GB2178246A - Bush mounting for minature electric motor rotor - Google Patents

Abstract

In order to prevent axial movement of an iron rotor core 2, a bush 1 is retained in position on motor shaft 3 by a curable electrically insulating coating film 4 applied to the bush, the shaft and the iron core, induction heating be used to effect curing. The bush may be of iron, steel or copper and is temporarily located by being a loose press fit so as not to damage the rotor assembly by for example being formed with a triangular bore. <IMAGE>

Description

SPECIFICATION Electric motor rotor This invention relates to electric motors and more particularly to rotors therefor.

In the manufacture of so-called "miniature" electric motors, it has previously been suggested to provide a bush press-fitted to the rotor shaft to hold the rotor in place (i.e. to prevent unnecessary axial displacement in the direction of the bush). However, since the bush itself must be press-fitted to the shaft (i.e. axially displaced thereon), the bush must be so constructed that nevertheless unwanted axial displacement of the rotor is prevented while the motor is running.

We have found that with a moulded synthetic resin bush, there is difficulty in obtaining positional accuracy. With a bush stamped or cut from non-ferrous metal (such as brass) sheeting, the bush does not cause damage to the rotor shaft and may still have an adequate resistance to axial displacement. However, non-ferrous metals are relatively expensive; our attempts to provide a bush stamped from inexpensive iron sheeting ran into difficulties in that press-fitting such a bush to the shaft tends to cause damage to the shaft, even to the extent of causing the shaft to bend, iron being significantly harder than non-ferrous metals. To effectively prevent unwanted axial displacement without causing damage to the shaft or causing the shaft to bend requires very careful control of the tolerances for the inside diameter of an iron bush and the outside diameter of the shaft.In the manufacture of miniature electric motors, such careful tolerance control is both difficult and leads to expense.

The present invention has ariser from our continuing work to overcome these difficulties.

In accordance with the present invention, we provide a rotor for an electric motor, comprising: an iron core; a rotor shaft; a bush mounted on the rotor shaft and adapted to limit axial movement thereof; and an electrically insulating coating film applied to said bush, said shaft and said iron core.

The invention is hereinafter more particularly described by way of example only with reference to the accompanying drawings, in which: Figures lA, 18 and 1C are side elevational views, partly in section, illustrating successive stages in the manufacture of an embodiment of rotor constructed in accordance with the present invention; Figure 2A is a longitudinal sectional view through an embodiment of bush useful in a rotor in accordance with the present invention; Figure 2B is an axial sectional view through a rotor shaft on which the bush of Fig. 2A is mounted; and Figure 2C is a view generally similar to Fig.

2A illustrating an alternative embodiment of bush also useful in embodiments of rotor in accordance with the present invention.

Referring first to Figs. 1A to 1C which show successive stages in the manufacture of an embodiment of rotor constructed in accordance with the present invention and suitable for use in a miniature motor, a bush 1 and a laminated iron core 2 are first mounted on a rotor shaft 3 as shown in Fig. 1 A. The bush 1 can be made of any material that can be subjected to high-frequency induction heating.

Suitable materials include iron, copper and alloys thereof, stainless steel, etc. We regard iron as the preferred material both in terms of its low cost and its good workability. Thus, the bush 1 is preferably made by stamping from an iron sheet. The bush 1 is set at a temporary mounting position on the shaft 3 with a relatively weak press-fitting force of 10 kg. wt., for example. Fitting the bush at a temporary mounting position with a relatively weak force in this way means that the tolerance for the inside diameter of the bush 1 need not be very strictly controlled in manufacture. This ease of manufacture coupled with the adoption of iron for the material of the bush 1 results in a strong bush produced at low cost. Fitting in this way without a substantial force avoids damage such as bending to the shaft 3.

As shown in Fig. 1 B, a coating film 4 is applied as a thin film of adhesive, such as epoxy resin with a thickness of 0. 15 mm. 9r more, for example, to the regions shown hatched in Fig. 1B (that is: to the side surface and coil-wound portion of the rotor iron core 2, and to the shaft 3 between the core 2 and bush 1 and over the portion of the bush 1 facing the rotor 2).

To set the adhesive, the coating film 4 must be cured and this is suitably achieved with high-frequency induction heating as will be explained below with reference to Fig. 1C.

To the rotor of Fig. 1B is first applied a commutator 5 which is pushed onto the rotor shaft 3 from the end opposite the bush 1 and fixed at a predetermined position thereon. A jig 7 is fitted to the shaft 3 at the bush end and a press-fitting rod 6 is fitted to the other end of the shaft, as shown in Fig. 1C. The press-fitting rod 6 is then pushed to the right in Fig. 1 C as shown by the arrow in- that Figure until the distance between a reference position at the extreme left of the commutator 5 and the right-hand end of the bush 1 reaches a predetermined separation Leo as shown in Fig. 1C.By this sequence of operations, the bush 1 is moved from its temporary mounting position to a predetermined position relative to the commutator and rotor in effect set at the distance L0 from the reference position at the extreme left-hand end of the commutator 5. The coating film 4 is allowed- to cure with the rotor in this condition.

The cured coated film 4 firmly bonds the bush 1 to the shaft 3 at a predetermined separation from the reference position at the extreme left of the commutator 5, and at the same time provides electrical insulation, as the material of the film 4 is an insulator.

The bush 1 preferably has a flanged shape as shown in the sectional view of Fig. 2A, produced by stamping an iron sheet with a thickness of the order of 1 mm. The bush hole is suitably formed in a generally triangular shape with each apex rounded, as shown in the view of Fig. 2B, rather than in a truly circular shape. Other non-circular (such as polygonal) shapes may be employed. Forming the bush hole with a shape such as that shown in Fig. 2B and without great dimensional accuracy nevertheless enables the bush 1 readily to be set a temporary position on the shaft 3 with a relatively weak press-fitting force. It will be appreciated that the bush is elastically supported on the shaft in this temporary position.

In place of a true flange such as that of Fig.

2A, the bush 1 may be formed in a generally eyelet shape as shown in Fig. 2C, but with a similar non-circular profile.

The relatively simple construction for a rotor in accordance with the present invention enables substantial advantages to be obtained.

The bush may be fitted in position initially with a relatively weak force. It is then coated with an adhesive to firmly fix the bush in position achieving high resistance to axial displacement thereof from the shaft and electrically insulating the bush as well. This simplifies tolerances in the manufacture of the bush which may be made of a ferrous metal, avoids damage to the shaft when the bush is press-fitted thereon, and enables the rotor coils to be wound right up to a position in contact with the bush, for the bush is insulated by the coating.

Claims (6)

1. A rotor for an electric motor, comprising: an iron core; a rotor shaft; a bush mounted on the rotor shaft and adapted to limit axial movement thereof; and an electrically insulating coating film applied to said bush, said shaft and said iron core.

2. A rotor according to Claim 1, wherein the bush is made of a metal.

3. A rotor according to Claim 1 or Claim 2, wherein the bush has a bush hole which is of a non-circular shape in axial cross-section.

4. A rotor according to any preceding Claim, wherein said bush is adapted to be fitted at a temporary mounting position on said shaft.

5. A rotor according to any of Claims 1 to 3, wherein resistance to positional displacement of said bush along said shaft is improved and electrical insulation is provided by curing said coating film.

6. A rotor for an electric motor, substantially as hereinbefore described with reference to and as shown in the accompanying drawings.