GB2273003A

GB2273003A - Miniature motor commutator having noise reduction construction

Info

Publication number: GB2273003A
Application number: GB9324457A
Authority: GB
Inventors: Takeshi Okumura
Original assignee: Mabuchi Motor Co Ltd
Current assignee: Mabuchi Motor Co Ltd
Priority date: 1992-11-30
Filing date: 1993-11-29
Publication date: 1994-06-01
Anticipated expiration: 2013-11-29
Also published as: CN1096141A; GB9324457D0; JPH06169555A; DE4339981A1; CN1034538C; GB2273003B

Abstract

The commutator (4) comprises a plurality of electrically conductive commutator pieces (42) formed into a segment shape and circumferentially spaced on the outer surface of a non-conductive core (41). In the prior art, burrs 44 are established on the "trailing edge" during machining of the commutator. To prevent the formation of burrs chamfers (47) are provided on at least one longitudinal outer edge of each commutator piece (42) facing a respective gap between adjacent pieces. <IMAGE>

Description

2273003 1 Miniature Motor This invention relates to miniature electric

motors of the type used for audio equipment and precision instruments. It is directed particularly at a reduction in the electrical and mechanical noise generated in the miniature motor.

In the manufacture of commutators for miniature electric motors, a plurality of part cylindrical conductive pieces are typically disposed, circumferentially spaced on a non-conductive core. The pieces are then machined, turned or otherwise worked, to obtain a true cylindrical surface for smooth contact with the motor brushes. This results in the formation of burrs at the longitudinal circumferential edges will damage the brushes if left. They must therefore be removed in a time consuming additional stage in the manufacturing process.

The present invention seeks to eliminate the additional stage referred to above in the manufacturing process for miniature motors. A miniature motor according to the invention has a tubular case with a closed and an open end; a permanent magnet fitted in the case; a case cap closing the open case end, and a rotor comprising an armature facing the permanent magnet and a commutator, the rotor being mounted in bearings disposed in the end cap and the closed case end, and the case cap including brushes making sliding contact with the commutator with input terminals electrically connected thereto. The commutator has a plurality of electrically conductive commutator pieces each formed into a segment shape and circumferentially spaced on the outer surface of a non- conductive core, each commutator piece having a chamfer on at least one longitudinal outer edge facing the gap between adjacent pieces. The chamfers may be formed on both longitudinal edges of each commutator piece, but one edge can be sufficient. The one edge will 2 be on the same circumferential side, normally the trailing side relative to the direction of commutator rotation when it is being machined, turned or otherwise worked to a smooth cylindrical surface.

The chamfers are formed on the longitudinal edges of 2 the commutator pieces prior to the commutator being machined, turned or otherwise worked to establish a smooth surface for engagement with the brushes. In this way, burrs are prevented from appearing during the mechanical machining or equivalent stage in the manufacturing process. The manufacturing costs can thus be reduced because a time consuming stage can be eliminated from the manufacturing process.

The invention will now be described by way of example and with reference to the accompanying schematic drawings wherein:

Figure 1 is a longitudinal part section view showing functional components of a typical known design of miniature motor; Figure 2 is an end view illustrating functional components of the commutator in the motor of Figure 1; Figures 3 and 4 are views similar to Figure 2, but showing burrs generated on the commutator, respectively by turning and milling; Figure 5 is an enlarged perspective view of a commutator in accordance with the present invention; Figure 6 is an enlarged perspective view illustrating a commutator piece from the commutator of Figure 5; Figures 7 and 8 are end view of commutators embodying the invention; and Figure 9 is a schematic diagram illustrating the shape of chips generated during turning a commutator embodying the invention.

Figure 1 illustrates a known example of a miniature motor of the type to which this invention relates. A 3 3 is motor case 1 made of a metallic material, such as mild steel, is formed into a bottomed hollow tubular shape, and has a permanent magnet 2 formed into an arc segment shape fitted to the inner circumferential surface thereof. The case 1 houses a rotor 5 comprising an armature 3 facing the permanent magnet 2 and a commutator 4. A case cap 6 made of an insulating material is fitted to the open end of the case, 1, and supports brushes 7 (only one is visible) whose free ends make sliding contact with the commutator 4. Input terminals 8 are electrically connected to the brushes 7. Bearings 9 and 10 disposed in the closed base of the case 1 and a central part of the case cap 6, rotatably support shafts 11 and 12 at the axial ends of the rotor 5.

The motor of Figure 1 operates in conventional manner as current is fed to the armature 3 via the input terminals 8, the brushes 7 and the commutator 4. The rotor is thus forced to rotate in the magnetic field formed by the permanent magnet 2, thereby driving external equipment (not shown) via the output shaft 11.

The outer circumferential surface of the commutator 4 in the miniature motor described above is formed to ensure smoothness and roundness and make smooth sliding electrical contact with the brushes 7. As can be seen from Figure 2, the commutator 4 has a plurality of commutator pieces 42 made of copper plate, for example, and formed into a segment shape, disposed at equal spaced intervals in the circumferential direction on the outer circumferential surface of a non-conductive cylindrical core 41.

To ensure roundness, the outer circumferential surface of the commutator pieces 42 may be turned by a cutting tool 13, as shown in Figure 3. As a consequence, burrs 44 tend to appear on the longitudinal outer edges facing gaps 43 between the commutator pieces 42 at least partly for the reason that the side surfaces of the commutator pieces 42 form virtual right angles with the 4 cutting direction of the cutting tool 13.

In a mould type commutator 4, burrs 45 also tend to form when the gaps 43 between the commutator pieces 42_ are created by means of a metal saw or a screw slotting cutter, after the outer circumferential surface of the commutator pieces 42 is turned, as shown in Figure 4.

If a commutator in which its pieces 42 are left with burrs 44 or 45 left unremoved is installed in a motor to make sliding contact with brushes 7, the contact between the brushes 7 and the commutator pieces 42 becomes unstable, increasing electrical and mechanical noise, and significantly deteriorating the performance of the miniature motor. Furthermore, the wear of the brushes 7 is accelerated, reducing the life of the brushes 7. If the burrs 44 and 45 peel or fall off the commutator pieces 42, while the motor is running, a short circuit could result.

The burrs 44 and 45 must therefore be removed. However, this involves much time and trouble leading to increased cost, because burrs have to be removed manually in a very limited space, usually using a fine file or a cutter. Moreover, since the removed burrs 44 and 45 mostly take the form of fine chips, they may adhere not only to the commutator 4 but also to other components of the motor, resulting in electrical troubles. As a result, cleaning is also required after the removal of burrs, and this further increases manufacturing cost.

As shown in Figures 5 and 6, a commutator piece 42 is formed of copper plate into a segment shape by press working. A plurality of such commutator pieces 42 are disposed circumferentially spaced on the outer surface of a core material 41 made-of a resin material, forming gaps 43 therebetween. A terminal piece 46, the free end of which is formed into an essentially U shape in longitudinal section, is integrally formed with the commutator piece 42 in such a manner as to protrude from the end of the commutator piece 42 to electrically W X -1 connect the end of a coil (not shown) of an armature (not shown). A chamfer 47 is provided on each longitudinal outer edge of the commutator piece 42. The plurality of commutator pieces 42 are fitted to the outer periphery of the core material 41 by a fixing member 48 engaging with the side of the terminal piece 46 of the commutator piece 42.

Figure 7 shows the chamfers 47 referred to above provided on both longitudinal outer edges of the commutator piece 42. Figure 8 shows a chamfer 47 provided on only one longitudinal outer edge of the commutator piece 42. It is on the same circumferential side of each commutator piece 42; the trailing edge in the intended direction of rotation of the commutator during machining.

In Figure 7, symbols Ild. and Ildll denote the outside diameters of the commutator pieces 42 before and after turning, respectively; 'It" a machining allowance on one side, and "cl' a chamfering size. That is, the chamfering size "cl' is made larger than the one-side machining allowance 'It" so that the chamfering size "cl' is left on the longitudinal outer edge of the commutator piece 42 even after the commutator piece 42 is turned from the outside diameter Ild,11 before turning to the outside diameter Ildll after turning. The chamfer 47 may be formed simultaneously with the press forming of the commutator piece 42 shown in Figure 6, or may be machined after the commutator 4 has been formed.

As shown in Figure 9, the cross section of a chip 14 generated by a cutting tool 13 is of an almost crescent shape. The thickness at the leading and trailing ends of the chip 14 becomes smaller than that of the central part corresponding to the amount of metal removal by the cutting tool 13 as a consequence of the chamfer 47 on the commutator piece 42. Thus, generation of the burr 44 shown in Figure 3 is inhibited.

As noted above, Figure 8 shows an example where the 6 chamfer 47 is provided on one longitudinal outer edge of each commutator piece 42. When turning the outer periphery of the commutator piece 42, the direction of rotation of the commutator piece 42 as a workpiece is almost invariably limited to one direction (in the direction shown by an arrow in the Figure, for example). Generation of burrs can therefore be prevented, as explained above, by providing the chamfer 47 only on the longitudinal outer edge on the downstream or trailing side in the machining direction.

It is not desirable to form the chamfer 47 in too large a size because vibration may be caused when the brushed cross the gaps between the commutator pieces 42 and 42. The size of the chamfer 47 should however, be made larger than at least the size left even after the outer circumferential surface of the commutator piece 42 or the gaps 43 between the commutator pieces 42 and 42 have been machined; that is, the one-side machining allowance.

As described above, the invention makes it possible to prevent the generation of unwanted burrs when the outer circumferential surface of the commutator piece is turned, and to inhibit the generation of electrical and mechanical noise, thus improving the performance of miniature motors. Furthermore, troublesome manual deburring operation, and associated cleaning operations can be eliminated, leading to reduced manufacturing cost.

IQ --i-

Claims

1. A miniature motor having a tubular case with a closed and an open end; a permanent magnet fitted in the case; a case cap closing the open case end; and a rotor comprising an armature facing the permanent magnet and a commutator, the rotor being mounted in bearings disposed in the end cap and the closed case end, and the case cap including brushes making sliding contact with the commutator with input terminals electrically connected thereto, wherein the commutator has a plurality of electrically conductive commutator pieces each formed into a segment shape and circumferentially spaced on the outer surface of a non-conductive core, each commutator piece having a chamfer on at least one longitudinal outer edge facing the gap between adjacent pieces.

2. A miniature motor according to Claim 1 wherein chamfers are provided on both longitudinal outer edges of each commutator piece.

3. A miniature motor according to Claim 1 wherein a chamfer is provided on the longitudinal outer edge on the same circumferential side of each commutator piece.

4. A miniature motor according to any preceding Claim wherein the commutator core is formed in a resin material.

5. A miniature invention substantially as described herein with reference to Figure 1 and Figures to 9 of the accompanying drawings.