GB2247232A

GB2247232A - Sintered carbon brushes

Info

Publication number: GB2247232A
Application number: GB9116464A
Authority: GB
Inventors: Isao Shibuya
Original assignee: Mabuchi Motor Co Ltd
Current assignee: Mabuchi Motor Co Ltd
Priority date: 1990-07-31
Filing date: 1991-07-30
Publication date: 1992-02-26
Also published as: DE4125346A1; JPH0488853A; GB9116464D0

Abstract

A carbon brush for electric motors is formed from pressure-formed and sintered graphite particles coated with a first layer of copper and a second layer of silver to impart improved contact resistance and wear resistance properties to the

Description

1 CARBON BRUSHES FOR MINIATURE MOTORS This invention relates to carbon

brushes for electric motors, and particularly such brushes for miniature electric motors having a permanent-magnet field. This invention seeks to improve the electrical contact resistance and wear resistance properties of carbon brushes that tend to deteriorate withtime.

Some known carbon brushes for miniature motors are made by a process in which graphite material is reduced to graphite powder, solidified by the addition of a binder, and then ground and screened to a composite powder. The composite powder is then mixed with metallic powder for imparting electric conductivity to the graphite powder as necessary, pressure-formed into the shape of a brush, and sintered to complete the process.

Another known graphite brush contains no binder, but is manufactured by subjecting graphite powder particles to a copper plating process, and pressure forming and sintering the graphite powder coated with a copper-plated layer into a brush.

In either of the known brushes referred to above, the graphite powder particles usually contain ashes, such as Si02, A1203, Fe203, MnO, MgO, Ti02, etc., having particle sizes of 1 to 500 microns. Since a high ash content of the graphite powder could adversely affect the electric conductivity and commutation properties of carbon brushes made therefrom, expensive treatment is needed to purify the powder to a high purity level. The copper-plated graphite brush, on the other hand, has a number of advantages such as high uniformity and low resistivity, and the ability to provide better wear resistance. However, when the copper-plated graphite brushes are used in high-temperature environments and to transmit relatively large electrical currents, the copper-plated layer can be readily oxidised, causing problems such as increased contact resistance with the commutator and reduced wear resistance.

1 2 According to the present invention, a carbon brush for an electric motor comprises pressure-formed and sintered graphite powder, the particles of the powder bearing a first layer of copper and a second layer of silver. The preferred composition by weight of the powder is substantially as follows:

graphite 50% copper plating 40 to 44% silver plating 6 to 10%.

The invention also provides a method of manufacturing a carbon brush for an electric motor comprising the steps of:

a) depositing a copper-plated layer as a first layer on graphite powder particles; b) depositing a silver-plated layer as a second layer over the copper-plated layer on the particles; c) pressure-forming said graphite powder particles bearing the first and second layers; and d) sintering the pressure-formed product.

Carbon brushes according to the invention are typically shaped for a particular motor and may therefore have a curved face at one end thereof for engaging the commutator of a said motor, and a projection at the other end thereof for mounting the brush on a resilient supporting member therefor. These features may be incorporated in the pressure-forming step of manufacturing process.

As noted above, the brushes of the invention are particularly, but not exclusively intended for use in miniature motors. Typically, such a motor has a permanent magnet forming the stator field in which the motor rotor bearing a suitable commutator rotates. The invention does of course extend to such motors using brushes of the type defined above.

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

i 1 I 1 1 1 I i i i i i I 3 Figure IA shows two brushes according to the invention as they could be mounted in an electric motor; Figure 1B is a block diagram illustrating a process according to the invention; Figure 1C shows a cross-section through a graphite powder particle embodying the invention; Figures 2A and 2B are graphs showing the change of contact resistance with time; Figure 3 is a table showing different compositions of graphite powder particles; Figures 4A and 4B are tables showing contact resistance and wear resistance after 150 hours running; and Figure 5 shows a cross-section through a graphite powder particle of the prior art.

As shown in Figure 1, each of two carbon brushes 4 is supported by an electrically conductive brush resilient member 5 in such a manner that the carbon brush 4 makes sliding contact with commutator segments 2 as the commutator 1 and motor shaft 3 rotate. Each carbon brush 4 is formed into a T-shape, as shown in the perspective view of the brush at A-1, and sintered. The brush is supported by the stem of the T being inserted into an opening (not shown) in the brush resilient member 5. The opposite surface of the brush is formed with a slightly curved surface which slides on the commutator segments 2 as the shaft 3 rotates.

The method of manufacturing carbon brushes of this invention is set out in Figure 1B and illustrated in Figure 1C. The method shown consists of a process 20 for grinding and screening graphite powder, a process 21 for plating or depositing a copper-plated layer 12 on a graphite particle 11 ground in the grinding/screening process 20 with a known non-electrolytic plating method, a process 22 for forming or depositing a silver-plated layer 13 on the copper-plated layer 12, a process 23 for pressure-forming the graphite powder with the copper and C 4 silver layers into the desired shape, and a process 24 for sintering the pressure-formed product.

On a carbon-brush 4 of the invention, the oxidation of the copper-plated layer 12 can be substantially inhibited when used in a high-temperature environment and in high-current conditions. As a result, the electrical contact resistance properties and wear resistance of the brush with respect to the commutator can be improved. These effects are demonstrated by the graphs of Figures 2A and 2B which show secular changes (time) in the contact resistance (mn) exhibited by four types of carbon brushes having the compositions set out in Figure 3, with the commutator in a miniature motor. Test specimen No. 1 is a carbon brush having only a copper-plated layer 12 formed on the graphite powder particle 11, generally as shown in Figure 5. In the test specimen the weight ratio of the copper-plated layer 12 to the graphite powder particle 11 is 50:50. Test specimen No. 2 is a carbon brush having only a silver-plated layer 13 on the graphite powder particle 11, with the ratio of the silver-plated layer to the graphite powder particle 11 being 50:50. Test specimen No. 3 is a carbon brush of this invention, with the ratio of the graphite power particle 11, the copper-plated layer 12 and the silver- plated layer 14 being 50:40:10. Test specimen No. 4 is also a carbon brush of this invention, with the ratio of the graphite powder particle 11, the copper-plated layer 12 and the silver-plated layer 13 being 50:44:6. In the test, contact resistance was measured 60 times in an hour on all the specimens, and the average contact resistance values for every 8 hours are shown in Figures 2A and 2B.

In Figures 2A and 2B, 01+11 denotes the contact resistance values for Test specimen No. 1, 1117 11 those for Test specimen No. 2, 110 19 those for Test specimen No. 3, and 11 011 those for Test specimen No. 4. The measurements plotted in Figure 2A were obtained with a current value of 2 Amps (current density: 10 A/cM2). In the graph of Figure 2B the measurements plotted were c obtained with a current value of 10 Amps (current density: 50 A/CM2). The motor speed was 10,000 rpm. The rate of wear after 150 hours of motor operation was also measured, and the results are shown, together with the average contact resistance values after 150 hours of operation, in Figures 4A and 4B, which correspond with Figures 2A and 2B respectively.

As is evident from Figures 2 and 4, Test specimens Nos. 2, 3 and 4 are far superior in contact resistance properties and wear rate properties, compared with specimen No. 1. This is particularly true when the current value is large. This is attributable to the fact that the graphite powder particle 11 or the copper-plated layer 12 covering the graphite powder particle 11 is covered with the silver-plated layer that is hard to oxidise.

Specimen No. 2 shows the best performance in the tests, but is too expensive to be commercially practicable because of the high silver content. However, the test results with specimens Nos. 3 and 4 demonstrate that miniature motors using the carbon brushes of this invention have good contact resistance properties and wear rate properties for commercial applications, and are also commercially practicable because of the low silver content.

The carbon brushes of the invention can be made using normal graphite powder particle containing more than 1% by weight of ash. However, lower ash contents are preferred and in this respect reference is directed to our Specification No. 2, 233,001. A double metalplated layer consisting of a copper-plated layer and a silver-plated layer as described herein may also be formed on graphite powder particles purified to approximately 99% to 99.5%.

C 6

Claims

1. A carbon brush for an electric motor comprising pressure-formed and sintered graphite powder, the particles of the powder bearing a first layer of copper and a second layer of silver.

2. A carbon brush according to Claim-1 qherein the composition by weight of the powder is substantially as follows:

graphite 50% copper plating 40 to 44% silver plating 6 to 10%.

3. A carbon brush according to any preceding claim having a curved face at one end thereof for engaging the commutator of a said motor, and a projection at the other end thereof for mounting the brush on a resilient supporting member therefor.

4. A carbon brush for an electric motor according to Claim 1 and substantially as described herein with reference to the accompanying drawings.

5. An electric motor includes carbon brushes according to any preceding Claim.

6. A method of manufacturing a carbon brush for an electric motor comprising the steps of:

a) depositing a copper-plated layer as a first layer on graphite powder particles; b) depositing a silver-plated layer as a second layer over the copper- plated layer on the particles; c) pressure-forming said graphite powder particles bearing the first and second layers; and d) sintering the pressure-formed product.

7. A method according to Claim 6 wherein the pressure-formed product of step c) has the form of a carbon brush with a curved face at one end thereof for engaging the commutator of a said motor, and a projection at the other end thereof for mounting the brush on a resilient supporting member therefor.

c 1 1 i 1 1 i 1 0 c 7

8. A method according to Claim 6 or Claim 7 including the preliminary step of purifying the graphite powder particles to a purity in the range 99% to 99.5%.

9. A method of manufacturing a carbon brush for an electric motor according to Claim 6 and substantially as described herein with reference to the accompanying drawings.

Published 1992 at The Patent Office. Concept House. Cardiff Road. Newport. Givent NP9 I RH. Furiher copies may be obtained from Sa)es Branch. Unit 6. Nine Mile Point. C%k7nfelinfach. Cross KeYS. Neikl)ort. NP1 7HZ. Printed by Multiplex techniques ltd. St Mary, Cray. Kent