US2448685A

US2448685A - Electrical contact brush

Info

Publication number: US2448685A
Application number: US533401A
Authority: US
Inventors: Ramadanoff Dimiter
Original assignee: Nat Carbon Co Inc
Current assignee: National Carbon Co Inc
Priority date: 1944-04-29
Filing date: 1944-04-29
Publication date: 1948-09-07
Anticipated expiration: 1965-09-07

Description

Patented Sept. 7, 194a ELECTRICAL CONTACT BRUSH Dimiter Ramadanofl, Cleveland, Ohio, assignor to National Carbon Company, Inc., a corporation of New York No Drawing. Application April 29, 1944,

' Serial No. 533,401

(Cl. 17l325) 2 Claims.

The invention relates to electrical contact brushes, and more specifically to carbon brushes for use in electrical machinery required to operate under conditions such as exist in the atmosphere at high altitudes.

Well-made brushes composed of natural or artiiicial graphite, or other commercial forms of carbon, or of mixtures of carbon and one or more 1 wheel, and instances are known where brushes" have worn out completely in less than an hour at these altitudes. The most suitable of the known brush compositions may in some instances last several hours; but at best the brushes can not be relied upon for long.

Modern aircraft use electrical motors, generators, and other electrical devices requiring contact brushes. Both military and civil aircraft are being designed to operate at great altitudes, and there is an urgent demand for electrical contact brushes that will operate dependably over a useful long life at such altitudes. The principal ob- J'ect of this invention is to meet this demand.

The reasons for the poor behavior of brushes at high altitudes are not altogether clear; but it seems certain that two important factors are the dryness and the low oxygen pressure of the air. A similar behavior has sometimes been observed, during periods of unusually dry winter weather, even at low altitudes in the cases of some brush compositions; but the problem has not been a serious one. By operating brushes in a test chamber in which the atmosphere can be controlled, it has been found that either a low moisture content or a low partial pressure of oxygen may 7 z Further support for this hypothesis, and a sug gestion that other film-forming materials might be substituted for moisture and oxygen, are found in the observation that new electrical apparatus, still containing traces of slightly volatile compounds in its insulation and paint, may not give trouble at high altitudes until after several weeks or months of use. This latter circumstance increases thehazard, because rapid brush wear may start without warning after a considerable period of satisfactory operation. In an airplane several hours from its base the brushes may suddenly start to wear at a rate which will destroy them in an hour.

One approach to the problem is to incorporate in the brush, or impregnate the brushes with, one or more substances which will establish and maintain a suitable film between the commutator or slip-ring andthe carbon brush. The choice of a suitable material is a difficult one, for much more is involved than the mere provision of a film-forming material. The material must be effective in'small amounts, because the interstitial storage space in a brush body is not very great. If it is to bean impregnant, it must be a liquid or liqueflable by melting or by solution to make impregnation possible, and it must have ability to penetrate thoroughly both inner and outer pores of the carbon. In any case, it must not evaporate rapidly, and it must not exude, for exudation may result not only in undue depletion of the impregnant but also in the formation of a surface layer which tends to pick up dirt and to interfere with the smooth operation of the brush in its holder. Finally, the film which the material supplies must have a low electrical resistance, must not pick up dirt nor decompose to deleterious products, and must be stable under a wide variety of conditions of atmosphere, temperature, and of mechanical and electrical stresses. Dozens of brush impregnants, including I such materials as paraffin wax, beeswax, carnaucause rapid brush wear, and it has also been the conditions existing at altitudes above 25,000

feet, and at the same time satisfied the-other requirements indicated.

'The principal object achieved by this invention is a carbon brush having improved life and good operating characteristics at altitudes well above 25,000 feet, for instance at 40,000 feet.

The invention is an electrical contact brush comprising carbon or graphite, which may inments thorium, cerium, and yttrium. Sometimes the commercial mixtures are byproducts of the manufacture of thorium or cerium and therefore contain but little of these elements. of any'of these mixtures, as well as of the commercially pure thorium, cerium, or yttrium, may be used in this invention. The particular compound used is not of controlling importance, but the ready availability of the oxides and salts particularly recommends their use. Fluorides are quite satisfactory.

The weight percentage of rare earth element compounds is not critical, but in general it is prei'erred that as much be added as possible without materially decreasing the strength and electrical conductivity of the brush. Rare earths or rare earth metal fluorides amounting to about 15% by weight of a graphite-copper brush is suggested as a suitable, but not limiting, proportion.

Insoluble compounds, such as the oxides, or

compounds of low solubility in common solvents,

may be added as dry powder to a powdered graphite, carbon, or graphite-metal molding mixture which is then formed, compacted, and heat treated in conventional manner. Soluble compounds,

Compounds such as the chlorides or acetates, are preferably incorporated by impregnating a completed brush with a solution of the compound and removing the solvent by evaporation. The impregnation can most conveniently be effected under vacuum. Such soluble compounds may then be converted to insoluble compounds, for example by heating or by treating with a suitable reagent such as an acid.

Brushes embodying this invention wear at only a very slow rate (in a particular typical instance 0.01 inch per hour on a brush containing about 15% rare earth oxides or fluorides) at the low pressures encountered at very high altitudes above 25,000 feet. Moreover, they are characterized by low friction, and low total contact voltage drop, ln sliding contact with copper.

I claim:

Electrical contact brush comprising a major proportion of a mixture of graphite and copper, and a minor proportion of at least one of the rare earths of the thorium, cerium, and yttrium groups, such rare earth or earths amounting'to about 15% by weight of the brush.

2. Electrical contact brush comprising a major proportion of a mixture of graphite and copper, and a minor proportion of at least one fluoride selected from the fluorides of the thorium, cerium, and yttrium groups of rare earth elements, such fluoride or fluorides amounting to about 15% by weight of the brush. 1

DIMITER RAMADANOFF.

' REFERENCES CITED The following references are of record in the filo of this patent:

UNITED STATES PATENTS Number v Name Date 1,039,522 Guay Sept. 24, 1912 1,147,422 Scott July 20, 1915 1,173,370 Molt Feb. 29, 1916 1,867,524 Orne July 12, 1932