US2518919A

US2518919A - Electrical contact brush

Info

Publication number: US2518919A
Application number: US69596A
Authority: US
Inventors: Earl L Mckinstry; Charles M Roseman
Original assignee: Union Carbide and Carbon Corp
Current assignee: Union Carbide Corp
Priority date: 1949-01-06
Filing date: 1949-01-06
Publication date: 1950-08-15
Anticipated expiration: 1967-08-15

Description

Patented Aug. 15, 1959 UNITED STATES PATENT OFFICE ELECTRICAL CONTACT BRUSH Earl L. McKinstry and Charles M. Roseman,

Cleveland, Ohio, assignors, by mesne assignments, to Union Carbide and Carbon Corporation, a corporation oi! New York Application January 6, 1949, Serial No. 69,596

1 Claim. (Cl. 171-325) This invention relates to electrical contact brushes for use in sliding contact with commutators or contact rings of electrical machinery.

Most such brushes are composed of carbon or graphite, often in admixture with a metal, usually copper. They are held in contact by a holder and spring which permit some motion of the brush to accommodate unevennesses in commutator or ring surface and to maintain constant contact as the brush is shortened by wear. To conduct the electrical current to or from the brush, a flexible wire or cable shunt is fastened to an end of the brush either by a rivet or bolt or by inserting an end of the shunt into a hole in the brush and tamping with metal powder or amalgam. This invention relates more particularly to tamped shunts.

The copper wire used in shunts is conventionally in the soft, fully annealed state. It has been observed that with conditions of extreme vibration of the brush and correspondingly rapid flexing of the shunt, mechanical failure almost always occurs first in the shunt. For instance, in controlled bench tests to failure of a large number of such brushes with shunts, 95% of the failures occurred in the shunt, at an average life of 1.6 millions of vibrations. This observation suggests the substitution of a stiffer shunt, for instance of work-hardened copper, and a similar test of brushes having full-hard copper shunts showed indeed that about 80% of the failures occurred by failure of the brush or amalgam; but no great advantage was thereby gained because the failures occurred at an average life of 1.5 millions of vibrations.

Prolonged vibration increases the electrical resistance of a tamped shunt connection many fold, long before failure of the brush or shunt.

The foregoing experimental observations suggest that real improvement might be obtained by using a hard copper shunt and strengthening the brush in conventional fashion by impregnating it with a synthetic resin. Attempts to achieve improvement in this way were disappointing, because increases in brush material strength only do not affect the mechanical and electrical properties of the tamped metal connection. Solutions of the usual phenol-formaldehyde, polyvinyl and ester resins applied to the finished brush connection, to try to obtain mechanical and chemical protection of the tamped metal and the immediately surrounding brush material, were unsuccessful because extreme dilution of the resins was required to obtain practical penetration into the connection.

We have now discovered that the improvement sought is obtained when furfuryl alcohol containing a polymerization catalyst is used as an impregnant for the tamping material and surrounding brush stock. After the brush cable connection is formed the resin-producing liquid is applied locally, as with a dropper. This mobile, self-hardening, undiluted resin-producing liquid, readily penetrates the brush and a very materially more durable product is obtained. Thus, in tests duplicating those mentionedabove, a large group of brush-shunt combinations embodying the features of hard drawn copper cable and furfuryl alcohol resin impregnated, tamped amalgam connections were so durable that 73% of them lasted 4.2 millions of vibrations before failure, these failures being in the copper shunt; only 27% of the failures took place in the brush or amalgam, and these failures occurred at an average life of 1.5 millions of vibrations. Furthermore, after one million vibrations, the resistance had increased, on the average, only 56%.

The accompanying drawing shows a partially cutaway view of a commutator made in accordance with the present invention.

The invention is a brush composed of graphite or carbon, or mixtures thereof with a metal such as copper, provided with a hard-drawn copper shunt secured by tamped metal powder or amalgam to an end of said brush, the tamping material, adjacent cable end and immediately surrounding brush material being impregnated with polymerized furfuryl alcohol. The amount of impregnant is not critical; but enough should be used to saturate the region in and about the shunt.

The degree of hardness of the copper shunt wire may be from one-quarter hard (20% cold reduction in area) to extra hard (75% cold reduction in area), hard (60% cold reduction in area) being preferred. Partially annealed drawn copper with residual hardness within this range is also satisfactory.

We have observed that one result of the use of a hard shunt wire is to decrease the amplitude of vibration whip of the shunt in service, and thus to reduce local strain concentrations. Accordingly, it is expected that in the course of several years of service experience with the invention it will be found that the service life will be even greater than indicated by the vibration tests mentioned hereinabove.

Other resin-producing liquids may be substituted for furfuryl alcohol, provided they are mobile, completely penetrate the tamping ma- 3 teriai and surrounding brush material, deposit a high percentage of resin in the finished connection, and will prevent a greater than two fold electrical resistance rise in the connection when it is heated continuously at 100 C. for 16 days.

We claim:

An electrical contact brush comprising a body and a shunt cable secured by tamped metal in a hole in said b dy; said body being composed oi material selected from the group consisting of carbon, graphite and mixtures of metal with graphite; the tamped metal, adjacent cable end and immediately surrounding brush material being impregnated with polymerized furfuryl alcohol; and said shunt cable being composed of 15 ,174,887

copper having a hardness within a range of one quarter hard to extra hard.

EARL L. McKINS'I'RY. CHARLES M. ROSEMAN.

REFERENCES CITED The following references are of record in the tile 01 this patent:

UNITED STATES PATENTS Number Name Date 1,485,942 Adams Mar. 4, 1924 2,066,176 Gix'vin Dec. 29, 1936 2,174,886 Kieter Oct. 3, 1939 Kiefer Oct. 3, 1939