Patent Publication Number: US-2021183554-A1

Title: Magnetic clamp device

Description:
FIELD OF THE INVENTION 
     The present invention relates to a magnetic clamp device. 
     A magnetic clamp device utilizing magnetic adsorption force is known with respect to mold-fixing of an injection molding machine. The magnetic clamp device is a technique for magnetically fixing a mold by fitting a plate made of a magnetic body on a platen. The plate can be switched between a magnetic circuit closed in the plate and a magnetic circuit passing through the mold by surrounding a magnetic pole member made of a magnetic body with a permanent magnet with non-reversible polarity, disposing a reversible magnet (alnico magnet) at the rear of the permanent magnet, and controlling a magnetic polarity of the alnico magnet by a coil. 
     Patent literature 1 discloses a magnetic clamp device wherein a circular magnetic pole member is accommodated in a cylinder that is formed by providing a circular accommodating recess on a base and arranging a plurality of permanent magnets with an arc-shaped cross section in a circle along the recess. Further, in patent literature 2, permanent magnets are incorporated in a clamp plate by being arranged on four sides around a square magnetic pole member. 
     PRIOR ART 
     Patent Literature 
     Patent literature 1: Japanese Patent Laid Open Publication No. 2017-144525 
     Patent literature 2: Japanese Patent No. 5385544 
     DISCLOSURE OF THE INVENTION 
     Problems to be Solved by the Invention 
     In patent literatures 1 and 2, when cores or steel blocks (hereinafter referred to each as a magnetic pole member) are installed in a permanent magnet, it cannot be assembled unless there is a gap between the members in advance by design. Although the gap is necessary for assembling, in the magnetic circuit that passes through the permanent magnet and the magnetic pole member, magnetic flux leaks due to this gap, and the magnetic resistance of the circuit has to be increased. 
     An object of the present invention is to provide a magnetic clamp device in which magnetic flux leaking from a magnetic circuit that passes through a permanent magnet and a magnetic pole member is reduced. 
     Means to Solve the Problem 
     A magnetic clamp device of the present invention includes a plate made of a magnetic body that magnetically clamps a mold, a permanent magnet disposed on a front surface side of the plate so as to surround a magnetic pole member, and a reversible magnet capable of reversing polarity on a rear surface side of the plate. The magnetic pole member is constituted by a plurality of magnetic pole pieces, and each of the magnetic pole pieces has a first lateral surface with a shape corresponding to the opposite permanent magnet and a second lateral surface at which adjacent magnetic pole pieces oppose each other. The second lateral surfaces of the adjacent magnetic pole pieces are arranged with a gap therebetween, the first lateral surface is in contact with the permanent magnet, and the magnetic pole pieces are held from the front surface side and the rear surface side of the plate. 
     Effects of Invention 
     According to the present invention, the magnetic pole pieces are free to move, and their relative positions to the permanent magnet are determined by adsorption by the permanent magnet. Therefore, each magnetic pole piece is adsorbed to the permanent magnet with a minimized gap to make contact. According to this, there is an effect that almost no magnetic flux leakage occurs between each magnetic pole piece and the permanent magnet. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A and 1B  are views showing a magnetic clamp device provided with a circular magnetic pole member.  FIG. 1A  is a front view,  FIG. 1B  is an exploded view. 
         FIG. 1C  is a demagnetized magnetic circuit diagram, and  FIG. 1D  is a magnetized magnetic circuit diagram. 
         FIGS. 2A and 2B  are views showing a magnetic clamp device provided with a square magnetic pole member.  FIG. 2A  is a front view and  FIG. 2B  is an exploded view. 
         FIGS. 3A, 3B and 3C  are views showing Example 1,  FIG. 3A  is an exploded view,  FIG. 3B  is a view showing a positional relationship between a magnetic pole member and a permanent magnet, and  FIG. 3C  is a view showing a state during assembly. 
         FIGS. 4A and 4B  are views showing Example 2,  FIG. 4A  is a view showing a positional relationship between a magnetic pole member and a permanent magnet, and  FIG. 4B  is a view showing a final positional relationship between the magnetic pole member and the permanent magnet. 
         FIGS. 5A, 5B, 5C, 5D and 5E  are views showing another embodiment. 
         FIGS. 6A, 6B, 6C, 6D, 6E, 6F, 6G, 6H, 6I and 6J  are views showing another division mode of a magnetic pole member. 
     
    
    
     PREFERRED EMBODIMENTS OF THE INVENTION 
       FIG. 1A  shows a plate PL of a magnetic clamp device provided with circular magnetic pole members. Cartesian coordinates X-axis, Y-axis are shown. A large number of magnet blocks  11  are arranged on a front surface of the plate PL. The plate PL is made of a magnetic body, and a large number of circular grooves  14  are provided on the front surface. A part surrounded by a groove  14  corresponds to a magnet block  11 . 
       FIG. 1B  is an exploded view taken along a line P-P in  FIG. 1A . The Z-axis is orthogonal to the X-axis and the Y-axis, and is a directional axis that extends from the front surface of the plate PL adsorbing the mold to the inside of the plate. In the figure, the plate PL is integrally provided with a portion D 1  having a small thickness of steel on the Z-axis direction side of the groove  14 , and a disc-shaped portion D 2  having a large thickness of steel further inside (functions as a pole member). A cylindrical permanent magnet  20  is inserted into the portion D 1  having a small thickness of steel from the back side of the plate PL. The permanent magnet  20  has, for example, S pole on the inner peripheral side and N pole on the outer peripheral side of a ring which is an outer shape of the permanent magnet. For example, a neodymium magnet can be used as a permanent magnet  20 . On the back side (Z-axis direction side) of the permanent magnet  20 , a reversible magnet  18  including a disc-shaped alnico magnet  16  and a coil  17  wound around the outside of the alnico magnet  16  is arranged. A disc-shaped cap  19  (magnetic body) is fitted into the plate PL from the back side (Z-axis direction side) of the reversible magnet  18 . Each magnet block  11  includes a portion D 2  functioning as a magnetic pole member, a permanent magnet  20 , an alnico magnet  16 , a coil  17 , and a cap  19 . The inner peripheral side of the permanent magnet  20  is magnetically coupled to the portion D 2 , and magnetically coupled to the outer peripheral side (outer yoke D 3 ) of the portion D 1  having a reduced thickness on the outer peripheral side. The outer yoke D 3  is fitted around the portion D 2 , the permanent magnet  20 , the alnico magnet  16 , the coil  17 , and the cap  19 . The alnico magnet  16  is magnetically coupled to the portion D 2  and the cap  19 , and an outer peripheral surface of the cap  19  is magnetically coupled to an inner peripheral surface of the outer yoke D 3 . Further, the portion D 4  of the groove  14  is made thinner than the other parts, so that it is easily magnetically saturated. Since the front surface of the plate PL is entirely covered with steel of the plate PL, the permanent magnet  20  and the reversible magnet  18  can be sealed from a work area for mounting the mold M 1 . The magnetic clamp device is a mono-mag type magnetic clamp device in which the inner peripheral side of the permanent magnet  20  is S pole and the outer peripheral side is N pole and a magnetic circuit is formed by one magnetic block  11 . 
       FIG. 1C  shows a state in which the magnetic clamp device is in a demagnetized state. The alnico magnet  16  is a magnet in which the front side (the left side in the drawing) of the plate PL is N pole and the back side is S pole. Magnetic flux passes through a magnetic circuit formed by the permanent magnet  20 , the outer yoke D 3 , the cap  19 , the alnico magnet  16 , and the portion D 2 . In this state, the magnetic flux does not leak to the front surface of the plate PL, and the mold M 1  is not adsorbed. 
       FIG. 1D  shows a state in which the magnetic clamp device is in a magnetized state. The magnetic pole of the alnico magnet  16  is reversed by applying a direct current to the coil  17  from the outside. The alnico magnet  16  is a magnet in which the front side of the plate PL is S pole and the back side thereof is N pole. The polarity of the alnico magnet  16  is reversed, and a direct current should be flowed for the time to hold the required magnetic flux. On the front surface side of the plate PL, both the permanent magnet  20  and the reversible magnet  18  are coupled to the portion D 2  as S pole. In a state in which the mold M 1  is pressed against the front surface of the plate PL, these magnetic fluxes pass through the mold M 1 . As a result, a magnetic circuit composed of the permanent magnet  20 , the outer yoke D 3 , the mold M 1  and the portion D 2 , and a magnetic circuit composed of the alnico magnet  16 , the outer yoke D 3 , the mold M 1  and the portion D 2  are formed. When the mold M 1  is lost, the magnetic force outward from the front surface of the plate PL is immediately lost by the magnetic force of the permanent magnet  20  because the alnico magnet  16  does not relatively have high holding power as a permanent magnet. 
     Although the magnetic clamp device in  FIGS. 1A, 1B, 1C and 1D  is formed into a mono-mag type magnetic clamp device by arranging the inner side of the permanent magnet  20  as S pole and the outer side thereof as N pole, a bi-mag type magnetic clamp device that forms a magnetic circuit with two adjacent magnet blocks can be configured if the inside of the permanent magnet  20 ′ is inverted to N pole and the outside to S pole. 
       FIGS. 2A and 2B  show a magnetic clamp device provided with a square magnetic pole member. The X axis, the Y axis and the Z axis are defined in the same manner as in  FIGS. 1A, 1B, 1C and 1D .  FIG. 2A  is a plan view, and  FIG. 2B  is an exploded view of one magnet block  11 . The magnet blocks  11  are arranged adjacent to each other on the front surface of the plate PL. The magnet block  11  includes a magnetic pole member  10 , a plurality of permanent magnets  20  arranged on the outer peripheral side of the magnetic pole member  10 , and an alnico magnet  16  arranged on the rear surface side of the magnetic pole member  10 . The magnetic pole member  10  and the alnico magnet  16  are formed in a square shape. The permanent magnets  20  between the magnet blocks  11  adjacent to each other in the X-axis direction and the Y-axis direction are also used for both of these magnet blocks  11 . Each of the magnet blocks  11  can be controlled into a magnetized state or a demagnetized state by the coil  17  wound around the alnico magnet  16  as in the example in  FIGS. 1A, 1B, 1C and 1D . The magnet block  11  is fixed to the plate PL by screwing a bolt  15  into a fixing hole  23  provided to the plate PL through a through hole  21  of the magnetic pole member  10  and a through hole  22  of the alnico magnet  16 . 
     In the magnet blocks  11  in  FIGS. 1A, 1B, 1C, 1D, 2A and 2B , the position of each magnet block  11  on the X axis and the Y axis is predetermined with respect to the plate PL. The permanent magnet  20  is arranged between the magnetic pole member  10  and the outer yoke D 3 , or between adjacent magnetic pole member  10  and magnetic pole members  10 . It is necessary to design the permanent magnet  20  to a slightly smaller size including an error so that it can be inserted around the magnetic pole member  10  without any trouble. This is because it is physically difficult to insert the permanent magnet  20  if the dimensions are designed so that there is no gap for fitting with the permanent magnet  20  in all directions around the magnetic pole member  10 . On the other hand, the direction of magnetic flux due to the permanent magnet  20  is in a plane consisting of the X axis and the Y axis, and therefore, there is no choice but to make a magnetic circuit through the gap between the magnetic pole member  10  and the permanent magnet  20  thus formed. 
     Examples of the present invention will be described below. The present invention aims to reduce the magnetic flux leaking from the magnetic circuit passing through the permanent magnet and the magnetic pole member, and will be described based on the following examples. 
     Example 1 
     Example 1 will be described below with reference to  FIGS. 3A, 3B and 3C . Those elements or members shown in  FIGS. 3A, 3B and 3C  having the same function as the reference numeral given in  FIGS. 1A, 1B, 1C and 1D  are designated by the same numeral. Example 1 is an embodiment in which the present invention is applied to a magnetic clamp device provided with a circular magnetic pole member in  FIGS. 1A, 1B, 1C and 1D .  FIG. 3A  is an exploded view of a magnet block  11 . The difference from the magnetic pole member  10  shown in  FIGS. 1A, 1B, 1C and 1D  is in the structure of a magnetic pole member  10 . In Example 1, the magnetic pole member  10  is divided into a plurality of magnetic pole pieces  101 ,  102 ,  103 ,  104 . In the embodiment of  FIGS. 1A, 1B, 1C and 1D , the portion D 2  that is a part of the plate PL has a function as a magnetic pole member, but in Example 1, the function is divided into the portion that receives magnetic flux from a permanent magnet  20  (hereinafter referred to as a magnetic pole member  10 ) and the portion of the front surface of the plate PL (front surface portion D 5 ). The front surface portion D 5  is a magnetic pole plate that continuously covers the magnetic pole pieces  101 ,  102 ,  103 ,  104  by one sheet. The magnetic pole member  10  is held between the front surface portion D 5  and the alnico magnet  16  by restricting the Z-axis movement of the magnetic pole pieces  101 ,  102 ,  103 ,  104 . 
     The magnetic pole pieces  101 ,  102 ,  103 ,  104  will be further described. The magnetic pole pieces  101 ,  102 ,  103 ,  104  are members that have the same shape as the magnetic pole member  10  of  FIGS. 1A, 1B, 1C and 1D  when they are assembled on a plane consisting of the X axis and the Y axis. In this example, it is circular. The magnetic pole member  10  is radially divided from the center thereof at equal angular intervals. Each of the magnetic pole pieces  101 ,  102 ,  103 ,  104  has a curved surface formed on a part of the side wall of the magnetic pole member  10 . This side surface (referred to as the first side surface R) is a curved surface that is as close to the inner diameter of the permanent magnet  20  as the design allows, and thus, when the first side surface R is magnetically adsorbed to the inner peripheral surface of the permanent magnet  20 , there is almost no gap between the first side surface R and the inner peripheral surface of the permanent magnet  20 . On the other hand, the other side surface (referred to as the second side surface T) faces the side surface of another adjacent magnetic pole piece. In  FIGS. 3A, 3B and 3C , the permanent magnet  20  is also divided into a plurality of arc-shaped permanent magnet pieces  201 ,  202 ,  203 ,  204 . 
       FIG. 3B  is a view showing a positional relationship between the magnetic pole member  10  and the permanent magnet  20 . The permanent magnet pieces  201 ,  202 ,  203 ,  204  are fitted onto the outer yoke D 3 . The magnetic pole pieces are installed inside the permanent magnet  20  that is a combination of a plurality of magnets to form a cylindrical shape.  FIG. 3C  is a view showing an assembly process. The curvature radius of a curved surface of the permanent magnet piece is substantially the same as the inner diameter of the outer yoke D 3 , and dimensioned so as to minimize the gap due to the difference with the inner peripheral diameter of the outer yoke D 3 . When the permanent magnet pieces  201 ,  202 ,  203 ,  204  are accommodated in the outer yoke D 3 , they are adsorbed to the outer yoke D 3  by their own magnetic force. 
     Next, when the magnetic pole pieces  101 ,  102 ,  103 ,  104  are inserted on the inner peripheral side of the permanent magnet  20 , the first side surface R of each magnetic pole piece is adsorbed to and abutted by the opposing inner peripheral surfaces of the permanent magnet  20  by a strong magnetic force. Since the magnetic pole pieces  101 ,  102 ,  103 ,  104  are free to move before being fixed by the bolt  15  described below, the position relative to the permanent magnet  20  is determined by the adsorption due to the permanent magnet  20 . Since the outer diameter of the arc of the permanent magnet piece is designed so as to substantially match the inner diameter of the outer yoke D 3 , the magnetic pole pieces  101 ,  102 ,  103 ,  104  are abutted to the permanent magnet  20  in a state that there is almost no gap. Therefore, the magnetic flux is hardly reduced due to the gaps between the magnetic pole pieces  101 ,  102 ,  103 ,  104  and the permanent magnet  20 . When the pole pieces are adsorbed and moved, the second side surfaces T do not interfere with each other. Further, although a gap is formed between the second side surfaces T, no magnetic circuit is planned for the magnetic flux to cross this gap, so that it does not lead to an increase in magnetic resistance. A fixing hole  23  having female screw threads is provided in the front surface portion D 5 , and the cap  19 , the alnico magnet  16 , and the magnetic pole member  10  are fixed to the front surface portion D 5  from the cap  19  side with the bolt  15 . Since the fixing in the Z-axis direction is made in close contact with each other by the bolt  15 , the gap that interferes with the magnetic flux in the Z-axis direction is minimized. 
     Although the permanent magnet  20  is divided into a plurality of arc-shaped permanent magnet pieces  201 ,  202 ,  203 ,  204  in Example 1, it may be a cylindrical permanent magnet that is not divided. In this case, a fitting gap is required between the permanent magnet  20  and the plate PL, but a fitting gap between the permanent magnet  20  and the magnetic pole member  10  can be reduced. 
     Example 2 
       FIGS. 4A and 4B  show views of a second embodiment. Those elements or members shown in  FIGS. 4A and 4B  having the same functions as the reference numerals given in  FIGS. 2A and 2B  are given the same reference numerals. This is an example in which the present invention is applied to a magnetic clamp device having the square magnetic pole member  10  of  FIGS. 2A and 2B . The magnetic pole member  10  is radially divided into four magnetic pole pieces  101 ,  102 ,  103 ,  104  from the center.  FIG. 4A  shows that the permanent magnets  20  are arranged around the magnetic pole members  10 .  FIG. 4B  shows the arrangement positions of the permanent magnets  20  and the pole pieces  101 ,  102 ,  103 ,  104  after assembling. The magnetic pole pieces  101 ,  102 ,  103 ,  104  are adsorbed to the permanent magnets  20 . In the present example, unlike Example 1, the contact surfaces of the permanent magnet  20  and the first side surfaces R of the magnetic pole pieces  101 ,  102 ,  103 ,  104  are not curved surfaces but flat surfaces. Therefore, contrary to Example 1, in fixing the permanent magnets  20 , the magnetic pole pieces  101 ,  102 ,  103 ,  104  and the plate PL to one another, if gaps are designed so that they can be fitted, the permanent magnets  20  arranged on the plate PL adsorb each of the magnetic pole pieces  101 ,  102 ,  103 ,  104 . According to this, it is possible to eliminate each gap formed between the permanent magnets  20  and the magnetic pole pieces  101 ,  102 ,  103 ,  104 . In this figure, it is illustrated that the second side faces T are largely separated and the magnetic pole pieces  101 ,  102 ,  103 ,  104  are greatly moved, but in actual dimensions, they are only slightly moved around the through holes through which the bolts are inserted. In Example 2, not shown in the figure, a magnetic plate for continuously limiting the movement of the magnetic pole pieces  101 ,  102 ,  103 ,  104  in the Z-axis direction by one sheet needs to be provided on the front surface side of the magnetic pole pieces  101 ,  102 ,  103 ,  104 . 
     Example 3 
       FIGS. 5A, 5B, 5C and 5D  show various structures for fixing the permanent magnets  20 , the magnetic pole members  10  (only the magnetic pole pieces  101  and  103  are shown because the magnetic pole pieces  102  and  104  are not shown in the figure) and the alnico magnets  16  on the plate PL. It should be noted that components having the same functions as the reference numerals given in other figures are given the same reference numerals.  FIG. 5A  shows a magnetic clamp device in which the front surface portion D 5  corresponding to Example 1 is integrated with the plate PL. This is a bi-mag type magnetic clamp device in which the width of the outer yoke D 3  is reduced so that the polarity of the permanent magnet  20  is constructed in S pole on the inner peripheral side and N pole on the outer peripheral side, and so that the polarity of the permanent magnet  20 ′ is constructed in N pole on the inner peripheral side and S pole on the outer peripheral side. 
       FIG. 5B  is different from the structure of  FIG. 5A  in that the plate PL is divided into a front surface part PL- 1  and a rear surface part PL- 2  from the middle in the Z-axis direction. 
       FIG. 5C  shows almost the same structure as  FIG. 5A , except that the plate PL and the cap  19  are located opposite to each other on the front side and the back side in the Z-axis direction.  FIG. 5C  shows a form in which the front side and the back side of  FIG. 5A  are opposite in the Z-axis direction. That is, the cap  19  is arranged on the front side, and a front surface member is arranged on the back side. The arrangement of the permanent magnet, the magnetic pole member and the alnico magnet is the same as the structure of  FIG. 5A . A seal is provided to isolate the permanent magnet from an environment on the front surface of the plate PL. 
       FIG. 5D  shows a structure in which caps  19   a  and  19   b  are provided on the front side and the back side in the structure of  FIG. 5C  to sandwich the permanent magnet  20 , the magnetic pole member  10  and the alnico magnet  16 . In  FIG. 5E , the plate PL is divided into the front surface part PL- 1  and the rear surface part PL- 2  from the middle in the Z-axis direction so as to sandwich across the plurality of permanent magnets  20 , magnetic pole members  10  and alnico magnets  16 . As apparent from  FIGS. 5A to 5E , the magnetic pole member  10  is divided into the magnetic pole pieces  101  to  104  so as to move freely move. Therefore, when the magnetic pole member  10  is adsorbed to the permanent magnet  20 , its mechanical position needs to be fixed. In these examples, each magnetic pole piece is covered with one magnetic plate (in each example, the cap  19  or the plate PL has the function) from the Z-axis direction to be held mechanically between the magnetic plate and the alnico magnet  16 . 
       FIGS. 6A to 6J  show another division form of the magnetic pole member  10 .  FIGS. 6A to 6C  show examples of other division forms of the circular magnetic pole member  10 ,  FIGS. 6D to 6F  show examples of other division forms of the square magnetic pole member  10 , and  FIGS. 6G to 6J  show examples of division forms of polygonal magnetic pole member. With respect to the polygonal magnetic pole member  10 , the polygonal permanent magnets  20  or a plurality of plate-shaped permanent magnets  20  arranged in a polygonal shape surround the periphery of the magnetic pole member  10  (however, it is not shown). 
     In the division form of  FIG. 6A , the center of division is eccentric, and the size of each pole piece  101 ,  102 ,  103  is different. In the division form of  FIG. 6B , the center of division is two places, and the size of each pole piece  101 ,  102 ,  103 ,  104  is different. The division form of  FIG. 6C  has a large defect part in the center. In each case, when the magnetic pole pieces  101 ,  102 ,  103 ,  104  are adsorbed to the cylindrical permanent magnet  20 , the second side surfaces divided by an inner dividing line Q of each magnetic pole piece  101 ,  102 ,  103 ,  104  are separated from each other. The magnetic pole pieces  101 ,  102 ,  103 ,  104  are adsorbed to the permanent magnet  20  without interfering with each other, respectively. 
     The division form of  FIG. 6D  is formed by being divided in the middle of two sides of a square (top and bottom opposing sides in the figure). In this configuration, the permanent magnet  20  is closely contacted up and down only, and may not be closely contacted left to right.  FIG. 6E  shows a form of division by an inner division line Q passing through corners of a square.  FIG. 6F  shows a form of division by an inner division line Q passing through corners of a square as in  FIG. 6E . In any of the forms, the second side surfaces divided by the inner division line Q of each magnetic pole piece are oriented away from each other, and the magnetic pole pieces are adsorbed to the permanent magnet  20  without interfering with each other. The inner division lines Q need only be set so that the second side surfaces do not interfere with each other when the magnetic pole pieces are adsorbed to the permanent magnet  20 , so that the inner division lines Q may be divided so that the defect part occurs in the magnetic pole member  10 . 
     The division forms of  FIGS. 6G and 6H  are formed by being divided by inner division lines Q passing through the vertices of a polygon. The division forms of  FIGS. 6I and 6J  are formed by being divided by inner division lines Q passing through the middle of a side of a polygon. In any of the forms, the second side surfaces divided by the inner division lines Q of each magnetic pole piece are separated from each other, and the magnetic pole pieces are adsorbed to the permanent magnet  20  without interfering with each other. 
     Although the caps  19 ,  19   a , and  19   b  are magnetic bodies that connect the magnetic circuit between the alnico magnet  16  and the outer yoke D 3 , the outer peripheral surfaces of the caps  19 ,  19   a ,  19   b  in the above mentioned examples are magnetically coupled to the inner peripheral surfaces of the outer yoke D 3 . On the other hand, also in the fitting of the caps  19 ,  19   a ,  19   b  and the outer yoke D 3 , it is necessary to previously provide gaps for fitting. Therefore, in order to make the gaps for fitting as small as possible, the caps  19 ,  19   a  and  19   b  may be divided into a plurality of divided pieces after the shapes of the caps  19 ,  19   a  and  19   b  are made as close as possible to the shape of the inner peripheral surface of the outer yoke D 3  as in the divided form of each magnetic pole member  10  shown in  FIGS. 3A, 3B and 3C ,  FIGS. 4A and 4B , and  FIGS. 6A to 6J . When the magnetic pole members  10  of  FIGS. 3A, 3B and 3C ,  FIGS. 4A and 4B , and  FIGS. 6A to 6J  are regarded as the caps  19 ,  19   a , and  19   b , and the magnetic pole pieces  101 ,  102 ,  103 , and  104  are regarded as divided pieces, respectively, a side surface that is as close as possible to the inner peripheral surface of the outer yoke D 3  to be connected of the side surfaces of divided pieces of the caps  19 ,  19   a , and  19   b  is defined as a third side surface, and side surfaces facing the side surfaces of another adjacent divided piece can be defined as fourth side surfaces. When the divided pieces of the caps  19 ,  19   a , and  19   b  form a magnetic circuit between the alnico magnet  16  and the outer yoke D 3 , the fourth side surfaces of the divided pieces are separated from each other and the third side surfaces are adsorbed to the outer yoke D 3 . 
     Each divided piece of the caps  19 ,  19   a , and  19   b  is fixed by pouring resin into each gap of the fourth side surfaces or pressing from behind after each third side surface is adsorbed to the outer yoke D 3 . 
     DESCRIPTION OF SYMBOLS 
     
         
         
           
               10  magnetic member 
               11  magnet block 
               14  groove 
               15  bolt 
               16  alnico magnet 
               17  coil 
               18  reversible magnet 
               19  cap 
               19   a ,  19   b  cap 
               20  permanent magnet 
               21  through hole 
               22  through hole 
               23  fixing hole 
               101 ,  102 ,  103 ,  104  magnetic pole piece 
               201 ,  202 ,  203 ,  204  permanent magnet piece