Carbon brush and method and material for the production thereof

Abstract

Carbon brush comprising a base body produced from carbon material by pressing and optionally sintering and a layer of a metallic material applied to an outer surface of the base body by pressing and possibly sintering, the metal layer being connectable to a metallic carrier by welding or soldering.

Description

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of PCT Patent Application No. PCT/EP04/014235 filed on Dec. 14, 2004.

BACKGROUND OF THE INVENTION

Carbon brushes for electric motors or generators must be secured to a support which ensures the necessary application pressure on the commutator or the like and permits the follow-up movement corresponding to the wear of the carbon brush. A carrier for the carbon brush which is particularly suitable because of its simplicity, cheapness and low weight is a leaf spring which in addition to the function of supporting and guiding can also perform the function of current transmission. However, the problem arises of securing the carbon brush to the leaf spring in such a manner that on the one hand the necessary mechanical strength in long-term operation and on the other the transmission of large currents is ensured.

From DE 102 07 406 A1 a carbon brush is secured to a leaf spring serving as carrier by welding or soldering and for this purpose is tin-plated, nickel-plated or coated with metal in another manner at the surface to be joined to the carrier.

From DE 40 40 002 A1 a carbon brush is known for a fuel pump motor which is soldered to a leaf spring serving as carrier and for this purpose has a solderable layer which consists of a copper layer and a tin layer applied thereto. However, with such metallizations, as a rule metal layers applied by electroplating, it is difficult to obtain a solder or weld connection between the carbon brush and leaf spring which has a mechanical, thermal and electrical load capacity adequate for long-term operation. In particular, the stress compensation between the carbon brush and the leaf spring involves problems due to the difference in modulus of elasticity and thermal coefficient of expansion.

From DE 2444957 A1 it is known to produce a carbon brush provided with a solderable or weldable metallic layer by jointly compressing a layer of pulverulent carbon-containing material and a layer of metal powder and then conducting a heat treatment. The metal powder consists of copper.

From U.S. Pat. No. 3,601,645 it is known to produce an electrical contact brush having a base body consisting of metal and carbon material and a metal layer undetachably connected to said base body by pressing and sintering a pulverulent metal-carbon material for the base body and a metal powder, wherein the metallic layer can consist of copper, iron or an alloy.

From DE 32 17 217 C2 it is known to produce a contact brush by pressing a bundle of metal-coated carbon fibres, wherein the metal coating of the carbon fibres can consist inter alia of copper, aluminium, tin, iron or an alloy thereof.

SUMMARY OF THE INVENTION

The invention is based on the problem of providing a carbon brush and a method and a material for the production thereof which permits a simple bonding to a leaf spring which can bear high mechanical and electrical loads.

BRIEF DESCRIPTION OF THE DRAWING

The FIGURE is a schematic view of a carbon brush welded to a leaf spring.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The invention will be explained hereinafter in detail with reference to an example of embodiment.

It is commercially of interest to weld carbon brushes to leaf springs because this enables a bond to be obtained through which high currents of more than 15 Å can be transmitted. This would enable this cheaper design to be used also for motor types which because of the high demands regarding the current strength in the blocking test or also in continuous operation hitherto could not be produced using this concept. Motors of this type, in which the carbon brush with small dimensions must transmit high currents, are for example actuators in motor vehicles, for example drive mechanisms for window lifters, seat adjustments, sunshine roofs, mirror adjustments and the like.

According to the invention a carbon brush is provided which apart from the usual one-layer or multi-layer base body of pressed and sintered carbon material has at least one outer layer of metallic material which ensures the weldability or solderability of the carbon brush, this metallic layer likewise being made by pressing and sintering and connected to the base body.

For welding or soldering a carbon brush a suitable welding method (for example ultrasonic welding, laser welding or the like) or soldering method must be applied and furthermore to produce the carbon brushes having a weldable metallic layer a material must be available which on the one hand in the powder-metallurgical processing bonds well to the carbon material of the base body of the carbon brush and on the other hand can be readily welded or soldered with the method chosen. It has been found that pure copper and its alloys as well as iron or steel are not suitable because their coefficient of thermal expansion and modulus of elasticity are not compatible with those of the carbon material of the base body of the carbon brush in the sintering method. Materials which are similar to the carbon material of the base body of the carbon brush cannot be employed either because due to the graphite component contained therein they cannot be welded.

According to the invention it has been found that a metallic material in the form of a powder mixture of a base metal component A, a ductile and low-melting metal component B and possibly further additives C meets both requirements at the same time, that is the good bonding by pressing and sintering with the functional carbon material of the base body of the carbon brush on the one hand and the good weldability or solderability on the other hand.

According to the invention component A constitutes the major part, preferably more than 80% of the material and preferably comprises copper or a copper alloy with for example Sn, Zn, Ni or Ag. It is preferably in powder form having a mean diameter D₅₀ of 15 to 25 μm.

Component B is an additional metal which has a higher ductility and lower melting point than the component A. Preferably, the melting point is 400° C. or less and the breaking elongation 5% or more. The component B consists preferably of Sn or an alloy of Sn with for example Cu, Ag, Sb. Zn, Ga, In, Bi or Pb. The metal component B may also consist of In and/or Bi or an alloy with In or Bi. Its proportion in the total powder mixture of the material is preferably 1.5 to 15% by weight. The additional metal B is present preferably in a mean grain size of D₅₀ of 15 to 35 μm.

In addition, the material may preferably contain one or more non-metallic organic and/or inorganic additives C in powder form. Possible additives of this type are for example stearates, graphite, talcum, SiC or A1₂O₃. Their mean grain size D₅₀ is preferably not more than 20 μm.

A preferred example of the composition of the material is (figures in percent by weight):

• • A: 92% Cu (D₅₀ 15-25 μm)
  • B: 7.5% Sn (D₅₀ 15-55 μm)
  • C: 0.5% graphite (D₅₀ ≦20 μm)

The material indicated can be excellently processed powder-metallurgically by pressing and sintering and bonds excellently to the functional layer (base body) of the carbon brush to form a metal layer having excellent welding and soldering properties. It has been found that in the pressing and sintering the low-melting and ductile metal B wets the base metal A but does not diffuse into the latter, or only to a slight extent.

It is thereby ensured that the predominant or at least an adequate part of the metal B is not alloyed to the metal A after the sintering but is present in pure form and thus the desired ductility of the metal layer is achieved. Due to the ductility of the material a stress compensation is achieved between the base body 1 of the carbon brush and the weldable metal layer 2 on the one hand and the leaf spring welded to the metal layer 2 on the other hand.

In other words, in the sintered metal layer 2 made by pressing and sintering the base metal A and at least the predominant part of the other metal B form two separate metal phases which although being intimately connected to each other by the pressing and sintering are not, however, substantially alloyed with each other.

The heat treatment, that is the sintering of the metal layer, is carried out at a temperature which is lower than the melting point of the base metal A but higher than the melting point of the further metal B. This is thus preferably a liquid sintering in which the further metal B is present in the liquid phase but the base metal A in the solid pulverulent phase. If the base metal A is copper and the further metal B tin, the sintering temperature is advantageously in the range from 300 to 700° C., preferably in the range from 400 to 500° C.

The non-metallic organic or inorganic additive C is not wetted by the metal B in the sintering process and thereby prevents any undesirably great diffusion of the metal B into the base metal A.

The attached drawing illustrates schematically a carbon brush according to the invention welded to a leaf spring.

The carbon brush comprises a base body 1 made in the usual manner in one or more layers from a conventional carbon material for carbon brushes, in particular a graphite mixture containing additives, by pressing and heat treatment (sintering). The excellently weldable or solderable metal layer 2, which is made from the aforementioned pulverulent material according to the invention likewise by pressing and possibly sintering, is undetachably connected or bonded to the base body 1. The base body 1 and weldable metal layer 2 may be simultaneously made and bonded together by pressing and sintering or the base body 1 may be prefabricated and the weldable metal layer 2 subsequently formed thereon by pressing and sintering.

The easily weldable metal layer 2 enables the entire carbon brush to be secured to a leaf spring 3 by welding, for example by weld spots 4 or for instance by a continuous weld seam.

The thickness of the sintered metal layer 2 depends on the requirements of the welding or soldering technique selected for connecting to the leaf spring 3. The thickness of the metal layer 2 preferably lies in the range from 0.2 to 5 mm. For a soldering connection to the leaf spring 2 as a rule a thickness of the metal layer 2 of less than 1 mm, in particular in the range from 0.2 to 0.5 mm, should suffice. For a weld joint as a rule a thickness of at least 1 mm, for example in the range from 2 to 4 mm, would be advantageous.

Claims

1. Carbon brush comprising a base body produced from carbon material and a metal layer which is applied to an outer surface of the base body and can be connected by welding or soldering to a metal carrier, the metal layer being a layer made from metal powder by pressing and optionally sintering and undetachably bonded to the base body by pressing and optionally sintering,

characterized in that the metal layer contains a base metal (A) and at least one further metal (B) of higher ductility and lower melting point than the base metal (A), the further metal (B) being present at least predominantly as a metal phase which is separate from the base metal (A) and is intimately bonded to the base metal (A) by pressing and optionally sintering but predominantly does not form an alloy therewith.

2. Carbon brush according to claim 1, characterized in that the further metal (B) is so chosen that at sintering temperature in the liquid phase it wets the base metal (A) but does not diffuse into said base metal, or only slightly.

3. Carbon brush according to claim 1, characterized in that the further metal (B) is composed of tin, indium or bismuth or an alloy thereof.

4. Carbon brush according to claim 1, characterized in that the further metal (B) has a melting point of not more than 400° C. and a breakage elongation of not less than 5%.

5. Carbon brush according to claim 1, characterized in that the further metal (B) is present in a proportion between 1.5 and 15% by weight of the metal layer.

6. Carbon brush according to claim 1, characterized in that the metal layer additionally contains an additive (C) of non-metallic, organic and/or inorganic material.

7. Carbon brush according to claim 6, characterized in that the non-metallic additive (C) contains stearates, graphite, talcum, SiC or Al2O3.

8. Carbon brush according to claim 1, characterized in that the metal layer thereof is connected to a carrier of metal by welding or soldering.

9. Carbon brush according to claim 8, characterized in that the carrier is a leaf spring.

10. Material for producing a weldable and/or solderable layer of a carbon brush comprising a powder mixture which contains a pulverulent main metal (A), in particular copper or copper alloy, and a pulverulent additive metal (B) of greater ductility and lower melting point than the main metal, the main metal (A) being copper or a copper alloy and the additive metal (B) being tin, indium or bismuth or an alloy thereof.

11. Material according to claim 10, characterized in that it contains a pulverulent additive (C) of non-metallic, organic and/or inorganic material.

12. Material according to claim 10, characterized in that the additive metal (B) is present in a proportion of 1.5 to 15% by volume of the powder mixture.

13. Method for producing a carbon brush comprising a base body of carbon material and a metal layer undetachably bonded thereto, comprising the steps:

a) producing the base body by pressing and sintering of a pulverulent carbon-containing material;

b) simultaneously with or following step a): producing the metal layer by pressing and sintering a pulverulent metallic material, the metal layer being bonded undetachably to the base body by pressing and sintering;

c) the pulverulent metallic material contains a mixture of a pulverulent base metal A and at least one pulverulent further metal B of greater ductility and lower melting point than the base metal A; and

d) the further metal B and the sintering temperature are so selected that during the sintering the further metal in the liquid phase wets the base metal A but does not, or only slightly, diffuse into the latter, whereby the further metal B is present at least predominantly as a metal phase separate from the base metal A and predominantly not alloyed therewith.

14. Method according to claim 13, characterized in that the main metal A comprises copper or copper alloy and the further metal B comprises tin, indium or bismuth or an alloy thereof.

15. Carbon brush according to claim 2, characterized in that the further metal (B) is composed of tin, indium or bismuth or an alloy thereof.

16. Carbon brush according to claim 2, characterized in that the further metal (B) is present in a proportion between 1.5 and 15% by weight of the metal layer.

17. Carbon brush according to claim 3, characterized in that the further metal (B) is present in a proportion between 1.5 and 15% by weight of the metal layer.

18. Carbon brush according to claim 4, characterized in that the further metal (B) is present in a proportion between 1.5 and 15% by weight of the metal layer.

19. Carbon brush according to claim 2, characterized in that the metal layer additionally contains an additive (C) of non-metallic, organic and/or inorganic material.

20. Material according to claim 11, characterized in that the additive metal (B) is present in a proportion of 1.5 to 15% by volume of the powder mixture.