US2409818A - Brush and art of commutation - Google Patents

Description

mach. slip rings, will perform satisfactorily through a Patented Oct. 2.2, 1946 UNITED. STATES ATEN'I OFFICE BRUSH AND ART OF COMIMUTATION Hugh H. Wikle, Cleveland Heights," Ohio, assignor to Superior Carbon Products, Inc., Cleveland, Ohio, a corporation of Ohio No Drawing. Application November 4, 1942, Serial No. 464,531

11 Claims. (01. l71-325).

Other problems have been concurrently present,

at both low and high altitudes,- in form of relatively high friction between the brushes and commutatcrs and sliprings, relatively high specific resistance in the body of the brush, relatively low current density and relatively low contact resistance. Factors. of economy. and facility of manufacture have been preserved and enhanced in my solution of these problems.

It is among the objects of my inventionto provide a brush which will have long and satisfactory life in service at high altitudes, particu larly altitudes in excess of 29,000 feet. Another object is to provide a brush capable of performing satisfactorily with a high current density without physical or electric deterioration or excessive sparking under a wide range of operating conditions including extreme conditions of altitude and temperature. .Alfurther object is to provide a bi which in its use in dynamoelectric es, as incontact-with commutators and strength, or with relatively increased physical strength. Another object is to providelan economical and facile method of making such brushes. Another object is to provide an improved method of commutation and an improved method of brush operation in respect to'wear, friction, contact resistancaisparking and the like. These andother objects of my invention will 'become more fully apparent inthe following [description of a preferred and certain modified forms of my invention. The essential characteristics are summarized in the claims. 7

WhileIQdo not care to be committed to any particular philosophical speculation as to the why or the wherefore of the manner in which my brushes or methods work and obtain their results, still I have made some observations and drawn some tentative conclusions which I be-' lieve will be helpful to those skilled in the art in their understanding and practice of .my inven-' tion. I mention and refer to these things without prejudice as tothemetes and bounds ofmy invention or patent'claims. I believe that in the ordinary use of the prior art brushes at or around sea level and in the ordinary range of temperatures in which dynamoelectric machines with brushes have been commonly used that the successful operation of the brush is coincident with the formation of or the deposition of a thinor microscopically thin film or coating on the com mutator or slip ring and perhaps also on the adjacent face of the brush. I believe that under these known prior artrconditions this film comprised largely cupreous or cupric oxide, particles of carbon or graphite and other constituents of the brush, impurities such as grease and dirt and perhapsalso oxygen, water vapor and ,other atmospheric gases. Whatever the constituents of such a film, it seems to have had the function of reducing friction between the brush and.commutator or slip ring, aiding in the maintainence of contact resistance and otherwise performing such a beneficent purpose that upon its substantial impairment or elimination, mechanical wear and abrasion increase very rapidly, 'contact resistance is lowered with accompanying electric disturbances such assparking, and other evidences of poor commutation appear. It appears to me that the presence of. oxygen and water vapor aid in or give facility to the formation of such film as I have described or at least that their. presence is consistent with conditions under which such a film seems to exist. I have observed that during attempted brush operation in anatrnosphere of dry'nitrogen the brush wear and abrasion increases noticeably and other undesirable conditions of commutation quickly appear. v r

One of the recognizable conditions at high altitude with the accompanying low temperature is the relative lack of oxygen and water vapor, and I believe that this deficiency may have much to do with the rapid and dangerous brush wear under this condition. My thought has been to provide a film or the equivalent in respect to the functions which I believe have been performed by the film above described, and which would not be dependent upon, among other things, the relatively abundant presence of. oxygen and phuror in a form whether free or compounded.

that is reactive with or adherent to the copper,

of the commutator or slip ring, that my desired results are obtained. By making sulphur avail" able to and at the brush and commutator or slip manner about to be described I have been able to achieve the objects of my invention.

I am aware on the one hand that the pres-- ence of relatively small amounts of sulphur. inv one form or another in prior art carbon brushes has been thought to be deleterious and effort has beenmad'e to eliminate it. I am also aware that relatively large quantitiesof sulphur have been included in the mix in the making of other types of prior art carbon brushes, but that in both instances the advantages which ,I have sought to obtain have been lost. I believe that the dis advantage'commonly recognized in the presence of asmall amount of sulphur and the absence of advantage in the prior practice of endeavoring to use large quantities of sulphur are wholly or partly due to using either or all carbon flour and pitch binders with .the sulphur and subjecting s'uchbrushes to such high temperatures during the baking thereof as to boil away or vaporize known as 4 /2 pound, meaning it contains about 4 /2 pounds of resin or solid material to every gal- I Ion-of solution including volatile solvents. The

mixture above described is mechanically mixed until substantially all parts of the graphite are fwettedwith the shellacsolution albeit the proportionof shellac to graphite issuch that'the Seemed wetness-is not clearly apparent to the eye. 'v'Aftera thorough mixture ofthese constituents the mixture is permitted to dry at room 7 temperaturepreferably being spread out in a relatively thin layer in appropriate drying pans until substantiallyall the volatile solvents of the shellac solution that will evaporate at room temperatures have passed off. When this mixture has been dried to this extent I, take 18 pounds ofmixture and add preferably 2 pounds of dry powdered sulphur. These ingredients are thoroughly mixed mechanically at room ternper- -atu re and then pressed, preferably in the form of a block'about 5 to 6 inches square and about A to 1% inches deep, depending on the thickness desired inthe finished brushes, under a gross pressure of about 200 tons, i. e., at about 6 to 'l-tons per square inch. This block of brush: material, from which the brushes of appropriate size 'and shape may subsequently be cut, is then li'eated slowly and gradually broughtup to a temperature of about 500. Fahrenheit and held fat about that temperature fora periodof about 5 orimproved contact resistance.

48 hours. After coolin the baked and molded mass will be found to be firmly bonded in a solid block, and will have satisfactory physical characteristics to facilitate its severance into desired sizes and shapes for brush use. I prefer, for reasons known to those skilled in the art, to control the thickness of the block, i.-e., its depth in the direction of application of molding pressure. and then cut or sever the block into sizes and shapes of finished brushes in such a way that .when the brush is placed in operative relation to the commutator, the direction of application of molding pressure will lie substantially tangentially of the commutator. ring and/or incorporating it in the brush in the r ,,the, preferred steps described above have given t satisfactory long life under high altitude condi- Brushes made in the preferred form and by tions. Moreover, the brushes embodying my invention have shown ability to carry current densities of the order of about amperes per square inch as compared with current densities of the order of 60 amperes per square inch in similar prior art brushes lacking the sulphur constituent. I'have also observed that brushes embodying my invention in its preferred form have as little as about one-half the specific resistance of such prior art brushes while retaining a high Moreover, the brushes embodying my invention reduce the'friction loss between the brush and'commutator or' slip ring up to aboutone-halffand do all these things without sparking or physical or electric disintegration or deterioration and without sacrifice of physical strength under the ordinary and normal conditions of use as well as under the extreme conditions of high altitude airplaneoperation above about 20,000 feet.

Inthe operation of brushes containing myinvention I have observed that at the beginning ofuse on a clean commutator, there appears to be formed on the commutator a dull chocolate colored polish or film which, as such, seemsto conditions the said visible film or polish is sup-' 7,

.plied by'the sulphur, and/or sulphur and binder c r-other mixtures or compounds including sulphur, which was contained in the initially ex-' posed bearing surface of the brush, and that the remaining constituents of the exposed surface wore so slowly under those sea level conditions, that the ingredients of the visible sulphur bearing'film were not exposed or released from within the brush at a rate rapid enough to be visible, aftertheir.ini-tialiappearance and 8,.to 10 hours wear. .(2) Under high altitude "conditions'the increased tendency to wear away the'bearing surface of. the brush removes the non-film form- .ing constituents'in the face of the brush thus exposing an additional increment of film forming ingredients which in turn retard or tend to eliminate the Wear. :A balance is thus provided'in which the very Wear of .the brushface, instead 'of. beginning a cycle of self-destruction asiinlthe prior art practice; constitutes the means-ofxselfpreservation within the precepts of. my invention.

observations of tests performed with my brush (under? or simulating. high altitude conditions? evi;

givenaboveis extended about a hundred'fold more or lessthe life of prior art'brushes.

In the practice of my invention I contemplate the admixture of sulphur in amounts varying substantially from the precise proportion above described without departing from the precepts of my invention, and I contemplate the use of com pounds and mixtures of sulphur as wellas com mercially powdered chemically pure sulphur so.

long as the change of proportion and the nature of the mixture of compounds do not substantially defeat the essential function and contribution to the mode of operation of my invention that is performed by the sulphur or its compounds, mixture or equivalents in the preferred example above given. Although I have indicated a pref erence for a10%. mixture of sulphur with the other constituents of my brush, I have varied the proportion from about to 15% sulphur with satisfactory results. I mention these variationsby Way of example rather than limitation. Those skilled in the art may be guided in the sameo'r different changes in proportions Within the precepts of my invention, having in mind that brush life at high altitudes tends to be increased, a I believe for the reasons given above, with the larger proportions of sulphur, although under some conditions of use I expect that further increases in the proportion of sulphur in a brush otherwise following my preferred form will'tend to restrict its utility in point of physical strength.

Again, I have observed within limited test con ditions that while my brush having 5% to sulphur substantially halves the specific resistance, compared with similar brushes lacking any sulphur, my brush with sulphur tends -to show some increase in specific resistance on the same comparison. Again,.while my brushhaving 5% to 10% sulphur appears to substantially equal the non-sulphur priorart brush in point of contact resistance, my brush with 15% sulphur has tended to show about one-third increase in contact resistance by the same comparison.

WhileI have employed shellac for a binder in the preferred form above mentioned I do not mean to exclude other chemically and/or me: chanically equivalent; binders. For guidance rather -thanlimitation I mentiona few of the characteristics and functions of the binder that I prefer. It performs its binding function, i. e., is cured and sets, 'at low temperatures in .relation to the vaporization or boiling point of sulphur so that sulphur is not driven from the mix during the baking step... It holds the sulphur in the finished brush iwhether 'mechanically or chemically or both in such a way that at least under the conditions of commutation a reaction between part of the sulphur and. the metal of the commutator is not substantially. impaired or inhibited. Preferably also it serves. as a Vehicle to carry the sulphur from the brush to the commutator or ringand tends to hold it in the film thereon; the binder thus also preferably entering into the contents of the film .and giving it adhesion to the commutator. It also appears that the binder should not, in the form it takes on the commutator or Wearing surface of the brush, be abrasive nor substantially diminish the lubricating effect of the film,

Even at the temperature of 500Fahrenheit that I have mentioned in the preferred form of my invention for baking or curing my product I believe there is a loss of about 10 to 15% of the sulphur fromthe original mix andI suggest.

that this tendency to loss be kept in mind. particularly where smaller percentages ofsulphur and/or higher baking temperatures are sought to be employed than I have suggested in my pre-.

ferred example. I

I Although I have specified the use of Ceylon graphite as a preferred form of'carbonorconducting element of the brush containing my invention my preference isto use graphite in the so-called flake or crystaline form. I prefer to use graphite as the carbon element of the brush especially where shellac is employed as the bind er, because I believe the absorbent or adsorbent characteristic of carbon flour for example, tends to impair the binding function oft-the; shellac. I do not exclude from my teaching, the use, of carbon in forms other than theform of graphite especially where binders that have the I other characteristics which I desire are available and are not subject to substantial impairment in their steps of bringing sulphur to or between the wear tor.

ing surfaces of brush and commutator or slip ring in a form and under conditions in which it will participate or aid in forming a wear resisting or lubricating film such as described above. tice this method by the employment of my preferred form of brush participating in the electrical operation of the machine, I do not exclude other ways or means of forming the sulphur bearing film between brush and commuta- For example a separate or thirdbr'ushj? within or without the electrical circuit, can be provided to bear on the commutator or ring in its wearing zone and can comprise sulphur and preferably a vehicle therefor such as shellac and preferably also some graphiteparticularly if the electrically active brushes lack these latter in gredients. l By way of further example of a practice of a form of my invention emphasizing the latter precepts, I have impregnated carbon brushes with sulphur and/ or with sulphur and shellac or other resinous substances, and in the use of such brushes obtained a film having the desirable characteristics mentioned above. Such brushes retain substantially their normal internal electrical characteristics with increased contact resistance and reduced friction between brush and commutator and improved wear resisting characteristics.

In this practice of my invention I first formthe carbon, carbon graphite, or carbon metal graphite brushes in the usual Way as with pitch binder, compression molding and relatively high temperatures baking, employing carbon flour in the first instance and mixtures of carbon flour and graphite, or powdered metal or both as may be desired to give the known characteristics of brushes inherent in these mixtures and well known processes of manufacture. For my illustrative pur pose such brushes are known to have certain mix- Although as indicated I prefer to prac porosity and absorptive or adsorptive qualities conducive to impregnation by additional. constituents... p .IaI have found thatsucha carbonpbrush or a molded and curedblock of such brush material in a slab x 6" x /2" when immersed in sulphurin the liquid state at a temperature of about ..respondingly require less time and pressure for the samev extent of. impregnation.

Similarly when less impregnation is sought, lower pressures for shorter periods .will provide a. convenient means for regulating the extent of impregnation v and. correspondingly reduce the proportion of sulphur absorbed by the brush. Instead of using sulphur in the hot liquid state I may also impregphur, impregnation can take place at room temperature with pressure applied to the bath in which the brush is immersed for a, suitable length of time within the precepts mentioned above. Those understanding the .art of. impregnating suchmaterials will also understand that the process may be hastened by first evacuating the air from the interstices of the brush or block of brush material about tobeimpregnated.

It'may on occasion be desirable to first impregnate'with a surplus of sulphur and thereafter reduce the sulphur content by baking the brush at temperatures varying from 500 to 1000 F. and by volatilizing or boiling ofi the excess sulphur. The time of such baking and the temperature will be controlled in relation to the extent to which it is desired to drive off the excess sulphur. By this means, the percentage of sulphur in the brush may be conveniently modified and controlled.

Within the practice of my invention involving the impregnation of carbon brushes with sulphur as above described, I also preferlto. im-

pregnate such brushes with resinous material such as shellac, which as mentioned in the preceding part of this specification seems to con- F 8 Within'the teaching of the foregoing example I do not mean to exclude-repetitive steps ofimpregnation at pressures and times differing substantially; from "those mentioned above.v

Neither do I exclude combining thesteps of ,impregnating with sulphur and impregnating with shellac or other resins where that may befoundv advantageous or expedient. For example where acommon vehicle or solvent or mixture thereof will carry both shellac and colloidal sulphurthey may both be forced into the interstices nin'one' operation or a series of operations involving both; the sulphur and shellac.

Whil I have described a preferred and certain modified forms of myinvention in the brush as an. article and the combination of a, brush with a commutator or slip ring and in respect to my method inventions allied thereto, I do not care to be limited to the example disclosed herein or in any manner other thanby the claims hereof. l

Iclaim: 1 15A brush comprising graphite,-shellac.,and about 5% to 15% sulphur. 2. A brush comprising graphite, shellac and about 5% to 10% sulphur, and having only about half the specific resistance of a lacking sulphur. H

' 3. A brush comprising graphite, shellac and sulphur in substantially. the proportion of 42, 9 and 3 to 9 parts respectively.

4. The method of making brushes whichconsists in forming a mechanical mixture of graphite, sulphur and shellac, pressing the constituents into a substantially solid mass and baking the mass ata temperature of about 500, F. for about 48 hours. Q

5. The method of'making brusheswhich consistsin forming a mechanical mixture of graphite, sulphur and a binder, pressing the constituents into a substantially solid mass, warming the mass slowly to atemperature of about 500 F; and baking the mass at about such temperature for about'48 hours..

6. The method of making carbon. brushes which consists in mixing carbon in one of its forms which conducts electricity, a binder ca.-

pable of taking its set or cure at a temperature lowerthan the volatilizin temperatureof sulphur, and sulphur, mixing such ingredients, and

compressing and curing the same without raising the temperature substantially above 500 F. 4 '7; A brush .comprisin 'carbon, a binder and about 5% to 15% sulphur. 8. A brush comprising graphite, a binder and about 5% to 15% sulphur. r

9. A carbon brush impregnated with from about 5% to 30% sulphur.

10. A carbon brush impregnated with a binder and from about 5% to 30% sulphur. 11..A carbon brush impregnated with shellac and from about 5% to 30% sulphur.

similar.- brush