Abstract:
A compact motor capable of being reflow-soldered, which can solve a problem of thermal demagnetization of a magnet with a simple structure, has magnets in a housing. At least a part of a surface to which the magnet is attached is separated by an acrylic adhesive sheet and heat resistant resin from metal members, such as a bracket and shaft forming part of the housing. The shaft is stainless steel having a low heat conductivity or an additional cover is used for the shaft to withstand reflow-soldering.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a compact motor used for audio systems such as a mini disk (MD) player or a silent call device of mobile communications apparatuses, and more particularly, to an improved compact motor capable of being reflow-soldered so direct soldering on a printed wiring board is possible. 
     2. Description of the Related Art 
     Recently, technology for reflow-soldering of electronic parts to reduce the manufacturing cost has been rapidly developed. However, the reflow-soldering to a compact motor is not possible because of thermal demagnetization generated due to the heating of a built-in magnet. In particular, the problem of thermal demagnetization becomes serious when a high performance magnet inevitable for miniaturization of a system is used. 
     Since an operation point permeance coefficient is not high in a compact motor mounted in portable devices, thermal demagnetization cannot be avoided. In the case of a neodymium magnet showing great thermal demagnetization, reflow-soldering can not be performed at all. Also, it is not possible to use a magnet of having high coercive force showing less thermal deterioration because of high manufacturing costs. 
     SUMMARY OF THE INVENTION 
     To solve the above problems, it is an objective of the present invention to provide a compact motor capable of being reflow-soldered by solving the thermal demagnetization problem of a magnet with a simple structure. 
     Accordingly, to achieve the above objective, there is provided a compact motor capable of being reflow-soldered which is formed by installing a magnet in a housing, in which at least part of a surface where the magnet is installed is separated from a metal member forming part of the housing. Thus, even when the installation portion of the housing is reflow-soldered, because heat is not directly transferred, thermal demagnetization can be reduced. 
     It is preferred in the present invention that the magnet is an axial gap type and attached to a part of the housing by a double-sided adhesive sheet as a separating means. Since a double-side adhesive sheet exhibits a thermal insulation effect, transfer of heat to the magnet is difficult. Also, since the sheet is thin and it is not desired that operation point permeance coefficient is sacrificed, it is preferable that an axial gap type is used. 
     It is preferred in the present invention that the magnet is a radial gap type using a shaft as part of a magnetic path and is incorporated in the shaft through heat resistant resin as a separating means. Since the magnet can be separated from the shaft which is a metal member, transfer of heat to the magnet is difficult. 
     It is preferred in the present invention that the shaft is formed of stainless steel having a relatively low heat conductivity. Accordingly, a thermal insulation effect of stainless steel can be expected. 
     It is preferred in the present invention that at least the housing exposed to the outside air is formed of a stainless steel plate having a low heat conductivity. Accordingly, a thermal insulation effect of stainless steel can also be expected. 
     It is preferred in the present invention that the housing is covered so as not to be exposed directly to the outside air except for an installation portion or terminal to be reflow-soldered at a printed wiring board. Since more thermal insulation effect can be obtained, it is possible to cope with one of which the reflow time is quite long. 
     It is preferred in the present invention that a shielding portion for preventing transfer of heat to a main body of the housing is installed at the installation portion or terminal and the shielding portion is covered by heat resistant resin. Thus, heat conduction to the main body of the housing is reduced and the strength of the shielding portion can be covered by heat resistant resin. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above objective and advantages of the present invention will become more apparent by describing in detail a preferred embodiment thereof with reference to the attached drawings in which: 
     FIG. 1 is a sectional view showing major portions of an axial gap coreless compact motor for generating vibration according to a first preferred embodiment of the present invention; 
     FIG. 2 is a plan view showing major portions of the rotor of the motor of FIG. 1; 
     FIG. 3 is a sectional view showing major portions of a motor according to a second preferred embodiment of the present invention; 
     FIG. 4 is a perspective view showing a modified example of a part of the second preferred embodiment of the present invention; and 
     FIG. 5 is a sectional view showing major portions of a radial gap compact motor for generating vibration according to a third preferred embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIG. 1, a bracket  1  forms part of a housing H which is a magnetized metal body and a shaft holder  1   a  is integrally formed with the bracket  1  at the center thereof. A shaft  2  of stainless steel is fixed by inserting one end in the shaft holder  1   a.  A rotor  3  which is eccentric is rotatably installed at the shaft  2 . 
     The eccentric rotor  3 , as shown in FIG. 2, has three air-core armature coils  3   a,    3   b  and  3   c  arranged to be eccentric to one side, at pitches of about 60°, of the flat panel commutator  5 , and is integrally formed with the flat panel commutator  5 . The commutator  5  includes a spark removing device  4   a  of resin  4  (refer to FIG. 1) which is heat resistant and has a low frictional coefficient. The resin having a low frictional coefficient also serves as a bearing. 
     A ring magnet  6  including neodymium facing the rotor  3  with a gap is attached to the bracket  1  by an acryl-based adhesive sheet A (product name: Sony Chemical T4000) having a thickness of 0.15 mm and using non-woven fabric as a base. Thus, the magnet 6 is separated from the bracket of the housing H. 
     A pair of brushes  7  are in sliding contact at an open angle of 90° with the inner side of the magnet  6 . The flat panel commutator  5  receiving a predetermined pressing force slidingly contacts the brushes  7  so that the brushes  7  receive electric power from the outside through a flexible power feeding sheet F. The leading end of the flexible power feeding sheet F is folded over a piece  1   b  protruding from the bracket  1  and soldering electrodes are exposed in three directions so that reflow can be performed easily. 
     A case  8  forming the other portion of the housing H has a tapered through hole  8   a  arranged at the center thereof in which the other end of the stainless steel shaft  2  is installed. A polyester film P is attached around the through hole  8   a  and slidingly contacts the eccentric rotor  3  through a projection  3   d  by a pressing force of the brushes  7 . Accordingly, the rotor  3  is always elastically biased toward the case  8  and is capable of rotating on the polyester film P so that the rotor  3  does not contact the case  8 . Also, because a gap is maintained to be constant, the position of rotation does not change so that the rotation of the rotor  3  is stable. 
     Thus, the rotor  3  is coupled to the bracket  1  by being inserted from the open end of the shaft  2  and the case  8  is capped thereon to cover the outer circumference of the bracket  1 , so that the assembly thereof is simplified. 
     The motor having the above structure is installed on the printed wiring board I together with other electronic parts through the acrylic adhesive sheet A using the non-woven fabric as a base member and having a thickness of about 0.15 mm, and then reflow-soldered. 
     Also, as a modified example of the present embodiment, the thermal insulation effect of stainless steel may be used by making at least a case side of the housing with weakly magnetized stainless steel. 
     FIG. 3 shows a second preferred embodiment of the present invention, in which the housing H is covered by resin J having an effect of thermal insulation and being capable of withstanding reflow-soldering, other than the power feeding sheet portion F and a caulking portion in the first preferred embodiment, to prevent direct exposure to the outside air. As the other portions are the same as those in the first preferred embodiment, the same reference numerals are used and the descriptions thereof are omitted. 
     FIG. 4 shows a modification of the second preferred embodiment of the present invention. Here, when a main body of the motor is installed at the printed wiring board by reflow-soldering, instead of using the acrylic adhesive sheet A, an installation leg portion  8   c  protrudes through a shielding portion  8   b  toward the side portion of the case  8  and the installation leg portion  8   c  is attached to the printed wiring board by reflow-soldering. 
     In this case, the shielding portion  8   b  is integrally formed with resin J which becomes a cover for reinforcement. Thus, the shielding portion  8   b  prevents conduction of heat to the main body of the case and resin J is used for reinforcement. 
     Also, in the present embodiment, to install the ring magnet  6  including neodymium at the bracket  1  which is a part of the housing, the acrylic adhesive sheet is used as a separating means. However, it may be possible that a plurality of small protrusions are provided at the part of the upper surface of the bracket so that the lower surface of the magnet is not supported by the entire bracket, owing to the small protrusions, thus enabling thermal insulation. 
     FIG. 5 shows major portions of a third preferred embodiment of the present invention. That is, a brush base  10  formed of relatively flexible heat resisting synthetic resin is fixedly inserted at one end of a pipe case  9  having both ends open. A case terminal  12  formed by outsert molding a bearing  11  of heat resistant synthetic resin is fixed by caulking at the other end the case  9  to form a housing H. These synthetic resins (for example, Smika Super LCP) exhibit a heat resistance of over 280° C. to bear dipping into soldering and a metal frame  13  for reinforcement is embedded in the brush base  10 . A part of the metal frame  13  is a circular electrode  14  in an axial direction and an electrode  15  used as a leg portion for attachment in a radial direction. A shaft  16  is stainless steel has both ends thereof fixedly supported by the brush base  10  and the bearing  11  of the case terminal  12 . A shaft fixing cut portion  16   a  is installed at the shaft  16  and a cylindrical magnet  18  is integrally formed with the cut portion  16   a  through a resin  17  which is heat resistant and slippery. 
     A cylindrical coreless coil  19  is arranged with a gap between the case  9  and the coil  19 . A pair of eccentric bearings  22  and  23  is arranged at both ends of the cylindrical coil  19  through the synthetic resin holders  20  and  21  and is supported at the shaft  16  by the bearings  22  and  23  for rotating. Further, a cylindrical commutator  24  is incorporated in the holder  20  at one side, and free ends of a pair of brushes  25  and  26  slidingly contact the commutator  24 . The base ends of the brushes  25  and  26  are installed at the brush base  10 . A base end  25   a  of the brush  25  at one side is pressed on the metal frame  13 , and simultaneously, a base end  26   a  of the brush at the other end is electrically connected to be commutator  24  when the brush base  10  is installed at the case  9  and simultaneously is soldered at an open end portion of the case  9 . 
     In the present embodiment, part of the case terminal  12  protrudes through the shielding portion in a radial direction, forming a leg portion  27  for attachment. Reference numeral  28  denotes a weight for generating vibration, which is integrally formed with the holders  20  and  21  and has a crescent moon shape and a thick profile. 
     Accordingly, as the magnet  18  does not directly contact the metal portion, rapid transfer of heat during reflow-soldering can be prevented. In particular, it is effective that both ends of the shaft  16  do not contact the metal portion. 
     Also, although a vibratory motor is described, a typical rotary motor such as a spindle motor, for an MD player and a motor for driving a pickup device, may be used. 
     It is noted that the present invention is not limited to the preferred embodiment described above, and it is apparent that variations and modifications by those skilled in the art can be effected within the spirit and scope of the present invention defined in the appended claims. 
     According to the invention, even when the installation portion of the housing is reflow-soldered, because heat is not directly transferred, thermal demagnetization can be reduced. 
     According to the invention, since a double-sided adhesive sheet provides a thermal insulation, transfer of heat to the magnet is difficult. Also, since the sheet is thin and it is not desirable that the operation point permeance coefficient be sacrificed, it is preferable that an axial gap be used. 
     According to the invention, since the magnet can be separated from the shaft, which is a metal member, transfer of heat to the magnet is difficult. 
     According to the invention, the thermal insulation effect of stainless steel is exploited. 
     According to the invention, since greater thermal insulation effect is obtained, it is possible to cope with a long reflow time. 
     According to the invention, heat conduction to the main body of the housing is reduced and the shielding portion can be covered by heat resistant resin.