Abstract:
A low-profile stepping motor has a stator assembly structured such that first and second coil units are disposed flush with each other horizontally and are sandwiched between upper and lower stator yoke pieces each comprising first and second stator yokes which are punched out integrally with each other, are shaped symmetrically, and which each have a plurality of pole teeth along its semi-circular inner circumference. The upper and lower stator yoke pieces are fixedly put together via the coils such that hollow-cylindrical protrusions formed in the upper and lower stator yoke pieces are inserted into through-holes of first and second coil bobbins from the upper and lower sides, respectively, and have their abutting end surfaces welded, bonded or otherwise fixed to each other, whereby the coil units are surely held between the upper and lower stator yoke pieces.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a stepping motor, and specifically to a low-profile stepping motor having its dimension reduced in an axial direction. 
   2. Description of the Related Art 
   Recently, in accordance with enhanced performance of various devices such as printers, facsimiles and floppy (Registered Trademark) disk drives, a stepping motor is required to be higher in torque, and lower in cogging torque and vibration. Under the circumstance, a PM (permanent magnet) stepping motor such as disclosed in Japanese Patent Application Laid-open No. Hei 10-127024 has been extensively used to meet the requirements. 
     FIG. 1  is a partly cutaway perspective view of a conventional PM stepping motor. As shown in  FIG. 1 , the PM stepping motor generally comprises: a stator assembly  6  composed of two stator units  6 A and  6 B; a rotor assembly  13 ; a face plate  1  punched out of a stainless steel plate; and a front bearing  2  made of oil impregnated alloy. The stator units  6 A and  6 B each comprise: stator yokes  3   a  and  3   b  each punched out of a soft magnetic steel plate and each having a plurality of pole teeth  10  bent up along its inner circumference; and a coil  4  formed such that a copper wire with polyurethane insulation is wound on a bobbin  5  housed in the stator yokes  3   a  and  3   b , and are attached back-to-back to each other. The rotor assembly  13  comprises: a rotary shaft  8 ; a cylindrical magnet  9  for magnetic field, having a plurality of magnetic poles at its circumferential surface, and having the circumferential surface opposing the pole teeth  10  of the stator yokes  3   a ,  3   b ; and a sleeve  12  to fasten the cylindrical magnet  9  and the rotary shaft  8  together such that the rotary shaft  8  is press fitted into the sleeve  12 , and the sleeve  12  with adhesive applied thereon is inserted into the cylindrical magnet  9 . The rotor assembly  13  thus structured has its outer circumference (of the magnet  9 ) magnetized for a predetermined number of magnetic poles, and is rotatably disposed inside the stator assembly  6  such that the rotary shaft  8  is supported by the front bearing  2  and a rear bearing (not shown in FIG.  1 ), and that the magnetic poles oppose the pole teeth  10  with a slight gap therebetween. 
   The stator unit  6 A is structured such that the pole teeth  10  of the stator yoke  3   a  mesh with the pole teeth  10  of the stator yoke  3   b  equidistantly from each other with a shift by an electrical angle of 180 degrees (half pitch). The stator unit  6 B is of the same structure as the stator unit  6 A. The stator units  6 A and  6 B thus structured are joined back-to-back to each other by molding with polymeric material or by plasma welding such that their respective pole teeth  10  and  10  are shifted from each other by an electrical angle of 90 degrees, whereby the rotor assembly  13  always starts its rotation in the same direction. 
   Recently, since the devices incorporating PM stepping motors are required to be downsized and lower-profiled, the PM stepping motors are also required to be further lower-profiled. The conventional PM stepping motor, however, is composed of two stator units joined to each other back-to-back thus making it extremely difficult or almost impossible to further reduce its profile dimension. And the conventional PM stepping motor uses four discrete stator yokes, which inevitably increases the number of the components and makes alignment troublesome. 
   SUMMARY OF THE INVENTION 
   The present invention has been made in view of the above circumstance, and it is an object of the present invention to provide a low-profile stepping motor which is of a simple structure and which has its dimension drastically reduced in the axial direction. 
   In order to achieve the object, according to a first aspect of the present invention, a low-profile stepping motor comprises a first stator unit, a second stator unit, and a rotor assembly. The first stator unit includes: a first upper stator yoke which has a plurality of pole teeth formed along its semicircular inner circumference, and which has a hollow-cylindrical protrusion with a top lid; a first lower stator yoke which has a plurality of pole teeth formed along its semicircular inner circumference and adapted to mesh with the pole teeth of the first upper stator yoke, and which has a hollow-cylindrical protrusion with a top lid positioned to oppose the protrusion of the first upper stator yoke thus configuring a pair of opposing protrusions; and a first coil unit which is fixedly sandwiched between the first upper and lower stator yokes. The second stator unit includes: a second upper stator yoke which is formed as one piece integrally with the first upper stator yoke, has a plurality of pole teeth formed along its semicircular inner circumference, and which has a hollow-cylindrical protrusion with a top lid; a second lower stator yoke which is formed as one piece integrally with the first lower stator yoke, has a plurality of pole teeth formed along its semicircular inner circumference and adapted to mesh with the pole teeth of the second upper stator yoke, and which has a hollow-cylindrical protrusion with a top lid positioned to oppose the protrusion of the second upper stator yoke thus configuring a pair of opposing protrusions; and a second coil unit which is fixedly sandwiched between the second upper and lower stator yokes and which is arranged horizontally flush with the first coil unit. And the rotor assembly is rotatably disposed in a circular open space defined by the pole teeth of the first and second stator units. With the structure above described, while the stepping motor can be significantly reduced in thickness, the coil unit can be easily attached to the stator yokes due to the protrusions, and the upper and lower stator yokes can be easily put together without any special structure such as studs. 
   According to a second aspect of the present invention, in the low-profile stepping motor of the first aspect, the protrusions are formed by drawing process. Consequently, the protrusions can be formed readily on the stator yokes. 
   According to a third aspect of the present invention, in the low-profile stepping motor of the first or second aspect, the pair of opposing protrusions have their respective top lids butting each other. Consequently, a closed magnetic path can be surely formed. 
   According to a fourth aspect of the present invention, in the low-profile stepping motor of the third aspect, the top lids each have a flat abutting surface. Consequently, a closed magnetic path can be effectively formed. 
   According to a fifth aspect of the present invention, in the low-profile stepping motor of the third aspect, the top lids each have a lug hole formed at its center. Consequently, the upper and lower stator yokes can be easily aligned to each other and also can be easily attached to the mount board. And the screw head can be well lodged in the hollow of the protrusion and does not stick out. 
   According to a sixth aspect of the present invention, in the low-profile stepping motor of the first aspect, the coil unit is fixedly attached to the pair of opposing protrusions such that the protrusions are fitted into a through-hole formed at the center of the coil unit respectively from upper and lower sides thereof. Consequently, the coil unit can be easily attached. 
   According to a seventh aspect of the present invention, in the low-profile stepping motor of the first or second aspect, one protrusion of the pair of opposing protrusions has a raised circular portion at its top lid, and the other protrusion thereof has at its top lid a recessed circular portion adapted to engage with the raised circular portion at the top lid of the one protrusion. Consequently, the upper and lower stator yokes can be easily aligned to each other by simply abutting the top lids of their respective protrusions against each other. 
   According to an eighth aspect of the present invention, in the low-profile stepping motor of the first or second aspect, one protrusion of the pair of opposing protrusions has a raised circular rim at its top lid, and the other protrusion thereof has at its top lid a circular hole adapted to engage with the raised circular rim at the top lid of the one protrusion. Consequently, the upper and lower stator yokes can be easily aligned to each other by simply abutting the top lids of their respective protrusions against each other. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a partly cutaway view of a conventional PM stepping motor; 
       FIG. 2  is an exploded perspective view of a low-profile stepping motor according to a first embodiment of the present invention; 
       FIG. 3  is a cross sectional view of the low-profile stepping motor of  FIG. 2 ; 
       FIG. 4  is an enlarged cross sectional view of two opposing hollow-cylindrical protrusions of the low-profile stepping motor of  FIG. 2 , showing how the low-profile motor is fixed to a mount board; 
       FIG. 5  is an enlarged cross sectional view of two opposing hollow-cylindrical protrusions of a low-profile stepping motor according to a second embodiment of the present invention; and 
       FIG. 6  is an enlarged cross sectional view of two opposing hollow-cylindrical protrusions of a low-profile stepping motor according to a third embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Preferred embodiments of the present invention will hereinafter be described with reference to the accompanying drawings. 
   Referring to  FIG. 2 , a low-profile stepping motor of the present invention generally comprises: a stator assembly  30  including first and second stator units  30 A and  30 B assembled to be horizontally flush with each other; and a rotor assembly  50  rotatably disposed inside the stator assembly  30 . 
   The first stator unit  30 A comprises: first upper and lower stator yokes  31   a  and  34   a ; and a first coil unit  40 A which is composed of a bobbin  41   a  having a through-hole  44   a  and a winding  42   a  provided around the bobbin  41   a , is sandwiched between the first upper and lower stator yokes  31   a  and  34   a , and which is provided with a terminal pin  43   a . The first upper stator yoke  31   a  is punched out of a soft magnetic steel plate, includes a plurality of pole teeth  32   a   1  to  32   an  punched out of the soft magnetic steel plate and bent up inward (downward in  FIG. 2 ) at its semicircular inner circumference, and has a protrusion  33   a  shaped hollow-cylindrical, having a top lid and extending inward (downward in  FIG. 2 ) to fit into an upper half of the through-hole  44   a  of the first coil unit  40 A. The first lower stator yoke  34   a  is punched out of a soft magnetic steel plate, includes a plurality of pole teeth  35   a   1  to  35   an  punched out of the soft magnetic steel plate and bent up inward (upward in  FIG. 2 ) at its semicircular inner circumference, and has a protrusion  36   a  shaped hollow-cylindrical, having a top lid and extending inward (upward in  FIG. 2 ) to fit into a lower half of the through-hole  44   a  of the first coil unit  40 A. 
   The second stator unit  30 B comprises: second upper and lower stator yokes  31   b  and  34   b  which are punched out as one piece integrally with the first upper and lower stator yokes  31   a  and  34   a , respectively; and a second coil unit  40 B which is composed of a bobbin  41   b  having a through-hole  44   b  and a winding  42   b  provided around the bobbin  41   b , is sandwiched between the second upper and lower stator yokes  31   b  and  34   b , and which is provided with a terminal pin  43   b . The second upper stator yoke  31   b  is punched out of a soft magnetic steel plate, includes a plurality of pole teeth  32   b   1  to  32   bn  punched out of the soft magnetic steel plate and bent up inward (downward in  FIG. 2 ) at its semicircular inner circumference, and has a protrusion  33   b  shaped hollow-cylindrical, having a top lid and extending inward (downward in  FIG. 2 ) to fit into an upper half of the through-hole  44   b  of the second coil unit  40 B. The second lower stator yoke  34   b  is punched out of a soft magnetic steel plate, includes a plurality of pole teeth  35   b   1  to  35   bn  punched out of the soft magnetic steel plate and bent up inward (upward in  FIG. 2 ) at its semicircular inner circumference, and has a protrusion  36   b  shaped hollow-cylindrical, having a top lid and extending inward (upward in  FIG. 2 ) to fit into a lower half of the through-hole  44   b  of the second coil unit  40 B. 
   The plurality of pole teeth  32   a   1  to  32   an  of the first upper stator yoke  31   a  and the plurality of pole teeth  32   b   1  to  32   bn  of the second upper stator yoke  31   b  are punched out simultaneously, and the plurality of pole teeth  35   a   1  to  35   an  of the first lower stator yoke  34   a  and the plurality of pole teeth  35   b   1  to  35   bn  of the second lower stator yoke  34   b  are punched out simultaneously. 
   Each stator yoke has pole teeth provided in number corresponding to the required number of steps per rotation of a stepping motor and arranged equidistant from one another. The pole teeth  32   a   1  to  32   an  of the first upper stator yoke  31   a  and the pole teeth  35   a   1  to  35   an  of the first lower stator yoke  34   a  mesh with each other with a shift by an electrical angle of 180 degrees, and the pole teeth  32   b   1  to  32   bn  of the second upper stator yoke  31   b  and the pole teeth  35   b   1  to  35   bn  of the second lower stator yoke  34   b  engage with each other with a shift by an electrical angle of 180 degrees. 
   In each of the upper and lower stator yokes of the first and second stator units  30 A and  30 B thus structured, two pole teeth adjacent to each other sandwiching one of a pair of slot cuts  37  or  38  (detailed hereinafter) are shifted from each other by an electrical angle of 90 degrees or 270 degrees. Specifically, in the first and second upper stator yokes  31   a  and  31   b , the pole tooth  32   a   1  is shifted by an electrical angle of 90 degrees or 270 degrees from the pole tooth  32   b   1  located adjacent to the pole tooth  32   a   1  sandwiching one slit cut  37 , and the pole tooth  32   an  is shifted by an electrical angle of 90 degrees or 270 degrees from the pole tooth  32   bn  located adjacent to the pole tooth  32   an  sandwiching the other slot cut  37 , and in the first and second lower stator yokes  34   a  and  34   b , the pole tooth  35   a   1  is shifted by an electrical angle of 90 degrees or 270 degrees from the pole tooth  35   b   1  located adjacent to the  35   a   1  sandwiching one slot cut  38 , and the pole tooth  35   an  is shifted by an electrical angle of 90 degrees or 270 degrees from the pole tooth  35   bn  located adjacent to the pole tooth  35   an  sandwiching the other slot cut  38 . 
   The first and second upper stator yokes  31   a  and  31   b  are punched out as one piece with the pair of slot cuts  37  above mentioned being formed right midway between the first and second upper stator yokes  31   a  and  31   b  thereby minimizing magnetic interflow therebetween so as not to have magnetic influence on each other, and the first and second lower stator yokes  34   a  and  34   b  are punched out as one piece with the pair of slot cuts  38  above mentioned being formed right midway between the first and second lower stator yokes  34   a  and  34   b  thereby minimizing magnetic interflow therebetween so as not to have magnetic influence on each other. The first and second upper stator yokes  31   a  and  31   b , and the first and second lower stator yokes  34   a  and  34   b  are formed of a soft magnetic steel plate, such as silicon steel plate, SECC (galvanized steel plate) and SUY (electromagnetic soft iron). 
   The rotor assembly  50  comprises: a cylindrical magnet  51  formed of ferrite magnet, rear-earth magnet and so forth, and magnetized at regular intervals; a rotary shaft  55  made of non-magnetic stainless steel; and a sleeve  52  made of aluminum, and is structured such that the cylindrical magnet  51  is adhesively fixed to the sleeve  52  which has the rotary shaft  55  press-fitted thereinto. The rotor assembly  50  is rotatably disposed inside the stator assembly  30  with a slight gap between the cylindrical magnet  51  and the pole teeth of the stator assembly  30 , and always starts rotating in the same direction by means of the first and second stator units  30 A and  30 B. 
   Referring to  FIG. 3 , the protrusions  33   a  and  36   a  are formed on the first upper and lower stator yokes  31   a  and  34   a , respectively, by drawing process, and lug holes  33   a   1  and  36   a   1  are provided centrally at respective top lids of the protrusions  33   a  and  36   a . And the protrusions  33   b  and  36   b  are formed on the second upper and lower stator yokes  31   b  and  34   b , respectively, by drawing process, and lug holes  33   b   1  and  36   b   1  are provided centrally at respective top lids of the protrusions  33   b  and  36   b.    
   The cylindrical protrusion  33   a  is fitted into the through-hole  44   a  of the first coil unit  40 A from the upper side, the protrusion  36   a  is fitted into the through-hole  44   a  of the first coil unit  40 A from the lower side, and the top lids of the protrusions  33   a  and  36   a  come in contact with each other. A positioning pin (not shown) is let through the lug holes  33   a   1  and  36   a   1 , and the protrusions  33   a  and  36   a  are connected to each other by bonding or welding, whereby the first upper and lower stator yokes  31   a  and  34   a  are put together with each other. When the protrusions  33   a   1  and  36   a   1  are connected to each other by bonding, an electrically conductive adhesive is applied to the top lids thereof, and when the protrusions  33   a   1  and  36   a   1  are connected by welding, welding heads are put inside the protrusions  33   a   1  and  36   a   1  for electric welding. 
   When the first upper and lower stator yokes  31   a  and  34   a  are put together as described above, the second upper and lower stator yokes  31   b  and  34   b  are also put together with each other in the same way as described above. 
   Since the top lids of the protrusions  33   a  and  36   a  are connected to each other, a part of the closed magnetic path is formed by the first upper stator yoke  31   a  together with the first lower stator yoke  34   a . Also, since the top lids of the protrusions  33   b  and  33   b  are connected to each other, a part of a closed magnetic path is formed by the second upper stator yoke  31   b  together with the second lower stator yoke  34   b.    
   A pole tooth  32   a  represents one pole tooth of the first upper stator yoke  31   a , and a pole tooth  35   b  represents one pole tooth of the second lower stator yoke  34   b . Resin is filled in a gap space between the meshing pole teeth for reinforcement thereby forming a resin portion  63 , which is preferable with respect to vibration. When resin is filled in the gap space between the meshing pole teeth forming the resin portion  63 , a resin wall  62  is formed simultaneously at the same process. An upper bearing  54  is fixed to the resin wall  62 , and the rotary shaft  55  of the rotor assembly  50  is let through the upper bearing  54 . Finally, a face plate  61 , which is punched out of a non-magnetic steel plate and which has a lower bearing  53  fixed thereto, is fixedly attached to the first and second lower stator yokes  34   a  and  34   b  by welding, bonding or crimping. The rotor assembly  50  is rotatably supported by the upper and lower bearings  54  and  53 . 
   Referring now to  FIG. 4 , the protrusion  33   a  of the first upper stator yoke  31   a  is fitted into the through-hole  44   a  of the coil unit  40 A from the upper side, and the protrusion  36   a  of the first lower stator yoke  34   a  is fitted into the through-hole  44   a  from the lower side. The first upper and lower stator yokes  31   a  and  34   a  are fixedly put together sandwiching the first coil unit  40 A such that the top lids of the protrusions  33   a  and  36   a  are in contact with each other and connected to each other by bonding or welding. The face plate  61  is fixedly attached to the first lower stator yoke  34   a . The protrusions  33   a  and  36   a  have on their respective top lids the lug holes  33   a   1  and  36   a   1  sized identical with each other and positioned concentric with each other, and the face plate  61  has a lug hole  61   a  sized identical with and positioned concentric with the lug holes  33   a   1  and  36   a   1 . The first upper and lower stator yokes  31   a  and  34   a  are fixedly attached to a mount board  65  such that a screw  67  goes through the lug holes  31   a   1 ,  36   a   1  and  61   a  and screws into the mount board  65 . 
   The second upper and lower stator yokes  31   b  and  34   b  are also fixedly put together sandwiching the second coil unit  40 B, and are fixedly attached, together with the face plate  61  fixed to the second lower stator yoke  34   b , to the mount board  65  in the same way as the first upper and lower stator yokes  31   a  and  34   a.    
   In the first embodiment of the present invention, while the low-profile stepping motor of the present invention can be reduced in thickness, the stator upper and lower stator yokes can be easily put together without any special structure such as a spacer, and the coil units can be easily attached to the stator yokes by means of the protrusions of the stator yokes. The stator yokes can be easily aligned to each other by inserting a pin through the lug holes formed at the top lids of the protrusions. 
   Since the protrusions have their respective top lids in contact with each other and connected to each other by bonding or welding, a closed magnetic path can be surely formed. And if the protrusions have a height identical with each other, a better magnetic connection can be achieved. 
   Also, the low-profile stepping motor of the present invention can be easily fixed to the mount board by simply inserting a screw through the lug holes formed at the top lids of the protrusions and at the face plate and driving the screw into the mount board. The protrusions are hollow, and the screw head can be well lodged therein without sticking out. 
   A second embodiment of the present invention will now be described with reference to FIG.  5 . As shown in  FIG. 5 , a hollow-cylindrical protrusion  73   a  with a top lid is formed on the first upper stator yoke  31   a  by drawing process, and the top lid has a circular portion  74  raised (downward in  FIG. 5 ) at its center. Also, a hollow-cylindrical protrusion  76   a  with a top lid is formed on the first lower stator yoke  34   a  by drawing process, and the top lid has a circular portion  75  recessed (downward in  FIG. 5 ) at its center. The recessed circular portion  75  formed at the top lid of the protrusion  76   a  is adapted to engage with the raised circular portion  74  formed at the top lid of the protrusion  73   a.    
   The protrusion  73   a  of the first upper stator yoke  31   a  is fitted into the through-hole  44   a  of the first coil unit  40 A from the upper side, and the protrusion  76   a  of the first lower stator yoke  34   a  is fitted into the through-hole  44   a  from the lower side and is positioned to be aligned to the protrusion  73   a  such that the recessed circular portion  75  at the top lid of the protrusion  76   a  engages with the raised circular portion  74  at the top lid of the protrusion  73   a . The protrusions  73   a  and  76   a  have their respective top lids in contact with each other and connected to each other by bonding or welding thereby firmly holding the first unit coil  40 A. The face plate  61  is fixedly attached to the first lower stator yoke  34   a . Lug holes  74   a  and  75   a  sized identical with each other and positioned concentric with each other are formed centrally at the circular portions  74  and  75 , respectively, and the face plate  61  has a lug hole  61   a  sized identical with and positioned concentric with the lug holes  74   a  and  75   a.    
   The first upper and lower stator yokes  31   a  and  34   a  are fixedly attached to the mount board  65  such that a screw  77  goes through the lug holes  74   a   1 ,  75   a  and  61   a  and screws into the mount board  65 . The second upper stator yokes  31   b  and  34   b  are also fixedly attached to the mount board  65  in the same way as the first upper and lower stator yokes  31   a  and  34   a.    
   In the second embodiment of the present invention, since the raised and recessed circular portions at the top lids of the protrusions are adapted to engage with each other as described above, the upper and lower stator yokes can be easily positioned to be aligned to each other by simply abutting the top lids of the protrusions against each other and can be easily put together without any special structure such as a spacer. And, the top lids of the protrusions are in contact with each other and connected to each other by bonding or welding, whereby a closed magnetic path is surely formed. 
   Also, the low-profile stepping motor of the present invention can be easily fixed to the mount board by simply inserting a screw through the lug holes formed at the top lids of the protrusions and at the plate and driving the screw into the mount board. The protrusions are hollow, and the screw head can be well lodged therein without sticking out. 
   A third embodiment of the present invention will now be described with reference to FIG.  6 . As shown in  FIG. 6 , a hollow-cylindrical protrusion  83   a  with a top lid is formed on the first upper stator yoke  31   a  by drawing process, and the top lid has a circular rim  84  raised (downward in  FIG. 6 ) at its center and defining a lug hole  84   a  thereinside. Also, a hollow-cylindrical protrusion  86   a  with a top lid is formed on the first lower stator yoke  34   a  by drawing process, and the top lid has a hole  85  at its center. The hole  85  is adapted to engage with the raised circular rim  84  formed at the top lid of the protrusion  83   a.    
   The protrusion  83   a  of the first upper stator yoke  31   a  is fitted into the through-hole  44   a  of the first coil unit  40 A from the upper side, and the protrusion  86   a  of the first lower stator yoke  34   a  is fitted into the through-hole  44   a  from the lower side and is positioned to be aligned to the protrusion  83   a  such that the hole  85  at the top lid of the protrusion  86   a  engages with the raised circular rim  84  at the top lid of the protrusion  83   a . The protrusions  83   a  and  86   a  have their respective top lids in contact with each other and connected to each other by bonding or welding thereby firmly holding the first coil unit  40 A. The face plate  61  has a lug hole  61   a  sized identical with and positioned concentric with the lug hole  84   a  and is fixedly attached to the first lower stator yoke  34   a.    
   The first upper and lower stator yokes  31   a  and  34   a  are fixedly attached to the mount board  65  such that a screw  87  goes through the lug holes  84   a  and  61   a  and screws into the mount board  65 . The second upper stator yokes  31   b  and  34   b  are also fixedly attached to the mount board  65  in the same way as the first upper and lower stator yokes  31   a  and  34   a.    
   In the third embodiment of the present invention, the raised circular rim and the hole at the top lids of the protrusions are adapted to engage with each other as described above, the upper and lower stator yokes can be easily positioned to be aligned to each other by simply abutting the top lids of the protrusions against each other and can be easily put together without any special structure such as a spacer. And, the top lids of the protrusions are in contact with each other and connected to each other by bonding or welding thereby surely forming a closed magnetic path. 
   Also, the low-profile stepping motor of the present invention can be easily fixed to the mount board without any special structure by simply inserting a screw through the lug holes formed at the top lids of the protrusions and at the face plate and driving the screw into the mount board. The protrusions are hollow, and the screw head can be well lodged therein without sticking out. 
   In the above embodiments of the present invention, the low-profile stepping motor is attached to the mount board with the face plate located toward the mount board, but may alternatively be attached to the mount board with the face plate located away from the mount board. In this case, the lug hole at the face plate may have a diameter large enough to allow the screw head to go therethrough thereby minimizing the length of the screw.