USRE39974E1 - Management of contact spots between an electrical brush and substrate - Google Patents
Management of contact spots between an electrical brush and substrate Download PDFInfo
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- USRE39974E1 USRE39974E1 US11/055,789 US5578905A USRE39974E US RE39974 E1 USRE39974 E1 US RE39974E1 US 5578905 A US5578905 A US 5578905A US RE39974 E USRE39974 E US RE39974E
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01H—ELECTRIC SWITCHES; RELAYS; SELECTORS; EMERGENCY PROTECTIVE DEVICES
- H01H1/00—Contacts
- H01H1/06—Contacts characterised by the shape or structure of the contact-making surface, e.g. grooved
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01R—ELECTRICALLY-CONDUCTIVE CONNECTIONS; STRUCTURAL ASSOCIATIONS OF A PLURALITY OF MUTUALLY-INSULATED ELECTRICAL CONNECTING ELEMENTS; COUPLING DEVICES; CURRENT COLLECTORS
- H01R39/00—Rotary current collectors, distributors or interrupters
- H01R39/02—Details for dynamo electric machines
- H01R39/04—Commutators
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01R—ELECTRICALLY-CONDUCTIVE CONNECTIONS; STRUCTURAL ASSOCIATIONS OF A PLURALITY OF MUTUALLY-INSULATED ELECTRICAL CONNECTING ELEMENTS; COUPLING DEVICES; CURRENT COLLECTORS
- H01R39/00—Rotary current collectors, distributors or interrupters
- H01R39/02—Details for dynamo electric machines
- H01R39/08—Slip-rings
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01R—ELECTRICALLY-CONDUCTIVE CONNECTIONS; STRUCTURAL ASSOCIATIONS OF A PLURALITY OF MUTUALLY-INSULATED ELECTRICAL CONNECTING ELEMENTS; COUPLING DEVICES; CURRENT COLLECTORS
- H01R39/00—Rotary current collectors, distributors or interrupters
- H01R39/02—Details for dynamo electric machines
- H01R39/18—Contacts for co-operation with commutator or slip-ring, e.g. contact brush
- H01R39/20—Contacts for co-operation with commutator or slip-ring, e.g. contact brush characterised by the material thereof
- H01R39/22—Contacts for co-operation with commutator or slip-ring, e.g. contact brush characterised by the material thereof incorporating lubricating or polishing ingredient
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01R—ELECTRICALLY-CONDUCTIVE CONNECTIONS; STRUCTURAL ASSOCIATIONS OF A PLURALITY OF MUTUALLY-INSULATED ELECTRICAL CONNECTING ELEMENTS; COUPLING DEVICES; CURRENT COLLECTORS
- H01R39/00—Rotary current collectors, distributors or interrupters
- H01R39/02—Details for dynamo electric machines
- H01R39/18—Contacts for co-operation with commutator or slip-ring, e.g. contact brush
- H01R39/24—Laminated contacts; Wire contacts, e.g. metallic brush, carbon fibres
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01R—ELECTRICALLY-CONDUCTIVE CONNECTIONS; STRUCTURAL ASSOCIATIONS OF A PLURALITY OF MUTUALLY-INSULATED ELECTRICAL CONNECTING ELEMENTS; COUPLING DEVICES; CURRENT COLLECTORS
- H01R43/00—Apparatus or processes specially adapted for manufacturing, assembling, maintaining, or repairing of line connectors or current collectors or for joining electric conductors
- H01R43/12—Manufacture of brushes
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
- Y10T—TECHNICAL SUBJECTS COVERED BY FORMER US CLASSIFICATION
- Y10T29/00—Metal working
- Y10T29/49—Method of mechanical manufacture
- Y10T29/49002—Electrical device making
- Y10T29/49009—Dynamoelectric machine
-
- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
- Y10T—TECHNICAL SUBJECTS COVERED BY FORMER US CLASSIFICATION
- Y10T428/00—Stock material or miscellaneous articles
- Y10T428/12—All metal or with adjacent metals
- Y10T428/12465—All metal or with adjacent metals having magnetic properties, or preformed fiber orientation coordinate with shape
-
- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
- Y10T—TECHNICAL SUBJECTS COVERED BY FORMER US CLASSIFICATION
- Y10T428/00—Stock material or miscellaneous articles
- Y10T428/12—All metal or with adjacent metals
- Y10T428/12993—Surface feature [e.g., rough, mirror]
Definitions
- This invention relates generally to the management of so-called contact spots through which, on a micro-scopic scale, electrical currents are conducted across interfaces of solids, whether between the two sides of switches or between sliding as well as stationary electrical brushes and their substrates, being mostly but not exclusively slip rings and commutator bars.
- the electrical brushes at issue include fiber brushes disclosed in the above-noted U.S. Pat. Nos. 4,358,699 and 4,415,635, and in U.S. Pat. No. 6,245,440. Additionally, they include foil brushes as described in the publication “Production and Performance of Metal Foil Brushes,” P. B. Haney, D. Kuhlmann-Wilsdorf, and H. G. F. Wilsdorf, WEAR, 73 (1981), pp. 261-282, which is also incorporated by reference, and ordinary monolithic brushes made of graphite or graphite-metal mixtures.
- the invention is also applicable to electrical switches for the reduction of resistance and sticking forces, as well as to devices for efficient heat transfer.
- the present invention includes the use of various technologies referenced and described in the above-noted U.S. Patents and Applications, as well as described in the references identified in the appended APPENDIX and cross-referenced throughout the specification by reference to the corresponding number, in brackets, of the respective references listed in the APPENDIX, the entire contents of which, including the related patents and applications listed above and the references listed in the APPENDIX, are incorporated herein by reference.
- monolithic graphite-based brushes cannot be reliably used, over extended periods of time, at current densities above about 30 Amp/cm 2 , nor at sliding speeds above about 25 m/sec. Further, as a coarse estimate, they waste about one watt per ampere conducted across the brush-substrate interface, i.e. the equivalent of one Volt, in terms of Joule and friction heat. Further, they emit significant intensities of electromagnetic waves (i.e., they are electrically very noisy so as to interfere with radio and similar signal reception), and finally they wear into a powdery debris that can be highly detrimental in electrical machinery, especially aboard submarines.
- the poor qualities of monolithic brushes arise from their small number of contact spots, namely in the order of ten per brush.
- the current flow lines in monolithic brushes are not rather uniformly distributed, as they are in metal fiber brushes, but they are “constricted [2]” at the few contact spots. This causes the corresponding “constriction resistance” that represents in the order of one third the resistance of monolithic brushes.
- metal fiber brushes does not only derive from their thousands of evenly distributed contact spots, but also from the fact that at their contact spots bare metal meets bare metal, ideally separated only by a double monomolecular layer of adsorbed water vapor.
- this most favorable type of lubrication which prevents cold-welding and accommodates the relative motion between brush and substrate at a “film resistivity” of only ⁇ F -1 ⁇ 10 ⁇ 12 ⁇ m 2 and average friction coefficient ( ⁇ ) of about 0.3, establishes itself automatically at any modest ambient humidity, provided that undue contamination with oils, etc., is avoided.
- monolithic brushes deposit a lubricating graphite layer through which the current must flow at much higher electrical film resistivity.
- Metal foil brushes [3] closely resemble metal fiber brushes except that they are composed not of substantially parallel fibers but of thin parallel foils. Consequently they typically have many fewer, but otherwise the same kind of, contact spots. Thus metal foil brushes are very similar to metal fiber brushes but cannot match their attainable current densities, sliding speeds and low power losses. At any rate, foil brushes are based on the same principle as metal fiber brushes, namely electrical contact to the substrate at a large number of microscopically small, bare metal-metal contact spots, optimally lubricated by a double monomolecular layer of adsorbed water. Hence, also, in terms of number of contact spots per unit working surface area (i.e., “contact spot density”), and mechanical load per contact spot, exactly the same theory applies to metal foil as to metal fiber brushes [4-6].
- foil brushes comprise a substantially smaller density of contact spots than well-constructed metal fiber brushes.
- foil brushes will be very superior to monolithic brushes but fall short of metal fiber brushes.
- the present invention presents methods for the management of the contact spots at brush-substrate interfaces of all three types of electrical brushes.
- the favorable impact of the management of contact spots according to the present invention may be the greatest on the performance of metal foil brushes which as a result may well become competitive with metal fiber brushes, but is expected to be significant to strong also for metal fiber and monolithic brushes.
- This management of the contact spots is mainly effected through suitable shaping and otherwise conditioning the substrate surface, and to a lesser extent also through modifications of the brush working surfaces, especially in connection with commutation.
- FIGS. 1A-1E show examples of schematic cross-sectional views of different profiles of brush substrates and resulting contact spots
- FIG. 2A shows a schematic perspective view of a tool with a wave-shaped cutting edge for cutting a grooving profile into a substrate
- FIG. 2B shows the tool of FIG. 2A in position during cutting the profile, which in this figure is rotated in, for example a lathe, as indicated by the arrows;
- FIG. 3A shows a fiber end encountering isolated flat asperities of closely similar elevation so as to form, in this case, four separate contact spots;
- FIG. 3B shows the situation comparable to FIG. 3A but for the case of foils instead of fibers and sliding on grooving instead of isolated asperities;
- FIGS. 4A-4F show schematic views of sections through different brushes, all except the foil brush in FIG. 4D including more than one type of fiber or foil, in position relative to substrate profilings adapted to them, except in FIG. 4E where the substrate is not shown;
- FIGS. 5A and 5B illustrates examples of corrugations in foils of foil brushes;
- the contact spot density at the brush-substrate interface is of paramount importance because it controls both, the electrical resistance across the interface and p trans , the critical brush pressure below which the average contact spot is elastic.
- p trans ⁇ 3 ⁇ 10 ⁇ 4 fH (1)
- ⁇ is the number of contact spots per fiber end (believed to be close to unity)
- f is the packing fraction already introduced (i.e., the fraction of brush volume occupied by fibers, the remainder being voidage)
- H the hardness of the brush fiber material, which generally should be smaller than, or at most equal to, the hardness of the substrate.
- n* ⁇ f/(1 ⁇ 2 ⁇ d 2 ) (2) with d the fiber diameter.
- the contact spot radius of curvature is emphatically not the average radius of the contact spot area. Rather, it is the radius of curvature normal to the average surface orientation. Namely, contact spots are formed where asperities of one side either meet an asperity of the other side or simply impact on a more or less flat area.
- contact spots are modeled as “Hertzian contacts,” i.e., as hard spherical asperities of radius r c impinging on a softer flat substrate of Meyer impression hardness H. Based on the derivation given in refs.
- the average local pressure at the contact spots is proportional to r c ⁇ 2/3 .
- the desirable elastic contact spots which give low wear rates are therefore favored by large asperity radii of curvature as well as high contact spot densities.
- the Hertzian model of contact spots is thus basically flawed in that, without special management in accordance with the present invention, the persistent asperities are located on the brushes, the softer side.
- the implications of this state of affairs are considered and found to lead to the desired possibility of manipulating contact spots so as to make them more numerous and/or to be associated with larger asperity radii of curvature.
- foil brushes perform very much like fiber brushes, as documented by Haney et al. already cited [3] and according to informal oral communication more recently by measurements at IAP, Dayton, Ohio.
- foil brush wear rates are expected to be correspondingly higher.
- r c the applicable asperity radii of curvature
- foil brushes could be made routinely competitive with metal fiber brushes if the number of contact spots per foil edge could be reliably, drastically increased as via the present invention.
- foil brushes are most easily made and are liable to be rather cheaper.
- foil brushes offer less flexibility in terms of cross-sectional shapes as well as brush holding and loading.
- the packing fraction of foil brushes can be made rather higher than the f'0.15 typical for metal fiber brushes, in fact depending on angle of attack up to near 100% [3]. This permits correspondingly higher current densities and also higher brush pressures which simplifies brush loading.
- the jury is still out in regard to commutation.
- Present best indications are that without modifications foil brushes arc more readily than fiber brushes do, and again without modifications are presumably less suitable for commutating applications than metal fiber brushes.
- measures for arc suppression at the trailing ends of brushes are more readily incorporated into foil than fiber brushes.
- the radii of asperity curvature, r c can now be chosen at will through the profiling of the troughs and crests of the grooving as indicated in FIG. 1 .
- Such grooving could be imparted to the substrates by optional means, as for example by machining with special cutters as clarified in FIGS. 2A and 2B .
- the foils might be 20 ⁇ m thick and run parallel to a grooving of 60 ⁇ m spacing, with a 45 ⁇ m depth from crest to valley.
- the expected contact spot density will be proportional to the “effective packing fraction” on account of foil thickness and directional cosine of the brush's angle of attack. E.g. a near-100% actual foil packing fraction at 30° angle of attack would yield an effective packing fraction of 50%.
- the brush resistance R B would, at same ⁇ -value and hence same expected dimensionless wear rate, be only one fifth as large as for the typical fiber brush. In the above example one could therefore increase the brush pressure, decrease the wear rate, increase the current density and/or decrease the total brush loss in any desired combinations.
- slip ring or commutator bars In order to achieve the outlined potential benefits of the invention, it will be mandatory to optimize the surface finish of the running surface, i.e., slip ring or commutator bars.
- the slip ring or commutator bar surfaces should be very smooth, best as of a light-optical metallographic polish and achievable by electropolishing, in order to exhibit as small deviations from the intended pattern of grooving and/or asperities as may be possible. This is for the reason that interlocking at contact spots and hence wear particle formation principally occur at irregularities.
- the substrate should be as hard as possible as may be consistent with an adequately low film resistivity.
- the substrate needs to retain its profiling for the life-time of the machine and, secondly, because, given smooth surfaces, brush wear rates are expected to decrease with increasing superficial hardness of the slip ring or commutator bars since interlocking becomes more difficult with rising hardness.
- the objective of this invention is to produce surface profiles and finishes on surfaces that are adapted to sliding electrical contacts, such that at otherwise same conditions they lead to a reduction of electrical brush resistance and/or wear in any combination.
- This objective requires different profiles and surface finishes for different brushes (i.e. metal foil, metal fiber, and monolithic graphite brushes) and/or conditions as follows:
- contact spot spacing and asperity radius of curvature will be comparable with the fiber diameter d or foil thickness d f for fiber and foil brushes, respectively, although one will try to reduce the spacing and increase the curvature.
- contact spot spacing this is so because for same ⁇ , i.e. same fraction of local pressure relative to the hardness of the brush material, and hence for same wear rate, the electrical brush resistance is inversely proportional to the number of contact spots.
- radius of curvature this is so because at same number of contact spots and same applied brush force an increase of asperity radius of curvature decreases the local pressure, and hence ⁇ and with it the wear rate (compare eq. 10).
- the average asperity spacing in sliding direction should be about 25 ⁇ m or less to generate at least two contact spots on the average fiber end and prevent the discussed vibrations as indicated in FIGS. 3A and 3B .
- future research will reveal the optimum choices for the asperity spacing in different conditions.
- optimal shapes of asperities, and thus contact spots namely preferably elongated in sliding direction so as to reduce the rate of brush resistance with sliding speed due to “aerodynamical lift” or “water planing”, that has already been introduced above.
- the quality of the surface finish of substrates constructed in accordance with the present invention is similarly important, for monolithic, fiber and foil brushes, as the shape of the profiling. This has three aspects: (i) Microscopic smoothness, (ii) hardness and (iii) resistance against oxidation and other chemical attack including corrosion. To discuss these in turn:
- foil and fiber brushes but not monolithic brushes
- metallic electrical conductivity including the foil or fiber ends
- the objective has been to configure the substrate so as to obtain an increase in contact spot density and/or optimal contact spot morphology beyond what would be established automatically by use of the same brushes.
- the means to this end is the shaping of substrates to provide “built-in” contact spots, either through grooving the substrate or providing it with isolated asperities.
- both can be done simultaneously and might be desirable in cases in which foil or fiber brushes comprise elements of two or more different sizes, e.g. “support fibers” 17 (i.e. minority mechanically stiffer fibers whose function is to protect the brush from being crushed) guided by grooves, and finer fibers responsible for most of the electric conduction exposed either to a smooth surface or to asperities between the grooves. This is indicated in FIG. 4A wherein the sliding direction is normal to the plane of the drawing.
- foil brushes i.e. comprising “support foils” guided in grooves.
- the brush would have to be run parallel to the foils as sketched in FIG. 4B while in the previous considerations foil brushes were thought of as running normal to the sliding direction or to be only moderately inclined to that orientation (e.g. as in FIG. 3 B).
- foil brushes Independent of selection of the discussed component of foil orientation, in order to obtain elastic compliance in the direction of load application, foil brushes will generally be run modestly or perhaps even strongly slanted against the substrate. That slant will mostly be in the “trailing” sense but can also be “leading” i.e.
- foil brushes may be run at any angle relative to the sliding direction ( FIG. 4D ) and their overall cross-sectional shape may be selected at will as also indicated in FIGS. 4E and 4F .
- foils may be teamed with fibers in “hybrid foil-fiber brushes” (FIG. 4 E).
- FIG. 4 E The discussed possible choices for foil and hybrid foil-fiber brush sliding, illustrated in FIG. 4 , could prove most useful in commutating applications. In that case the support foils or fibers could in fact be insulating and simply serve the function of reducing the jarring as the brush crosses the gaps between the commutator bars.
- At least some of the fibers in-between the foils could be very fine fibers of tungsten or stainless steel or similarly arc-resistant material to serve the function of “lightning rods”, i.e., provide preferential sites for arcing, thereby protecting the majority current conducting fibers from arc damage.
- the stratagem of “lightning rod” fibers has already been used successfully with metal fiber brushes were the “lightning rod” fibers were crowded at the trailing ends of the brushes as indicated in FIG. 4 F.
- foils could be dimpled or corrugated in a regular or irregular manner; in the case of corrugations, the not necessarily straight crests could be oriented in any desired manner relative to the intersection line between foil and substrate.
- one's freedom of choice is somewhat restricted by the need for relatively easy sliding between neighboring foils so that the brush as a whole does not become too hard.
- a certain degree of foil brush stiffness in selected directions may be desired so as to resist Lorentz forces on account of the sometimes large magnetic field strengths within a motor.
- the stiffness of the individual fiber can be somewhat regulated, as a function of orientation relative to the sliding direction via its cross-sectional shape, e.g. tubular or flattened to various degrees, including shapes that are intermediate between fibers and foils, with the long axis, say, in the plane containing the sliding direction or at right angles thereto.
- the invention is applicable to heat transfer in the same manner as to current transfer, and to electrical switches as to sliding electrical contacts. Physically, electrical and heat conduction rise and fall together on account of the Wiedemann-Franz law (compare ref. 2).
- Commutation generally raises the wear rates of brushes by about a factor of two but may be relatively more detrimental to metal fiber brushes [24,25]. This is partly due to the mechanical fatiguing caused by the high-frequency jarring and thus momentary pressure increases at transitions between bars and insulators or, worse, air gaps. Such jarring can be largely eliminated through filling the gaps with insulating material at as nearly smooth surface leveling between bars and insulators as may be possible. Additionally it is believed that the jarring can be reduced by the use of support foils, especially when guided in continuous grooves as suggested in FIGS. 4B and 4F .
- Such support foils need not necessarily be electrically conductive. Namely, the more important and less tractable part of extra wear in commutation arises from arcing due to rapid current density changes. Such arcing is mostly concentrated at the trailing edge of brushes but according to recent observations in our laboratory can have a minor component also at the leading brush edge, namely on account of “current closing” as the leading edge of the brush makes first contact with the next commutator bar.
- the described arcing damages monolithic brushes through eroding away brush material. This causes the roughly doubling of the wear rate through commutation observed for monolithic brushes, already mentioned.
- the end-result is a characteristic “leopard skin patterning” of fused groups of fiber ends separated by zones of relatively undamaged fibers in a morphology resembling the spots on a leopard's fur.
- the brush is strongly mechanically hardened on account of leopard skinning, will increasingly begin to bounce and in consequence be subject to even more arcing.
- FIGS. 4D-4F are not meant to be exhaustive but rather to give general indications. Specifically, any one edge may exhibit one or more extra corners, they may include sharp peaks or cusps and they may be composed of straight and curved sections in any combination. At this point we know that the shapes indicated in FIGS. 4D-4F are helpful. No doubt in the future other shapes will be found to be equally good and better, but in any event depend on the more controlled and gradual change of contact spot numbers and distribution per commutator bar. However, for success of this method, the lateral (or “cross”) resistance within the brush should be adequately high. For copper fiber brushes a controllable cross resistance can be imparted by very simply heating in air. Say, twenty minutes heating in air at 130° C. may yield a few ohms of cross resistance in a 1 cm 2 brush. This is also important for inhibiting circulatory/eddy currents that can waste energy in the presence of strong magnetic fields.
- the brushes may be subject to strong magnetic fields which act with the corresponding Lorentz forces on the current-carrying fine brush elements, i.e. the individual fibers or foils.
- Those magnetic fields vary with the mode of operation of the machine. i.e. are not simply constant, and they add to or subtract from the deliberately applied mechanical brush force.
- the electrical fiber brush resistance, R B is almost totally due to the unavoidable surface film that separates the two sides, with a specific resistance of ⁇ F .
- R B A B ( ⁇ F /K 2 ) ⁇ (E/p B ) 2 (d/r c ) 2 /(70 ⁇ f) ⁇ 1/3 (18)
- K 2 >1 is a factor not far from unity which takes account of “peripheral” electron tunneling about contact spots [5,6,14] which will be neglected.
- metal fiber brushes can be, in fact should be best operated at significantly higher brush pressures in combination with much lower ⁇ -values than accepted hitherto.
- Lorentz force variations will represent a percentage-wise smaller perturbation of brush pressures.
- Total brush losses may be decreased or alternatively higher currents and speeds be attained at same loss per ampere conducted.
- brush wear is virtually eliminated.
- FIGS. 1A-1E show examples of schematic cross-sectional views of different profiles of brush substrates and resulting contact spots.
- the bush substrates have surface irregularities, i.e., the surface of the brush substrate is not perfectly smooth. These surface irregularities may include asperities and/or grooves.
- the asperities and grooves may be regular, i.e., regular grooving or a regular pattern of asperities.
- FIG. 1A is an illustration of a simple sinusoidal grooving seen normal to sliding direction and at the same time clarifies the meaning of the parameters A, ⁇ and r c .
- FIG. 1A is an illustration of a simple sinusoidal grooving seen normal to sliding direction and at the same time clarifies the meaning of the parameters A, ⁇ and r c .
- FIG. 1B shows the same grooving but with flattened crests as would impart an increased asperity radius r c in the plane of the drawing to foil brushes when run with the foils parallel to the plane of the drawing.
- FIG. 1C shows the profile of a grooving, again seen normal to sliding direction, particularly suitable for foil brush operation, in which both the crests and troughs are flattened to the effect that both of the corresponding radii of curvature are increased.
- FIG. 1D is a grooving profile that would be suitable for the operation of metal fiber brushes whereby the fiber ends run in the troughs.
- the flattened shape of the troughs provides a correspondingly large r c value for the asperities at the fiber ends.
- FIG. 1E shows a grooving profile including overhangs. Over-hangs should be avoided, firstly because of their potential for catching wear debris which then can damage the brushes in the course of sliding and, secondly, because of their potential for wearing away fiber ends as indicated.
- FIGS. 2A and 2B clarify the means whereby grooving as in FIGS. 1A to 1 C, and by implication many others, can be produced. Specifically, FIG. 2A shows a schematic perspective view of a tool with a wave-shaped cutting edge for cutting a grooving profile into a substrate. FIG. 2B shows the tool of FIG. 1F in position during cutting the profile into the substrate which in this figure is rotated, e.g., in a lathe, as indicated by the arrows.
- FIG. 3A shows a fiber end encountering isolated flat asperities of closely similar elevation so as to form, in this case, four separate contact spots. Note that in FIGS. 3A and 3B the size of the contact spots is greatly exaggerated. In fact, the total contact spot area will typically amount to only fractions of one percent of the working surface. Also note that on FIG. 3B the two foils of a foil brush are seen sliding in an orientation mildly inclined against the foil normal, on a sinusoidal substrate grooving of the type of FIG. 1 A.
- FIGS. 4A-4F show schematic views of sections through different brushes, all except the foil brush in FIG. 4D comprising more than one type of fiber or foil, in position relative to substrate profilings adapted to them, except in FIG. 4E where the substrate is not shown.
- the sliding direction is normal to the plane of the drawing unless otherwise indicated by corresponding arrows labeled v.
- FIG. 4A is the cross-sectional view of the tips of two support fibers oriented normal to the average interface area that are guided in relatively deep grooves, and between them parallel regular fibers that run on an otherwise smooth substrate but studded with microscopically smooth isolated asperities. Equivalently, FIG.
- FIG. 4A can be read as showing the cut edges of two support foils running in grooves and thinner regular foils running on an otherwise smooths substrate that is studded with microscopically smooth asperities.
- FIG. 4B is a schematic view of part of the leading edge of the same foil brush depicted in FIG. 4A but in a cut through the foils parallel to and above the average surface level of the substrate.
- FIG. 4C shows the same type of brush as in FIG. 4A , i.e.
- FIG. 4D is a schematic cross-sectional view parallel to the substrate plane of the cross-section of a foil brush of lozenge-shape, running on commutator bars with separated asperities.
- FIG. 4E is a hybrid foil/fiber brush, i.e. parallel foils with fibers between them, whose cross-section and sliding direction is similarly shaped as in FIG.
- FIG. 4F is the cross-sectional view of a brush of rounded cross-section composed of support foils, regular foils and/or fibers, a zone of increased electrical resistivity at the trailing end, and “lightning rod” fibers concentrated near the trailing end of the brush, sliding along commutator bars which, together with the insulators between the bars, are profiled with grooves for the guidance of the support fibers.
- FIGS. 5A and 5B show examples of corrugations in foils of foil brushes so as to either increase their mechanical compliance via horizontal corrugations as in FIG. 5A or wavy slanted corrugations in FIG. 5 B.
- foil corrugations for the purpose of modifying the mechanical stiffness of foils.
- such corrugations should be parallel and coordinated among neighboring foils so as to reduce mutual friction between the foils that would tend to reduce brush compliance. Note that in line with FIG. 3B the foils could be arbitrarily inclined to the sliding direction.
- p B 7N/cm 2
- interference of uncontrolled Lorentz forces with brush force application is expected to be negligible.
- Profiling a brush substrate, whether slip ring, commutator bars or other, with a grooving is straightforward and unproblematic since it can be done with an appropriately shaped, tool in relative motion.
- This may take the form of turning the substrate material as the work piece in a lathe, as sketched in FIG. 2 B.
- the tool may machine just one groove at a time to be repositioned for making the next groove and on.
- the cutting edge of the tool will advantageously be shaped for the simultaneous cutting of multiple grooves as indicated in FIG. 1 G. Shaping of such tools may be done by any available means, e.g.
- the substrates be very carefully shaped into the desired cylindrical surface or other overall shape, so as to minimize run-out. This is important because the wear rate sharply increases with the run-out, i.e. variations of surface from the rotation axis per revolution. Such run-out should always be kept below 0.001 inches i.e. about 25 ⁇ m.
- the substrate includes a surface irregularities shaped and dimensioned to provide multiple contact spots to plural current conducting elements.
- the surface irregularities include asperities and/or grooves.
- the number of asperities per square centimeter i.e., the density (D) of asperities
- the contact spots on the current conducting elements are preferably on average less than 100 d apart, where d is the average diameter of the current conducting elements in the brushes (i.e. fibers or foils). Further, it is preferable to have between 1 and 10 contact spots per fiber [4-6].
- grooves are preferably dimensioned such that they provide contact spots for foils less than 100 d apart along the individual foil. Moreover, for the thickest foils, it is preferable to have relatively dense groove spacings. Thus, with 10 ⁇ m ⁇ d ⁇ 200 ⁇ m, groove spacings ( ⁇ , see FIG. 1 ) for foil brushes are preferably within the inclusive range of 10 ⁇ m to 1000 ⁇ m. The same values are suitable for fiber brushes.
- groove widths for guiding fibers, support fibers and lengthwise sliding foils are preferably moderately larger than the fiber or foil diameters, i.e. between 10 ⁇ m and 200 ⁇ m, assuming that support fibers, too, are not thicker than 200 ⁇ m.
- Groove depths (A, FIG. 1A ) preferably compare to, or are moderately larger than, the spacing ⁇ , i.e. again between 10 ⁇ m and 200 ⁇ m and for the thickest foils perhaps as large as 1 mm.
- a surface radius of curvature (r c ) of the surface irregularities are preferably related to d as 2 ⁇ r c /d ⁇ 10, or 2 d ⁇ r c 10 d.
- the surface radius of curvature are preferably within the inclusive range of 20 ⁇ m to 2 mm.
- r c is not restricted except that, on account of too rapid wear, surface radius of curvature's well below 10 ⁇ m are preferably avoided for substrates that are significantly harder than the brush material.
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Abstract
Description
ptrans=α3×10−4fH (1)
where α is the number of contact spots per fiber end (believed to be close to unity), f is the packing fraction already introduced (i.e., the fraction of brush volume occupied by fibers, the remainder being voidage), and H the hardness of the brush fiber material, which generally should be smaller than, or at most equal to, the hardness of the substrate. Next, the number of contact spots per unit area of fiber brush working surface is
n*=αf/(½πd2) (2)
with d the fiber diameter. Eq. 1 follows if the asperity radius of the contact spots is assumed to be
rc=d/2 (3)
which is a reasonable assumption and has so far been borne out by experimental evidence. In that case the average contact spot pressure is, in the case of elastic contact spots,
elpc(β)=β1/3H=0.004β1/3E (4)
assuming that Young's modulus is related to the hardness as (very approximately) E=H/0.004, and where β is the ratio of the actual brush pressure to the transition brush pressure, i.e.,
β=pB/ptrans (5)
Note that, remarkably, ptrans (eq. 1) is independent of fiber diameter. This arises because, as stated, the asperity radius has been assumed to be d/2 (eq.3) Correspondingly, for copper brushes with α=1, E=1.1×1011N/m2 and H=5×108N/m2,
cnptrans=2cmpseld=15f(N/cm2)≦4(N/cm2)≈6 lb/in2 (6)
With negligible brush body and construction resistances, then, in the realm of elastic contact spots the fiber brush resistance is found as
RB=σF/AC=σF(elpC/pB)/AB<σF(HfpB)/AB (7)
where σF (mostly σF=10−12 Ωm2) is the specific resistivity, i.e. the resistance of unit area, of the film separating the two sides at the contact spots, as already introduced above. Or, including the above assumptions on contact spot number and asperity curvature,
RBAB.0.034[mΩcm2]/(fβ2c) (8)
el=1.1(rcP/nE)1/3 (9)
Hence the average pressure over the area of the average elastic contact spot is, with H=0.004E and n* the contact spot density i.e. number of contact spots per unit area of interface,
elpc=P1/3E2/3(n1/3×1.12rc 2/3)=0.26(pBE2/n*rc 2)1/2≈10.44(pBH2/n*rc 2)1/3 (10)
ptrans=9=10−4Hn*rc 2=3.5×10−4En*rc 2 (11)
The desirable elastic contact spots which give low wear rates are therefore favored by large asperity radii of curvature as well as high contact spot densities.
-
- Case 1: Two mated absolutely flat surfaces would ideally yield perfect mechanical contact over the whole macroscopic area, at a uniform local pressure equal to the macroscopically applied pressure. This ideal situation, in which electrical contact resistance would be essentially eliminated barring insulating surface films, is intrinsically unstable. Namely, any accidental local pressure increase will concentrate friction heat and cause local thermal expansion. The ensuing “thermal mounding”, if significant, relieves the pressure in the vicinity of the evolving mound which in turn increases the local pressure thereby more strongly relieving the pressure in its vicinity on to local separation of the surfaces about the mound. Thereby the thermal mound has been converted into a contact spot which further concentrates not only friction heat but now also the current and, hence, local Joule heat generation. The end result of this cycle of instability due to self-excited thermal mounding is a few large contact spots, and in the extreme limit just one large contact spot (compare ref. 20). Still, this instability becomes important only at significant local heating. More typically, in sliding contacts the number of contact spots ranges about ten.
- Case 2: A soft brush material sliding across a hard substrate studded with densely spaced, sharply peaked asperities. In this case the friction coefficient as well as wear are liable to be unacceptably large since the spikes would essentially plow through the brush material, admittedly at negligible electrical contact resistance but probably high friction and definitely high wear rate.
- Case 3: A relatively hard substrate material shaped with intermediate asperity radii in accordance with the present invention and mated with a softer or at most equally hard brush material, will form a number of contact spots equal to the number of asperities within the interfacial area, provided the spots are not too closely spaced so as to merge and cause thermal mounding. The essential characteristic is therefore that under elastic loading, isolated contact spots will form that are separated by regions not in load-bearing contact. This requires a sufficient depth of the profiling. At the same time, and as already explained, as large asperity radii as compatible with the highest number of separate contact spots and still adequate local pressure is desirable since at given macroscopic brush pressure this increases ptrans. In the case of grooving in conjunction with foil and fiber brushes, the spacing of the contact spots in sliding direction is self-limiting to the foil diameter and to the fiber diameter, respectively, regardless of the shape of the substrate. Correspondingly the contact spots will not have radial symmetry but exhibit different asperity radii in different direction, and the above considerations apply only to the lateral profile of the grooving. In either case, given the contact spot density and average radius of curvature, the equations previously derived for fiber brushes apply appropriately since they depend only on the density and asperity radius of curvature [4-6]. Hence for β below unity the local pressure at the contact spots will fall below the (Meyer) impression hardness of the softer side, the more so the larger the asperity radius. The estimate for the optimal asperity radius as well as average asperity spacing as generally comparable with the foil diameter, and similarly for the depth and spacing of grooves, is thereby justified, but detailed experiments will be needed to locate the optimum conditions from case to case. Herein the shape of isolated asperities is of minor importance except that elongated contact spots will be more resistant against “aerodynamical lift” or “water planing” and therefore desirable.
- Case 4: Elastically flattened contact spots. As to the limits on cross-sectional shapes of the profiles, begin by considering a prismatic sinusoidal shape, i.e.
u=A sin(2x/λ)x. (12)
as in FIG. 1A. On a substrate of Young's modulus E, that profile will be flattened by pressure [21]
PA=xEA/λ (13)
For the choice of λ=A=df, the permissible pressure before contact spots centered on the crests of a sine-wave profile would merge is therefore, with a hardness of H=0.004 E in accordance with eq.4,
PA,A=λ=πE=(x/0.004)H=800H (14)
i.e. very much larger than any conceivable local contact spot pressure. And even less liable to be flattened would be a profile with the same period and amplitude but crests that are flatter than according to a sine function. This modification of the profile, indicated inFIG. 1B , is desirable because the curvature at the crests of a sinusoidal profile, which determines the asperity curvature rc is
(d2u/dx2)x=λμ=A(2x/λ)2 (15a)
yielding a local radius of curvature for λ=A=d (or df, respectively), of
rcA=λ2/2πA=0.16 d (15b)
This is significantly smaller than the desired radius of curvature, namely rcA>=0.5 d in accordance with eq.3. It follows that the profile of grooves (and by interference of isolated asperities) should be flattened at the crests compared to a sine function, as indicated in FIG. 1B.
-
- (i) In order to minimize wear (and presumably also the coefficient of friction), the surface finish ought to be as microscopically smooth as possible, as was already mentioned above. Such microscopically smooth finishes prevent microscopic interlocking at contact spots which generates wear particles. An obvious method for achieving the desired high smoothness on a microscopic scale is mechanical polishing, such as buffing with a soft textile cloth, felt or plush, or with chamois leather, most typically in conjunction with some polishing agent, e.g. alumina or diamond powder. Alternatively, electrolytic polishing may be used. However, any such smoothing may give rise to nano-sized unseen layers of high electrical resistance. If this occurs these have to be removed before use, e.g. through electrolysis or through annealing in a protective or reducing atmosphere such as argon, CO2 or hydrogen, with or without moisture addition, as practical experience will indicate.
- (ii) While low wear rates of the brushes is highly desirable, in practice virtually no wear of the substrate can be tolerated. This is a well-known requirement for the ordinary substrates (i.e. not deliberately profiled slip rings and commutator bars in electric machinery) but it is even more essential for profiled substrates in accordance with the invention; otherwise their profiling, i.e. groovings and/or isolated asperities, that will typically be only up to a few tens of micrometers high, will be worn off. This means that the profiled surfaces have either to be made of an intrinsically harder material than the low-concentration copper alloys normally used for slip rings and commutator bars, e.g. of stainless steel or nickel or brass or titanium or other suitable metal, or that the surface be plated with a thin hard layer, or both. A diamond-like coating has already been developed for this purpose under sponsorship of the Annapolis Navy laboratory. However, recent tests on this coating in our laboratory have been very disappointing, partly for the reason that the diamond-like coating is composed of small particles which can be readily dislodged, and partly because of a much too high intrinsic electrical resistivity. What is needed, instead, is a locally very smooth, hard surface finish of a conductive material. In accordance with the present invention, the desired surface finish can be made of TiN (titanium nitride) or related metal nitrides (e.g. zirconium nitride or chromium nitride) which are characterized by their bright metallic gold luster. These are increasingly used on cutting tools, such as drill bits, which testifies to their great hardness as well as tenacity under severe wear conditions, both of these essential properties for the intended use on electrical brush substrates. Note here that the metallic luster of these platings is due to conduction electrons and thus is a token of their intrinsic high electric conductivity. By contrast, diamond-like coatings, while vary hard, tend to be transparent since they lack conduction electrons. Correspondingly diamond coatings, even if heavily implanted with charge carriers, tend to have poor or no intrinsic electrical conductivity, thereby making them unsuitable on substrates for electrical brushes. However, preliminary experiments have revealed a tendency of TiN (and by inference similar coatings) to form an invisible oxidized surface layer of an unacceptably high electrical resistivity that preclude their use in the open atmosphere. It is expected that such surface layer formation can be prevented by the use of a reducing protective atmosphere, such as hydrogen.
-
- (iii) Corrosion and oxidation resistance is desired so as to be able to operate the brush-substrate combination in the open atmosphere. The discussed metal-nitride platings are produced at elevated temperatures (typically at and above 500° C.), and exhibit the desired high chemical stability in addition to hardness and electrical conductivity. It is for this reason that bathroom fixtures plated with metal nitride have been nationally advertised with a life-time guarantee against tarnishing and corrosion, specifically by the Moen Company. Even so, the already discussed invisible insulating surface films on TiN and similarly other corrosion resistant hard materials such as stainless steel, chromium and nickel, preclude the successful use of such coatings in the open atmosphere, as already indicated.
3.6°[m/sec]/Ψ°≦vmax≦89.4°[m/sec]/Ψ° (16)
with vmax in meters per second, depending on numerical assumptions made. Thus for a desired speed of vmax=150 m/sec, as would be applicable to future maglev (magnetically levitated) trains, the maximum slope of the substrate in sliding direction which still permits full tracking with elastic fibers and contact spots, lies between 0.024° and 0.60°. Evidently, then, the opportunities for preformed isolated asperities which do not form multiple contact spots per fiber end as in
Ψ°≈45°/vmax (17)
with again vmax in [m/s], a 10° slope cannot be tracked above 4.5 m/sec.
RBAB=(σF/K2){(E/pB)2(d/rc)2/(70αf)}1/3 (18)
were K2>1 is a factor not far from unity which takes account of “peripheral” electron tunneling about contact spots [5,6,14] which will be neglected. By the use of a more accurate expression for ptrans than eq.1, extracted from ref. 6, this may be rewritten
RBAB=870 σF(d/rc)2/(β2/3αf) (19)
Furthermore, β=½ is accepted as an upper limit compatible with long wear life.
RBAB/(RBAB)n=(σF/10−12Ωm2)(d/rc)2(1/2βselect)2/3/4=0.157(σF/10−12Ωm2)(d/rc)2(1/βselect)2/3 (20)
or with eq.4
RBAB/(RBAB)n=0.157(σF/10−12Ωm2)(d/rc)2(H/el Pselect)2=2.52×10−6(σF/10−12Ωm2)(d/rc)2(E/el Pselect)2 (21)
PB=βselectptrans=βselect4.6×10−4αEf(rc/d)2=βselect6.9[N/cm2](rcd)2 (22)
And similarly the specific brush resistance correlated with elpselect is found from eq.21. By controlling rc/d i.e. the microscopic smoothness of the substrate, one is therefore able to choose, for any desired local contact spot pressure, i.e. βselect, the correlated brush pressure and brush resistance.
βselect=(elpselect/0.004E)2=(1.5×108/0.004×1011)3=0.05 (23a)
or similarly using the correlation with the hardness of H=5×108N/m2 for copper,
βselect=(elpselect/H)3=(1.5×108/5×108)3=0.027 (23b)
RBAB/(RBAB)n=1.57(d/rc)2βselect −2/3 (24)
so that it falls below the standard case for rc/d above =4, whereas the relative friction loss,
LM/LM=μpB/(μstpBst)=0.02pB/(0.3×1.5N/cm2)=0.044pB=0.3βselect(rc/d)2[N/cm2] (25)
rises above unity only for rc/d>10 or so. It follows, then, that in this particular example above rc/d about 4 and up to rc/d at least 10 conditions are excellent for not only counteracting Lorentz force variations but, beyond this, to lower the combined electrical and friction brush losses. In general, already rc/d values of 2 and above will cause marked improvement in brush performance.
-
- (i) In this day and age of micro-chips, doubtlessly many related methods exist which could be utilized for the present purpose, mostly probably based on a combination of lithography and etching. These are liable to be the most cost-effective in the long run since they are adaptable to automation by the use of methods which have long since been developed by the computer industry and might be similarly utilized for grooving as well as formation of separate asperities. Insulating or high-resistance surface layers, which may remain on the substrate after completion of the profiling, will have to be removed as a last step, as already indicated for the case of electrolytic polishing or buffing.
- (ii) Laser cutting is another method. It is expected to be adaptable to a wide range of shapes but probably to be fairly expensive.
- (iii) More traditionally, one could make the desired asperity-covered surfaces by spraying an aerosol of liquid metal, e.g. copper or nickel or chromium etc., on the heated, pre-shaped substrate. In this, one will have to experimentally determine suitable droplet sizes, spraying velocities and temperatures to achieve the desired asperity size, shape and density. By spraying the liquid metal aerosol vertically onto the substrate, roughly rotationally symmetrical asperities will be obtained, while spraying at an angle will cause elongated asperities.
- (iv) Separated asperities, and in particular asperities strongly elongated in sliding direction, could be readily formed on substrates by a totally different method as follows: Through suitable casting of suitable alloys, followed by mechanical working such as rolling or drawing and/or heat treatments as may be suitable, one may produce harder precipitates or eutectic lamellae of desired form, size and density dispersed in a softer matrix. Then, after careful overall shaping through some cutting process, e.g. turning on a lathe, grinding, milling etc., one may produce a metallographic “relief” polish which lets the precipitates project above the average surface to the desired height so as to form the asperities. This metallographical relief polish can be done either mechanically, i.e. by buffing or polishing on a soft textile material such as cloth or felt, or a real or artificial chamois leather already mentioned, typically with the aid of fine alumina or diamond powder. Alternatively it could be done by electrolytic polishing. And, again, if a remnant insulating layer should remain after the polish it must be removed, e.g. through electrochemistry, mild etching or annealing in an inert or reducing atmosphere, as already indicated above.
- [1] D. Kuhlmann-Wilsdorf, D. D. Makel and G. T. Gillies, “Continuous Metal Fiber Brushes”, U.S. patent application, . . . .
- [2] R. Holm. “Electrical Contacts—Theory and Applications” 4th edition (Springer Berlin/N.Y., 1967).
- [3] P. B. Haney, D. Kuhlmann-Wilsdorf and H. G. F. Wilsdorf, “Production and Performance of Metal Foil Brushes”, WEAR, 73 (1981), pp. 261-282
- [4] D. Kuhlmann-Wilsdorf, “Uses of Theory in the Design of Sliding Electrical Contacts”, ICEC-IEEE Holm 91 (37th. Holm Conference on Electrical Contacts, IEEE, Chicago, Oct. 6-9, 1991), pp. 1-24.
- [5] D. Kuhlmann-Wilsdorf, “Electrical Fiber Brushes—Theory and Observations”, ICEC-IEEE Holm 95 (41st. Holm Conference on Electrical Contacts, IEEE, Montreal, Canada, Oct. 2-4, 1995), pp. 295-314.
- [6] D. Kuhlmann-Wilsdorf, “Metal Fiber Brushes” (
Chapter 20, pages 943-1017, in “Electrical Contacts: Principles and Applications”, Eds. Slade/Lee/Witter/Horn/Shobert, Marcel Dekker, N.Y.), 1999. - [7] Y.J. Chang and D. Kuhlmann-Wilsdorf, “A Case of Wear Particle Formation Through Shearing-Off at Contact Spots Interlocked Through Micro-Roughness in ‘Adhesive’ Wear”, Wear 120 (1987), pp. 175-197.
- [8] Yu Jun Chang and Doris Kuhlmann-Wilsdorf, “Comparison of Wear Chip Morphology with Different Models of ‘Adhesive’ Wear”, in “Approaches to Modeling of Friction and Wear”, (Eds. F. F. Ling and C. H. T. Pan, Springer, New York 1988), pp. 118-124.
- [9] J. L. Young, “Mixing of Material During Sliding With and Without Lubricants”, M. S. Thesis, Dept. of Materials Science and Engineering, Univ. of Virginia, October 1998.
- [10] R. McNab and P. Reichner, “Environment and Brushes for High-Current Rotating Electrical Machinery” U.S. Pat. No. 4,227,708, July 1981.
- [11] P. Reichner, “Metallic Brushes for Extreme High Current Applications”, Electrical Contacts—1979. 25th Holm Conf., Chicago, Ill. 1980, pp.191-197.
- [12] P. Reichner, “High Current Tests of Metal Fiber Brushes”, Electrical Contacts—1980, 26th Holm Conf. Chicago, Ill., 1980, pp.73-76.
- [13] C. M. Adkins III and D. Kuhlmann-Wilsdorf, “Development of High-Performance Metal Fiber Brushes I—Background and Manufacture”, in Electrical Contacts—1979 (Proc.25th Holm Conf. on Electrical Contacts, Ill. Inst. Techn., Chicago, Ill., September 1979), pp. 165-170.
- [14] C. M. Adkins III and D. Kuhlmann-Wilsdorf, “Development of High-Performance Metal Fiber Brushes II—Testing and Properties”, ibid, pp. 171-184.
- [15] C. M. Adkins III and D. Kuhlmann-Wilsdorf, “Development of High-Performance Metal Fiber Brushes III—Further Tests and Theoretical Evaluation”, in “Electrical Contacts—1980” (Proc. 26th Holm Conf. on Electrical Contacts, Ill. Inst. Techn., Chicago, Ill., September/October 1980), pp. 67-72.
- [16] D. Kuhlmann-Wilsdorf, “What Role for Contact Spots and Dislocations in Friction and Wear?”, D. Kuhlmann-Wilsdorf, WEAR, 200 (1996), pp. 8-29.
- [17] D. Kuhlmann-Wilsdorf, “A Versatile Electrical Fiber Brush and Method of Making”, U.S. Pat. No. 4,358,699 granted Nov. 9, 1982.
- [18] C. Gao and D. Kuhlmann-Wilsdorf, “Adsorption Films, Humidity, Stick-Slip and Resistance of Sliding Contacts”, ICEC—IEEE Holm 90 (1990 Holm Conference on Electrical Contacts, IEEE, Montreal, Aug. 20-24, 1990, IEEE, Piscataway, N.J. 1990), pp. 292-300.
- [19] C. Gao and D. Kuhlmann-Wilsdorf, “Experiments on, and a Two-Component Model for, the Behavior of Water Nano-Films on Metals”, in “Thin Films: Stresses and Mechanical Properties II”, (Mater. Res. Soc. Symp Proc., 188, Eds. M. F. Doerner, W. C. Oliver, G. M. Pharr and F. R. Brotzen, Mater. Res. Soc., Pittsburgh, Pa., 1990), pp. 237-242.
- [20] R. A. Burton (editor): “Thermal Deformation in Frictionally Heated Systems”, (Elsevier Sequoia, Lausanne/N.Y., 1980).
- [21] R. A. Burton, Wear, Vol.59, 1980, p.1.
- [22] D. Kuhlmann-Wilsdorf, D. D. Makel, N. A. Sondergaard and D. W. Maribo, “On the Two Modes of Operation of Monolithic Ag-C Brushes”, Electrical Contacts-1988, reprinted in IEEE Trans. Comp. Hybrids and Manuf. Techn., 12 (1989) pp. 237-245.
- [23] C. Gao and D. Kuhlmann-Wilsdorf, “Observations on the Effect of Surface Morphology on Friction and Sliding Modes”, in “Tribology of Composite Materials”, (Eds. P. K. Rohatgi, P. J. Blau and C. S. Yust, ASM Intl., Materials Park, Ohio 1990, pp. 195-201.
- [24] D. Kuhlmann-Wilsdorf and D. Alley, “Commutation with Metal Fiber Brushes”, Proc. 1987 Intl. Current Collector Conf., Nov. 16/17, Austin, Tex. (Ed. J. H. Gully.
- [25] D. Kuhlmann-Wilsdorf and D. M. Alley, IEEE Trans. Components, Hybrids and Manuf. Techn., 12 (1989), pp. 246-253.
- [26] D. Kuhlmann-Wilsdorf, D. D. Makel, N. A. Sondergaard and D. W. Maribo, “Interfacial Temperatures and Transition of Surface Films on Monolithic Silver-Graphite Brushes Sliding on Copper”, Electric Contacts, (Proc. 14th International Conf. on Electric Contacts, Paris, France, Jun. 20-24, 1988, SEE), pp. 47-53.
- [27]D. Kuhlmann-Wilsdorf, D. D. Makel, N. A. Sondergaard and D. W. Maribo, “Friction, Wear and Interfacial Temperatures in Metal-Graphite Composites Cast Reinforced Metal Composites (Eds. S. G. Fishman and A. K. Dhingra, ASM International, Metals Park, Ohio, 1988), pp. 347-359.
- D. Kuhlmann-Wilsdorf and D. D. Makel, “Microstructural Instability in Metal-Graphite Lubrication Films”, in “Metastable Microstructures: Principles, Design and Applications” (Eds. D. Banerjee and L. A. Jacobson, Vedama Books International, new Delhi, 1992; Proc. U.S.-India Workshop on Metastable Microstructures, Goa, India, Mar. 30, 1998-Apr. 2, 1989) pp.257-274.
Claims (77)
2≦rc/d≦10,
2≦rc/d≦10,
2500/cm2≦D≦107/cm2,
10 μm≦λ≦1000 μm.
20 μm≦rc≦2 mm.
20 μm≦rc≦2 mm.
10 μm≦(λ)≦1000 μm.
20 μm≦rc≦2 mm.
2500/cm2≦D≦107/cm2,
20 μm≦rc≦2 mm.
10 μm≦(λ)≦1000 μm.
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EP3287857B1 (en) * | 2016-08-26 | 2019-04-03 | The Swatch Group Research and Development Ltd. | Method for obtaining a zirconia item having a metallic appearance |
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Also Published As
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US20020060506A1 (en) | 2002-05-23 |
US6753635B2 (en) | 2004-06-22 |
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