Electric motor - Patent Review 4259603

What is claimed is:

1. An electric motor comprising a stator assembly, a rotor assembly including a plurality of rotor magnets, a detecting winding assembly and motor housing means containing and relatively positioning said stator assembly, rotor assembly and detecting winding assembly; said stator assembly including stator windings, a control circuit substrate fixed relative to said stator windings so as to be positioned within the field of said rotor magnets, circuitry on said control circuit substrate for controlling a driving current supplied to said stator windings, and means mounted on said control circuit substrate for detecting the rotational position of said rotor magnets; said rotor assembly including a shaft rotatably journalled in said stator assembly, a magnet casing secured to said shaft and within which said plurality of rotor magnets are fixed, and a detector magnet secured to said shaft; and said detecting winding assembly being arranged in face-to-face relationship with said detector magnet for generating detecting signals in response to the rotation of said detector magnet with said shaft.

2. An electric motor according to claim 1; wherein said means mounted on said control circuit substrate for detecting the rotational position of said rotor magnets includes a plurality of position detector elements secured on said control circuit substrate in a fixed relationship, said stator assembly further includes a bobbin about which said stator windings are wound and being fixed positionally with respect to said control circuit substrate whereby said position detector elements are positioned in a fixed predetermined relationship with respect to said stator windings by means of said control circuit substrate and bobbin.

3. An electric motor according to claim 2; wherein a holder provided with recesses for receiving said position detector elements is secured to said control circuit substrate, said recesses being formed with a configuration so as to allow regulation of the position of said elements in a circumferential and radial direction for proper positioning of said elements with respect to said rotor magnets.

4. An electric motor according to claim 1; wherein said rotor assembly comprises a magnet casing having magnets secured therein and further includes an annular groove to receive an adhesive to firmly secure said magnets in said magnet casing.

5. An electric motor according to claim 4; wherein an annular magnet holder having a configuration accommodating the inner diameter of the magnet casing is arranged in and secured to the magnet casing and said plurality of magnets forming said rotor assembly are disposed respectively between pairs of positioning means provided at a side of said magnet holder.

6. An electric motor according to claim 1 wherein said detecting winding assembly is disposed outside the bottom of said magnet casing and a speed detecting winding therein is disposed concentrically with said detector magnet for detecting a voltage induced therein by the rotation of said detector magnet.

7. An electric motor comprising a stator assembly, a rotor assembly including a plurality of rotor magnets, a detecting winding assembly and motor housing means containing and relatively positioning said stator assembly, rotor assembly and detecting winding assembly;

said stator assembly including stator windings, a control circuit substrate having circuitry thereon for controlling a driving current supplied to said stator windings, means mounted on said current circuit substrate for detecting the rotational position of said rotor magnets, a bearing housing, an annular recess in said bearing housing having a reference surface and having a hook-shaped cross section, a bobbin about which said stator windings are wound, an end portion on said bobbin having serrations, and a compression spring interposed between said bearing housing and said bobbin, said annular recess, serrations and spring being arranged so that said serrations on said bobbin are urged by said spring into engagement with said reference surface of the annular recess to position said bobbin with respect to said bearing housing and stator windings;

said rotor assembly including a shaft rotatably journalled in said stator assembly, a magnet casing secured to said shaft and within which said rotor magnets are fixed, and a detector magnet secured to said shaft; and

said detecting winding assembly being disposed within said motor housing means in face-to-face relationship with said detector magnet for producing detecting signals in response to the rotation of said detector magnet with said shaft.

8. An electric motor according to claim 7; wherein said bobbin has, along the inner periphery of one end thereof an annular recess, and a holding member has a flange which is press-fitted in said annular recess and grips said control circuit substrate between the flange of said holding member and the end of said bobbin.

9. An electric motor according to claim 7; wherein said bobbin has recesses extending longitudinally at one end thereof and opening at inner and outer sides of the bobbin, and said control circuit substrate has recesses corresponding to said recesses of said bobbin and being disposed along the inner periphery, ends of the stator windings wound on said bobbin extending outwards through said recesses of said bobbin and control circuit substrate.

10. An electric motor according to claim 7; wherein said control circuit substrate includes a tongue at one area along the periphery thereof, connector means is secured to said tongue for supplying electric power to said circuitry, and said motor housing means has an open end thereof with a longitudinally extending elongated recess receiving said tongue, said recess having a width substantially equal to that of said tongue so that the disposition of said tongue within said recess orients said stator assembly in a circumferential direction with respect to said motor housing means.

11. An electric motor according to claim 10; wherein said motor housing means has a circumferentially extending slot contiguous to said elongated recess, said circumferentially extending slot having a length greater than the width of said tongue, said recess having a bearing surface engaged with said tongue, and a stopper having a cross-section substantially the same as the configuration of said slot is engaged with a press-fit within said slot and thereby fixes said tongue against said bearing surface in said elongated recess.

12. An electric motor according to claim 7; wherein said bearing housing has a tubular opening at one end, a bearing member rotatably supporting said shaft and being press-fitted in said tubular opening, said motor housing means has a bracket for closing an end thereof, said bearing housing is clinched at said one end to said bracket, and said one end of the bearing housing has a stepped bore having an inner diameter slightly larger than the outer diameter of said bearing member press-fitted in said tubular opening of said bearing housing.

13. An electric motor comprising a stator assembly, a rotor assembly including a plurality of rotor magnets, a detecting winding assembly and motor housing means containing and relatively positioning said stator assembly, rotor assembly and detecting winding assembly;

said stator assembly including stator windings, a control circuit substrate fixed relative to said stator windings so as to be positioned within the field of said rotor magnets, circuitry on said control circuit substrate for controlling a driving current supplied to said stator windings, and means mounted on said control circuit substrate for detecting the rotational position of said rotor magnets;

said rotor assembly further including a shaft rotatably journalled in said stator assembly, a magnet casing secured to said shaft, said plurality of rotor magnets being fixed within said magnet casing, a detector magnet secured to said shaft, an annular groove containing an adhesive which firmly secures said plurality of rotor magnets in said magnet casing, a ring having a pair of opposed flanges in the circumferential periphery of said magnet casing and balance weight means for controlling the balance of said rotor assembly mounted between said pair of opposed flanges; and

said detecting winding assembly being disposed within said motor housing means in face-to-face relationship with said detector magnet for producing detecting signals in response to rotation of said detector magnet with said shaft.

14. An electric motor according to claim 12; wherein said motor housing means and said bracket include cooperatively engaging projections and recesses concurrently determining the radial, circumferential and thrust-directional position of said bracket relative to said motor housing means, and a pair of inclined projections are provided one on said bracket and the other on said motor housing means in the vicinity of said recesses for restraining interlock with each other.

15. An electric motor comprising a stator assembly, a rotor assembly including a plurality of rotor magnets fixed therein, a detecting winding assembly and motor housing means which contains and relatively positions said stator assembly, rotor assembly and detecting winding assembly;

said stator assembly including stator windings, a control circuit substrate fixed relative to said stator windings so as to be positioned within the field of said rotor magnets, circuitry on said substrate for controlling a driving current supplied to said stator windings, and means mounted on said control circuit substrate for detecting the rotational position of said rotor magnets;

said rotor assembly including a shaft rotatably journalled in said stator assembly, a magnet casing secured to said shaft and within which said rotor magnets are fixed, said magnet casing having a bottom adjacent an end wall of said motor housing means, and a detector magnet secured to said shaft;

said detecting winding assembly being disposed concentric with said detector magnet and including a plurality of cores, a speed detecting winding on said plurality of cores, a holder supporting said speed detecting winding and having a plurality of flexible integral detent levers thereon, said plurality of flexible detent levers having an inner surface accommodating an outer periphery of said cores, and a positioning pawl on each of said detent levers extending inwardly toward said cores; and

said detector magnet and detecting winding assembly being disposed between said bottom of said magnet casing and said adjacent end wall of said motor housing means.

16. An electric motor according to claim 15; wherein said holder has a plurality of latching segments along an outer periphery for engagement with said motor housing means, each of said latching segments having, at an end thereof, a latching pawl projecting radially outwardly and cooperating with a plurality of respective receiving openings formed in the side wall of said motor housing means in the vicinity of said adjacent end wall thereof.

17. An electric motor according to claim 15; wherein said detecting winding assembly includes a bobbin having a hub and a pair of opposed flanges located at respective ends of said hub, and a winding wound around said hub and having lead ends, one of said flanges being provided with projections guiding said lead ends of said winding.

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BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to an electric motor and, more particularly, to such an electric motor in which speed detecting magnets are provided outside the bottom of a rotor casing and a speed detecting winding is provided concentrically with the detecting magnets and wherein the driving current of the motor is controlled by a voltage induced in the detector winding in response to the rotation of the detector magnets.

2. Description of the Prior Art

Tape drive capstans of tape recorders are generally driven by electric motors which ideally have a minimum fluctuation in rotational speed with a maximum service life. It has been the recent trend to employ brushless motors which do not have a commutator and brush for such purposes. However, in the conventional brushless motor, a motor torque generating section consisting of stator windings, rotor magnets and a detector element for detecting the rotational angular position of the rotor magnets is provided separately from a control circuit section which controls current supply to the stator windings in response to the detecting signals from the detector element. Such brushless motors also have a considerable number of lead wires for connecting the two sections. Thus it is tedious to assemble and connect such brushless motors and the space requirements necessary to accommodate such motors becomes prohibitive. It is also a problem in that the manipulation of such a motor is cumbersome.

There are also additional disadvantages in that, where such a conventional brushless motor is used as a servomotor, it is the practice to utilize the voltage induced in the driving windings by the rotation of the driving magnets of the rotor as the signals corresponding to the speed of rotation. This results in the fact that the induced voltage includes substantial ripple components due to fluctuation of driving current when the number of poles is small. The ripple components must be eliminated by the use of other means, thereby requiring a complex circuit structure. Further, the magnitude of the output of the induced voltage tends to be affected by temperature variation so that it is often difficult to control the motor speed with a high degree of accuracy. Notwithstanding the presence of these disadvantages, it is essential for high quality performance to control with absolute accuracy the rotational speed of electric motors employed in audio devices such as tape recorders and record players.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a novel electric motor immune from the various disadvantages present in conventional electric motors.

Another object of the invention is to provide an electric motor in which assembling and connecting operations of the component blocks are easy to perform.

A further object is to provide an electric motor which is compact in size and is effectively mounted within a small space.

An additional object is to provide an electric motor which is effective in controlling the rotational speed thereof with an optimum accuracy without causing objectionable speed fluctuation.

A still further object is to provide an electric motor in which position detector elements for detecting the rotational position of the rotor are positioned with a high degree of accuracy with respect to the rotor windings.

Yet another object is to provide an electric motor in which rotor magnets mounted within a magnet casing are easily associated with the magnet casing.

Another object is to provide an electric motor in which a rotation detector section to be arranged within a motor housing is mounted therein in a simple manner, with each component part thereof being accurately positioned.

Another object is to provide an electric motor in which the motor housing is connected easily to a bracket to which is attached a circuit control substrate plate and which receives or supports the rotor and stator.

Another object is to provide an electric motor in which a bobbin wound with stator windings can be easily fitted by a simple fitting means to a bearing housing mounted integrally to the bracket, without resorting to adhesives or other attachment means.

Yet another object is to provide an electric motor in which the bobbin wound with rotor windings is detachably fitted in the bearing housing thereby assuring easy access to the control circuit plate for maintenance and inspection.

Further and other objects and advantages of the present invention will become apparent on reading the following description with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an exploded perspective view of an electric motor according to the present invention;

FIG. 2 is a schematic cross-sectional view illustrating the manner in which the stator block of the electric motor according to the present invention is assembled;

FIG. 3 is an exploded perspective view showing the manner in which the bracket forming part of a stator block is assembled with the bearing housing;

FIG. 4 is a perspective view of the bracket and bearing housing as assembled;

FIG. 5 is an exploded perspective view representing the manner in which a yoke is mounted relative to the bobbin forming part of the stator block;

FIG. 6 is an exploded perspective view illustrating how the control circuit substrate is fitted with the bobbin forming part of the stator block;

FIG. 7 is an exploded perspective view showing the manner in which the assembly of bracket and bearing housing is assembled with the assembly of bobbin and control circuit substrate;

FIGS. 8(A)-8(C) are cross-sectional views illustrating the sequence in which the bearing housing is fitted within the bobbin;

FIG. 9 is a perspective view, partly exploded, of a holder for supporting Hall elements;

FIG. 10 is a view similar to FIG. 9 but illustrating another embodiment of Hall element holder;

FIGS. 11(A) and 11(B) are cross-sectional views of essential parts showing the manner in which the bearing housing is secured to the bracket;

FIG. 12 is an exploded perspective view of the rotor block according to the present invention;

FIG. 13 is a perspective view thereof as assembled;

FIG. 14 is a perspective view illustrating the manner in which the magnets are fixed within the magnet casing of the rotor block;

FIG. 15 is a cross-section of the magnets and magnet casing as assembled;

FIGS. 16-18 are cross-sectional views of alternate embodiments showing mounting magnets in the magnet casing;

FIG. 19 is an exploded perspective view showing the structure of the detector block of the electric motor according to the present invention;

FIG. 20 is an enlarged perspective view of a guiding projection provided on the flange of a hub of the detector block;

FIG. 21 is a perspective view of a holder for the above-mentioned detector block;

FIGS. 22(A) and 22(B) are cross-sectional views showing the manner in which a rotation detector section is fitted in the holder;

FIG. 23 is a perspective view of the rotation detector section and holder as assembled;

FIG. 24 is a perspective view of a motor housing of an electric motor according to the present invention;

FIG. 25 is an enlarged perspective view of an essential part of the motor housing and bracket at the juncture thereof illustrating how they are positioned relative to each other;

FIG. 26 is an enlarged perspective view showing the motor housing and bracket as engaged together;

FIGS. 27(A) and 27(B) are cross-sectional views representing the sequence of assembly of the rotation detector section and motor housing;

FIG. 28 is an exploded perspective view showing the manner in which a stopper is attached to the motor housing assembled with a bracket;

FIG. 29 is a perspective view of an electric motor according to the invention as completed by assembling all the component parts thereof; and

FIG. 30 is a cross-sectional view thereof.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

As seen in FIG. 1, an electric motor according to the present invention comprises a stator block or assembly 100, a rotor block or assembly 200, a detector winding block or assembly 300 and a motor housing 400.

The stator block 100 best seen in FIG. 2 consists of a bracket 101 which is positioned within the motor housing 400, a bearing housing 102 press-fitted or clinched to bracket 101 at a central aperture 105, driving windings 103 wound over a bobbin 107 snap-fitted over bearing housing 102, and a control circuit substrate 104 mounted on a bobbin 107. Formed on central aperture 105 (FIG. 3) along the periphery thereof are provided a plurality of recesses 106 to assure smooth clinching of the bearing housing 102.

Bearing housing 102 is formed of a cylindrical portion 110 having a thin wall thickness defining a tubular opening 113 and at the upper portion there is formed a jaw 111 on which bracket 101 rests when cylindrical portion 110 is inserted through central aperture 105 of bracket 101. A pair of bearings 114 and 115, respectively, are press fit at each end of tubular opening 113 of bearing housing 102. The thin-walled cylindrical portion 110 of bearing housing 102 is forced to flare out laterally cooperating with jaw 111 whereby bracket 101 is gripped between them and is firmly clinched to the bearing housing 102.

An annular slot or groove 116 is provided at the opposite end of the bearing housing 102 along the exterior periphery thereof. Slot 116 has a hook-shaped cross-section. As is apparent from FIG. 2, annular slot 116 has a cross-sectional shape defined by a plane 117 perpendicular to the peripheral plane of bearing housing 102, a parallel plane 118 adjacent the plane 117 and an inclined plane 119 located next to the parallel plane 118.

As shown in FIG. 5, bobbin 107 which is mounted around bearing housing 102, includes a cylindrical portion 121 and a flange 122 extending outwardly from the cylindrical portion 121 with both of these being preferably made from a rigid material as required, such as bakelite or synthetic resins. At the lower end of the cylindrical portion 121, there are formed serrations 123 having an inner diameter slightly smaller than that of the cylindrical portion 121. As best seen in FIG. 2, serrations 123 have a cross-sectional configuration defined by a vertical plane 125, which is situated adjacent to a horizontal end surface 124 of cylindrical portion 121, and an outwardly inclined plane 126 contiguous thereto. An annular yoke 108, made from a resilient magnetic material and which is adapted to be wound with driving windings, is engaged within flange 122 and around the cylindrical portion 121.

A winding coil retainer member 141 (FIG. 5) is fitted at the other end of cylindrical member 121 of bobbin 107 opposite flange 122. Winding retainer member 141 includes a flange 142 and a tubular projection 143 and is engaged with the cylindrical portion 121 of bobbin 107 in such a manner as to cover the upper end of annular yoke 108, disposed about bobbin 107. Thus a bobbin 107 is formed on which driving windings 103 are wound. The driving windings 103 are wound over and through an undercut 144 of flange 142 of winding retainer 141, on the one hand, and over and through an undercut 145 of flange 122, on the other hand. Thus windings 103 are wound over flange 142 of winding retainer 141 and flange 122 located at the other end of the bobbin 107, as shown in FIG. 6.

The cylindrical portion 121 of bobbin 107 is assembled with control circuit substrate 104 by means of a holder 129 in such a manner that the opening 135 of the control circuit substrate 104 which has an inner diameter slightly greater than the outer diameter of holder 129 is first brought into registration with an annular recess 127 formed at the uppermost interior portion of the cylindrical portion 121. Annular recess 127 has an inner diameter slightly smaller than the outer diameter of holder 129 so that holder 129 is pressed down by the substrate 104 into the annular recess 127. The holder, on being inserted, is deflected in the inward direction and is resiliently held in the recess 127.

At the top of the winding coil retainer 141 a protrusion 134 is provided which is adapted to engage with a recess 137 of the control circuit substrate 104 whereby relative positioning of the plate 104 with respect to the bobbin can be assured.

The control circuit substrate 104 connected to the bobbin 107 in the manner described when compared with the conventional way in which adhesives or screws are used, provides certain advantages in that the assembling operation is made more simple and effective and maintenance and inspection can be easily conducted by disassembly of the circuit parts without undue difficulty.

In addition, a coil spring 131 is provided between the jaw 111 of the bearing housing 102 and a radially extending flange 130 of holder 129. The spring 131 urges the horizontal end surface 124 of the serrations 123 against the vertical surface 117 of the annular slot 116 absorbing unnecessary clearance of the bobbin 107 relative to the bearing housing 102 and assuring proper positioning between them. The vertical surface 117 and horizontal end plane 124 together form a reference plane of mount for these two parts.

With reference now to FIGS. 8A-8C, a description will now be given to the sequence in which the bobbin 107 is mounted on the bearing housing 102. Spring 131 is first engaged about bearing housing 102 and housing 102 is then inserted from above into the bobbin 107. As this stage, the annular slot 116 faces against the serrations 123 as shown in FIG. 8A. On pressing the bearing housing 102 down against the action of the spring 131, the serrations 123 are brought into engagement with the lower end of the bearing housing 102 so that the inclined surface 126 is expanded outwardly, as shown in FIG. 8B. Upon further downward movement of housing 102, the serrations 123 resume their original position as they snap into the annular slot 116, as shown in FIG. 8C. After the engaging movement is completed, the bobbin 107 will be subjected to the action of the spring 131 so that the horizontal end surface 124 of serrations 123 bears against the vertical surface 117 of the annular recess 116. Thus, the position of the bobbin 107 in the thrust direction relative to the bearing housing 102 is maintained stable. This positioning of the bobbin 107 is accomplished simply and quickly without employing conventional adhesives or screws.

Easy connection of driving windings 103, which are wound over the bobbin 107, to the circuitry provided on the control circuit substrate 104, is accomplished through the assembly of circuit substrate 104 and bobbin 107 in the manner described below. The top of the cylindrical winding retainer 141 includes four recesses 133 corresponding to the number of leads 132 (FIG. 6) of the driving windings 103. Each recess 133 extends in the longitudinal direction to form an opening through the tubular portions 143 of retainer 141. As noted above, at least one projection 134 is provided at the top of the retainer 141 and it fits within recess 137 which has a size substantially equal to that of the projection 134 to accommodate that projection. Four additional recesses 136 are provided along the inner periphery of opening 135 of control circuit substrate 104 in such a manner as to face with the four recesses 133. Control circuit substrate 104 is mounted on the bobbin 107, with recess 137 engaging the projection 134 to properly locate substrate 104 with respect to the bobbin 107. Thus, the recesses 133 of bobbin 107 are located in proper registration with the recesses 136 of the control circuit substrate 104 to form passages from the outer area of the bobbin to the area above substrate 104. In assembly, the leads 132 of the driving windings 103 are first drawn together through the recesses 133 of the winding retainer 141 toward the interior of the tubular portion of winding retainer 141 and are passed through aperture 135 of control circuit substrate 104. After projection 134 is engaged within recess 137 of substrate 104, leads 132 appear above the control circuit substrate 104 via the passages formed by the recesses 133 and recesses 136 on substrate 104 thereby enabling soldering of leads 132 to a printed circuit 138 provided on control circuit substrate 104.

The control circuit substrate 104 is also provided with a position detector assembly 150 for detecting the rotational position of the rotor magnets arranged within the rotor block 200 (See FIGS. 9 and 10). The position detector assembly 150 includes three Hall position detector elements 151 mounted on the control circuit substrate 104 by a holder 152. Control circuit substrate 104 is provided with a pair of positioning holes 154 (FIG. 6) which receive positioning pins 153 extending from holder 152. The positioning holes 154 are located in a predetermined positional relationship with respect to recess 137 which engages projection 134 of winding retainer 141. Holder 152 serves to support the three Hall elements 151 in a relationship equally angularly spaced apart from each other at an angle of 30.degree. as well as in a predetermined positional relationship relative to the control circuit substrate 104. In other words, the holder 152 is provided with three recesses 155 for locating the Hall elements 151 which are arranged in sequence at 30.degree. in the direction X indicated by the arrow in FIGS. 9 and 10 of the drawing. Positioning pins 153 are also located in a predetermined position with respect to these recesses 155. It is also necessary to assure the proper positioning of Hall elements 151 in a radial direction, e.g., in the direction of arrow Y of the drawing, if they are to face with the end surface of the rotor magnets. To this end, holder 152 is provided with stop elements 156 (FIG. 10) to which the Hall elements 151 are secured to prevent radial displacement of the elements.

After Hall elements 151 are located and fixed in recesses 155 of holder 152, and positioning pins 153 are located in positioning holes 154 of control circuit substrate 104 to fix holder 152 on substrate 104, the Hall elements are properly positioned in a predetermined angular and radial relationship with respect to the control circuit substrate 104.

As the relative position between the positioning holes 154 and recess 137 is fixed on control circuit substrate 104, Hall elements 151 are positioned correspondingly in a predetermined relationship relative to bobbin 107, simply by engaging projection 134 with recess 137 to assembly substrate 104 relative to bobbin 107. Since bobbin 107, as described above, supports yoke 108 wound with driving windings 103 and, in addition, since the relative position between driving windings 103 and bobbin 107 is fixed, the Hall elements 151 will automatically be mounted in a given fixed position with respect to the driving windings 103. The radial position of the Hall elements is also fixed in such a position as to enable normal detection of the magnetic poles of the rotor magnets.

As is clear from the foregoing description, the positions of Hall elements 151 are fixed with respect to the driving windings 103 in a given positional relationship through holder 152, control circuit substrate 104 and bobbin 107. Thus, since positioning has been made with a high degree of accuracy between the recesses 155 of the holder 152, the positioning pins 153, projection 134 and the driving windings 103, positioning of the Hall elements 151 and the driving windings 103 is obtained automatically with corresponding high accuracy by a simple assembly operation.

Bearings 114 and 115, which support shaft 203 of rotor block 200, are press fit within tubular opening 113 of bearing housing 102 at either end thereof, respectively. Preferably, upper bearing 114 is placed as close to the uppermost position of the bearing housing as possible where the bearing housing is clinched to bracket 101, in order to reduce the length of the free end of shaft 203 which extends beyond the bearing to avoid the effect of eccentric load caused by pulley and the like. However, the inner race of bearing 114 may be subjected to strain developed by the clinching operation and may tend to detract from the smooth rotation and support of shaft 203. To avoid this possibility, a stepped portion 161 (FIGS. 11A and 11B) is formed at an end of bearing housing 102 where the clinching is effected. Stepped portion 161 has an inner diameter larger than the outer diameter of the bearing 114 and 115 so that a clearance of d (FIG. 11A) is maintained between tubular opening 113 and cylindrical portion 110 as illustrated in FIG. 11A. The clinching operation is carried out using a forming tool 162 in such a manner as to expand the end of the bearing housing 102 to fold the end of the bearing housing over bracket 101 (See FIG. 11B). During the course of clinching, the inner diameter of stepped portion 161 is not reduced beyond the bore of the bearing housing 102 and, thus, no strain is imparted to bearing 114. Thus, smooth rotation and support of shaft 203 is ensured by the bearings 114 and 115.

As shown in FIGS. 12 and 13, rotor block or assembly 200 comprises a magnet casing 201 of iron alloy, a series of four spaced-apart driving magnets 202 mounted in magnet casing 201, a shaft 203 carrying a washer 220 and a detector magnet 204 mounted on shaft 203 outside magnet casing 201. A holder 205 to support shaft 203 is mounted at the bottom of magnet casing 201. Magnets 202 are supported in magnet casing 201 in equally spaced orientation by a magnet holder 206 which is press fitted into the bottom of magnet casing 201. Adhesives are preferably also applied to magnet holder 206 in order to ensure a solid securement to magnet casing 201. Magnet holder 206 has an annular configuration and is provided with four upstanding projections 207 equidistantly spaced apart from each other to separate driving magnets 202 which are placed upon annular holder 206 between adjacent projections 207. An adhesive is applied to secure the magnets to magnet casing 201. After magnet holder 206 and driving magnets 202 have been assembled in magnet casing 201, a balance ring 209 having an annular groove 208 is mounted along the periphery of the magnet casing 201. Groove 208 is intended to receive balance weights 210 (FIG. 13) for balancing the rotation of the rotor block. To complete the assembly, shaft 203 is passed through holder 205 secured to magne