US2946907A - Instant filming brush - Google Patents

Description

July 26, 1960 A. c. TITUS INSTANT FILMING BRUSH Filed Dec. 2, 1957 [NI cHZ'Of. A/befii' C 7737.45,

ff/l? Attorney United States Patent ()fifice 2,946,907 INSTANT FILMING BRUSH Albert C. Titus, Schenectady, N.Y., assignor to General Electric Company, a corporation of New York Filed Dec. 2, 1957, Ser. No. 700,009

8 Claims. (Cl. 310-228) The invention described herein relates to carbon brushes and more particularly to an improved cored brush capable of applying a film to the surface of a current transferring device, such as a commutator or slip ring, when subjected to operation under very high altitude conditions.

The Savage Patent No. 2,736,830, assigned to the same assignee as the present invention, discloses the problems associated with obtaining efilcient collection of current in a dynamoelectric machine when operated under conditions of either high or low altitude where the humidityv is very low. In accordance with the Savage invention, an instant filming brush is provided with multiple engaging surfaces of different materials wherein one material, such as graphitic carbon or mixtures of carbon and a metallic component, as copper or silver, permits commutation of current under low altitude, high humidity conditions. The other material in the brush provides for commutation under high altitude, low humidity conditions, e.g., 40,000 feet or higher, and comprises, for example, molybdenum disulfide and silver. The molybdenum disulfide and silver are inserted in the brush in the form of a single or plurality of cores. This basic invention disclosed in the Savage patent, permitted for the first time, dependable operation of dynamoelectric machines in aircraft designed .for operations in excess of 40,000 feet. It is to be noted that an ordinary carbonaceous brush will dust away within a matter of minutes under the low humidity conditions prevailing at these altitudes.

Continued development of the instant filming brush of the Savage patent, including research and testing under actual operating conditions, revealed that the cores of molybdenum disulfide and silver streaked the collector ring or commutator surfaces and that the current transferring surface sometimes became slightly grooved, and in consequence, the grooves which were formed were reflected mirror-like in the face of the brush where the track of the core stood out in relief. This grooving occurs under the cathode or negative brush when operated at sea level with normal conditions of humidity and pressure prevailing.

Several different theories have been advanced as to the cause of grooving, one being the formation of an oxide, such as molybdenum trioxide, created by oxidizing the molybdenum disulfide of the core at a few hundred degrees centigrade. This is a well known white abrasive material which may be instrumental in causing grooving. It is capable of being dissolved in alkaline or basic solutions, as well as in certain acidic solutions. Another theory is that another oxide, molybdenum blue, is formed,

having the same detrimental effects. Associated with the 2,045,907 Patented July 26, 1960 fer and above certain contact drop levels. These may be effective in reacting with or abrading the copper or other metallic parts of the current transferring device to form grooves beneath the core sections of the brush.

In further explanation of these theories, in spite of the presence of the locally reducing conditions associated with cathode flow of electrons between the current transfer surface and the molybdenum disulfide thereon or in the core, the molybdenum disulfide may first be oxidized by the air, or by oxygen obtained from electrolysis of the moisture film on the current transfer surface. This oxidation may be due to slightly dissolved molybdenum trioxide, and the product formed may be displaced slightly in the moisture film and thus be reduced to the molybdenum blue by the electrons from the cathode, or by products formed thereon, such as hydrogen sulfide. The presence of moisture in the air, or even in the wet inert atmosphere, is essential in wetting the molybdenum disulfide and also in permitting the formation of the sulfuric acid which accompanies the molybdenum blue. This acid may be important in dissolving any molybdenum trioxide before its reduction by the electric current. The sulfuric acid itself may be formed by the oxidation of migrating hydrogen sulfide at the anode, or oxidizing pole, where electrons leave the molybdenum disulfide surface. The blue oxide can also be formed by laboratory processes, in an acid solution of molybdenum trioxide by means of a chemical reducing agent to form a colloidal blue solution or dispersed precipitate. It has also been formed as a fine solid blue crystal mass by directing a blast flame at an angle on molybdenum trioxide and thereby causing the blue crystals to form on clean carbon beyond the reacting white oxide or on a clean porcelain surface. In this method of formation, the product resembles blue soot.

Whatever the theory however, grooving does occur and the primary object of my invention is to incorporate in the brush, and particularly in the inserted cores, a substance capable of eliminating the undesirable grooving of brush contacting surfaces.

In carrying out my invention, I provide an instant filming brush of the type having a main body of graphite provided with inserted cores extending the operating length of the brush. I modify the core material of molybdenum disulfide and silver by adding a substance capable of preventing the formation of abrasive substances and/or neutralizing the formation of acids resulting from operation under both high and low humidity conditions. In a modification, such oxides or acids as may be formed are thinned out and thereby made ineffective by the addition of graphite to the core material, or by adding other constituents, as silver for example, in amounts sufficient to thin out the oxides or acids.

While the specification concludes with claims particularly pointing out and distinctly claiming the subject matter which I regard as my invention, it is believed the invention will be better understood from the following description taken in connection with the accompanying drawing in which:

Figure l is a partial view of a commutator and brush having cores of instant filming material positioned therein as illustrated by dotted lines in the drawing;

Figure 2 is a sectional view of a brush illustrating the arrangement of cores therein; and

Figure 3 illustrates the manner in which cores may be distributed across the face of the brush.

Referring now to the drawing wherein like reference characters designate like or corresponding parts throughout the several views, there is shown a carbon brush 10 bearing against the conductive segments 12 of a commutator 14. The brush has stranded pigtail conductors 16 connected thereto in a well known manner, as by riveting as indicated at 18.

The main body of the brush consists of unimpregnated graphitic carbon or mixtures of carbon and a.

metallic component such as copper and silver. Recesses or cavities 20 are formed in the brush and arranged to extend away from the contact-engaging surface of the brush and such recesses are charged with a special solid commutating material or core 22 chosen as hereinafter specified, which is adapted to engage the moving surface of a current transfer device under conditions of low humidity and low atmospheric pressures without either excessive wear or overfilming.

The inserted material on core 22 comprises acompound containing an element having chemical affinity for carbon, such, for example, as oxygen, sulfur, a halogen, nitrogen, or phosphorus. Such materials ordinarily are introduced as a compound of a metal and the chosen element which is reactive chemically to carbon, for example, as a sulfide, a halide, an oxide, or as a salt of a metal. Molybdenum disulfide is a preferred example of such material. Among the sulfides, particular attention is called in addition, to cadmium sulfide, lead sulfide, and silver sulfide. Among the halides, cadmium iodide and calcium fluoride are examples. Various oxides, in particular, Zinc oxide, lead oxide,'and magnesium oxide, are suitable as lubricating adjuvants. .Molybdenum disulftde in comparison with other suitable materials possesses the advantages of relatively high electrical conductivity, substantial immunity from shrinkage upon firing and low h i e ngth bet e n p a ar ye of its c yst s- In the n rmal situ ion n oun d. he on po on h br s su ac c l s cur ent from the s p ring or commutator when the machine is operated under high humidity conditions, with a film being applied to the contacting surfaces by moisture in the The core material wears away readily under these conditions, thereby causing the untreated carbon .to carry most of the load. The life of the carbon at this time is substantially the same as any other carbonaceous brush operated in high humidities. At high altitudes, however, the reverse operation occurs where the core collects cl cntand the carbon wears away rapidly. As is evident, both the carbon and core material collect current when used in intermediate humidity atmospheres. As previously mentioned, the independent functions performed by the brush materials incorporated in a singular structure, has permitted operation of dynamoelectric machines in both high and low tremes f humidity n i on e ts no p s ble prio to th S e inve tions d scussed abo b sh of the typ Pr usly d cr d m y r a e roo ng nder h sh'h m d y c n t n s f ope i n y r u of t d p e of an oxide abrasive substance such as molybdenum trioxide, or one of molybdenum blue, which is ground into the commutator surface, with accompanying acceleration of the wear rate. It is believed that formation of sulfuric acid, accompanying the molybdenumblue, during the time the machine is operated in high humidity atmospheres, may also contribute to grooving. In-order to counteract or nullify such grooving, it has been found that the-addition of basic compounds capable of reacting with acids, such as barium carbonate or sodium silicate,

4 to the inserted core material, proves particularly efliective. The active basic additives, such as sodium silicate, may also prevent the formation 'of molybdenum trioxide in which the latter, if formed, would dissolve. The active basic additives may change conditions such as to prevent the formation of the molybdenum blue. Also, the addition of graphite, either with or without the use of such basic compounds, serves to thin out the acid and/or the resulting oxides, so as to limit grooving effectively. The

barium carbonate may also operate to minimize the adverse elfects which may result from formation of oxides.

Silver also may be employed as a thinning agent.

Having in mind the various theories postulated above,

various tests were carried out in which the basic compounds were combined with the molybdenum disulfide and silver of the cores in an effort to counteract grooving of the current transferring surface. 7

Brushes comprising a carbon body having core compositions in the proportion indicated-below were tested on a slip ring positioned in a small chamber containing nitrogen, and on a D.-C. machine commutator in a large vacuum chamber, for simulating very high altitudes. The tests also were performed under sea level atmospheric conditions. The predominant changes in core composition resided in varying the percentage of barium carbonate, sodium silicate and graphite inv each of the cores in brushes undergoing tests. Each commutator core brush had 0.078 inch core diameter with two offset rows of three cores each along the face of the brush, giving 6 core paths. A divided polarity slip ring test for bringing out any grooving tendency utilized brushes having two cores in line to provide a single core track. The addition of the anti-grooving chemicals sometimes lowered the molybdenum disulfide concentration sufficiently to allow dusting to take place in the more severe dry nitrogen slip ring tests where the polarity was not divided by having brushes of each polarity. Brush thicknesses were 0.440 inch for the commutation brushes and 0.500 inch for the divided polarity slip ring test brushes.

It will be apparent that the teachings herein are equally applicable to carbon brushes of other designs utilizing molybdenum disultide as an element to obtain filming of a brush contacting surface, since the molybdenum disulfide is the agent which assists in producing detrimental substances discussed above.

Obviously, larger diameter cores, such as 0.100 inch cores, can be used for increasing the percentage of molybdenum disulfide in the core track on a slip ring or commutator, without reducing the ratio of anti-grooving additive to molybdenum disulfide. To maintain the area ratio of carbon tracks to core tracks substantially con stant, the number of cores are then reduced from that used in a particular application with 0.078 inch cores. Alternatively, compositions with sufiicient anti-grooving material can be used for preventing grooving, but the amount of antiagrooving additive must be small enough to allow the molybdenum disulfide to be present in adequate concentration for the needs of the application.

Tabulation of test results follow, wherein rounded numbersrepresent average brush life in hours per A inch of brush wear, assuming dry nitrogen as representing high altitude in the divided polarity slip ring test.

B B G BBQNA Slip Ring Test- Commutator Test Pe Perur /$4 M in u s/l4" cent Percent cent Negative 1 Mils Ag BaCO MOS] Grooved Grooved Dry Sea 40,000 Sea Lev.

Nitrogen Lev. ft.

(27 hours) Analysis of the above indicates that the barium carbonate additive is more effective for use on commutators than on slip rings where representative values obtained were 0.4 and 1.9 mils, respectively, in the 25% silver case where I above table, the crucial time period for checking grooving at sea level is 100 hours, in addition to a preliminary eight hour run at sea level pressure. The time periods of 200 hours for the slip ring test and 100 hours for the com- 2.5% barium carbonate was used. In the slip ring tests, 5 mutator test apply also to the following tables identified the depth of grooving indicated is that made by a negaas Sodium Silicate, Graphite, Barium Carbonate and tive brush which generally grooved to a greater degree Graphite and Sodium Silicate and Graphite. The inthan the positive brushes. In order to provide a base for dication (27 hours) appearing at the bottom of the above comparing a prior art brush of the type having a body Barium Carbonate table means that the last test was of carbon with cores of molybdenum disulfide and silver, 1 terminated at 27 hours. Likewise, the notation (35 hrs.) with these test brushes, which are of the same type but appearing in the Barium Carbonate and Graphite table including anti-grooving additives in the cores, it has been means that the test appearing immediately thereabove was found that the prior art brush grooved a copper surface terminated at 35 hours.

SODIUM SILIOATE Slip Ring Test- Commutator Percent Hours/$4 Test in Hours/ Percent E uiv. Percent Mils M Mils Silver So .511. Moly. Negative Grooved QHgO Disult. Grooved Nitrogen Sea 40,000 Sea Lev. ft. Lev.

0 1.5 98.5 500 500 2 1. 5 93. 5 500 500 1. a 250 250 1. a 5 a 92 1,000 500 0.0 250 250 grooved 5 s as 500 500 0.1 0 75 1, 000 500 s. 5 5 500 2 55; 25 1.5 73.5 1,000 500 2.5 500 1,000 0.2 25 3 72 1,000 500 0.3 500 500 0.2 25 4 71 250 Wear 3.8-5.4 25 5 70 350 700 s 500 500 0. 3 50 3 47 1. 000 500 0. 2 500 500 0. 2

10 mils over a peiiod of 200 hours at sea level. This represents the crucial time period for checking grooving in the divided polarity slip ring test. It is to be noted, however, that in a similar test, grooving was considerably less for the same operating period and under the same test conditions. The grooving is thus a variable phenomenon. For the commutator test listed on the right side of the The addition of sodium silicate as an anti-grooving agent appears to produce the best results in the range of 25% silver in the core. In attempting to find an acceptable range of sodium silicate, difliculty was experienced in the 4% and 5% tests which possibly may have been influenced by summer humidity effects on the sodium silicate. The 3% sodium silicate, 25% silver level, appears to give the most satisfactory results.

"(N) although all types are completely interchangeable.

BARIUM OARBONATE AND GRAPHITE Siip Ring Test- Commutator Hours/ A" Test in Hours/ Percent Percent Percent Percent Mils Mils Ag BaO 0; Graph- M053 Negative Grooved its 1 Grooved Nitrogen Sea 40,000 Sea Lev.

Lev. it.

4 5 25 (N) 66 800 1, 000 0. 2 350 700 0.9, 0.6 and ss. 4 6 20(N) 1, 300 700 3. 5 700 less 0 than 5. 3. 5 7 33 56. 5 1, 500 1, 000 O 3. 5 10 33 53. 5 2. 000 l, 000 0 500 700 0. 25. 3. 5 10 33(N) 53. 5 l, 000 1, 000 0 500 400 0.35. 4 12 20 (N) 64 1, 000 1, 000 4.0 250 500 0.

(35 hrs.) 8. 5 l7 33(N) 46. 5 350 700 0.6

Diflerent types of graphite; both artificial and natural. Natural graphite is marked with (N) although all types are completely interchangeable.

SODIUM SILIOATE AND GRAPHITE Slip Ring Testcommutator Hours/)4" 1 Test in Hours! Percent Percent Percent Percent Mils Negative 54" I Mils Ag SodSil. Graph- MOS: Grooved Grooved QHgO ite Nitrogen Sea 40,000 Sea Lev. it. Lev.

3.5 3.5 33 60 500 1,000 0.1 (0.8 positive 7 i grooving).

4. A current carrying brush comprising the combination' of a body portion consisting essentially of graphitic carbon and having one or more auxiliary members positioned therein for depositing a film on a contacting surface under low humidity conditions; said auxiliary members comprising an admixture of a major component of An unusually severe test (standard non-divided polar-' ity slip ring test in dry nitrogen) was conducted also on brushes of the type appearing in the last table, except the core diameter was changed from 0.078 inch to 0.100 inch, and the number of cores reduced from five to four to provide separate tracks in a brush of 0.437 inch thickness. This brush as tested gave 1600 to 4600 hours life per inch of wear in a dry nitrogen atmosphere. At sea level atmosphere, the brush life was 1900 to 3000 hours per inch of wear.

In view of the above, it will be apparent that many modifications and variations are possible in light of the above teachings. It therefore is to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as, specifically described.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. A current carrying brush comprising the combination of a body portion consisting essentially of graphitic carbon and having one or more auxiliary members positioned therein for depositing a film on a contacting surface under low humidity conditions; said auxiliary members comprising an admixture of a major component of a compound having chemical afiinity for carbon and minor components of a solid metal dispersed therein and a base compound capable of neutralizing an acid and preventing the formation of an oxide of said metal.

2. A current carrying brush comprising the combination of a body portion consisting essentially of graphitic carbon and having one or more auxiliary members positioned therein for depositing a film on a contacting surface under low humidity conditions; said auxiliary members comprising an admixture of a major component of sulfide of a metal and minor components of a metalchosen from the group consisting of silver and copper and a base compound oapable of neutralizing an acid and preventing the formation of an oxide of said first-named metal.

3. A current carrying brush comprising the combination of a body portion consisting essentially of graphitic carbon and having one or more auxiliary members positioned therein for depositing a film on a contacting sur-' face under the low humidity conditions; said auxiliary members comprising an admixture of a major component of molybdenum disulfide and minor components of silver and graphite.

molybdenum disulfide and minor components of silver, graphite, and a base compound capable of neutralizing an acid and preventing the formation of an oxide of molybdenum.

A current carrying brush comprising the combination of a body portion consisting essentially of graphitic carbon and having one or more auxiliary members positioned therein for depositing a film on a contacting surface under low humidity conditions; said auxiliary members comprising an admixture of a major component of molybdenum disulfide and minor components of silver, graphite, and barium carbonate.

6.. A current carrying brush comprising the combination of a body portion consisting essentially of graphitic carbon and having one or more auxiliary members positioned therein for depositing a film on a cont-acting surface'under low humidity conditions; said auxiliary members com-.

denum disulfide and minor components of silver and a base compound capable of neutralizing an acid and preventing the formation of an oxide.

7. A current carrying brush comprising the combination of a body portion consistingessentially of graphitic carbon and having one or more auxiliary members positioned therein for depositing a film on a contacting surface under low humidity conditions; said auxiliary members comprising an admixture of a major component of molybdenum disulfide and minor components of silver and barium carbonate.

8. A current carrying brush comprising the combination of a body portion consisting essentially of graphitic carbon and having one or more auxiliary members positioned therein for depositing a film on a contacting surface under low humidity conditions; said auxiliary members comprising an admixture of a major component of molybdenum dlsulfide and minor components of silver and sodium silicate.

References Cited in the file of this patent UNITED STATES PATENTS 0 745,673 Germany May 19,

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