JPH0989637A - Structure and method for assembly of magnet unit for electronic force balance - Google Patents

Abstract

PROBLEM TO BE SOLVED: To position and fix a magnet and a pole piece to a yoke with good accuracy without using an adhesive. SOLUTION: A screw hole 22a is formed in the central position in the bottom part of a yoke 22, and bolt insertion and passage holes 24a, 26a are formed in the central position of a magnet 24 and in the central position of a pole piece 26. Then, the magnet 24 and the pole piece 26 are tightened and fixed, in a stacked state, to the center in the inner bottom part of the yoke 22 via a bolt 28 which is composed of a nonmagnetic material.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an assembling structure and an assembling method of a magnet unit used in an electromagnetic balance type electronic balance.

[0002]

2. Description of the Related Art An electromagnetic balance type electronic balance generates an electromagnetic force corresponding to an applied load by supplying an electric current to a force coil arranged in a magnetic field, and an electric power supplied at that time generates an electromagnetic force. Measure the load. This kind of electronic balance is capable of measuring minute mass, but for this purpose, a mechanism for transmitting a load and a member for generating an electromagnetic force are required to have high accuracy.

FIG. 6 shows an example of an electromagnetic force generating mechanism of the electromagnetic balance type electronic balance. In the figure, a disk-shaped magnet 2 is arranged at the central bottom portion of a bottomed cylindrical yoke 1 made of a magnetic material, and a pole piece 3 having substantially the same shape is arranged on the magnet 2. Three
A magnet unit is configured by arranging a bobbin 4 in the shape of an inverted hat around which a force coil 5 is wound in a space d formed between the yoke 1 and the yoke 1.

The upper end side of the bobbin 4 is connected to the beam 6, and in order to generate an electromagnetic force F that opposes the pushing-down force of the load WT of the weighing object transmitted to the bobbin 4 via the beam 6, The current is supplied to the force coil 5 and the WT
The load of the object to be weighed can be obtained from the quantity of electricity for which F = F. In such a weighing means, a magnetic field is formed in the axial direction of the magnet 2. When an N pole and an S pole are formed in the axial direction of the magnet 2 as shown in the drawing, this magnetic field is formed by passing through the pole piece 3 and the yoke 1 as shown by an arrow B.

In this case, the gap d has a large resistance to the transmission of the magnetic force lines, and therefore, in order to secure a high magnetic flux density, it is effective to make the width of the gap d as small as possible. Therefore, the width of the gap d is usually 2
The force coil 5 is arranged in this narrow portion, which is very small such as about mm. When the width of the gap d is set to about 2 mm as described above, the allowance for the change in the width of the gap d in the circumferential direction needs to be 0.1 mm or less.

However, since the circularity of the yoke 1 and the pole piece 3 is formed with extremely high precision by cutting or the like, the uniformity of the width of the gap d is eventually determined by the axes of the yoke 1, the magnet 2 and the pole piece 3. It depends on how accurately the hearts are matched. Therefore, conventionally, the magnet 2 is attached to the central bottom of the yoke 1 by using the fixing jig 7 shown in FIG.
The pole piece 3 was fixed with the adhesive 8.

However, in the method of assembling the magnet unit using the fixing jig 7 and the adhesive 8 as described above,
There are the following technical problems, and improvements have been desired.

[0008]

That is, as a method of fixing the magnet 2 and the pole piece 3, the adhesive 8 is exclusively used. As the adhesive 8, a two-component resin such as an epoxy resin or a polyacrylate resin is usually used. However, in the case of the most reliable epoxy resin, the curing time is as long as 24 hours, and during that time. , The fixing jig 7 must be left as it is inserted as a weight, and the pole piece 3 can be adhered onto the magnet 2 only after the magnet 2 is first fixed to the yoke 1. Since it takes a very long time to complete the assembly of a product, it has a drawback of being inferior in productivity.

In this case, the curing time can be shortened, but if the curing time is shortened, the two liquids must be mixed frequently, so that it does not contribute much to the improvement in productivity. If the magnet 2 and the pole piece 3 have the same diameter and are to be bonded at once,
Both can be expected to improve productivity to some extent if they are inserted into the yoke 1 while being fitted inside the jig 7, but if such means is adopted, as shown in an enlarged view in a part of FIG. However, the adhesive 8 sometimes squeezes out more or less, and the squeezed adhesive 8 is solidified inside the jig 7 and becomes very unsatisfactory, so that it cannot be actually used.

Therefore, at the time of mass production, at least two kinds of jigs 7 must be prepared, and when the jigs 7 are worn by being used many times, the above-mentioned allowable dimension is required. Since it may exceed the tolerance, it is necessary to replace it with a new one on a regular basis. Since these jigs 7 are required to have high mechanical accuracy and are expensive, it is economically burdensome to prepare a plurality of jigs 7 of this kind and periodically replace and discard them. .

Furthermore, the present inventors have confirmed that in this type of precision electronic balance, the influence of expansion and contraction of the adhesive 8 due to changes in humidity has a great influence on the weighing. Of the adhesives 8 of this type, the epoxy resin has the lowest expansion coefficient and generally causes no problem. However, for example, when the weighing accuracy is on the order of 0.1 mg or less, the measuring accuracy is lowered due to the influence of the expansion and contraction of the adhesive 8.

Further, in the above-mentioned assembling method, the bobbin 4
Since the magnet 3 cannot be magnetized until the assembled magnet unit is completed, there is a process limitation. In addition to these problems, two-component resin adhesive 8
Has a strong odor, so it has a great impact on the work environment.
It was not suitable for work in a tightly closed room or for long hours.

The present invention solves the above problems and provides an assembling structure and an assembling method of a magnet unit for an electronic balance, in which a magnet and a pole piece can be accurately positioned and fixed to a yoke without using an adhesive. It is intended to be provided.

[0014]

In order to achieve the above-mentioned object, in the invention according to claim 1, a magnet is arranged in the bottom center of a bottomed cylindrical yoke made of a magnetic material,
In a magnet unit in which a pole piece is arranged on top of it, and a bobbin wound with a force coil is arranged in the space formed between the pole piece and the yoke, a screw hole is formed at the center of the bottom of the yoke. In addition, by forming a bolt insertion hole at the center position of the magnet and the pole piece, by fastening and fixing the magnet and the pole piece in a laminated state at the center of the inner bottom portion of the yoke via a bolt made of a non-magnetic material. By eliminating the waiting time for the adhesive to cure and dry, workability and productivity have been greatly improved. Further, in the invention according to claim 2, by forming a fitting groove for the installation guide, which is concentric with the screw hole in the inner bottom portion of the yoke and matches the outer diameter of the magnet, the magnet of the magnet with respect to the yoke is formed. The center positioning can be performed without using an insertion jig. further,
In the invention according to claim 3, in the bottom of the pole piece,
By forming a fitting groove for the installation guide that is concentric with the bolt insertion hole and matches the outer diameter of the magnet, the pole piece can be positioned with respect to the magnet without using an insertion jig. . In the method invention of claim 4, the outer diameter matches the inner diameter of the yoke,
The inner diameter of the lower side matches the outer diameter of the magnet and the pole piece, and the insertion jig with the upper center opened is used. Then, insert a non-magnetic material bolt through the opening into a bolt insertion hole opened at the center position of the magnet and pole piece, and form the male screw tip at the center of the bottom of the yoke. By screwing into the screw hole and fastening and fixing the magnet and the pole piece to the inner bottom portion of the yoke in a laminated state, each component can be accurately positioned and arranged when a bolt is used as the fixing means.

[0015]

Preferred embodiments of the present invention will be described below in detail with reference to the accompanying drawings. 1 to 3
1 shows a first embodiment of an assembly structure of an electronic balance magnet unit and an assembly method thereof according to the present invention. FIG. 1 shows a positional relationship at the time of assembling each component constituting the magnet unit.
Each of the parts constituting the above is composed of a bottomed cylindrical yoke 22 made of a magnetic material, a cylindrical magnet 24 installed and fixed on the inner bottom surface of the yoke 22, and laminated on the upper surface of the magnet 24 and having the same diameter. And a pole piece 26.

All of these are integrally laminated and fixed by a single bolt 28. The bolt 28 includes a head portion 28a, a shaft portion 28b and a male screw portion 28 at the tip thereof.
c, which are formed from a non-magnetic material.
Correspondingly, the screw hole 22 is formed at the center of the bottom of the yoke 22.
a is formed so as to penetrate vertically, and bolt insertion holes 24a and 26a are formed at the centers of the magnet 24 and the pole piece 26 so as to penetrate therethrough.

The screw hole 22a corresponds to the male screw portion 28c of the bolt 28, and the insertion hole 24a.
The inner diameters of a and 26a are set to be slightly larger than the diameter of the shaft portion 28b of the bolt 28. Further, a dish-shaped recess 26b is formed at the top of the pole piece 26 so that the head portion 28a of the bolt 28 can be accommodated in the recess 26b.

The yoke 22 and the pole piece 26 are formed by cutting, and the screw hole 22a and the insertion hole 2 are formed.
The center of 6a is accurately centered according to its machining accuracy. Further, the yoke 22 and the ball piece 26 are preliminarily plated with chrome for the purpose of surface protection, but since the screw hole 22a and the insertion hole 26a are penetrated during the immersion of the plating solution, the plating solution is prevented from flowing. By making it favorable, there is also a secondary effect of preventing non-uniformity of the plating thickness.

The magnet 24 is a press-molded body of sintered metal, which is also the insertion hole 24 depending on the precision of the mold.
A is accurately centered and the outer shape is also processed with high accuracy. Magnet 2 configured in this way
4, the pole piece 26 is inserted into the yoke 22 using the insertion jig 30. FIG. 2 shows a state in which the magnet 24 and the pole piece 26 are positioned and fixed in the yoke 22 by using the insertion jig 30. In the figure, the outer diameter of the insertion jig 30 matches the inner diameter of the yoke 22 and the outer diameters of the magnet 24 and the pole piece 26 inside the lower portion thereof in order to accurately match the centers of the respective parts. A support concave portion 30a having an inner diameter and a height capable of accommodating both of them is formed, and a hole 30b for inserting the bolt 28 having a smaller diameter is continuously formed in the upper portion of the support concave portion 30a. It was done.

Of course, this insertion jig 30
Chamfering is applied to the corners at the insertion position, so that the ease of insertion work is taken into consideration. When fixing the unit 20, in the state shown in FIG. 2, the bolt 28 is inserted into the insertion holes 24a and 26a with the spring washer 32 interposed.
Assembling is completed with the centers of the magnet 24 and the pole piece 26 accurately aligned with the center of the yoke 22 by being screwed into the screw holes through.

FIG. 3 shows a completed assembly state in which the insertion jig 30 is pulled out. In this assembled state, the bolt 28
Is applied between the recessed portion 26b of the pole piece 26 and the screw hole 22a of the yoke 22 and is applied to the entire upper and lower surfaces of the magnet 24 as a holding pressure.
4 is accurately held at the center position without locally applying stress.

Before the screwing of the bolt 28,
The adhesive resin is threaded through the hole 30 into the screw hole 22a and the insertion hole 24.
It may be dropped in a and 26b to prevent the bolt 28 from coming off. In any case, immediately after the fastening work of the bolt 28, the insertion jig 30 is pulled out, and the bobbin around which the force coil is wound is assembled to complete the magnet unit 20.

FIG. 4 shows a second embodiment of the present invention. In the figure, the same or corresponding parts as those in the first embodiment are designated by the same reference numerals, and only newly added portions are designated by different reference numerals. In the figure, the inner bottom of the yoke 22 is formed with a fitting groove 40 concentric with the screw hole 22a and having an inner diameter equal to the outer diameter of the magnet 24.

The pole piece 26 is composed of the magnet 2
4 is set larger than the outer diameter, and the insertion hole 2
A fitting groove 42 for positioning that is concentric with the magnet 6a and that matches the outer diameter of the magnet 24 is formed. Therefore,
In this embodiment, by fitting the magnet 24 into the fitting groove 40 of the yoke 22, the magnet 24 is aligned with the center of the yoke 22. Further, by stacking the pole piece 26 on the upper portion thereof, the fitting groove 42 of the pole piece 26 can be fitted on the upper portion of the magnet 24 and the center thereof can be aligned with the center of the magnet. By tightening the bolts 28 in this state, the centers of the respective parts can be accurately positioned and fixed. It is needless to say that the edges of the fitting grooves 40 and 42 are chamfered in order to ensure workability during fitting.

In addition to the above, as an assembly structure without using an insertion jig, various reference positioning mechanisms such as the insertion of reference pins can be considered, but in consideration of the workability of each part, etc. Therefore, it is desirable to make the required minimum positioning shape. FIG. 5 is a graph comparing the span value of the electronic balance using the magnet unit according to the present invention obtained in the first embodiment described above with that using the magnet unit bonded with an acrylate resin and an epoxy resin.

The measurement conditions are as follows: the temperature of each electronic balance is turned off, the temperature is kept at 25 ° C. and the humidity is 80% for about 2 days, and then the span of the electronic balance is set in a standard chamber kept at 23 ° C. and 45%. The value was measured. As is clear from the results shown in this figure, in the case of using the acrylate adhesive, the span value changes about 4 times as much as that of the product of the present invention with the passage of time, and the expansion and contraction of the resin due to humidity changes. It shows that the influence on the measured value is high.

Although the epoxy resin has not reached the level of performance problem, the effect of humidity is still recognized as compared with the product using the present invention, and the bolt fastening structure of the present invention is observed. It was confirmed that this magnet unit had the least effect on humidity.

[0028]

As described in detail above with reference to the embodiments, the invention according to claim 1 eliminates the conventional waiting time for the curing and drying of the adhesive, thereby significantly improving workability and productivity. In addition, it is possible to prevent the influence of the expansion and contraction due to the humidity of the resin on the measurement, and to prevent the deterioration of the working environment due to the use of adhesive.

Further, according to the second and third aspects of the invention, the positioning accuracy of the magnet with respect to the yoke can be secured without using an insertion jig. Further, the method of the present invention according to claim 4 has an advantage that the positional accuracy of each portion can be sufficiently secured during the fastening work with the bolt.

[Brief description of drawings]

FIG. 1 is an exploded perspective view showing a component assembling arrangement of a first embodiment of an assembling structure of an electronic balance magnet unit and an assembling method according to the present invention.

FIG. 2 is a cross-sectional view showing a state in which the magnet and the pole piece are positioned and arranged in a yoke.

FIG. 3 is a sectional view showing a completed state of the assembly.

FIG. 4 is a sectional view showing a completed state of assembly in the second embodiment of the present invention.

FIG. 5 is a graph comparing a span value of an electronic balance using a magnet unit according to the present invention with a span value of a conventional magnet unit by resin bonding.

FIG. 6 is an explanatory sectional view showing an electromagnetic force generating mechanism of an electromagnetic balance type electronic balance to which the present invention is applied.

FIG. 7 is a cross-sectional view showing a conventional magnet unit assembly structure by resin bonding.

[Explanation of symbols]

 20 magnet unit 22 yoke 22a screw hole 24 magnet 24a bolt insertion hole 26 pole piece 26a bolt insertion hole 28 bolt 30 insertion jig 40, 42 fitting groove

Claims (4)

[Claims] 1. A magnet is arranged at the center bottom part of a bottomed cylindrical yoke made of a magnetic material, and a pole piece is arranged on the magnet, and in a space formed between the pole piece and the yoke, In a magnet unit in which a bobbin wound with a force coil is arranged, a screw hole is formed at the center position of the bottom of the yoke, and a bolt insertion hole is formed at the center position of the magnet and the pole piece. An assembly structure of a magnet unit for an electronic balance, wherein the yoke is fastened and fixed in a laminated state at a center of an inner bottom portion of the yoke via a bolt made of a non-magnetic material. 2. The electronic device according to claim 1, wherein a fitting groove for an installation guide, which is concentric with the screw hole and which matches the outer diameter of the magnet, is formed in the inner bottom portion of the yoke. Assembling structure of magnet unit for balance. 3. A fitting groove for an installation guide, which is concentric with the bolt insertion hole and matches the outer diameter of the magnet, is formed at the bottom of the pole piece. 2. The assembly structure of the magnet unit for an electronic balance described in 2. 4. An insertion jig having an outer diameter that matches the inner diameter of the yoke, an inner diameter that matches the outer diameter of the magnet and pole piece, and an opening at the center is used to stack the magnet and pole piece on the inner circumference. The insertion jig is inserted into the inner circumference of the yoke in a state of being held, and then a bolt of a non-magnetic material is inserted through the opening into a bolt insertion hole opened at the center position of the magnet and the pole piece, A method for assembling a magnet unit for an electronic balance, comprising screwing a tip screw portion into a screw hole formed in a center of a bottom portion of the yoke, and fastening and fixing the magnet and the pole piece to an inner bottom portion of the yoke in a laminated state.