the counterweight 40 is determined, as shown in FIG. 7, the impeller body 20 and the hub 30 are incorporated into the shaft 7 of the actual machine. In the mounting example of the counterweight shown in the same figure, the counterweight 40 is configured by a set of eight sheets appropriately selected from a plurality of types of shims P1 to P5.

In the procedure of assembling the impeller 10, first, a necessary amount of counterweight 40 is fixed to the hub 30 by the spring pin 50, the hub 30 is mounted on the shaft 7 of the actual machine, and fixed by the hub fixing nut 11. After that, with the fixing pin 60 interposed between the facing surfaces of the impeller body 20 and the hub 30, the impeller body fixing bolt 12 is tightened while the impeller body 20 is fitted to the hub 30 by the spigot fitting from the front. 10 is fixed to the tip of the shaft 7.

Next, a method of manufacturing the impeller 10 and an operation/effect of the slurry pump 100 including the impeller 10 manufactured by the method will be described.
In the slurry pump 100, when the slurry pump 100 is in operation, the shaft 7 is rotationally driven by driving the motor. As a result, when the impeller 10 rotates in the casing 2, the pressure of the fluid to be fed is increased, and the slurry serving as the fluid to be fed can be transported from the suction port 3n of the casing 2 toward the discharge port 4t.
Here, in the impeller 10 of this embodiment, as shown in FIG. 1, a cavity 22 is formed inside the boss of the impeller body 20, the balancer 21 is integrally formed on the inner surface of the cavity 22, and the inner surface of the hub 30 is mounted. A removable counterweight 40 is attached to the portion 33 to adjust the balance. Therefore, the balance adjustment can be performed in the non-wetted portion, and the number of balance adjustment steps can be significantly reduced as compared with the balance adjustment by cutting work, and the manufacturing cost can be reduced.

Here, a comparative example will be described with reference to FIGS. In the description of the comparative example, configurations corresponding to those in the above-described embodiment will be denoted by the same reference numerals, and description thereof will be omitted as appropriate.
As shown in FIG. 8, in the slurry pump 100B of the comparative example, the impeller 110 of the comparative example is cantilevered in the casing 2. The impeller 110 includes an impeller body 120 having a cavity 122 inside a boss opening rearward, and a hub 130 engaged so as to cover an opening of the cavity 122 of the impeller body 120.

In the comparative example, as shown in FIGS. 8 and 9, the impeller body 120 is different from the above embodiment in that the cavity 122 does not have a thick portion corresponding to the balancer 21 in the above embodiment. .. Further, the comparative example is different in that the balancer 121 is integrally formed with the hub 130.
In this comparative example, a balancer 121 is integrally formed with the hub 130 as shown in FIGS. 8 and 10 in order to cancel the imbalance due to the single blade having an asymmetrical shape. In the comparative example, the hub 130 is provided with a boss portion 30a that is provided in an axially protruding manner on the surface of the impeller body 120 facing the cavity 122 side. The balancer 121 of the comparative example is integrally formed with the hub 130 by radially projecting a part of the boss portion 30a so as to form a substantially crescent-shaped weight along the circumferential direction.

In the case of the structure like the impeller 110 shown in the comparative example, the balancer 121 is formed at a position other than the liquid contact surface, and thus it is suitable as a balance weight for a slurry pump, like the impeller 10 of the present embodiment.
However, in the configuration shown in the comparative example, the balancer 121 is integrally formed with the hub 130, and when manufacturing the impeller, after measuring the balance of the impeller material, the balancer 121 is cut to perform balance adjustment. There is. Therefore, there is room for improvement in that it takes a lot of time for the balance adjustment work.

On the other hand, in the impeller 10 of the present embodiment, as described above, the detachable counterweight 40 is attached to a portion other than the liquid contact surface. Therefore, compared with the balance adjustment work by cutting as shown in the comparative example, the balance adjustment man-hours are significantly reduced, and the manufacturing cost can be reduced.
Particularly, in the impeller 10 of the present embodiment, the balancer 21 is formed on the hub 30 when the impeller body 20 and the hub 30 are combined in a state where the counterweight 40 is not provided and a static balance around the rotation axis is achieved. Since the mounting portion 33 of the counterweight 40 is provided so as to be located above, the workability in the balance adjustment process is extremely excellent.

Further, according to the present embodiment, as described above, the counterweight 40 is formed of the plate member, and has the weight side mounting hole 41 that penetrates the surface 43 of the plate and the peripheral surface shape of the plate in plan view. And a pair of hook portions 42 formed as.
When the mounting portion 33 of the counterweight 40 is arranged so that the hub side mounting hole 31 formed at a position facing the mounting hole 41 and the plate surface 43 of the counterweight 40 face the mounting portion 33 in the axial direction. And a concave engaging portion 32 with which the hook portion 42 is engaged.
The counterweight 40 is mounted on the mounting portion 33 of the hub 30 by inserting the spring pin 50 into the hub-side mounting hole 31 from the weight-side mounting hole 41, which improves the mounting workability in the balance adjusting process. In addition to being excellent, the mounted state of the slurry pump 100 during operation is also stable and will not come off due to centrifugal force.

Further, in the method of manufacturing the impeller of the present embodiment, the cavity 22 is provided inside the boss that opens rearward, and the balancer 21 is integrally formed on the inner surface of the cavity 22, and the opening of the cavity 22 of the impeller body 20. The counter weight 40 is mounted on the mounting portion 33 on the inner surface on the cavity side of the hub 30 by using the hub 30 engaged so as to cover the portion, so that the weight is attached to a portion other than the liquid contact surface. Can be attached. As a result, the number of man-hours for balance adjustment is significantly reduced, and the manufacturing cost can be reduced, as compared with the conventional balance adjustment by cutting.
In particular, in the method for manufacturing the impeller of the present embodiment, when the impeller body 20 is not provided with the counterweight 40 and the impeller body 20 and the hub 30 are combined to achieve a static balance around the rotation axis, the counterweight 40 Since the balancer 21 is provided so that the mounting portion 33 is located on the upper side, it is excellent in workability in the balance adjusting process.

In the method for manufacturing the impeller of the present embodiment, the step of adjusting the balance is performed by combining the impeller body 20 and the hub 30 on the rotating shaft and maintaining the static balance with the mounting portion 33 of the counterweight 40 positioned above. At the same time, since the required amount of the counter weight 40 to be attached is determined, the number of man-hours for balance adjustment is significantly reduced, which is excellent in reducing the manufacturing cost.
In the impeller manufacturing method of the present embodiment, the counterweight 40 is composed of a plurality of types of shims having different thicknesses, and the balance is adjusted by adjusting the type or number of the plurality of types of shims P1 to P5. It is excellent in adjusting the mass of the counterweight 40 so as to correspond to the mass determined by the temporary balance adjusting material m.

As described above, according to the method of manufacturing the impeller 10 and the slurry pump 100 including the impeller 10 manufactured by the method, the man-hours for adjusting the balance of the impeller 10 can be reduced and the manufacturing cost can be reduced.
The method for manufacturing the impeller according to the present invention and the slurry pump including the impeller manufactured by the method are not limited to the above embodiments, and various modifications can be made without departing from the spirit of the present invention. Of course.

1 Frame 2 Casing 3 Front Cover 4 Casing Main Body 5 Back Cover 6 Adjustment Plate 7 Shaft 8 Shaft Sealing Section 9 Shaft Support Section 10 Impeller 11 Hub Fixing Nut 12 Impeller Main Body Fixing Bolt 20 Impeller Main Body 21 Balancer 22 Cavity 23 Single Blade 24 Bolt Insert Hole 25 Hub mounting part 26 Hub mounting pin hole 30 Hub 31 Hub side mounting hole 32 Engagement part 33 (Counterweight) mounting part 34 Mounting surface 35 Back blade 36 Shaft part 37 Disc part 38 Screw part 39 Pin hole 40 Counterweight 41 Weight side mounting hole 42 Hook part 43 Plate surface 50 Spring pin 60 Fixing pin 100 Slurry pump

Claims (7)

An impeller body provided with a cavity inside a boss that opens rearward; and a hub engaged so as to cover the opening of the cavity of the impeller body,
The impeller body has a balancer integrally formed on the inner surface of the cavity,
An impeller, wherein the hub has a counterweight for balance adjustment mounted on a mounting portion formed on an inner surface of the hollow side. The balancer is provided so as to position the mounting portion when a static balance around the rotation axis is achieved by combining the impeller body and the hub without the counterweight. Impeller described in. The counterweight is formed of a plate member, and has a weight side mounting hole that penetrates the surface of the plate, and a hook portion formed as a peripheral surface shape of the plate in a plan view,
The mounting portion has a hub-side mounting hole formed at a position facing the mounting hole, and the hook portion engages when the surface of the plate of the counterweight is axially opposed to the mounting portion. Has a joint and
The impeller according to claim 1 or 2, wherein the counter weight is mounted on the mounting portion by inserting a spring pin from the weight side mounting hole into the hub side mounting hole. A pump comprising the impeller according to any one of claims 1 to 3. A cavity is provided inside the boss that opens rearward and a balancer is integrally formed on the inner surface of the cavity, and a hub engaged so as to cover the opening of the cavity of the impeller body is used.
A method of manufacturing an impeller, characterized in that a counterweight is mounted on a mounting portion formed on an inner surface of the hub on the cavity side to adjust the balance. As the impeller body, when the balancer combines the impeller body and the hub in a state where the counterweight is absent to maintain a static balance around the rotation axis, the counterweight mounting portion is positioned above. Using the one provided in
In the step of adjusting the balance, the impeller body and the hub are combined on a rotating shaft, and the required amount of the counterweight is attached while maintaining the static balance in a posture in which the mounting portion of the counterweight is located on the upper side. The method for manufacturing an impeller according to claim 5, which is determined. 7. The method of manufacturing an impeller according to claim 5, wherein the counterweight is composed of a plurality of types of shims having different thicknesses, and the balance is adjusted by adjusting the type or number of the plurality of types of shims.

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