HK1066335B - Stepping motor - Google Patents

Description

Stepping motor

Technical Field

The present invention relates to a motor, such as a stepping motor, and a structure for fastening a stator to a housing.

Background

Hybrid stepping motors, for example, are generally constructed in a structure in which a stator having a coil wound around an annular stator lamination is located in a small air gap around a rotor, wherein magnets are attached to the rotor lamination, the stator lamination being a lamination pressed from a thin iron plate, and similarly, the rotor lamination is a lamination pressed from a thin iron plate. In this type of stepping motor, when current is supplied to the coils, the magnetic poles of the rotor are connected to the magnetic poles of the stator, causing the rotor to be held at a specific position where the rotor is rotated stepwise by changing the phase of the current supplied to the coils.

In general, a motor, including the above-described stepping motor, has a protective housing that houses a rotor and a stator, wherein a shaft serving as an output shaft and connected to the center of the rotor is held by the housing by a bearing, such as a ball bearing, so as to rotate freely. As is well known, a casing structure includes a structure having individual casings formed by being joined into individual unit casing sections, wherein the casings are separated in the axial direction of a shaft, and a structure in which stator laminations of a stator are inserted between different casing sections such that the stator is fixed in the casings when the casing sections are bolted together. (see, for example, patent reference 1 and patent reference 2)

The above-described motor disclosed in the above-listed publication is complicated in connection operation, in which the two housing sections are bolted together while the stator is held in a state in which the stator is interposed between the two housing sections, and at the same time such a motor tends to increase in the number of parts. Furthermore, since a step of manufacturing bolt holes and threaded holes for bolting the respective housing sections together is required, there is a problem of a large number of manufacturing steps. Furthermore, metal particles generated when the unit is fastened by turning the bolt can enter a small air gap between the rotor and the switch, causing problems in the application of rotation. These problems tend to lower the production efficiency, and thus development of techniques capable of solving these problems is required. Moreover, in the structure in which the parts are fastened together with bolts, there is a risk that the bolts become loose over a long period of operation, tend to cause rattling sounds, and oscillate at the position where the stator is held.

Disclosure of Invention

Accordingly, an object of the present invention is to provide a motor capable of reducing the number of manufacturing steps and parts while making it easier to fasten housing [ segments ] to each other, and capable of preventing a malfunction in rotation caused by the generation of metal particles as the respective parts are fixed together, thereby providing a motor capable of not only improving production efficiency but also securing long-term stability in which a stator is fixed.

The motor according to the present invention has an inner housing and an outer housing of cylindrical shape spaced in the direction of the rotation axis of the motor, a shaft supported by bearings on these housings so as to be freely rotatable, a rotor combined with the shaft as a single unit, and a ring stator assembled facing the rotor and wound with coils, wherein not only the inner housing is mounted and fixed in the outer housing and the stator is inserted between the housings, the outer housing and the inner housing are fixed to each other by caulking, bonding or welding, and in order to increase the holding force of the stator by the housing, the outer housing and/or the inner housing is provided with a force application part, wherein the outer case has a large diameter portion for accommodating and mounting the inner case, a small diameter portion, and an inner space formed by a step portion; wherein the step portion has an inner surface that transitions from the large diameter portion to the small diameter portion and abuts the stator for accommodating a part of the coil of the stator; when the stator contacts the inner surface of the step portion, the coil is disposed in the inner space of the small diameter portion, an elastic member is generated either from the inner surface of the step portion or by straining the step portion itself, and when the stator is inserted between the inner case and the outer case, the elastic member is elastically deformed to increase a holding power of the stator inserted between the inner case and the outer case.

The housing of the electric machine according to the invention is configured as an outer housing, an inner housing mounted therein, wherein the stator is held between the housings, and the housings are fixed together by riveting, gluing or welding. Therefore, the housings are not fastened together with bolts, and the various problems caused by the conventional bolt connection described above are solved. In other words, the work related to fastening the housings together is simplified, and not only is the number of parts required because the bolts are omitted reduced, but also the operation of performing bolting the housings together with the bolts is omitted, thereby making it possible to reduce the amount of work in the manufacturing process. And since there is no metal particle generated when the bolt is rotated at the bolt node, a rotation failure (e.g., seizure) due to the metal particle can be eliminated. For the above reasons, the production efficiency is improved.

The stator is fixed by being held between the outer housing and the inner housing, but the application of the spring portion fitted on at least one of the housings will keep the stator firmly held in the housing. This enables the stator to maintain stability over time.

The rotation stopping means for the stator may be assembled in the present invention. Special means include means of cooperating projections and recesses on the stator and the outer housing. For example, a recessed portion is formed on the stator, and a protruding portion that fits the recessed portion is formed on the outer housing. The inner housing may also be used as a housing with recesses and protrusions matching the stator, or two inner and outer housings may be used in parallel.

The outer case in the present invention has a large diameter portion at an opening side into which the inner case is fitted and a small diameter portion at an end surface, wherein a bearing surface supporting the stator is constituted at an inner surface of a step portion which is a transition from the large diameter portion to the small diameter portion, wherein at least a part of the coil is in a space of the small diameter portion. By contacting the stator with the bearing surface of the outer cylinder, the stator is firmly held between the housings, and it is held in a firmly held state. The spring portion is effective if it is formed on the bearing surface or if the bearing surface itself is strained into the spring portion. Further, disposing at least a part of the coil in the space of the small diameter portion makes it possible to increase the space utilization in the housing, and thereby it is possible to reduce the size of the unit. Further, the shape of the bearing surface is formed so as to be in close contact with the side surface of the stator, so that it is possible to increase the heat radiation area for radiating the heat generated by the coil and to have the spring force, to improve the heat conductivity to the heat radiation part (to reduce the heat pocket) by improving the assembly, and to have the overall effect of improving the heat radiation effect.

The present invention includes a structure in which a connecting portion for connecting the motor to other portions is fastened to an end plate of a small-diameter portion of an outer case, wherein the connecting hole is formed at a position corresponding to a step portion of the outer case for the connecting portion. For example, the connection portion comprises a metal plate having a through-hole or a threaded hole or the like therein for connecting the motor to another material at the connection hole. A connection hole is formed at a position corresponding to the stepped portion of the housing to provide a clearance space for the screw. If there is no step portion formed at the end plate of the outer case, it would be necessary to form the connection hole after projecting the connection portion in the axial direction of the outer case; however, this is not necessary in the present invention. Thus, space can be saved.

Further, at least one bearing in the present invention includes a form fixed in a state of being outwardly projected in an axial direction, wherein each bearing is fixed in the outer housing and the inner housing. In this structure, the height of the motor can be reduced (as removing the bearings in the axial direction) without changing the distance between the bearings, in other words, the bearing span of the bearings fixed to the housing is reduced, so that a small form factor can be produced. Conversely, the bearing span can be increased, thereby making the rotation of the rotor much more stable.

When the bearing is fixed in the outer housing in a manner projecting outward in the axial direction, as described above, the bearing with the above-described connecting portion can be protected. The protection in this case is very useful because it covers the bearing and reduces the portion exposed to the outside, so when the bearing protrudes from the outer housing to the outside, dust and other particles are prevented from entering the motor, force is prevented from being directly applied to the bearing, and physical impact thereto is prevented. When the protective portion covering the bearing on the outer housing also protrudes, the protective portion can serve as a center positioning for aligning the positions of the outer portions.

The outer and inner housings according to the invention may be constructed from a thin sheet metal process, whereby such a method is considered from the viewpoint of easy handling and suitability for mass production. Further, the present invention can be applied to a stepping motor.

The above aspects, advantages and features are merely representative of embodiments. It is to be understood that it is not to be taken as limiting the invention as defined by the claims. Additional features and advantages of the invention will be apparent from the description and drawings, and from the claims.

Drawings

The present invention is illustrated by way of example, and not by way of limitation, in the figures of the accompanying drawings and in which like reference numerals refer to similar or corresponding parts and in which:

fig. 1 is a top view of a stepper motor according to an embodiment of the present invention.

Fig. 2 is a cross-sectional view taken along section II-II in fig. 1.

Fig. 3 is a partial cross-sectional view of the rotor.

Fig. 4(a) is a bottom view of the stator and fig. 4(b) is a top view of the stator laminations.

Fig. 5 is a view showing a front case, wherein (a) is a cross-sectional view along section 5A-5A.

Fig. 6 is a view showing the elastic member and its arrangement in the front housing, in which fig. 6(a) is a longitudinal cross-sectional view, and fig. 6(b) is a view showing a transverse cross-section.

Fig. 7 is a view showing a front case, in which fig. 7(a) is a top view, and fig. 7(B) is a cross-sectional view taken along a section 7B-7B.

Fig. 8 is an enlarged cross-sectional view of the abutting portion in the embodiment.

Fig. 9(a) is a longitudinal cross-sectional view showing the structure of an embodiment in which the elastic material has been changed, and fig. 9(b) is an enlarged cross-sectional view in an integrated state.

Fig. 10 is a longitudinal cross-sectional view of various embodiments according to embodiments of the present invention.

Detailed Description

An embodiment of the present invention applied to a stepping motor will be described below with reference to the drawings. Fig. 1 is a top view of a stepper motor according to an embodiment of the present invention. Fig. 2 is a cross-sectional view taken along section II-II in fig. 1. In fig. 2, 10 is a rotor, 20 is a stator disposed around the rotor 10, and 30 is a housing covering the rotor 10 and the stator 20.

As shown in fig. 2, the housing 30 is constituted by a cylindrical outer housing (hereinafter referred to as front housing) 40 and an inner housing (hereinafter referred to as rear housing) 50 mounted on the opening side of the front housing 40 at the bottom center of the drawing in combination. As can be seen from fig. 1, a square front plate (connecting portion) 31 is fixed to the top surface of the front case 40 by means of, for example, welding. For example, a through hole is formed by a drilling process at the corner of the front plate 31, and in order to screw the step to the outer member, a screw hole 31a (hole for attachment) is machined in the hole. The front plate 31 is constituted by a drilling process or the like so that the small projection 31b faces the front housing 40 side. For example, the front plate 31 and the cases 40 and 50 are composed of an elastic plate material such as a galvanized steel plate by a press molding process, which is useful from the viewpoint of ease of machine fabrication and suitability for mass production.

A pair of top and bottom ball bearings (axel bearings) 60 and 61 are mounted to the centre of the housing 30 where the rotor 10 is assembled so that it can rotate freely on the shaft 15 at these bearings 60 and 61. As shown in fig. 3, the rotor 10 is fixed by being held between two disk-shaped rotor laminations 11, and similarly, disk-shaped permanent magnets 12 are inserted and held therebetween. A sleeve 13 projects through the centre of the rotor 10 and is rigidly secured therein, where a shaft 15 passes through the sleeve 13 and is rigidly secured therein. The multiple teeth 11a are formed at a uniform pitch around the outer periphery of each rotor lamination 11, wherein each rotor lamination 11 is fixed to the permanent magnet 12 by an adhesive or the like in such a manner that the teeth 11a are offset from each other by an amount equal to one-half of the pitch. The tight fastening between the rotor lamination 11 and the bushing 13 and the shaft 15 is also made by means of, for example, an adhesive.

As shown in fig. 2, the shaft 15 rigidly coupled to the rotor 10 is fitted into inner rings (not shown) of ball bearings 60 and 61, wherein the ball bearings 60 and 61 are coupled to the center of the front housing 40 and the center of the rear housing 50, respectively. The outer ring (not shown) of the ball bearing 60 is press-fitted to the cylindrical hub 44a formed in the center of the rear housing 40, where the outer ring (not shown) of the bottom ball bearing 61 is press-fitted to the inside of the cylindrical hub 53a formed in the center of the rear housing 50. In this case, a portion (e.g., about 1/3 in length in the axial direction) of the top ball bearing 60 extends from the front housing 40 to the top. A cover portion 31c covering the protruding portion of the ball bearing 60 is formed in the center of the front plate 31. This cover portion 31c serves as a centering of the position of the outer portion of the array.

As shown in fig. 2, a spring 70 for providing pressure in the axial direction against the top and bottom ball bearings 60 and 61 and for determining the position of the shaft 15 and the rotor 10 in the axial direction by applying upward pressure is located between the outer ring of the bottom ball bearing 61 and the bottom rotor lamination 11. In this manner, when the rotor 10 is assembled in a state in which the rotor is freely rotatable in the housing 30, the shaft 15 passes through the front housing 40 and the front plate 31 and protrudes a certain distance without protruding from the housing 50.

As shown in fig. 4(a), the stator 20 is equipped with an annular stator lamination 21 and a coil 25 wound on the stator lamination 21. As shown in fig. 4(b), a plurality of stator poles 22 are formed at a uniform pitch around the circumference on the inner circumference of the stator lamination 21, wherein a plurality of pole teeth 22a are provided on the inner circumferential surface of each stator pole 22 in the circumferential direction. A pair of ring-shaped insulators 23 (see fig. 2) covering each stator pole 22 are fitted into the stator lamination 21 from each side, wherein a coil 25 is wound on each stator pole 22 covered by these insulators 23.

As shown in fig. 2 and 4(a), a terminal cover 23a is formed on the bottom insulating member 23 with the terminal passage 24 in the terminal cover 23. The terminal channel 24 has several terminals 24a, wherein these terminals 24a are connected to the coil 25 and from the outside to the power and control wires 26. A plurality of grooves 21a (rotation stopping means) projecting in the axial direction are formed around the outer periphery of the stator lamination 21.

As shown in fig. 2, the stator 20 is held in a front case 40 and a rear case 50, and the front case 40 and the rear case 50 are divided into several parts in the axial direction so that the stator is held in a state of being concentric with the outer periphery of the rotor 10 located in the case 30. The front case 40 and the rear case 50 will be described below.

As shown in fig. 5, the front case 40 has, on the primary opening side (bottom side), a large diameter portion 41 and a small diameter portion 42 formed on the top edge side concentric with the large diameter portion 41, wherein the transition between the large diameter portion 41 and the small diameter portion 42 passes through a horizontal annular stepped portion 43. A top plate portion 44 forming an end surface of the front housing 40 is constituted adjacent to the small diameter portion 42, wherein the aforementioned boss 44a is formed in the center of the top plate portion 44.

The inner surface of the step portion 43 is a bearing surface 43a for the stator lamination 21 in which the top of the coil 25 is located in the inner space 45 of the small diameter portion 42. A notch 41a is formed in the large diameter portion 41 as a clearance space for the aforementioned terminal cover 23 a. Further, on the inner peripheral surface of the large diameter portion 41, there is a protruding portion (rotation stopping means) 41b at a position of 180 ° with respect to the notch 41 a. The inner diameter of the large diameter portion 41 is set to the outer diameter of the stator lamination 21 plus a specified insertion tolerance.

Also, as shown in fig. 6(a) and 6(b), a plurality of (three in this case) elastic pieces (springs) 46 are formed so as to project downward to the step portion 43, as shown in fig. 1 and 5(b), the elastic pieces 46 being constituted with equal spaces around the outer periphery of the step portion 43. These elastic pieces are formed with two slits in the tangential direction, and constitute a portion between which the elastic pieces are inserted.

On the other hand, as shown in fig. 7, the rear case 50 has a bottom portion 51 and a cylindrical portion 52 extending in an upward direction from the periphery of the bottom portion 51. A through hole 51b sufficient for the aforementioned bottom edge portion of the shaft 15 extending from the bottom ball bearing is formed in the center of the bottom 51, wherein the periphery of the through hole 51b forms an outer peripheral groove 53 in the molding process. The boss is formed inward of the outer peripheral groove 53. A notch 51a is also formed in the cylindrical portion 52 of the rear housing 50 as a clearance for the terminal cover 23 a. The height (e.g., the height in the axial direction) of the rear housing 50 or, in other words, the height of the cylindrical portion 42 is approximately half the height of the large-diameter portion 41 of the front housing 40, just as much as the thickness of the stator laminations 21. The outer diameter of the cylindrical portion 52 of the rear housing is set to a size such that it can be fitted just inside the large-diameter portion 41 of the front housing 40 including tolerance.

In the stepping motor of the embodiment of the present invention, as shown in fig. 2, the cylindrical portion 52 of the rear housing 50 is fitted into the bottom of the inner side of the large diameter portion 41 of the front housing 40 so that the tip end surface of the outer peripheral portion of the stator lamination 21 is in a state of being in contact with the bearing surface 43a of the front housing 40, as shown in fig. 8, in which the elastic pieces 46 of the stepped portion 43 are pushed to be elastically deformed, in which the bearing surface 43a is in contact with the front housing 40, and the bottom surface thereof is in edge contact with the top surface of the cylindrical portion 52 of the rear housing 50. In this manner, the stator lamination 21 is held between the housings 40 and 50, and thereby the stator 20 is securely held. Also, the top of the coil 25 is located in the inner space of the small diameter portion 42 of the front housing 40. A projection 41b on the front housing 40 is fitted with one of the slots 21a in the stator lamination 21 to prevent rotation, and the terminal cover is fitted into the recesses 41a and 51a of the housings 40 and 50, and the lead wires 26 are connected to the outside. Rear housing 50 is finally securely held to large diameter portion 41 of front housing 40 around the outer periphery of bottom portion 51 by caulking, adhesive welding, or the like.

Each of the screw holes 31a and the front plate 31 is located on the stepped portion 43 of the front housing 40. As described above, a part of the cover 51c of the front plate 31 covers the protruding portion of the ball bearing 60.

The step of assembling the stepping motor is explained next. The top of the shaft 15, which is first firmly fastened to the sleeve 13 and the rotor 10, is inserted into the ball bearing 60 fitted on the front housing 40. Next, the stator 20 is fitted around the rotor 10, and the terminal cover 23a is fitted into the recess 41 a. One slot 21a of the stator lamination is fitted with the projection 41b of the front housing 40 by fitting the terminal cover 23a into the recess 41 a.

Next, the spring 70 is fitted to the bottom side of the shaft 14, wherein the bottom of the shaft 15 is fitted into the rear housing 50 when the rear housing 50 with the ball bearing 61 mounted thereon is fitted into the front housing 40. At this time, the terminal cover 23 is fitted into the recess 51a in the rear housing 50. Moreover, the rear housing is urged into the front housing 40 with the respective resilient tabs 46 urging the stator laminations 21 against the bearing surfaces 43a and causing the stator laminations 21 to be securely held between the housings 40 and 50. Next, with this state maintained, the rear case 50 is riveted to the front case 40. Note that means other than riveting, spot welding, bonding or other rigid attachment may be used. The close connection of the front plate 31 to the front housing 40 may be completed in advance, or may be completed at the last. Also, an adhesive material is sometimes used at the mounting portions of the stator lamination and the front housing.

The stepping motor of the embodiment of the invention is assembled by the steps.

In the embodiment of the present invention, the rear case 50 is mounted into the front case 40, and then riveting is performed in order not only to fix the cases 40 and 50 to each other, but also to hold the stator 20 between the cases 40 and 50 to fix it thereto. Since the housings 40 and 50 are firmly fixed to each other without using bolts conventionally used, the operation of fixing these to each other is more simplified. Also, since the bolts are omitted, not only is the number of parts reduced but also since it is not necessary to perform lathe machining on the housings 40 and 50 in order to bolt the housings 40 and 50 together, the manufacturing step can be omitted. Further, since there are no metal particles generated by turning the bolt, rotation defects (such as seizure) due to these particles can be eliminated. As a result, the manufacturing efficiency is improved.

The stator lamination 21 of the stator is elastically urged in the direction of the rear housing 50 by the elastic portion 46 formed on the step portion 43 of the front housing 40, wherein the stator is held between the front housing 40 and the rear housing 50 and fixed thereto. The elastic force of the elastic portion 46 compensates for a dimensional change occurring in the gap between the front case 40 and the rear case 50 due to vibration, impact or thermal expansion, and always exerts a strong force on the stator 20, thereby firmly holding the stack [ SIC ]20 between the cases 40 and 50. Also, even if the force of the rear case 40 against the stack [ SIC ]20 is not uniform, the elastic portion 46 has elastic characteristics, thereby being able to compensate for the non-uniformity. Therefore, the stator 20 is securely held between the housings 40 and 50 with high reliability without being affected by such things as vibration, impact, thermal expansion, assembling method or the like, and such a state is maintained, so that it is possible to ensure stable and safe fixing of the stator 20 for a long time.

Also, press-fitting the stator laminations 21 onto the bearing surface 43a of the front housing 40 causes the stator 20 to be securely held between the housings 40 and 50, which is maintained in a securely held state. Further, positioning the coil 25 in the inner space of the small diameter portion 42 of the front case 40 makes it possible to improve the utilization of the space in the case 30 and to reduce the size of the unit.

The screw hole 31a of the front plate 31 is formed on the step portion 43 of the front housing 40 such that the step portion 43 has a clearance for the small projection 31b caused by a bore hole adapted to form the screw hole 31a and to provide clearance for a screw, not shown, which is screwed into the screw hole 31 a. Therefore, it is not necessary to always project the front plate 31 to the outside of the front housing 40 in the rotational direction in order to secure a space for the screw and the small projection 31b, so that space can be saved. Note that the head of the screw may be located in a space above the stepped portion 43, and the screw may be rotated, for example, with a wrench, and thus the screw hole 31a on the front plate 31 may be simply referred to as a through hole without drilling.

A top ball bearing 60 is assembled to the front housing 40. The boss 44a protrudes above a certain length, whereby the height of the housing 30 can be reduced without changing the distance between the top and bottom ball bearings 60 and 61, or in other words, the unit can be made thinner without changing the bearing span. Conversely, the bearing span may be extended, thus making it possible to increase the stability of the rotation of the rotor 10. The top ball bearing 60 protruding upward from the front housing 40 is protected by being covered by the cover portion 31c of the front plate 31, and thus the chance of dust and contaminants entering the housing 30 is reduced and direct force and impact on the ball bearing 60 are prevented. Also, the cover portion 31c may also serve as a center positioning of a position aligned with the peripheral component.

In the above-described embodiment, in order to firmly hold the stator 20 between the housings 40 and 50, one example of the spring portion according to the present embodiment is the elastic portion 46, wherein, in addition to the elastic portion 46, a structure as shown in fig. 9(a) may be used. In other words, the stepped portion 43 in the front case 40 may be drawn in advance toward the top front plate 31 before assembly, in a tensioned state rotated by an angle toward the top diagonal line facing the outer periphery. If so, when assembling the stator 20, the front housing 40 is fixed by caulking, bonding, welding or the like around the periphery of the rear housing 50, in which state the step portion 43 is elastically deformed in the downward direction. Fig. 9(b) shows an assembled state in which the stepped portion 43 constantly attempts to recover to the front plate 31 side, and therefore the rear housing 50 is affected against the pushing of the front plate 31 side. Therefore, the peripheral portion of the stator 20 is held firmly, firmly and with high reliability between the housings 40 and 50, wherein the elastic force automatically compensates for vibration, impact, thermal expansion, or impact against other pieces, whereby the self-compensating stator 20 by the elastic force is maintained in a state of being held firmly and with high reliability even in the case of vibration, impact, thermal expansion, or impact against other pieces, or the like.

Fig. 10 shows a modified embodiment of the stepping motor of the above-described embodiment, in which the stepping motor in this case is provided with a torsion spring 71 made of a wound spring instead of the aforementioned spring 70. Furthermore, the rotor 10 is firmly fastened directly to the shaft 15 without the bushing 13. Although this stepping motor has a slightly different structure from that shown in fig. 2 described above, the basic mechanism is the same, and not only the rear housing 50 is fitted into the front housing 40 but also the stator 20 is held between the housings 40 and 50, and therefore the result is the same.

As described above, according to the motor of the present invention, since the assembly is performed by holding the outer housing and the inner housing of the stator by caulking, bonding, or welding, it is possible to simplify the connection of the housings to each other while reducing the number of parts and the number of manufacturing steps and preventing a rotation defect (such as seizure) caused by metal particles generated when the housings are assembled to each other, and for the reasons described above, the production efficiency is improved. Moreover, since the spring portion that increases the force with which the housing holds the stator is used in at least one housing, the present invention enables the characteristic of stable assembly to be ensured throughout the extension.

For the convenience of the reader, the above description has focused on a representative one of all possible embodiments, which are illustrative of the principles of the invention and which represent the best modes contemplated for carrying out the invention. Other variations and modifications not described are also possible. For example, several of the embodiments described may, in various instances, combine elements of different embodiments or combine elements of the embodiments described herein with other modifications and variations not expressly described. Many undescribed variations, modifications and variations are intended to be included within the scope of the following claims and their equivalents.

Claims (8)

1. An electric machine comprising:

a cylindrical inner housing and a cylindrical outer housing divided in a rotational axis direction of the motor, wherein the inner housing has a cylindrical portion manufactured to be fitted into the outer housing, and the inner and outer housings are fixed to each other by caulking, bonding, or welding;

a shaft supported by bearings on the outer housing and the inner housing, respectively, to allow the shaft to freely rotate;

a rotor combined with the shaft as a single unit;

a ring-shaped stator wound with a coil facing the rotor, wherein the inner housing is fixed within the outer housing, and the stator is inserted between and pushed by the outer and inner housings, the stator being fixedly held by forces of the inner and outer housings contacting opposite ends of the stator, respectively; and

wherein the outer case has a large diameter portion for accommodating and mounting the inner case, a small diameter portion, and an inner space formed by a step portion; wherein the step portion has an inner surface that transitions from the large diameter portion to the small diameter portion and abuts the stator for accommodating a part of the coil of the stator; when the stator contacts the inner surface of the step portion, the coil is disposed in the inner space of the small diameter portion, an elastic member is generated either from the inner surface of the step portion or by straining the step portion itself, and when the stator is inserted between the inner case and the outer case, the elastic member is elastically deformed to increase a holding power of the stator inserted between the inner case and the outer case.

2. The electric machine of claim 1, wherein a rotation stop is provided for the stator.

3. An electric machine as claimed in claim 2, wherein the rotation stop means is formed by the cooperation of a recess of the stator and a projection of the outer housing and/or the inner housing.

4. The motor of claim 1, wherein a connecting portion having holes vertically aligned with said step portion for connecting said motor to an outer portion is formed to be rigidly fixed to an end surface of said small diameter portion of said outer housing.

5. The electric machine according to claim 4, wherein at least one of said bearings on said outer housing and at least one of said bearings on said inner housing are fixed in an outwardly axially protruding state.

6. The electric machine according to claim 5, wherein the at least one bearing on the outer housing projecting axially outward is protected by a connecting portion rigidly fixed to the end surface of the small-diameter portion of the outer housing.

7. The electric machine of claim 1 wherein the outer housing and the inner housing are made of sheet metal.

8. The motor of claim 1, wherein the motor is a stepper motor.