GB2185154A

GB2185154A - Mounting miniature electric motors

Info

Publication number: GB2185154A
Application number: GB08630226A
Authority: GB
Inventors: Mitsuru Kitahara
Original assignee: Mabuchi Motor Co Ltd
Current assignee: Mabuchi Motor Co Ltd
Priority date: 1985-12-18
Filing date: 1986-12-18
Publication date: 1987-07-08
Also published as: CN86108616A; GB2185154B; GB8630226D0; KR900003397Y1; JPS62101355U; KR870011398U; HK34891A

Abstract

An electric motor has a generally cup-shaped motor case 1 comprising a generally cylindrical side wall and a base. The base is provided with threaded through holes 11 enabling the motor to be mounted to an external apparatus by its base. A thin film seal 13 formed of adhesive resin is provided on the inside surface of the motor case so as at least to cover the openings of the through holes on the inside of the motor case base. <IMAGE>

Description

SPECIFICATION Electric motor This invention relates to electric motors.

As will become apparent from the description below, the present invention neatly overcomes a problem which results from the conventional manner in which so-called "miniature" electric motors are commonly mounted to an external structure.

In accordance with the present invention, we provide an electric motor having a generally cup-shaped motor case comprising a generally cylindrical sidewall and a bottom, the bottom being provided with threaded through holes enabling the motorto be mounted to an external apparatus by its bottom, a thin film seal formed of adhesive resin being provided on the inside surface of the motor case so as at least to coverthe openings of said through holes on the inside ofthe motor case bottom.

The invention is hereinafter more particularly described by way of example only with reference to the accompanying drawings, in which Figure 1A is a cross-sectional view of part ofthe motor case of an embodiment of electric motor constructed in accordance with the present invention; Figure lBis an overall perspective view of the motor case of Figure 1 A; Figure2 is a side elevational view, partly in section, illustrating a typical electric motor of conventional design to which the present invention may be applied; and Figure3 is a sectional viewsimilarto Figure 1A illustrating a previous proposal for overcoming the problem described in more detail below and which the present invention solves.

Referring first to Figure 2 ofthe accompanying drawings, in which is somewhat schematically illustrated a typical construction for a "miniature" electric motor of the kind in which the present invention can be applied with advantage, it will be seen that the motor has a generally cup-shaped motor case 1 having a generally cylindrical sidewall and a bottom. An end closure 2, here formed of synthetic resin material closes the open end ofthe casei.Theshaft3mounting rotor4 is supported in bearings 5 respectively mpunted in the motor case bottom and in the end closure 2.The stator is provided by a permanent magnet 6 fixedly mounted in position on the inner side ofthe cylindrical side wall ofthecase 1.The rotorshaft3also mountsa commutator 7 which is in operative sliding contact with brushes 8 electrically coupled to terminals 9 which extend externallyofthe motor case end closure for coupling in an external electric circuit.

The motor is fitted to a mounting plate 10, shown in dashed lines, on external apparatus with which the motor is to be employed. The motor is held in position by threaded fasteners received in threaded holes 11 in the motor case bottom.

We have found that in mass production of miniature electric motors, the formation ofthreaded through holes such as those shown at 11 with atap or the like leaves so-called "threading burrs" which may be leftin the threaded holes 11. Thereafter, when screws or other threaded fasteners are inserted into the threaded through holes 1 1,the threading burrs may separate from the holes and enterthe interiorofthe motorcase. This leadsto various problems. For example, if a burr comes between the shaft 3 and bearing 5, the bearing may seize. If a burr infiltrates between the magnet 6 and rotor 4, this may cause a lock-up.If a burr enters between commutator7 and brushes 8, commutation troublewill result, and the surface ofthe commutator 7 may be damaged, as may be the brush 8, these usually being formed of relatively thin flexible metal strips. Burrs may also damage the rotor coils. If this happens, there may be a partial short circuit in the rotor coils with magnetic loss and hence loss of motor efficiency.

These problems are not new and proposals have been made previously to avoid them.

In particular, the motor case may be subjected to a deburring process operating either mechanically or chemically. However, this introduces at least two additional steps, namely the debu rring process itself and in addition a necessityforatotal inspection of the entire run of motor cases. As a result increased man hours are spent in production. Even so, it has proved difficult to ensure that burrs are completely removed from all the threaded holes.

The alternative is to adoptthe construction shown in Figure 3 where the threaded hole is formed blind in effect by forming a protrusion 12 on the inside surface of the motorcase bottom! However, not only does this result in increased axial dimensionsforthe motor, which is undesirable for "miniature" electric motors where space is at a premium, but also the drawing operation for forming the protrusion 12 also presents an unnecessary additional step and/or complicates manufacture of the case.

Despite the existence of the problem, and the drawbacks in the accepted solutions, until the present invention, we believe that no adequate solution to this problem has existed.

Referring nowto Figures 1Aand 1B,itwill beseen thatthe motor case construction of Figure 1A is essentially the same as that shown in Figure 2 (and indeed the entire motor can be the same as in Figure 2) with the exception that a thin-film seal 13 is provided on the inside surface of the bottom ofthe motor case 1 so as at least to coverthe opening on the inside of the motor case bottom ofthethrough hole 1 lIThe seal 13 is formed from an adhesive resin material, which may be provided in the form of a tape or of a thin-film ring applied to the motor case.

Alternatively, an adhesive resin thin-film layer may coverthe entire inside surface of the motor case including the regions of the oper7ings forthe through holes 11.

By providing a thin-film seal 13 of adhesive resin material, any threading burrs which separate from the threaded hole 11 as a motor-mounting screw 14 is screwed into the hole 11 are held by the seal 13 and prevented from entering the motor case interior whether or notthe leading end ofthescrew 14 reachesthethin-film seal 13. Indeed,sincethe material ofthe seal 13 has a degree of flexibility and resilience, the screw 14 may have a length such asto protrude into the interior of the motor case if the seal 13 were absent.

As can be seen in Figure 1 B, the thin-film seal 13 in this case is formed as a preformed ring of adhesive resin material provided so asto cover at leastthe partofthe motor case where the through holes 11 open through the bottom.

Application ofthethin-film seal can readily be automated so that, notonlyarethethreading burr problems encountered heretofore overcome, but they are overcome in a neat simple and efficient manner from a production viewpoint.

Claims

1. An electric motor having a generally cup-shaped motor case comprising a generally cylindrical sidewall and a bottom, the bottom being provided with threaded through holes enabling the motor to be mounted to an external apparatus by its bottom, a thin film seal formed of adhesive resin being provided on the inside surface of the motor case so as at least to cover the openings of said through holes on the inside ofthe motor case bottom.

2. A motor according to Claim 1, wherein said seal is provided by applying the adhesive resin to the entire inside surface of the motor case.

3. A motor according to Claim 1, wherein the said seal is provided by applying adhesive resin to a selected portion only of the inside surface ofthe motor case including the regions ofthe said openings of the through holes.

4. A motor according to Claim 1, wherein said seal is provided by applying the said adhesive resin in the form of a tape to substantially the entire inside surfaceofthe motor case.

5. A motor according tq Claim 1, wherein the said seal is provided by applying the said adhesive resin in the form of a tape to a selected portion ofthe inside surface ofthe motor case, including the region ofthe said openings ofthe through holes.

6. Amotoraccording to Claim 1,wherein the said seal is provided by applying the adhesive resin formed into a thin film ring to the inside surface of the motor case wherebyto cover at least an annular portion of the inside surface ofthe bottom ofthe motor case including the region of the said openings ofthethrough holes.

7. An electric motor substantially as hereinbefore described with reference to and as shown in Figures 1A and 1 B of the accompanying drawings.