GB2161029A - Motor - Google Patents

Abstract

A generally cylindrical motor case is generally closed at the axial end thereof. A first bearing retainer 1 is provided at this axial end and has a generally cylindrical shape protruding from the axial end of the case. A portion 19 of the protruding generally cylindrical shape is tapered at a predetermined angle into a frusto-conical shape. A first bearing 2 is held in position in the first bearing retainer by press- fitting into the generally cylindrical shape and into the frusto-conical portion. A motor case cover (4 Fig. 2 not shown) closes the opposite open axial end of the cylindrical motor case. A second bearing retainer (5) is provided on the motor case cover. A second bearing (6) is mounted in the second bearing retainer, the rotary shaft (8) of the motor being rotatably supported in the first and second bearings. <IMAGE>

Description

SPECIFICATION Motor This invention relates to rotary electric motors.

So-called "miniature electric motors" commonly have a generally cylindrical motor case which is closed at one axial end, with a bearing mounted in the said axial end. A second bearing is provided on a motor case cover for the open end of the cylindrical casing. We have found that with this arrangement difficulties may readily arise in the fitting and accurate location of the bearings. The present invention has arisen from our work in seeking to improve upon prior proposals in this field.

According to the present invention, we provide a rotary electric motor, comprising: a generally cylindrical motor case which is generally closed at one axial end thereof; a first bearing retainer being provided at said one axial end, having a generally cylindrical shape protruding from said one axial end but being provided with a portion thereof tapered at a predetermined angle into a frusto-conical shape; a first bearing held in position in said first bearing retainer by press-fitting into said generally cylindrical shape and into said frusto-conical portion; a motor case cover closing the opposite open axial end of said cylindrical motor case; a second bearing retainer provided on said motor case cover; and a second bearing mounted in said second bearing retainer; the rotary shaft of the motor being rotatably supported in said first and second bearings.

The invention is hereinafter more particularly described by way of example only with reference to the accompanying drawings, in which: Figure 1A shows a sectional view through a bearing retainer in one embodiment of electric motor in accordance with the present invention; Figure 1B is a sectional view through a bearing adapted for press-fitting into the bearing retainer of Fig. 1A in the direction of the arrow shown; Figure 1 C shows the bearing press-fitted into the retainer; Figure 2 is a somewhat schematic side elevational view of an embodiment of electric motor of the kind in which the bearing and retainer combination of Figs. 1 A to 1 C can usefully be employed, the motor being shown partially in section; and Figures 3A, 3B and 3C show three different prior bearing retention arrangements as seen in both plan and cross-sectional view in each case.

Referring first to Fig. 2 which shows a miniature rotary electric motor of a generally conventional design, it will be seen that it has a first bearing retainer 1 of simple cylindrical form mounting a bearing 2 of simple cylindrical form in one axial end wall of a generally cylindrical motor case 3. The opposite open end of the motor case is closed by a motor case cover 4 of composite construction, having an outer metallic surface and an inner surface formed of an electrically insulating material such as a synthetic resin material in which is integrally moulded a bearing retainer 5 for a second bearing 6. The motor case cover 4 also mounts brush gear including integral terminals 7 which extend to the exterior of the motor case cover.The rotor shaft 8 is supported in the bearings 2 and 6 and mounts a commutator 1 3 with which brushes 10 of the brush gear make contact. The stator is provided by permanent magnets 11 mounted on the interior of the cylindrical motor case cover. Rotor windings 1 2 are provided on the rotor shaft 8 in conventional fashion.

In this arrangement, the bearing 2 is held in position simply by being press-fitted into the bearing retainer 1. This requires that the external diameter of the bearing and the internal diameter of the bearing retainer 1 must differ in dimension in such a way as to allow a close fit to be formed. We have found that in practice, in mass production of miniature electric motors, machining errors make it difficult consistently to obtain the optimum dimensons.

This results in situations which we may describe as under- or over-fitting. In the case of over-fitting it may be difficult to insert the bearing into its retainer at all. On the other hand, with under-fitting, the bearing 2 tends to detach from the retainer 1 which is highly detrimental to the motor. For this reason, the practice, heretofore, has been to design for a slight excess of over-fitting to ensure an adequate press-fit. However, this may result in deformation of the bearing as well as producing adverse effects in regard to the axial hole for the rotor shaft.

We briefly describe below with reference to Figs. 3A, 3B and 3C, three prior proposals which have sought to overcome these inconveniences.

Referring first to Fig. 3A, in this prior proposal, the contact area between the bearing 2 and the inner surface of the retainer 1 was limited by providing an annular shouldered or castellated portion 14. Adverse effects on the dimensions of the shaft hole were limited by providing an annular recess 1 5.

However, to produce bearings of this construction involves significant labour and attention to troublesome dimensional controls.

The arrangements of Figs. 3B and 3C, while avoiding expensive labour charges in the construction of the bearing 2, have other disadvantages. In Fig. 3B an inwardly extending annular flange 1 6 defining a central opening 1 7 prevents the bearing 2 from being dislodged. In Fig. 3C a similar effect is achieved by crimping lugs 1 8. These measures involve additional forming steps, such as drawing, drilling or crimping. Additionally, in the case of Fig. 3C, the crimping operation may result in deformation of the bearing retainer 1 or of the bearing 2.

Referring now to Figs. 1 A, 1 B and 1 C, an embodiment of electric motor in accordance with the present invention has a bearing retainer 1 provided on the closed axial end of the motor case 3 as shown in Fig. 1A. The bearing retainer 1, as in each of the arrangements of Fig. 2 and Figs. 3A, 3B and 3C, is generally cylindrical in shape. However, it is provided with a portion 1 9 thereof which is tapered at a predetermined angle 20 into a frusto-conical shape. The co-operating bearing 2 has an axial shaft hole 21. The bearing is also generally cylindrical in shape, but is provided with chamfered portions 22 which facilitate press-fitting of the bearing 2 into the retainer 1.As shown in Fig. 1 B, the circumferential edge of the bearing is chamfered at both axial sides thereof; however, the bearing may be chamfered only on the side thereof which serves as the leading side as the bearing is press-fitted into the retainer.

We have found that in miniature electric motors utilising this bearing and retainer combination, the bearing 2, when press-fitted into the retainer 1 in the direction of the arrow shown in Fig. 1A is firmly held in position as shown in Fig. 1C. As the bearing 2 is pressfitted in the direction shown, when the leading end face of the bearing reaches the tapered portion 19, the inside diameter of that portion tends gradually to be expanded, so as automatically to exert a strong crimping force upon the bearing 2. It is this that provides the strength preventing the bearing 2 from becoming detached. The exact strength depends on the angle of inclination 20.In an experimental comparison between motors constructed in accordance with the present invention having an inclination angle 20 of 20 degrees and motors having an otherwise similar construction but without any tapered portion 19, showed that the motors with the tapered portion had a substantially greater resistance to removal of the bearing from the retainer. Taking an average of fifty samples in each case, we found that the force necessary to remove the bearing in the motor constructed in accordance with the present invention was 58.1 kg force, while for the motors having a simple cylindrical bearing retainer, the force necessary was 28.85 kg force (i.e.

about half that for motors in accordance with the present invention).

We accordingly prefer to set the angle of inclination of the taper at 20 degrees.

In addition to this increased resistance to removal of the bearing from the retainer, we find that the construction of Figs. 1A to 1C has other advantages, namely: (i) dimensional controls during machining may be eased since minor manufacturing errors relating to the outside diameter of the bearing 2 or the inside diameter of the bearing 1 are in effect absorbed by the provision of the tapered portion 19; (ii) the shaft hole 21 and the outside diameter of the bearing retainer are free from deformation and dimensional changes which may arise with the arrangements of Figs. 3A, 3B or 3C, as explained above; and (iii) since the alignment of the motor shaft can be adjusted at the tapered portion 19, a plain cylindrical bearing can be used in place of a self-aligning bearing (that is one having a part spherical outer surface) as was commonly necessary previously.

It will be appreciated that the construction illustrated in Fig. 1A, 1B and 1C is very easy to produce.

Claims (7)

1. A rotary electric motor, comprising: a generally cylindrical motor case which is generally closed at one axial end thereof; a first bearing retainer being provided at said one axial end, having a generally cylindrical shape protruding from said one axial end but being provided with a portion thereof tapered at a predetermined angle into a frusto-conical shape; a first bearing held in position in said first bearing retainer by press-fitting into said generally cylindrical shape and into said frusto-conical portion; a motor case cover closing the opposite open axial end of said cylindrical motor case; a second bearing retainer provided on said motor case cover; and a second bearing mounted in said second bearing retainer; the rotary shaft of the motor being rotatably supported in said first and second bearings.

2. A motor according to Claim 1, wherein said frusto-conical portion is tapered from said generally cylindrical portion of said first bearing retainer by an inclination angle of 20 degrees.

3. A motor according to Claim 1 or Claim 2, wherein the circumferential edge of the first bearing is chamfered at least on the side thereof which serves as the leading side as the first bearing is press-fitted into the said first bearing retainer.

4. A motor according to Claim 3, wherein the frusto-conical portion of the first bearing retainer is tapered at an inclination angle which is less than the corresponding inclination angle of the chamfered portion.

5. A motor according to any preceding claim, wherein the frusto-conical portion is so formed as to grip the first bearing resiliently.

6. A rotary electric motor substantially as herein before described with reference to Figs.

1 A to 1 C of the accompanying drawings.

7. A rotary electric motor substantially as herein before described and as shown in Fig.

2, modified as shown in Fig. 1C.