Abstract:
A vibration motor includes a rotary shaft, an armature core secured to the rotary shaft and having a plurality of salient poles, an armature coil wound around each of the salient poles except at least one salient pole, a frame serving as an outside cover, a plurality of magnets secured to the frame and having respective different polarities, and a weight adapted to rotate together with the rotary shaft and disposed eccentrically, wherein the weight is disposed inside the frame and secured to the armature core in such a manner that at least a part of the weight is inserted in a hollow provided in at least the one salient pole having no armature coil.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a vibration motor, and in particular to a vibration motor to be used for providing a physical sensation in a portable communication device such as a mobile phone and a PHS (Personal Handy-phone System) and a game device. 
     2. Description of the Related Art 
     For notifying a user of an incoming call in communication devices such as a mobile phone, two methods are available. In one method, sound is generated at the time of receiving the incoming call to acoustically inform the user of the incoming call, and in the other method, vibration is generated by a built-in vibration motor to physically inform the user of the incoming call. These two methods are both incorporated in the communication devices and can be switched over as appropriate. 
     Also, in order to enhance entertaining property in game devices, some game devices transfer vibration generated in steps with progresses of games by vibration motors embedded in the game devices to operators of the game devices. 
     A vibration motor of this type is configured to be secured on a circuit board in the device and, vibration is generated, for example, by a rotating eccentric weight attached to a motor shaft and the generated vibration is transferred to a housing of the device. 
     A vibration motor is mounted in a small-sized device such as a portable communication device in many cases and, therefore, miniaturization of the vibration motor itself is required. Conventionally, an eccentric weight is attached to an end of a motor shaft to generate vibration. Such configuration presents a problem that the eccentric weight is mounted outside a motor frame and, therefore, when the vibration motor is embedded in the portable communication device or the like, it is necessary to form an appropriate gap in a portion around the eccentric weight, which, as a result, causes complicated design. 
     To solve the above problem, in Patent Reference 1 (Japanese Patent Application Laid-open No. Hei 11-289734) or Patent Reference 2 (Japanese Patent Application Laid-open No. Hei 8-256449), inventions are disclosed in which, in order to be able to generate vibration without mounting the eccentric weight outside the motor frame, one of three salient poles being a magnetic pole of an armature core is removed and the remaining two salient poles are made to have eccentricity to generate vibration. 
     However, in the configuration as disclosed in Patent References 1 and 2 in which one of the three salient poles is removed, there is a problem that a cogging phenomenon increases and actually the motor would not rotate or sufficient output from the motor can not be obtained. 
     SUMMARY OF THE INVENTION 
     In view of the above, it is an object of the present invention to provide a vibration motor capable of stably rotating to generate vibration without causing an increase in a cogging phenomenon and without the need for providing an eccentric weight outside a motor frame. 
     According to an aspect of the present invention, there is provided a vibration motor which includes: a rotary shaft; an armature core secured to the rotary shaft and having a plurality of salient poles; an armature coil wound around each of the salient poles except at least one salient pole; a frame serving as an outside cover; a plurality of magnets secured to the frame and having respective polarities alternately different from each other; and a weight adapted to rotate together with the rotary shaft and structured to be eccentrically weighted with respect to the rotary shaft, wherein weight includes a main body portion, a first holding portion and a second holding portion such that the main body portion is sandwiched between the first holding portion and the second holding portion, is disposed inside the frame, secured to the armature core in such a manner that a part of the second holding portion is inserted in a hollow provided in the at least one salient pole having no armature coil, and secured to the rotary shaft by means of the first holding portion. 
     In the aspect of the present invention, the main body portion, the first holding portion and the second holding portion may be integrated with one another. 
     In the aspect of the present invention, the main body portion and the second holding portion may be integrated to each other, and the main body portion and the first holding portion may be engagingly connected to each other. 
     In the aspect of the present invention, the main body portion, the first holding portion and the second holding portion may be separated from one another, the main body portion and the first holding portion may be engagingly connected to each other, and the main body portion and the second holding portion may be engagingly connected to each other. 
     In the aspect of the present invention, the weight may be eccentrically weighted with respect to the rotary shaft to an angle range where the armature coil is disposed. 
     In the aspect of the present invention, the outermost radius of the weight is larger than the radius of a circle defined by radially inward surfaces of the magnets. 
     In the aspect of the present invention, the frame may have a cross section in which two plane portions are provided. 
     In the aspect of the present invention, the plane portion may be attached to a mounting board. 
     In the aspect of the present invention, the armature coils may make a V-connection. 
     With the above configurations, the vibration motor is capable of stably rotating to generate vibration without causing an increase in the cogging phenomenon and without the need for providing the eccentric weight outside the motor frame. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other objects, advantages, and features of the present invention will be more apparent from the following description taken in conjunction with the accompanying drawings in which: 
         FIG. 1  is a schematic axial cross-sectional view of a motor with a commutator according to a first embodiment of the present invention; 
         FIG. 2  is a schematic radial cross-sectional view of the motor with a commutator, taken from line I-I of  FIG. 1 ; 
         FIG. 3  is a perspective view of a weight shown in  FIG. 1 , seen from above; 
         FIG. 4  is a perspective view of the weight shown in  FIG. 1 , seen from below. 
         FIG. 5  is a side view of the weight shown in  FIG. 1 , seen from a projection thereof; 
         FIGS. 6A and 6B  are diagrams to explain a V-connection of an armature coil for the motor with a commutator shown in  FIG. 1 , wherein  FIG. 6A  schematically shows a state where brushes are in touch with commutator units and  FIG. 6B  shows an equivalent circuit corresponding to the connection state; 
         FIG. 7  is a schematic axial cross-sectional view of a motor with a commutator according to a second embodiment of the present invention; 
         FIG. 8  is a perspective view of a weight of  FIG. 7  seen from above; 
         FIG. 9  is a perspective view of a first weight holding member shown in  FIG. 7  seen from below; 
         FIG. 10  is a schematic axial cross-sectional view of a motor with a commutator according to a third embodiment of the present invention; 
         FIG. 11  is a perspective view of a weight of  FIG. 10  seen from above; 
         FIG. 12  is a perspective view of a first weight holding member shown in  FIG. 10  seen from below; 
         FIG. 13  is a schematic axial cross-sectional view of a motor with a commutator according to a fourth embodiment of the present invention; 
         FIG. 14  is a perspective view of a weight of  FIG. 13  seen from above; 
         FIG. 15  is a perspective view of a second weight holding member of  FIG. 13  seen from below; 
         FIG. 16  is a schematic axial cross-sectional view of a motor with a commutator according to a fifth embodiment of the present invention; 
         FIG. 17  is a perspective view of a second weight holding member of  FIG. 16  seen from below; 
         FIG. 18  is a schematic axial cross-sectional view of a motor with a commutator according to a sixth embodiment of the present invention; 
         FIG. 19  is a perspective view of a weight shown in  FIG. 18  seen from below; and 
         FIG. 20  is a schematic radial cross-sectional view of a variation example of the first embodiment in which the frame of the motor with a commutator has a different shape. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Best modes of carrying out the present invention will be described in further detail using various embodiments with reference to the accompanying drawings. In each of the following embodiments, the present invention is applied to a motor with a commutator as an example of a vibration motor of the present invention. 
     First Embodiment 
     Now, referring to  FIGS. 1 through 6B , description is made about a motor with a commutator as a vibration motor according to a first embodiment of the present invention. 
       FIG. 1  is a schematic axial cross-sectional view of the motor with a commutator (hereinafter referred to simply as “motor” as appropriate)  1  according to the first embodiment of the present invention. 
       FIG. 2  is a schematic radial cross-sectional view of the motor  1  shown in  FIG. 1 , taken from line I-I of  FIG. 1 . 
     In the motor  1 , a commutator unit  3  and an armature core  2  are mounted on a rotary shaft  5  thereby constituting a rotor as a whole. The rotary shaft  5  is detachably disposed and rotatably supported via bearings  8  attached to a frame  6 . Magnets  7  are attached to the frame  6  thereby constituting a stator as a whole. 
     The armature core  2  is configured such that a required number of magnetic plates each having punched three salient poles are stacked on one another, and only two out of the three salient poles are each wound with an armature coil  4 . While the present invention does not depend on the number of poles of the motor, the above described three-pole motor is used as an example in this embodiment. One pole, out of the three poles, is not wound with the armature coil  4  and has a shape and structure different from those of other poles, that is, one hollow  2   a  is formed in the pole not provided with the armature coil  4 . 
       FIGS. 3 and 4  are perspective views of a weight  9  shown in  FIG. 1 , seen from above and below, respectively, and  FIG. 5  is a side view of the weight  9 . Referring to  FIGS. 3 ,  4  and  5 , the weight  9 , which is configured to be weighted eccentrically with respect to the rotary shaft  5 , integrally includes: a main body  9   b  shaped substantially semi-cylindrical; a main body holding block  9   d  shaped substantially semi-cylindrical, disposed so as to have its flat surface integrally communicating with a flat surface of the main body  9   d  and having a smaller radius and a smaller axial length than the main body  9   b ; and a projection  9   a  extending axially from an axial end of the main body portion  9   b . The main body  9   b  has a Japanese folding fan-like cross section, has a radius substantially equal to a radius of the armature core  2  and has its center coinciding substantially with a center of the rotary shaft  5 . The projection  9   a  is inserted in the hollow  2   a  of the armature core  2  and the weight  9  is thereby secured to the armature core  2 . 
     A through-hole  9   e  is formed to axially extend and to be radially centered at a portion where the main body  9   b  communicates with the main body holding block  9   d , and the rotary shaft  5  is penetrated through the through-hole  9   e . A groove  9   c  is formed to axially extend at the flat surface of the main body  9   b , and the rotary shaft  5  is guided along the groove  9   c.    
     A material for the weight  9  is determined by giving priority to a specific gravity of the material and the weight  9  is formed by sintering or MIM (Metal Injection Molding) process on, for example, a tungsten alloy. 
     The weight  9  is attached to the motor  1  in a manner that the rotary shaft  5  is penetrated through the through-hole  9   e , and the projection  9   a  is inserted into the hollow  2   a  of the armature core  2  so as to come into contact with the axial end of the armature core  2 . The weight  9  may be secured to the rotary shaft  5  by press-fitting the rotary shaft  5  into the through-hole  9   e . Alternatively, the weight  9  may be secured to the rotary shaft  5  by caulking the weight  9  after inserting the rotary shaft  5  into the through-hole  9   e.    
     The armature coil  4  is wound around only two poles out of the three poles of the salient poles to realize V-connection. 
       FIGS. 6A and 6B  are diagrams to explain the V-connection of the armature coil  4  for the motor  1  shown in  FIG. 1 , wherein  FIG. 6A  schematically shows a state where brushes are in touch commutators and  FIG. 6B  shows an equivalent circuit corresponding to the connection state. 
     In the V-connection, the armature coils  4  are connected to commutator units  3  and brushes  11  are disposed in touch with the commutator units  3  as shown in  FIG. 6A . 
     In the motor  1  of the first embodiment of the present invention, the shape of the armature core  2  disposed in a manner to face the magnets  7  is the same as a shape of a conventional armature core in which coil is wound around all of three poles thereof. Accordingly, the motor  1  has an advantageous effect that a cogging torque is not increased. Further, in the motor  1  of the first embodiment, the salient pole having the hollow  2   a  into which the projection  9   a  of the weight  9  is inserted is not wound with the armature coil  4 . Accordingly, the motor  1  has another advantageous effect that not only the number of winding processes but also the amount of magnet wires is reduced. 
     Second Embodiment 
     Next, referring to  FIGS. 7 through 9 , description proceeds to a motor with a commutator as a vibration motor according to a second embodiment of the present invention. 
       FIG. 7  is a schematic axial cross-sectional view of the motor with a commutator  101  according to the second embodiment of the present invention. 
     In the second embodiment, components similar to those of the first embodiment are designated by like reference numerals and their detailed descriptions are omitted accordingly. 
     According to the second embodiment, a weight  109  serving as an eccentric weight to vibrate the motor  101  is secured to the rotary shaft  5  not by itself but by means of a weight holding member  112  provided separately from the weight  109 . 
       FIG. 8  is a perspective view of the weight  109  shown in  FIG. 7  seen from above, and  FIG. 9  is a perspective view of the weight holding member  112  shown in  FIG. 7  seen from below. 
     Referring to  FIG. 8 , the weight  109  includes a semi-cylindrical main body (corresponding to the main body  9   b  of the first embodiment)  109   b  which has on its flat surface an axially extending groove  109   c  along which the rotary shaft  5  is guided, and a projection  109   a  extending axially from an axial end of the main body  109   b . The main body  109   b  has a Japanese folding fan-like cross section, has a radius substantially equal to a radius of the armature core  2  and has its center approximately coinciding with a center of the rotary shaft. A notched portion  109   f  is formed at the arc edge of the main body  109   b.    
     A material for the weight  109  is determined by giving priority to a specific gravity of the material and the weight  109  is formed by sintering or MIM processes on, for example, a tungsten alloy. 
     Referring to  FIG. 9 , the weight holding member  112  integrally includes: a large radius semi-cylindrical portion  112   a ; a small radius semi-cylindrical portion  112   c  having its flat surface integrally communicates a flat surface of the large radius semi-cylindrical portion  112   a ; and a boss  112   b  extending axially from an axial end of the large radius semi-cylindrical portion  112   a . A through-hole  112   d  is formed to axially extend and to be radially centered at a portion where the large radius semi-cylindrical portion  112   a  communicates with the small semi-cylindrical portion  112   c , and the rotary shaft  5  is penetrated through the through-hole  112   d.    
     The weight holding member  112  can be made of a material having a small specific gravity and is formed by sintering or MIM process on, for example, steel, copper, or an alloy thereof. 
     The projection  109   a  of the weight  109  is inserted into the hollow  2   a  of the armature core  2  so as to be in contact with the axial end of the armature core  2 , and the rotary shaft  5  is penetrated and pushed through the through-hole  112   d  of the weight holding member  112  thereby allowing the boss  112   b  of the weight holding member  112  to be geared to the notched portion  109   f  of the weight  109 , and thus the weight holding member  112  is press-fitted to the weight  109 . Alternatively, the weight  109  may be secured to the weight holding member  112  by caulking the weight holding member  112  after the weight holding member  112  is geared to the weight  109 . 
     As a result, the weight  109  is secured to the rotary shaft  5  by the boss  112   d  of the weight holding member  112  being geared to the notched portion  109  of the weight  109 . 
     According to the motor  101  of the second embodiment, the weight  109  has a simple structure compared with the weight  9  of the first embodiment due to no through-hole formed therein. The motor  101  of the second embodiment therefore has an advantageous effect that the weight  109  can be readily produced. 
     The weight holding member  112  requiring the through-hole  112   d  through which the rotary shaft  5  is penetrated is different in function and separated from the weight  109 . The weight holding member  112  can therefore be made of a material different from that of the weight  109 . This extends a range of choice of the material. In other words, since a proportion of mass as a requirement for the weight holding member  112  is small, the weight holding member  112  can also be made of a material having a small specific gravity such as steel, copper, an alloy thereof, and the like. 
     The material having a small specific gravity such as steel, copper, an alloy thereof and the like is small in hardness, compared with a tungsten alloy. Therefore, a load can be reduced which is imposed on the weight holding member  112  when it is caulked to be secured to the rotary shaft  5 . Consequently, stress imposed on the rotary shaft  5  also is reduced. As a result, a problem that the rotary shaft  5  is bent at the caulking process is not easily caused to occur. Accordingly, a yield of the motor  101  can be improved. 
     Moreover, although the motor  101  of the second embodiment requires the weight holding member  112  as an additional component, the weight holding member  112  has a small axial length and therefore can be readily formed by sintering or MIM (Metal Injection Molding) process on the material thereof. 
     In the motor  101  of the second embodiment of the present invention, the shape of the armature core  2  disposed in a manner to face the magnets  7  is the same as a shape of a conventional armature core in which coil is wound around all of three poles thereof. Accordingly, the motor  101  has an advantageous effect that a cogging torque is not increased. 
     In the interim, in the conventional motor in which the eccentric weight is secured externally using only the rotary shaft, it is necessary to increase the load on the eccentric weight, for example, when caulking the eccentric weight to be secured to the rotary shaft in order to prevent the eccentric weight from free wheeling during the rotation of the rotary shaft. On the other hand, in the motor  101  of the second embodiment, the projection  109   a  of the weight  109  is inserted into the hollow  2   a  of the armature core  2 , and the weight  109  can thereby be prevented from free wheeling. The load imposed on the weight  109  can be decreased, when the weight  109  is caulked to be secured to the rotary shaft  5 . As a result, the rotary shaft  5  is hard to bend; when the weight  109  is secured to the rotary shaft  5 . Accordingly, a yield of the motor  101  can be improved. 
     Further, in the motor  101  of the second embodiment, the salient pole having the hollow  2   a  through which the projection  109   a  of the weight  109  is inserted is not wound with the armature coil  4 . Accordingly, the motor  101  has another advantageous effect that not only the number of winding processes but also the amount of magnet wires is reduced. 
     Third Embodiment 
     Next, referring to  FIGS. 10 through 12 , description proceeds to a motor with a commutator as a vibration motor according to a third embodiment of the present invention. 
       FIG. 10  is a schematic axial cross-sectional view of the motor with a commutator according to the third embodiment of the present invention. 
     In the third embodiment, components similar to those of the first embodiment are designated by like reference numerals and their detailed descriptions are omitted accordingly. 
     According to the third embodiment, a weight  209  serving as an eccentric weight to vibrate a motor  201  is secured to the rotary shaft  5  not by itself but by means of a weight holding member  212  provided separately from the weight  209 . 
       FIG. 11  is a perspective view of the weight  209  shown in  FIG. 10  seen from above, and  FIG. 12  is a perspective view of the weight holding member  212  shown in  FIG. 10  seen from below. 
     A material for the weight  209  is determined by giving priority to a specific gravity of the material and the weight  209  is formed by sintering or MIM processes on, for example, a tungsten alloy. 
     Referring to  FIG. 11 , the weight  209  includes: a semi-cylindrical main body (corresponding to the main body  9   b  of the first embodiment)  209   b  having on its flat surface an axially extending groove  209   c  along which the rotary shaft  5  is guided; a projection  209   a  disposed on an axial end of the semi-cylindrical main body  209   b ; and a boss  209   g  formed at the other axial end of the semi-cylindrical main body  209   b . The semi-cylindrical main body portion (large-diameter portion)  209   b  has a Japanese folding fan-like cross section, has a radius substantially equal to a radius of the armature core  2  and has its center approximately coinciding with a center of the rotary shaft  5 . 
     Referring to  FIG. 12 , the weight holding member  212  includes a large radius semi-cylindrical portion  212   a  and a small radius semi-cylindrical portion  212   c . A through-hole  212   d  is formed to axially extend and to be radially centered at a portion where the large radius semi-cylindrical portion  212   a  communicates with the small semi-cylindrical portion  212   c , and the rotary shaft  5  is penetrated through the through-hole  212   d . Also, a recessed portion  212   e  having a cross section corresponding to and slightly larger than a cross section of the boss  209   g  of the weight  209  is formed at an axial end of the large radius semi-cylindrical portion  212   a.    
     The projection  209   a  of the weight  209  is inserted into the hollow  2   a  of the armature core  2  so as to be in contact with the axial end of the armature core  2 , and the rotary shaft  5  is penetrated and pushed through the through-hole  212   d  of the weight holding member  212  thereby engaging the recessed portion  212   e  of the weight holding member  212  with the boss  209   g  of the weight  209 , and thus the weight holding member  212  is press-fitted to the weight  209 . Alternatively, the weight holding member  212  may be press-fitted to the weight  209  by caulking the weight holding member  212  after the weight holding member  212  is geared to the weight  209 . 
     As a result, the weight  209  is secured to the rotary shaft by the recessed portion  212  of the weight holding member  212  engaging with the boss  209   g  of the weight  209 . 
     According to the motor  201  of the third embodiment, the weight  209  has a simple structure compared with the weight  9  of the first embodiment due to no through-hole formed therein. The motor  201  of the third embodiment therefore has an advantageous effect that the weight  209  can be readily produced. 
     The weight holding member  212  requiring the through-hole  212   d  through which the rotary shaft  5  is penetrated is different in function and separated from the weight  209 . The weight holding member  212  can therefore be made of a material different from that of the weight  209 . This extends a range of choice of the material. In other words, since a proportion of mass as a requirement for the weight holding member  212  is small, the weight holding member  212  can also be made of a material having a small specific gravity such as steel, copper, an alloy thereof, and the like. 
     The material having a small specific gravity such as steel, copper, an alloy thereof and the like is small in hardness, compared with a tungsten alloy. Therefore, a load can be reduced which is imposed on the weight holding member  212  when it is caulked to be secured to the rotary shaft  5 . Consequently, stress imposed on the rotary shaft  5  also is reduced. As a result, a problem that the rotary shaft  5  is bent at the caulking process is not easily caused to occur. Accordingly, a yield of the motor  101  can be improved. 
     Moreover, although the motor  201  of the third embodiment requires the weight holding member  212  as an additional component, the weight holding member  212  has a small axial length and therefore can be readily formed by sintering or MIM (Metal Injection Molding) process on the material thereof. 
     In the motor  201  of the third embodiment of the present invention, the shape of the armature core  2  disposed in a manner to face the magnet  7  is the same as a shape of a conventional armature core in which coil is wound around all of three poles thereof. Accordingly, the motor  201  has an advantageous effect that a cogging torque is not increased. 
     In the interim, in the conventional motor in which the eccentric weight is secured externally using only the rotary shaft, it is necessary to increase the load on the eccentric weight, for example, when caulking the eccentric weight to be secured to the rotary shaft in order to prevent the eccentric weight from free wheeling during the rotation of the rotary shaft. On the other hand, in the motor  201  of the third embodiment, the projection  209   a  of the weight  209  is inserted into the hollow  2   a  of the armature core  2 , and the weight  209  can thereby be prevented from free wheeling. The load imposed on the weight  209  can be decreased, when the weight  209  is caulked to be secured to the rotary shaft  5 . As a result, the rotary shaft  5  is hard to bend, when the weight  209  is secured to the rotary shaft  5 . Accordingly, a yield of the motor  201  can be improved. 
     Further, in the motor  201  of the third embodiment, the salient pole having the hollow  2   a  through which the projection  209   a  of the weight  209  is inserted is not wound with the armature coil  4 . Accordingly, the motor  201  has another advantageous effect that not only the number of winding processes but also the amount of magnet wires is reduced. 
     Fourth Embodiment 
     Next, referring to  FIGS. 13 through 15 , description proceeds to a motor with a commutator as a vibration motor according to a fourth embodiment of the present invention. 
       FIG. 13  is a schematic axial cross-sectional view of the motor with a commutator according to the fourth embodiment of the present invention. 
     In the fourth embodiment, components similar to those of the first and the second embodiments are designated by like reference numerals and their detailed descriptions are omitted accordingly. 
     According to the fourth embodiment, a weight  309  serving as an eccentric weight to vibrate a motor  301  is secured to the rotary shaft  5  not by itself but by means of (first) weight holding member  112  provided separately from the weight  309 . Further, the weight  309  is not directly secured to the armature core  2  by itself but via another (second) weight holding member  313  interposed between the weight  309  and the armature core  2 . 
       FIG. 14  is a perspective view of the weight  309  shown in  FIG. 13  seen from above, and  FIG. 15  is a perspective view of the second weight holding member  313  shown in  FIG. 13  seen from below. In this connection, the first weight holding member  112  of this embodiment is structured the same as the weight holding member  112  in the second embodiment and therefore has the same reference numerals. 
     A material for the weight  309  is determined by giving priority to a specific gravity of the material and the weight  309  is formed by sintering or MIM processes on, for example, a tungsten alloy. 
     The weight  309  includes a substantially semi-cylindrical main body  309   b  having on its flat surface an axially extending groove  309   c  along which the rotary shaft  5  is guided. A notched portion  309   f  is formed at the arc edge at one axial end of the main body  309   b  and geared to a boss  112   b  of the first weight holding member  112 . A notched portion  309   h  is formed at the arc edge at the other axial end of the main body  809   b . The main body  309   b  of the weight  809  has a Japanese folding fan-like cross section, has a radius substantially equal to a radius of the armature core  2  and has its center approximately coinciding with a center of the rotary shaft  5 . 
     The second weight holding member  313  integrally includes: a large radius semi-cylindrical portion  313   b ; a small radius semi-cylindrical portion  313   d  wherein a through-hole  313  is formed to axially extend and to be radially centered at a portion where respective flat surfaces of the large and small radius semi-cylindrical portions  313   b  and  313   b  communicate with each other; a boss  313   c  disposed at the arc edge at one axial end of the large radius semi-cylindrical portion  313   b ; and a boss  313   a  disposed at the other axial end of the large radius semi-cylindrical portion  313   b . The boss  313   a  of the second weight holding member  313  is to be inserted into the hollow  2   a  of the armature core  2  thereby securing the second weight holding member  313  to the armature core  2 , while the boss  313   c  is to be geared to the notched portion  309   h  of the weight  309 . 
     The second weight holding member  14  can be made of a material having a small specific gravity and is formed by sintering or MIM process on, for example, steel, copper, or an alloy thereof. 
     The rotary shaft  5  is penetrated through the through-hole  313   e  of the second weight holding member  313 , the boss  313   a  of the second weight holding member  313  is inserted into the hollow  2   a  of the armature core  2  so as to be in contact with the axial end of the armature core  2 , then the weight  309  is pushed toward the second weight holding portion  313  with the groove  309   c  guiding the rotary shaft  5  thereby gearing the notched portion  309   h  of the weight  309  to the boss  313   c  of the second weight holding member  313 , further the rotary shaft  5  is penetrated through the through-hole  112   d  of the first weight holding member  112 , and finally the first weight holding member  112  is pushed toward the weight  309  thereby gearing the boss  112   b  of the first weight holding member  112  to the notched portion  309   f  of the weight  309  thus press-fitting the first weight holding member  112  to the weight  309 . Alternatively, the first weight holding member  112  may be secured to the weight  309  by caulking the weight holding member  112  after the weight holding member  112  is geared to the weight  309 . 
     As a result, the weight  309  is secured to the armature core  2  via the second weight holding member  313  and at the same time secured to the rotary shaft  5  by means of the first and second weight holding members  112  and  313 . 
     According to the motor  301  of the fourth embodiment, it is not necessary to form a through-hole in the weight  309 , and so on. the weight  309  has a still simpler structure than the second and third embodiments due to no through-hole nor a projection formed therein. As a result, the motor  301  of the fourth embodiment has an advantageous effect that the weight  309  can be readily produced. 
     The first and second weight holding members  112  and  313  each of which requires the through-hole through which the rotary shaft  5  is penetrated are different in function and separated from the weight  309 , respectively. The first and second weight holding members  112  and  313  can therefore be made of materials different from that of the weight  309 . This extends a range of choice of the materials. In other words, since a proportion of mass as a requirement for the first and second weight holding members  112  and  313  is small, the first and second weight holding members  112  and  313  can also be made of a material having small specific gravity such as steel, copper, an alloy thereof, and the like. 
     The material having a small specific gravity such as steel, copper, an alloy thereof and the like is small in hardness, compared with a tungsten alloy. Therefore, a load imposed on the first and second weight holding members  112  and  313  at the time of being caulked to be secured to the rotary shaft  5  can be reduced. Consequently, stress imposed on the rotary shaft  5  also is small. As a result, a problem that the rotary shaft  5  is bent at the time of being caulked is not easily caused to occur. Accordingly, a yield of the motor  301  can be improved. 
     Moreover, although the motor  301  of the fourth embodiment requires the weight holding members  112  and  313  as additional components, the first and second weight holding members  112  and  313  have a small axial length and therefore can be readily formed by sintering or MIM (Metal Injection Molding) process on the materials thereof. 
     In the motor  301  of the fourth embodiment of the present invention, the shape of the armature core  2  disposed in a manner to face the magnets  7  is the same as a shape of a conventional armature core in which coil is wound around all of three poles thereof. Accordingly, the motor  301  has an advantageous effect that a cogging torque is not increased. 
     In the interim, in the conventional motor in which the eccentric weight is secured externally using only the rotary shaft, it is necessary to increase the load on the eccentric weight, for example, when caulking the eccentric weight to be secured to the rotary shaft in order to prevent the eccentric weight from free wheeling during the rotation of the rotary shaft. On the other hand, in the motor  301  of the fourth embodiment, the boss  313   a  of the second weight holding member  313  is inserted into and secured in the hollow  2   a  of the armature core  2  while the weight  309  is pushed radially toward the rotary shaft  5  by the boss  313   c  of the second weight holding member  313 . The weight  309  can thereby be prevented from free wheeling. The load imposed on the weight  309  can be decreased when the weight  309  is caulked to be secured to the rotary shaft  5 . As a result, the rotary shaft  5  is hard to bend when the weight  309  is secured to the rotary shaft  5 . Accordingly, a yield of the motor  301  can be improved. 
     Further, in the motor  301  of the fourth embodiment, the salient pole having the hollow  2   a  through which the boss  313   a  of the second weight holding member  313  is inserted is not wound with the armature coil  4 . Accordingly, the motor  301  has another advantageous effect that not only the number of winding processes but also the amount of magnet wires is reduced. 
     Fifth Embodiment 
     Next, referring to  FIGS. 16 and 17 , description proceeds to a motor with a commutator as a vibration motor according to a fifth embodiment of the present invention. 
       FIG. 16  is a schematic axial cross-sectional view of the motor with a commutator according to the fifth embodiment of the present invention. 
     In the fifth embodiment, components similar to those of the first and the second embodiments are designated by like reference numerals and their detailed descriptions are omitted accordingly. 
     According to the fifth embodiment, a weight  309  serving as an eccentric weight to vibrate a motor  401  is secured to the rotary shaft  5  not by itself but by means of a first weight holding member  112  provided separately from the weight  309 . Further, the weight  309  is secured directly to the armature core  2  not by itself but via a second weight holding member  413  interposed between the weight  309  and the armature core  2 . 
       FIG. 17  is a perspective view of the second weight holding member  413  shown in  FIG. 16  seen from below. In this connection, the weight  309  of this embodiment is structured the same as the weight  309  of the fourth embodiment, and also the first weight holding member  112  of this embodiment is structured the same as the first weight holding member  112  of the second embodiment. 
     The second weight holding member  413  can be made of a material having a small specific gravity and is formed by sintering or MIM process on, for example, steel, copper, or an alloy thereof. 
     The second weight holding member  413  integrally includes: a main body  413   b  formed in a segmented cylinder having a Japanese folding fan-like cross section defining a smaller angle than 180 degrees; a square portion disposed to extend radially from a center of the segmented cylinder; a boss  413   a  disposed at an axial end of the square portion; and another boss  413   c  disposed at an axial end of the main body  413   b  facing in the direction opposite to the axial end of the square portion having the boss  413   a  wherein the two bosses  413   a  and  413   c  are positioned circumferentially opposite to each other. 
     The boss  413   a  is inserted into the hollow  2   a  of the armature core  2  thereby securing the second weight holding member  413  to the armature core  2 . The boss  413   c  is geared to the notched portion  309   h  of the weight  309 . Further, a through-hole  413   d  is formed at a portion where the main body  418   b  communicates with the square portion, and the rotary shaft  5  is penetrated through the through-hole  413   d.    
     The rotary shaft  6  is penetrated through the through-hole  413   d  of the second weight holding member  413 , the boss  413   a  of the weight holding member  413  is inserted into the hollow  2   a  of the armature core  2  so as to be in contact with the axial end of the armature core  2 , then the weight  309  is pushed toward the second weight holding member  413  thereby gearing the notched portion  309   h  of the weight  309  to the boss  413   c  of the second weight holding member  413 , and further the rotary shaft  5  is penetrated through the through-hole  112   d  of the first weight holding member  112  and pushed toward the weight  309  thereby gearing the boss  112   b  of the first weight holding member  112  to the notched portion  309   f  of the weight  309  thus press-fitting the first weight holding member  112  to the weight  309  Alternatively, the first weight holding member  112  may be secured to the weight  309  by caulking the weight holding member  112  after the second weight holding member  112  is secured to the weight  309 . 
     As a result, the weight  309  is secured to the armature core  2  via the second weight holding member  418  and at the same time is pushed radially to the rotary shaft  5  by means of the boss  112   b  of the first weight holding member  112  and the boss  413   c  of the second weight holding member  413 . 
     In the fifth embodiment, the weight  309  is positioned in the same direction circumferentially with respect to the rotary shaft  5  as the salient pole around which the armature coil  4  is wound. Consequently, the armature coil  4  serves as part of the eccentric weight, thus enabling the vibration to be increased. 
     According to the motor  401  of the fifth embodiment, it is not necessary to form a through-hole in the weight  309 , and so on. the weight  309  has a simpler structure than any of the above-described embodiments due to no through-hole formed therein and also to the simpler configuration. As a result, the motor  401  of the fifth embodiment has an advantageous effect that the weight  309  can be readily produced. 
     The first and second weight holding members  112  and  413  each of which requires the through-hole through which the rotary shaft  5  is penetrated are different in function and separated from the weight  309 . The weight holding members  112  and  413  can therefore be made of materials different from that of the weight  309 . This extends a range of choice of the materials. In other words, since a proportion of mass as a requirement for the first and second weight holding members  112  and  413  is small, the weight holding members  112  and  413  can be made of a material having small specific gravity such as steel, copper, an alloy thereof, and the like. 
     The material having a small specific gravity such as steel, copper, an alloy thereof and the like is small in hardness, compared with a tungsten alloy. Therefore, a load can be reduced which is imposed on the first and second weight holding members  112  and  413  when it is caulked to be secured to the rotary shaft  5 . Consequently, stress imposed on the rotary shaft  5  also is reduced. As a result, a problem that the rotary shaft  5  is bent at the caulking process is not easily caused to occur. Accordingly, a yield of the motor  401  can be improved. 
     Moreover, although the motor  401  of the fifth embodiment requires the first and second weight holding members  112  and  413  as additional components, the weight holding members  112  and  413  have a small axial length and therefore can be readily formed by sintering or MIM (Metal Injection Molding) process on the materials thereof. 
     In the motor  401  of the fifth embodiment of the present invention, the shape of the armature core  2  disposed in a manner to face the magnets  7  is the same as a shape of a conventional armature core in which coil is wound around all of three poles thereof. Accordingly, the motor  401  has an advantageous effect that a cogging torque is not increased. 
     In the interim, in the conventional motor in which the eccentric weight is secured externally using only the rotary shaft, it is necessary to increase the load on the eccentric weight, for example, when caulking the eccentric weight to be secured to the rotary shaft in order to prevent the eccentric weight from free wheeling during the rotation of the rotary shaft. On the other hand, in the motor  401  of the fifth embodiment, the boss  413   a  of the second weight holding member  413  is inserted into and secured in the hollow  2   a  of the armature core  2  while the weight  309  is pushed toward the rotary shaft  5  by the boss  413   c  of the second weight holding member  413 . The weight  309  can thereby be prevented from free wheeling. The load imposed on the weight  309  can be decreased, if the weight  309  is caulked to be secured to the rotary shaft  5 . As a result, the rotary shaft  5  is hard to bend when the weight  309  is secured to the rotary shaft  5 . Accordingly, a yield of the motor  401  can be improved. 
     Further, in the motor  401  of the fifth embodiment, the salient pole having the hollow  2   a  through which the boss  413   a  of the second weight holding member  413  is inserted is not wound with the armature coil  4 . Accordingly, the motor  401  has another advantageous effect that not only the number of winding processes but also the amount of magnet wires is reduced. 
     Sixth Embodiment 
     Next, referring to  FIGS. 18 and 19 , description proceeds to a motor with a commutator as a vibration motor according to a sixth embodiment of the present invention. 
       FIG. 18  is a schematic axial cross-sectional view of the motor with a commutator according to the sixth embodiment of the present invention. 
     In the sixth embodiment, components similar to those of the first embodiment are designated by like reference numerals and their detailed descriptions are omitted accordingly. 
     Referring to  FIG. 18 , a projection  509   a  of a weight  509  serving as an eccentric weight to vibrate a motor  501  is inserted into the hollow  2   a  of the armature core  2  thereby securing the weight  509  to the armature core  2 . 
       FIG. 19  is a perspective view of the weight  509  shown in  FIG. 18  seen from below. 
     A material for the weight  509  is determined by giving priority to a specific gravity of the material and the weight  509  is formed by sintering or MIM processes on, for example, a tungsten alloy. 
     The weight  509  has a similar structure to the weight  9  of the first embodiment. Referring to  FIG. 19 , the weight  509  integrally includes: a main body  509   b  shaped substantially semi-cylindrical; a main body holding block  509   d  shaped substantially semi-cylindrical, disposed so as to have its flat surface integrally communicating with a flat surface of the main body  509   d  and having a smaller radius and a smaller axial length than the main body  9   b ; and a projection  509   a  extending axially from an axial end of the main body  509   b . The main body  509   b  has a Japanese folding fan-like cross section, has a radius larger than of a circle defined by radially inward surfaces of the magnets  7  and has its center coinciding substantially with a center of the rotary shaft  5 . The projection  9   a  is inserted in the hollow  2   a  of the armature core  2  and the weight  9  is thereby secured to the armature core  2 . 
     A through-hole  9   e  is formed to axially extend and to be radially centered at a portion where the main body  9   b  communicates with the main body holding block  9   d , and the rotary shaft  5  is penetrated through the through-hole  9   e . A groove  9   c  is formed to axially extend at the flat surface of the main body  9   b , and the rotary shaft  5  is guided along the groove  9   c.    
     In the motor  501  of the sixth embodiment, the outermost radius of the weight  509  is larger than the radius of the inner circumferential plane of the magnets  7 . In other words, the radially outermost portion of the weight  509  is made closer to the inner circumferential surface of the frame  6  than in the first embodiment to such an extent that an appropriate gap is still provided from the inner circumferential surface of the frame  6 . Thus, the weight  509  is made larger and heavier compared with in the first embodiment and therefore vibration can be increased. 
     Moreover, needless to say, configurations realized by making the outermost radius of the weight  509  larger than the radius of the inner circumferential plane of the magnets  7  can be applied to any one of the embodiments described above. 
     In each of the above embodiments, the frame  6  has a circular cross section as shown in  FIG. 2 , however, the present invention is not limited to this structure and the frame  6  may have an oval cross section. Alternatively, a frame  606  which has a cross section including two flat planes as shown in  FIG. 20 , may be used as a variation example derived from  FIG. 2  and incorporated in a motor  601 . 
     In  FIG. 20 , components similar to those of  FIG. 2  are designated by like reference numerals and their detailed descriptions are omitted accordingly. 
     The motor  601  is different from the motor  1  shown in  FIG. 2  in that a magnet  607  is disposed at the flat planes. Thus, the motor  601  can achieve advantageous effects of reducing the motor dimension and also easing motor disposition design. 
     Further, the motor  601  is further advantageous in that no further consideration needs to be taken about the space around the weight. 
     Moreover, the motor  601  generates vibration in a different direction, which can improve a physical sensation of vibration. 
     Furthermore, according to the motor  601 , the frame  606  has plane portions and therefore can be mounted flatly on a substrate. Thus, the motor  601  can be used as an SMD (Surface Mount Device). 
     Additionally the features of the embodiments described above may be combined as appropriate, which still falls within the scope of the present invention. 
     While this invention has thus far been described in conjunction with only several embodiments thereof, it will now be readily possible for those skilled in the art to put this invention into various other manners. 
     For example, in each of the above-described embodiments, a motor with a commutator is used as an example of a vibration motor of the present invention. However, the present invention can be applied to any vibration motors other than the motor with a commutator