GB2173648A

GB2173648A - Electric motors and their assembly

Info

Publication number: GB2173648A
Application number: GB08602389A
Authority: GB
Inventors: Michio Hoshino
Original assignee: Mabuchi Motor Co Ltd
Current assignee: Mabuchi Motor Co Ltd
Priority date: 1985-02-22
Filing date: 1986-01-31
Publication date: 1986-10-15
Also published as: KR900003401Y1; CN1005749B; CN86100417A; DE3604584A1; JPH079580Y2; HK36091A; JPS61141971U; KR860006861A; GB8602389D0; GB2173648B; DE3604584C2

Abstract

Electrical connection is provided in an electric motor between the brush terminal 5 and the motor case 1 by means of a U-shaped electrically conductive bridge piece 13 separate from the terminal. One leg of the bridge piece is inserted into the terminal aperture from the inside of an end cap or motor case cover and the other leg of the U-shape is trapped between the peripheral surface of the end cap or motor case cover and the internal surface of the casing. <IMAGE>

Description

SPECIFICATION Electric motors and their assembly This invention relates to electric motors and to their assembly.

In so-called "miniature electric motors" there is occasionally need to earth or ground one or more of the brushes. Heretofore this has generally required that the brush terminal to be grounded be connected to part of an electrically conductive motor case constituting the motor stator using lead wires, for example. This involves cumbersome operations such as cutting the lead wire to an appropriate length, providing connecting lugs at both ends of the length of wire and connecting the wire to the motor case and to the terminal by soldering or other means. Such time consuming steps provide poor working efficiency. Additionally, the lead wires may be obtrusive depending on the location in which the motor is employed in other apparatus.

Our British Patent 2013042 proposes a solution to this problem and has achieved a fair measure of success. The said British Patent proposes the use of a generally U-shaped electrically conductive bridge piece which has one leg inserted into a terminal aperture by which a terminal extends through the end cap of the casing and its other leg inserted between engaging portions of the end cap and the casing.

In practice, however, we have found that when the motor is provided with locating means in the form of inter-engaging projections and notches in the end cap and the motor casing to allow the angular position of the brush gear relative to the permanent magnets to be selected (so as to enable advancement or retraction of commutation), as a practical matter, the location of the projections/ notches mean that we can often only ground one of the terminals. Moreover, insertion of the Ushaped bridge piece can give rise to unwanted contact between the bridge piece and an internal component of the motor leading to an unwanted short circuit. Finally, in this prior construction, the bridge piece is visible from the exterior of the motor which is not desirable.

The present invention seeks to provide an alternative and improved construction which achieves a similar object.

In accordance with a first aspect of the present invention, we provide an electric motor comprising: a rotor carrying motor windings; a generally cylindrical casing formed of an electrically conductive material; at least one permanent magnet mounted internally of the casing to act as the stator; an end cap or motor case cover formed of an electrically non-conductive material and carrying brush gear for the motor, which brush gear includes at least one terminal portion extending through a terminal aperture of said end cap or motor case cover for connection to an external electric circuit, the end cap or motor case cover having a surface at its periphery engaged with the interior surface of said casing at one axial end thereof; and an electrically conductive bridge piece separate from the terminal portion, the bridge piece being generally U-shaped in configuration, one leg of said U-shape being inserted into said terminal aperture from the inside of the end cap or motor case cover and being secured in position in said terminal aperture, and the other leg of said U-shape being trapped between the engaging peripheral surface of the end cap or motor case cover and the internal surface of the casing.

According to a second and alternative aspect of the present invention, there is provided a method of assembling an electric motor of the kind comprising a rotor carrying motor windings, a generally cylindrical casing formed of an electrically conductive material, at least one permanent magnet mounted internally of the casing to act as the stator, and an end cap or motor case cover formed of an electrically non-conductive material and carrying brush gear for the motor, which brush gear includes at least one terminal portion extending through a terminal aperture of said end cap or motor case cover for connection to an external electric circuit, the method of assembly being such that the said terminal is earthed to the generally cylindrical casing, the methd being characterised in that the end cap or motor case cover is provided with a surface at its periphery adapted to engage with the interior surface of the casing at one axial end thereof, and in that an electrically conductive bridge piece separate from the terminal portion, which bridge piece is generally U-shaped in configuration has one leg of said U-shape inserted into said terminal aperture from the inside of the end cap or motor case cover and secured in position therein and has its other leg trapped between the said peripheral surface of the end cap or motor case cover and the internal surface of the casing as the end cap or motor case cover is engaged with the internal surface of the casing as the motor is assembled.

The invention is hereinafter more particularly described by way of example only with reference to the accompanying drawings, in which: Figure 1 is an exploded perspective view illustrating in somewhat schematic form the motor case, end cap or motor case cover, brush gear and associated parts of an embodiment of electric motor constructed in accordance with the present invention; Figure 2 is an enlarged sectional view of the peripheral portion of the motor case and the end cap or motor case cover before these parts are fitted together; and Figure 3 is a similar sectional view illustrating the motor case and end cap or motor case cover in fully assembled condition of the motor.

In the drawings, the rotor assembly including the shaft, windings and commutator are all omitted in order better to illustrate other parts of the motor.

The rotor assembly may take an entirely conventional form.

As will be seen from the exploded view of Figure 1, the motor includes a generally cylindrical motor case 1 made of an electrically conductive material such as iron. Brush gear for the motor includes brushes 2, 2' in which brushes proper 3, 3' are supported from brush bases 4, 4' provided with integral terminal holes in an end cap or motor case cover for connection in an external electric circuit, as will be explained below. The brushes further include fixing portions 6, 6' bent out of the plane of the brush bases 4, 4' respectively on the side away from the terminal portions 5, 5'. The end cap or motor case cover 7 is provided with terminal apertures 8, 8' through which the brush terminal portions 5, 5' extend when the brush fixing portions 6, 6' are fitted to respective brush mounting bosses 9, 9' of the end cap or motor case cover 7.The end cap or motor case cover 7 has a circumferentially extending peripheral surface 10 which is adapted to engage with the interior surface of the motor case 1, as will be explained.

The drawings also show an electrically conductive bridge piece 13 of generally U-shaped configuration and with one leg longer than the other. The bridge piece 13 is formed of a resilient metallic strip and the two legs diverge somewhat from each other as shown in Figure 2 in the unbiased condition of the bridge piece 13. The outer sides of the two legs are provided with projections P to ensure firm electrical contact respectively with the inside surface of the motor case and with the terminal portion 5 in use of the bridge piece, as will be explained.

In assembling the motor, the bridge piece 13 is first mounted on the end cap or motor case cover by inserting the longer leg of the bridge piece 13 into the terminal aperture 8 in which the terminal portion 5 is already located. At its end the longer leg of the bridge piece 13 has a projection U on the inner side of the leg which is located in a corresponding notch 16 in the terminal aperture 8.

The projection P on the outer side of this longer leg tends to bite into the terminal portion 5 at the same time ensuring good electrical contact.

The outer peripheral surface 10 of the end cap or motor case cover 7 is provided with a shallow groove 15 for accommodating the shorter leg of the bridge piece, though under this condition (that is before inter-engagement of the end cap or motor case cover with the motor case 1), the shorter leg, because of its divergency from the larger leg, does not actually lie in the groove 15. Formation of the groove 15 leaves a shouldered portion 14 on the surface 10.

With the U-shaped member 13 mounted on the end cap or motor case cover 7, and having first assembled the other components of the motor, namely at least one permanent magnet mounted internally of the casing 1 and the rotor assembly comprising the rotor shaft, windings and commutator, the end cap or motor case cover 7 is fitted to the motor case. As shown in Figure 1, the end cap or motor case cover 7 has a projection X enabling selective positioning of the brushes relative to the commutator by locating the projection X with a selected one of several notches Y formed on the motor case 1. The peripheral surface 10 of the end cap or motor case cover 7 is thus brought into engagement with the inside surface of the motor case 1.

By this movement the short leg of the U-shaped bridge piece 13 is pushed into the shallow groove 15 and the projection P on the outer side of this short leg is forced into the material of the inside surface of the motor case 1, so ensuring good electrical contact between the motor case 1 and the bridge piece 13. The depth of groove 15 is chosen such that no gap remains in the fuily assembled condition shown in Figure 3 when the motor case 1 overlies both the short leg of the bridge piece 13 and the shouldered portion 14.

Thus it will be seen that the U-shaped bridge piece 13 provides electrical connection between the motor case 1 and the brush terminal portion 5.

It should be noted particularly that not only is the bridge piece 13 in the described embodiment anchored in the terminal hole by means of its projection U, but the direction in which the parts are assembled serves, if anything, to more firmly fix the bridge piece 13 in position and to enhance the electrical connection between the bridge piece and the terminal portion and between the bridge piece and the motor case respectively. Moreover, the existence of projections and notches X and Y will not interfere with mounting of the bridge piece. There is no possibility of the bridge piece interfering with other parts of the motor and making inadvertant electrical contacts. In external appearance, a motor with the bridge piece has exactly the same appearance as a motor without the bridge piece.

Claims

1. An electric motor comprising: a rotor carrying motor windings; a generally cylindrical casing formed of an electrically conductive material; at least one permanent magnet mounted internally of the casing to act as the stator; an end cap or motor case cover formed of an electrically non-conductive material and carrying brush gear for the motor, which brush gear includes at least one terminal portion extending through a terminal aperture of said end cap or motor case cover for connection to an external electrical circuit, the end cap or motor case cover having a surface at its periphery engaged with the interior surface of said casing at one axial end thereof; and an electrically conductive bridge piece separate from the terminal portion, the bridge piece being generally U-shaped in configuration, one leg of said U-shape being inserted into said terminal aperture from the inside of the end cap or motor case cover and being secured in position in said terminal aperture, and the other leg of said U-shape being trapped between the engaging peripheral surface of the end cap or motor case cover and the internal surface of the casing.

2. A motor according to Claim 1, wherein the electrically conductive bridge piece is formed from a U-shaped resilient metallic strip, its said one leg being longer than its said other leg and the two said legs diverging slightly from each other in the unbiased condition of the said bridge piece.

3. A motor according to Claim 2, wherein a projection is provided on each said leg on the side away from the other said leg, the said projections being adapted to hold the bridge piece in position.

4. A motor according to Claim 2 or Claim 3, wherein the end of said one leg on the side thereof towards the other said leg is provided with a projection for locating the said one leg in said terminal aperture.

5. A motor according to Claim 4, wherein the inside surface of said terminal aperture is provided with a corresponding notch for accommodating the said projection on the end of said one leg.

6. A motor according to any of Claims 2 to 5, wherein a groove is provided on said engaging peripheral surface for accommodating the said other leg.

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

8. A method of assembling an electric motor of the kind comprising a rotor carrying motor windings, a generally cylindrical casing formed of an electrically conductive material, at least one permanent magnet mounted internally of the casing to act as the stator, and an end cap or motor case cover formed of an electrically non-conductive material and carrying brush gear for the motor, which brush gear includes at least one terminal portion extending through a terminal aperture of said end cap or motor case cover for connection to an external electric circuit, the method of assembly being such that the said terminal is earthed to the generally cylindrical casing, the method being characterised in that the end cap or motor case cover is provided with a surface at its periphery adapted to engage with the interior surface of the casing at one axial end thereof, and in that an electrically conductive bridge piece separate from the terminal portion, which bridge piece is generally Ushaped in configuration has one leg of said Ushape inserted into said terminal aperture from the inside of the end cap or motor case cover and secured in position therein and has its other leg trapped between the said peripheral surface of the end cap or motor case cover and the internal surface of the casing as the end cap or motor case cover is engaged with the internal surface of the casing as the motor is assembled.

9. A method substantially as hereinbefore described with reference to the accompanying drawings for assembling an electric motor.