Abstract:
There is provided a stepping motor including: a stator assembly; a rotary shaft which includes a lead screw portion and a plain portion with a rotor magnet disposed thereon, and which has both ends thereof rotatably supported by respective bearings; and a thrust mechanism which includes a resilient member provided in a recess formed at one end of the rotary shaft positioned toward the lead screw portion, and adapted to give an axial thrust force to the rotary shaft, and a point-contact member provided between the resilient member and one bearing of the respective bearings, the one bearing rotatably supporting the one end of the rotary shaft positioned toward the lead screw portion. Thus, the thrust mechanism is located out of and apart from the stator assembly which includes coils to radiate heat, and therefore is prevented from suffering influences from the heat radiated by the coils.

Description:
[0001]     This is a U.S. national stage of PCT Application No. PCT/JP2004/015027 filed on Oct. 12, 2004, claiming priority from Japanese Patent Application No. 2003-404756 filed on Dec. 3, 2003. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The present invention relates to a stepping motor as an actuator for use in a floppy disk drive, various office automation equipments, and the like, and more particularly to a stepping motor with a lead screw.  
       BACKGROUND OF THE INVENTION  
       [0003]     Recently, as the dimensional scale-down and the cost reduction of various electronic apparatuses are demanded, there is also a demand for stepping motors mounted in the apparatuses to become smaller, to deliver higher performance, and to be priced lower. To deal with such a demand, Japanese Patent Application Laid-Open No. H8-186959 discloses a stepping motor with a lead screw, which has a simple structure and still ensures a rotational accuracy good enough to prohibit deterioration of performance of reading and writing information on the disk of a floppy disk drive.  
         [0004]      FIG. 1  is a partly cross-sectional view of such a stepping motor as described above. A stepping motor  60  shown in  FIG. 1  for use in a floppy disk drive generally includes a stator assembly  61  including an A-phase stator unit  61   a  and a B-phase stator unit  61   b , and a rotary shaft  64  including a plain portion with rotor magnets  65  concentrically fixed thereon and a lead screw portion  66  integrally continuous with the plain portion. A protection cap  62  is fixedly attached to the rear end of the stator assembly  61 , and a hole  62   a  is formed at the center of the protection cap  62 . One end of the rotary shaft  64  positioned toward the lead screw portion  66  is rotatably supported at a pivot bearing  68  by means of a ball  67 , while the other end of the rotary shaft  64  positioned toward the plain portion is rotatably supported at a pivot bearing  63  by means of balls  69  and  70 . Here, a thrust mechanism is structured and operates such that the pivot bearing  63  is movably fitted into the hole  62   a  of the protection cap  62  so as to be disposed inside the stator assembly  61 , and is axially thrust by a spring  71  thereby axially thrusting the rotary shaft  64  by way of the balls  69  and  70 . The stepping motor  60  further includes a positioning bush  72  and a front plate  73 . For the purpose of cost reduction, the protection cap  62 , the pivot bearing  63 , and the positioning bush  72  are made of synthetic resin.  
         [0005]     Synthetic resin may be used also for forming a lead screw, and  FIG. 2  shows a stepping motor  80  incorporating a lead screw formed of synthetic resin. The stepping motor  80  in  FIG. 2  includes a stator assembly  94  composed of an A-phase stator unit  94   a  and a B-phase stator unit  94   b , and a rotary shaft  81  structured such that a metal pipe  82  has its inner and outer surfaces continuously covered by a synthetic resin  83  molded so as to form a plain portion toward one end of the metal pipe  82  and a lead screw portion  84  toward the other end thereof. A rotor magnet  85  is fixedly disposed on the plain portion of the rotary shaft  81  structured as described above. One end of the rotary shaft  81  toward the lead screw portion  84  is rotatably supported by means of a ball  86  at a bearing  88  attached to a bracket  87 , and the other end of the rotary shaft  81  toward the plain portion is rotatably supported at a pivot bearing  90  by means of balls  91  and  92 . A protection cap  89  having a hole at its center is attached to the B-phase stator unit  94   b  by means of bosses  96  and adapted to protect the pivot bearing  90 . Here, a thrust mechanism is structured and operates such that the pivot bearing  90  is movably fitted inside a yoke  95  of the B-phase stator unit  94   b , also movably fitted into the hole of the protection cap  89 , and is axially thrust by a spring  93  thereby axially thrusting the rotary shaft  81  by way of the balls  91  and  92 . The stepping motor  80  further includes a positioning bush  97 . The protection cap  89 , the pivot bearing  90 , and the positioning bush  97  are made of synthetic resin for cost reduction.  
         [0006]     In the stepping motor  60  with a lead screw shown in  FIG. 1 , when the rotary shaft  64  with the magnets  65  is rotated, heat is radiated from coils disposed inside the stator units  61   a  and  61   b  and is kept inside the stator assembly  61  which is almost hermetically closed with a minute clearance provided between the pivot bearing  63  and the protection cap  62  for allowing the pivot bearing  63  to be axially moved by force of the spring  71 . Consequently, the pivot bearing  63 , which is made of synthetic resin and susceptible to high temperature, undergoes a dimensional change due to the heat kept inside the stator assembly  61 , and if the outer dimension of the pivot bearing  63  is changed to increase, the clearance provided between the pivot bearing  63  and the protection cap  62  is diminished to disappear, which causes generation of friction torque therebetween and therefore prohibits the pivot bearing  63  from freely moving thus failing to duly transfer the thrust force of the spring  71  to the rotary shaft  64 . If the clearance is set larger in consideration of the problem, then the positioning accuracy is sacrificed, and the pivot bearing  63  is caused to jolt thereby making noise. Further, the spring  71  to generate the thrust force can be affected by the heat, thus worsening the problem.  
         [0007]     In the stepping motor  80  with a lead screw shown in  FIG. 2 , when the rotary shaft  81  with the magnet  85  is rotated, heat is radiated from coils disposed inside the stator units  94   a  and  94   b  and is kept inside the stator assembly  94  which is almost hermetically closed with minute clearances provided respectively between the pivot bearing  90  and the yoke  95  and between the pivot bearing  90  and the protection cap  89  for allowing the pivot bearing  90  to be axially moved by force of the spring  93 . Consequently, the pivot bearing  90 , which is made of synthetic resin and susceptible to heat, undergoes a dimensional change due to the heat kept inside the stator assembly  94 , and if the outer dimension of the pivot bearing  90  is changed to increase, the clearances provided between the pivot bearing  90  and the yoke  95  and between the pivot bearing  90  and the protection cap  89  are diminished to disappear, which causes generation of friction torque therebetween and therefore prohibits the pivot bearing  90  from freely moving thus failing to duly transfer the thrust force of the spring  93  to the rotary shaft  81 . If the clearances are set larger in consideration of the problem, then the positioning accuracy is sacrificed, and the pivot bearing  90  is caused to jolt thereby making noise. Further, the spring  93  to generate the thrust force can be affected by the heat, thus worsening the problem.  
       SUMMARY OF THE INVENTION  
       [0008]     The present invention has been made in light of the above problems resulting from a thrust mechanism being disposed within a stator assembly which includes coils to radiate heat, and inside which the heat from the coils is kept, and it is an object of the present invention to provide a stepping motor, in which thrust force to axially move a rotary shaft is not influenced by heat radiated from stator coils thus improving motor performance.  
         [0009]     In order to achieve the object described above, according to an aspect of the present invention, there is provided a stepping motor including: a stator assembly; a rotary shaft which includes a lead screw portion and a plain portion with a rotor magnet disposed concentrically thereon so as to face an inner circumference of the stator assembly, and which has both ends thereof rotatably supported by respective bearings; and a thrust mechanism. The thrust mechanism is provided at one end of the rotary shaft positioned toward the lead screw portion, and is structured such that a resilient member is provided in a recess formed at the one end of the rotary shaft positioned toward the lead screw portion, and a point-contact member is provided between the resilient member and one bearing of the respective bearings, the one bearing rotatably supporting the one end of the rotary shaft positioned toward the lead screw portion. With the structure described above, thrust force is given by the resilient member to the rotary shaft in the axial direction.  
         [0010]     Since the thrust mechanism to give an axial thrust force to the rotary shaft is disposed out of and apart from the stator assembly having coils to radiate heat, the relevant components of the thrust mechanism, even if made of heat-susceptible materials, are kept free from influences of the heat radiated by the coils. Consequently, a constant thrust force can be stably generated and transmitted to the rotary shaft without rattling noises generated by the jolting movement resulting from the sacrifice of the positioning accuracy, and the performance of a stepping motor is enhanced. In the aspect of the present invention, the resilient member may be a coil spring, and the point-contact member may be a spherical body made of steel. Consequently, the thrust mechanism can be simply structured with reduced cost, and this enhances the assembly efficiency thereby achieving cost reduction of the stepping motor. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]      FIG. 1  is a partly cross-sectional view of a conventional stepping motor;  
         [0012]      FIG. 2  is a partly cross-sectional view of another conventional stepping motor;  
         [0013]      FIG. 3  is a side view (partly sectioned) of a stepping motor according to a first embodiment of the present invention;  
         [0014]      FIG. 4  is a side view (partly sectioned) of a stepping motor according to a second embodiment of the present invention; and  
         [0015]      FIG. 5  is a side view (partly sectioned) of a stepping motor according to a third embodiment of the present invention.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0016]     Preferred embodiments of the present invention will be described with reference to the accompanying drawings.  
         [0017]     Referring to  FIG. 3 , a stepping motor  1  with a lead screw according to a first embodiment of the present invention includes a stator assembly which is composed of an A-phase stator unit  9  and a B-phase stator unit  12 , and a rotary shaft  2  which is structured such that a metal pipe  3  has its inner and outer surfaces continuously covered by a synthetic resin  4  molded so as to form a plain portion toward one end of the metal pile  3  and a lead screw portion  5  toward the other end thereof, thus the plain portion and the lead screw portion  5  are formed integrally with each other. A rotor magnet  6  is fixedly disposed on the plain portion of the rotary shaft  2  structured as described above. Among the most suitable materials for the synthetic resin  4  are, for example, polyphenylene sulfide (PPS) and polyoxymethylene (POM).  
         [0018]     The A-phase stator unit  9  includes a bobbin  7  which has a coil  8  wound therearound, and which is provided with terminal pins  13 , and further includes yokes  15  and  16  which have respective pole teeth  15   a  and  16   a  at their inner circumferences. The B-phase stator unit  12  includes a bobbin  10  which has a coil  11  wound therearound, and which is provided with terminal pins  14 , and further includes yokes  17  and  18  which have respective pole teeth  17   a  and  18   a  at their inner circumferences. The A-phase and B-phase stator units  9  and  12  structured as described above are disposed so as to surround the magnet  6 , such that the pole teeth  15   a ,  16   a ,  17   a  and  18   a  face the outer circumferential surface of the magnet  6 .  
         [0019]     A pivot bearing  19  made of synthetic resin and having a circular recess  21  at the center is attached at one side of the B-phase stator unit  12  by means of bosses  20 , and a bracket  23  having a positioning bush  22  attached at its proximal end and having a bearing  24  attached at its distal end is attached at one side of the A-phase stator unit  9 .  
         [0020]     The rotary shaft  2  has a recess  25  at an end thereof toward the lead screw portion  5 , a resilient member  26  such as a coil spring is housed in the recess  25 , and the end of the rotary shaft  2  toward the lead screw portion  5  is rotatably supported at the bearing  24  by means of a point-contact member  27  disposed at one end of the resilient member  26 . The rotary shaft  2  has a recess  28  at an end thereof toward the plain portion, a ball  29  is disposed in the recess  28 , a plurality of balls  30  are disposed in the recess  21  of the pivot bearing  19 , and the end of the rotary shaft  2  toward the plain portion is rotatably supported at the pivot bearing  19  by means of the balls  29  and  30 . The point-contact member  27  may be a spherical or semispherical body made of metal or ceramic and, in the present embodiment, is constituted by a spherical body made of steel that is preferable in terms of cost. The resilient member  26  and the point-contact member  27  in combination make up a thrust mechanism which, while the point-contact member  27  is supported by the bearing  24 , is adapted to give thrust force to the rotary shaft  2  in the axial direction.  
         [0021]     The balls  29  and  30  are made of steel, ceramic, or the like, wherein steel is preferable in terms of cost. The pivot bearing  19  and the bearing  24  may be made of metal, but when they are made of synthetic resin, cost is reduced, slidability is enhanced, and sliding noise is lowered. Polyphenylene sulfide (PPS), polyoxymethylene (POM), and the like are among the synthetic resin suitable for forming the components.  
         [0022]     The positioning bush  22  may preferably be made of, for example, polyphenylene sulfide (PPS), polybuthylene terephthalate (PBT), and the like.  
         [0023]     As described above and as shown in  FIG. 3 , in the stepping motor  1 , the thrust mechanism is provided out of the stator assembly so as to be located apart from the coils  8  and  11  as heat sources, and therefore does not suffer influences from the heat radiated by the coils  8  and  11 . Consequently, consideration in determining the clearance about the thrusting means is not necessary, and therefore the positioning accuracy is not sacrificed thus preventing the noise problem while ensuring a stable thrust force applied to the rotary shaft  2  in the axial direction. As a result, the performance of the stepping motor is enhanced.  
         [0024]     Referring to  FIG. 4 , a stepping motor  40  with a lead screw according to a second embodiment of the present invention has the same structure as the stepping motor  1  shown in  FIG. 3  except a pivot bearing  41 , and description will be made with focus put on the difference using the same reference numbers for the same components.  
         [0025]     In the stepping motor  40  according to the second embodiment, a rotary shaft  2  has a recess  28  at an end thereof toward a plain portion, a ball  43  is partly housed in the recess  28 , the aforementioned pivot bearing  41  has a hemispherical recess  42  shaped so as to partly enclose the ball  43 , and the end of the rotary shaft  2  toward the plain portion is rotatably supported at the pivot bearing  41  by means of the ball  43 , while the support mechanism for the end of the rotary shaft  2  toward a lead screw portion  5  has the same structure as that of the stepping motor  1  of  FIG. 3 , that is to say, the rotary shaft  2  has a recess  25  at an end thereof toward the lead screw portion  5 , a resilient member  26  such as a coil spring is housed in the recess  25 , and the end of the rotary shaft  2  toward the lead screw portion  5  is rotatably supported at a bearing  24  by means of a point-contact member  27  disposed at one end of the resilient member  26 .  
         [0026]     Thus, in the stepping motor  40 , the plurality of balls  30  shown in the stepping motor  1  of  FIG. 3  are eliminated thereby reducing the number of components, and at the same time the ball  43  can be duly positioned and set with less difficulty. The ball  43  may be made of steel, ceramic, and the like, wherein steel is preferable in terms of cost. The ball  43  may be in point-contact with the spherical surface of the recess  42  at one point or multiple points. In case of point-contact at multiple points, if the pivot bearing  41  is made of synthetic resin, the slidability is assured without applying grease, but friction loss can be reduced with grease application.  
         [0027]     Referring to  FIG. 5 , a stepping motor  50  with a lead screw according to a third embodiment of the present invention has the same structure as the stepping motor  1  shown in  FIG. 3  except a pivot bearing  51 , and description will be made with focus put on the difference using the same reference numbers for the same components.  
         [0028]     In the stepping motor  50  according to the third embodiment, a rotary shaft  2  has a recess  28  at an end thereof toward a plain portion, a ball  53  is partly housed in the recess  28 , the aforementioned pivot bearing  51  has a circular recess  52  with a plane bottom, and the end of the rotary shaft  2  toward the plain portion is rotatably supported at the pivot bearing  51  by means of the ball  53  making point-contact with the plane bottom surface of the recess  52 , while the support mechanism for the end of the rotary shaft  2  toward a lead screw portion  5  has the same structure as that of the stepping motor  1  of  FIG. 3 , that is to say, the rotary shaft  2  has a recess  25  at an end thereof toward the lead screw portion  5 , a resilient member  26  such as a coil spring is housed in the recess  25 , and the end of the rotary shaft  2  toward the lead screw portion  5  is rotatably supported at a bearing  24  by means of a point-contact member  27  disposed at one end of the resilient member  26 .  
         [0029]     Thus, in the stepping motor  50 , the plurality of balls  30  used in the stepping motor  1  of  FIG. 3  are eliminated thereby reducing the number of components, and at the same time the ball  53  is in point-contact with the plane bottom surface of the recess  52  at one point thus reducing the loss resulting from friction. The ball  53  may be made of steel, ceramic, and the like, wherein steel is preferable in terms of cost.  
         [0030]     The pivot bearings  41  and  45  can be made of metal, but if they are made of synthetic resin as the pivot bearing  19 , cost is reduced, slidability is enhanced, and sliding noise is lowered. Polyphenylene sulfide (PPS), polyoxymethylene (POM), and the like are among the synthetic resin suitable for forming the components.  
         [0031]     The ball  29 ,  43  or  53  may be fitted into the recess  28  formed at the end of the rotary shaft  2  toward the plain portion, or may be welded to the end of the rotary shaft  2  toward the plain portion. Alternatively, the end of the rotary shaft  2  toward the plain portion may be shaped hemispherical thereby replacing the ball  29 ,  43  or  53 .  
         [0032]     Also, the lead screw is formed of synthetic resin for reducing weight and improving slidability in the embodiments described above, but the present invention is not limited to such a structure, and the lead screw may be made of metal.  
         [0033]     And, the rotary shaft is structured such that the lead screw portion is formed integrally with the plain portion in the embodiments described above, but may alternatively be structured such that the lead screw portion is discretely produced and fixedly jointed to the plain portion by an appropriate method.  
       INDUSTRIAL APPLICABILITY  
       [0034]     The present invention discloses a stepping motor to function as an actuator for use in a floppy disk drive and various office automation equipments, thus providing industrial applicability.