GB2062365A

GB2062365A - Electric motor

Info

Publication number: GB2062365A
Application number: GB8032720A
Authority: GB
Original assignee: Mabuchi Motor Co Ltd
Current assignee: Mabuchi Motor Co Ltd
Priority date: 1979-10-11
Filing date: 1980-10-10
Publication date: 1981-05-20
Also published as: HK65385A; JPS5657661U; GB2062365B; DE3038135A1

Abstract

Heat generated in the metal bearing for the shaft of an electric motor which has a case with an end plate 3 of synthetic resin is dissipated by means including a metal sheet 7 having a portion thereof conforming to and in contact with part or all of the outer surface of the bearing. Sheet 7 may extend to the metal casing 8 and is preferably disc-shaped, but may be of a spoke-form. Variations are described w.r.t. Figs. 4-8 (not shown), e.g. sheet 7 may be outside the casing. Fins facilitate cooling. <IMAGE>

Description

SPECIFICATION Electric motor This invention relates to electric motors.

In recent years, small electric motors have been increasingly used in a wide variety of applications, and accordingly more and more sophisticated performances have been required for small electric motors to meet these new and differing applications. The development of small electric motors which satisfy the performance requirements is of great concern to the trade.

Among the requirements is high-speed rotation. In small motors rotating at high speed, motor bearings made of brass, gun metal, sintered oilimpregnated alloy and other metals tend to generate a considerable amount of heat.

As we shall explain below with reference to Figures 1, 2A and 2B, conventional small electric motors suffer from the problem af build up of heat at high speed rotation. Previous suggestions for overcoming this problem of heat build up have not, in our view, proved a success. The present invention represents a wholly new approach to the problem. In accordance with the present invention, there is provided an electric motor, provided with a motor case; an end plate therefor made of synthetic resin; a bearing made of metal for rotatably supporting the motor shaft; and means for dissipating heat generated in the bearing by rotation of the motor, the said means being formed from metal sheet, and having a portion thereof conforming to and in contact with part or all of the outer surface of the bearing.

The invention is hereinafter more particularly described by way of example only with reference to the accompanying drawings, in which: Fig. 1 is a cross-section illustrating an example of a fixed bearing used in a conventional small electric motor; Fig. 2A is a similar cross-section of an example of a self-aligning bearing used in a conventional small electric motor; Fig. 2B is a front view of the same viewed in the direction shown by arrow a in Fig. 2A; Figs. 3 and 4 are cross-sections of motors in accordance with this invention illustrating the provision of heat dissipating means for each of the two types of bearings shown in Figs. 1 and 2A and B; Figs. 5A, and 5C are longitudinal cross-sections of other embodiments of electric motor in accordance with this invention at the portion shown by arrow b in Fig. 3 or 4;; Fig. 6A is a front elevation of the bearing portion of another embodiment of electric motor in accordance with this invention, as seen in the direction of arrow c in Fig. 6B; Fig. 6B is a longitudinal sectional view of the bearing portion of the motor of Fig. 6A; and Figs. 7 and 8 are similar longitudinal crosssections of further embodiments of motor in accordance with this invention.

Particularly, where a motor case end plate to which the bearing is fitted is made of synthetic resin, as shown in Figs. 1 and 2A and 2B, the resin material around the bearing is apt to be softened or deformed due to the heat generated in the bearing. In Fig. 1 a conventional fixed bearing 1, and in Figs. 2A and 2B a conventional self-aligning bearing 2 are embedded in a motor case end plate 3. The self-aligning bearing 2 has a part-spherical external configuration and is seated in a partspherical cavity of the end plate 3 so as to be alignable with the direction of a motor shaft (not shown) inserted into a bearing hole 4. A bearing retainer 5 having claws 5-1, 5-2... is provided to prevent the bearing 2 from separating from the end plate 3. In the arrangements of Figs.

1 and 2A and 28, frictional heat is generated in the motor bearing 1 or 2 as the motor shaft rotates in the bearing hole 4. The frictional heat is increased with increasing rate of revolution of the motor. The frictional heat generated, however, is hardly transmitted at all to the motor case end plate 3 because the end plate 3 to which the bearing 1 or 2 is fitted is made of synthetic resin, which is a poor conductor of heat, and only a small amount of heat is dissipated from the end surfaces of the bearing 1 or 2 exposed to the outside air.

Consequently, as the motor rotates at higher speeds and the frictional heat is increased, the temperature of the bearing 1 or 2 is gradually increased to an extent that the end plate portion making contact with the bearing 1 or 2 is softened. With a further increase in the temperature, deformation of that portion may result. When this happens, the motor can no longer withstand high-speed rotation due to misalignment of the bearing 1 or 2 with the motor shaft.

Of the possible solutions which have been considered, namely: (a) Increasing the heat capacity and heat dissipating area of the bearing by increasing the wall thickness thereof; (b) Increasing the contacts area of the bearing with the motor shaft; (c) Increasing the surface area of the bearing exposed to the outside air and making the bearing of a complex shape to improve heat dissipation; or (d) Manufacturing the motor case end plate from metal, such as a die casting; none is in our view wholly satisfactory. All involve increased manufacturing costs. In particular, in small electric motors where brushes are often mounted on the motor case end plate, making the motor case end plate out of metal, as suggested in (d) above, would require additional insulating means for the brushes.

We shall now explain, with reference to Figs. 3 to 8, how the present invention enables the heat problem to be overcome by simple and effective means in a novel fashion.

In Figs. 3 to 8 like parts are identified by the same numerals as in Figs. 1, 2A and 2B.

In the Fig. 3 arrangement heat is dissipated from a fixed type bearing 1 of a small electric motor by means of a metal sheet having a good heat conductivity (such as of copper or aluminium) shaped to define a portion 6 which conforms to and is fitted in contact with the outer surface of the motor bearing 1, and a heat dissipating portion 7 integral with portion 6 and extending outwardly therefrom along the inner surface of the motor case end plate 3, made of synthetic resin. The edge of the heat dissipating portion 7 is bent to make good contact with motor case 8, which is.

made of metal. As the motor shaft (not shown) which is rotatably supported by bearing hole 4 rotates at high speed, the heat generated in the bearing 1 is transmitted to the heat dissipating portion 7 via portion 6 of the heat dissipator in contact with the bearing surface and then to the motor case 8 to dissipate from the surface thereof.

Although the heat dissipating portion 7 itself of course takes part in dissipating part of the heat, the motor case 8 has a much higher heat dissipating effect because it covers a substantial part of the motor and has a substantial surface area. It is preferable to form the heat dissipating portion 7 into a disc shape such that the heat dissipating portion 7 extends from the entire periphery of the bearing contact portion 6 to reach the motor case 8 because this both improves heat transmission to the motor case 8 and also enhances the heat dissipating effect of the heat dissipating portion 7 itself due to the increased area thereof. The abovementioned heat dissipating effect, however, is not reduced so appreciably by forming the heat dissipating portion 7 in a spokewise structure extending radially between the bearing contact portion 6 and the motor case 8.There is no problem in insulating brushes since part of the heat dissipating portion 7 may be cut out to provide clearance for the brushes.

Fig. 4 shows a heat dissipating arrangement for a self-aligning bearing 2. There is no need for a detailed description of this embodiment since its basic features are essentially the same as those of the embodiment shown in Fig. 3 except that the shape of the contact portion 6 is of different configuration conforming to the outer surface of the bearing 2, and in that a bearing retainer 5 is provided for preventing the bearing 2 from becoming detached.

In the Figs. 3 and 4 arrangements the edge of the heat dissipating portion 7 is bent to make good contact with the motor case 8. Other arrangements are possible as shown in Figs. 5A, 5B and 5C. Fig. 5A shows an arrangement where the edge of the heat dissipating portion 7 is bent in the opposite direction (i.e. away from the end plate 3) to make contact with the motor case 8.

Fig. 5B shows an arrangement where the edge of the heat dissipating portion 7 is butted against the motor case 8, without bending, to make contact therewith Fig. 5C shows an arrangement where the heat dissipating portion 7 terminates short of and does not make contact with the motor case 8.

With the arrangements of Figs. 5A and 5B, essentially the same effect as in Figs. 3 and 4 can be accomplished by ensuring good contact of the heat dissipating portion 7 with the motor case 8.

The Fig. 5C arrangement relies soley on the heat dissipatiion effected by the head dissipating portion 7 itself. Such being the case, the area of the heat dissipating portion 7 must be appropriately selected depending on the service conditions of the motor.

In Figs. 3 to 5C, the heat dissipating portion 7 is provided inside the motor case. The heat dissipating portion 7, however, may be provided outside the motor when there is ample space. Figs.

6A to 8 show heat dissipating arrangements of this kind. In each case a portion 6 of a heat dissipator formed from metal sheet conforms to and is in contact with part of the outer surface of the bearing 1 , the heat dissipating portion 7 being integral with the contact portion 6. By providing the heat dissipating portion 7 on the outside of the motor where there is an ample space, it can be formed in various shapes to increase the available heat dissipating area thereof. Figs. 6A to 8 show embodiments where the heat dissipating portion 7 is formed into a generally cup shape. Needless to say, the shape of the heat dissipating portion 7 is not limited to the cup shape shown in Figs. 6A to 8, but may be of other shapes having larger heat dissipating area. Since a wide variety of such shapes can be easily conceived, description is un necessary.Futhermore, the heat dissipating portion 7 may be extended along the outer surface of the end plate 3 to the motor case 8 (as shown in Fig. 3) to cause the heat dissipating portion 7 to make contact with the motor case 8.

In addition, the heat dissipating effect can be increased by providing fins (not shown) on the heat dissipating portion 7 for each of the embodiments shown in Figs. 3 to 8. The shape of the heat dissipating portion 7 and the size of the heat dissipating area thereof are suitably selected in accordance with the service conditions of the motor.

The several arrangements described above with reference to Figs. 3 to 8 all provide for a substantial heat dissipating effect in bearings in a small electric motor and enable such motors to use synthetic resin (which is advantageous in terms of mouldability, electrical insulating properties and material costs) for their end plates without hindrance, and thereby allow small electric motors capable of withstanding high load and high-speed rotation to be produced at lower manufacturing costs.

Claims

1. An electric motor, provided with a motor case; an end plate therefor made of synthetic resin; a bearing made of metal for rotatably supporting the motor shaft; and means for dissipating heat generated in the bearing by rotation of the motor, the said means being formed from metal sheet, and having a portion thereof conforming to and in contact with part or all of the outer surface of the bearing.

2. An electric motor as claimed in Claim 1, wherein the motor case is constructed of meta!; the heat dissipating means making contact with the motor case.

3. An electric motor as claimed in Claims 1 or 2, wherein the heat dissipating means is provided on the outer surface of the motor case end plate.

4. An electric motor as claimed in Claim 1, wherein the heat dissipating means is provided on the outer side surface of the motor case end plate and is formed into a generally cup shape.

5. An electric motor as claimed in Claim 2, wherein the heat dissipating means extends over substantially the entire internal surface of the motor case end plate.

6. An electric motor substantially as hereinbefore described with reference to and as shown in the accompanying drawings.