CA2325683A1 - Improvement in wrapped ring technology for manufacture of large diameter slip ring bases - Google Patents

Abstract

In a wrapped ring manufacturing process for slip ring assemblies, the assembly is heated to expand a gap between ends of the slip ring. A conductive material is then used to fill the expanded gap, to assure compression of the conductive material and to eliminate cracks therein. The filled gap is electroplated with a precious metal to form a layer of material of uniform hardness, wear resistance, electrical conductivity and environmental resistance around the entire perimeter of the slip ring.

Description

r Title: IMPROVEMENT 1N WRAPPED RING TECHNOLOGY FOR
MANUFACTURE OF LARGE DIAMETER SLIP RING
BASES
Inventors: Glenn B. Lawson, Stephen R. Cole, Anthony L. Bowman and Michael J. Day.
Technical Field This invention relates generally to methods of manufacturing electrical slip ring assemblies. More particularly, the inventive method relates to construction of a slip ring assembly base having a plurality of conductive rings tlzeroabewt, arid sssoso opoosfion113. to oa isaprevosaeat is a method for ouoh manufacture known in the art as wrapped ring tCChnOlOgy_ B SClC~l011aG1. Alt Electrical slip rings are well known in the art for communicating elecuical signals becweea srrucnual members which are rotatable relarive to each other.
A useful method of manufacturing large diameter electrical slip ring bases, cvmu~ztoztly referred to as wrapped ring technology, is disclosed in Bowman et al. US patent 5,054,189, the contents of which are hereby incorporated by rcfennce. As described therein, a slip ring assembly may include an annular base mrmbcr having a plurality of conductive rings surmunding as outer circumfercntial face thereof, and a series of electrically conductive brushes arranged on another structural rncmber, wherein one of the annular base mEmbcr and the structural member is relatively rotatable about the other. In such a configuration, the conductive rings are contacted by the conductive brushes t~o form a plurality of electrical connections between the two structural members.
As disclosed in the prior patent, a method of manufacture of the slip ring base may include forming a series of annular grooves in the circumfercntial outer face of an angular base, for receiving rings of electrically conductive material. Linear conductive material is cut to lengths, forming stilpt~ itch ur w~i~l~ ~uLWwuci~ally uVlieo~rvuds to tlao circauufcrcnco oftlae oasbcr face of the annular base, to form a plurality of electrically conductive rings.
First ends of respective rugs are anchored at respective points on respective grooves, sad the respective rings are pressed into engagement within the respective grooves (e.g., by progressively exerting s rolling pressure along the lengths of the rings around the circumference of the outer face). The other ends of the respective rings are secured to the slip ring base. In one application of the wrapped ring technology, the conductive strips are pre plated with a conductive material, such as silver.
One advantage of the wrapped ring technology over known solid ring technologies is that, in the wrapped ring technology, inexpensive strip-form xnaterisl can be used, instead of the more expensive solid rings. .
Hovvover, while slip ring assemblies manufactured by the vurapped ring toolmssol,ogy ~dreoribmd abrsr. sars. "enW 1, cnrh peepmhlipc emf~Pr FrflTn T1~P
followiz~g def cien.cxes.
ha. fbe known art, thv accumulation ofmanufacturiug tolerances results in an inevitable gap between the ends of the conductive slip rings.
A small gap between the ring ands is, indeed, generally desirable to assure that the ring ends do not contact during the rolling operation and prevent the ring from properly sitting in the groove. Howcwer, the gap resultitlg from the lmown manufacfizring process is a physical discontinuity having technical conscqucnccs oa durability of the brushes, on the quality of electrical connection between the conductive rings and the brushes, and on transmission of signals thcrcbctwcrn.
A problem arising from the c~dstcace of the gap is the accumulation therein of wear debris, whether from the brushes, rings, or other materials.
Once su~cicnt wear debris accuxnulatos in the gap between the ring ends, such debris can cause an increase in electrical resistance between the brush and the slip ring. Increased contact resistance may occur when the debris is dislodged .
onto the ring coratact surface, for example. Increased eontaot resistance may introduce electrical noise into signals transmitted between the slip rings and the biushcs, or may attenuate signal transmission therebetween, Ia one embodiment of a wrapped slip ring descn'bed at Fig. l l in Hirao et al. US patent 5,224,138, beginning and end portions of a metal tape 264.
forming a slip ring are coupled by solder 272. While such an approach may appear to overcome the above described technical problems, in fact the enlnt~inn rrP.s~tPa ~n s~~i~iitinns~l trnht~tins~i rrc~hlem ref Snlder SmBaritlB.
Solder used in the manner shown in the pzioz azi will s~orxeaz outo the adjacent ring surface and, as solder is not a precious metal, with oxitdize with undesirable ehaxaetezitstxes_ An aecuz~ttulation ofthe smeared oxides will cause increased elec~ri.cal noise, si.g~oaficantly degrading the ability of the ring to a function with low voltage, low cun~nt signals, While accumulation of oxide on a slip riug supplying power to CT scanner X-ray tubes may be acceptable, such accumulation is unaeeeptabie for slip rings operating at low level signai voltages.
Thus, use of solder between ring ends is as unacceptable solution which replaces one noise problem in signal ttaasznission by another, similar, problem.
Moreover, solder will wear at a faster rate than the silver. Accordingly, after extended operation, a rut will devdop in the solder.
The potential for these or other possible technical problems is one of the chief drawbacks to the wrapped ring technology relative to solid ring technologies, However, as noted above, the wrapped ring technology has err economic advantage over solid ring technologies. This economic advantage becomes more pronounced as the diameter of the device increases.
There is thus a need in the prior. art for an improved method of manufacture of slip ring assemblies which maintains the economic advantage of wrapped ring technologies over solid ring technologies, but which overcomes the known deficiencies of lanown wrapped ring technologies.
Disclosure of Invention It is accordingly an ObJect of the prc~c~rt ,ialvo,uUi,u4~t w uvcmuum tho deficiencies of the prior ar~_ _ More specifically, it is an object Qf the invention to provide an improved method for manufacture ot'slzp zing asscmblios, which maintains the econormc advantage of wrappca ring tcchnolugy uvrs ~vli~l ruy eGUlmulvgy, while overcoming the disadvantages of known wrapped ring technologies.

_ It is s particular object ofthe invention to provide an improved wrapped ring technology for manufacture of slip ring assemblies.
It its a more specific object of the inventlon to provide as improved me't'hod for 'maxxu~a,cttu~c of slip ring bases overcoming prior art problems az~isiuo~g from existence of gaps between ends of slip rings.
It is still a more particular object of the invention to provide an improved method for manufachu~c of a slip ring base by filling a gap between ends of slip rings with a solid conductive mai~cxial.
It is yet a more specific object of the invention to provide an improved method for manufacture of a slip ring bast, whera~n the assembly is heated to a tentperatuze dhosen in order to reduce tensile forces acting a filler material added to a gap between ends of the ring.
It is still a ~oo~ore specific obj ect of the invention to provide an improved method for manufacfiire of a slip ring base, wherein the assembly is heated to a iempccaturc Wuscu iu uW w trivvial~ wLUpr~osaiYO fvscca oa a fillor iaatorinl added to a gap bMween cads of the ring.
It is another object ofthe invention to provide an improved method for manufacture of a slip ring base, wherein a filler material is added to the gap be~wor,~r llLC ClitL'i of a ai~~br awl day auafswco urtla.:.:a.~e, w_'3 tla~, gap arc plotxid with a precious metal.
It is yet another object ofthe invention to provide an improved method for manufacture of a slip ring base wherein a mismatch in thermal expansion characteristics of the ring material and base material is ~reduced_ It is still a further object of the iuavendon to provide an improved method far msaufacture of a slip riztg base having slip rings plated with a E

hc"i;er,~lpr rnn~inrtnr, whprp!itt ~ fillCT rOlatC~91 1.11C1Ud~g a DOIY~,eIAC
1'eSin.
filled either with the same particular conductor or with a different conductor is added to the gap between the ends of a ring.
It is still another obj ect of the invention to proride an improved method for znanufacturc of s slip ring base having slip rings plated with a precious metal, wherein a filler material including a polymeric resin filled with the same precious metal is added to the gap between the ends of a ring.
Zt is yet a further obj ect of the invention to provide an improved method for ~aaauufactun of a slip ring base having plated slip rings wherein a filler m.atezial is added to the gap between the ends of a ring and the filled gap is over plated with the same material as the slip rings.
It should be appreciated, how~.wcr, that while the disclosure refers specifically tv addition to a gap of specific filler materials including epoxy resin and a noble metal, in the form of silver powder for example, the method desczl'bed heroin may be implemented with any materials now known ~ or hezeinafter davclapcd which conform to the invcativc concept and are suitable for the purposes herein set forth.
In accordance with the invention, there is thus generally provided a method of asaaufscturing a slip ring, including various combinations of steps such as wrapping conductive strips onto an insulating base, heating the assembly to a predetermined temperature thereby to expand the gap between tizo oad~ of the oondztetivo ottip gad, whih ~~ $ap is aalarged, filling the gpr with a rigid conductive filler material. The above combination of steps znay also include curing of the gap filler, to the extent necessary.
In accordance with a specific feature of the invemtiorr, in the heating step the assembly is heated to a temperature determined in accordance with a maximum temperature to which the slip ring will be exposed is its subsequent utilization.
In accordance with one aspect of the invention, in the heating step the assembly is heated to a temperature equal to or above a maximum texnperatuze to which the slip ring will be exposed in its subsequent uulizatiion_ 'Ja aocordaace with another aspect ofthe invention, in the heating step the assembly is heated to a temperature deterrniaed in accordance with a relation between the amount of mismatch between the thermal expansion characteristics of the ring material and the base material, on the one ha.~ad, and the maximum temperature to which the slip ring will be exposed in its subsequent utilization on the other hand.
Ia accordance with still another aspect of the invention, thcxc is also implemented a step of electroplating over the filled gap with precious rrxctal to form a layer of material of uniform hardness, wear resistance, electrical conductivity, and environmental resistance around the entira zipg pezizneter.
These and other objects, features and advantages of tJae pz~esent invention will become readily apparent to those skilled in the axt from the following descriptive and drawings, wherein there is shown and desc~bed a preferred embodiment of the invention, simply by way of illustration and not of limitation of one of the beet modes (and altcmatlve ernbofiments) suited to oasry out the invention. The invention itself is set forth iut the clai~zos appended hereto. As will be realized upon examination of the specification and dravrings and from praetlce of the same, the present invcnrion is capable of still other, different, embodiments and its several details arc capable of modifications in -'-" "' - CA 02325683 2000-11-10 various obvious aspects, all without departing from the scope ofthe invention as recited is the claims. Accordingly, the drawings and the descriptions provided herein are to be regarded as illust~'at~ive in nature and not as rest<ictivc of the invention.
BRIBF DESCR1PTYON OF THL DR.AWJNG
The accompanying dsswings, incoxpvratod into and forming a part of the specification, illustrate several aspects of a prcfcrrcd embodiment of the present invention and, together with the description, strut to explain the principles of the invention. Ia the drawings:
FIG. i is a sectional view of a la~.own annular slip ring base and an apparatus for manufacture thereof in accoz~dauace with the prior arty and FIG. 2 is a sectional view taken along the line 2-2 ofFig. I illustrating an annular slip ring base msaufactuxed in. accordance with an embodiment of the inventive concept_ Best Mode for CarNinl~ Out the Invention In order better to appreciate the present impxovemcnt, the following provides a description of a known method of manu~acturc of annular base members for slip ring assemblies, in aceoxdancc with the lmown wrapped ring technology.
Referring now to the dzawings, in Figure 1 there is shown a sectional view of a known annular base mepaber 10 for an electrical slip ring assembly having a plurality of conductive rings placed thcrcaround to carry both signal voltages and opcrarlng power. A sCriCa of mha,livcly wialm circuinfcszaltinl grooves 12 are provided in a portion of the outer circumferential surFace 14 of base member 10. A series of relatively narrower circumferential grooves 16 may be machined in another portion of surface 14. As known in the art, the wr~'~ sr,~;~iuu~sl ~Lares of gcomves 12 sued 16 xxUay be selected to sub3taatiahy conform to the shape of conducdve strips tai be mounted therein and the $bcr brushes ro be used in connection with those strips.
Ia Fig_ 1, only one groove, designated by reference numeral 12a, is shown as having a conductive strip partially placed therein.
As lanown in the art, the ends of conductive strips placed in the grooves may be anchored by pins or bolts, which may be accessed from the inner circumference surface 15 of the annular base member through a series of openings, one of which xnay be provided for tech groove.
Individual lengths of conductive znatcrial form canductivc strips to be pressed into grooves i2. One such snip is shown at 20. Each conductive strip may be formed from a roll of conductive material such as brass or copper.
As seen in Fig_ 1, snap 20 ~oaay have a groove 21 formed in the outer su~aee thereof the groove 21 adapted to receive one of the seines of brushes to be mounted on a rotating portion of the oloctrical slip ring assembly (not shown). The groove 21 may be pressed into the material by passing the material through a conventional rolling machine. The material may them be cut to lcngtb. and mounting studs may be braacd to the cads of the ring. As known in the art, the rin~a formed by strips 20 may be plated with a Isycr of a conductive material. Silver is a known plating material used in the art.
In accordance with the known method, each conductive strip or ring, such as strip 20, is first fastened at one end thereof to one of a plurality of openings (not shown) provided in the gmvves 12 ofthe annular base member.
Coaduotive strip 30 i.E then presr fit into grnnvn 17a by mcar~s of rwllittg pressure exerted by fixture 30. Fixtuz'c 30 includes a pressure roller member having an outer circumfcrcnfiial surface 33 shaped similarly to groove 21.
Tlxe Suctiire fiuthcr includes a franc 34 having vertical support members 36 and 38 joined to horizontal support members 40 and 42. The vertical mcmbot~ have vertically eloagatrd apcain~ fnrrnPr~ therein (not shown). fOr receiving an adjustable arm 4~ whioh rotatably carries roller member 32 in a bifurcated end thereof, shown at 45. Arm 44 may be threadedly engaged with vertical member 3 B to provide an adjustable lateral positioning of roller member 32, while the elongated shapes of the openings formed is vertical members 36 sad 38 provide for proper vertical positioning of the pz~essure roller member. A support roller 46 may be mtatably mounted is a bifurcated end 49 of a second adjustable arm 48, which also Gxteads through the openings ire vertical members 36 and 3 8. Arm 48 may be throadedly adjusted to provide proper lateral support on the exterior circuznferential surface and to ensure that roller member 32 is carved in ~s trot Lorizoatsl position.
A pair of cylindrical support roller provide proper positioning lateral pressure on the interior circutnferentiat surface of annular base member 15_ One such roller is shown at SOa, in front of hari2ontal support me~aaber 40. A
second such roller may be provided behind support membez 40_ Thus, the rollers arc mtatably mounted to an outer'"T" shaped end 41 of support member 40. Support member 40 also has laterally attached thereto a pain of vertical support rc~llc~ t~ facilitate motion of the entire fixture 30 about the circumference of annular base rnembcr 10. One such vrcrtical support roper is shown at 52a. positioned in front of support member 40. A second such roller tray be provided behind support merrtbcr 40.
In forming each ofthe conductive rings 20 into its respective groove I2 a, one end is attached to the annular base zncmber 10. At that point, fixture is placed in engagement with sauular bast member 10 as shown in Fig. 1.
Roller member 32 is adjusted to exert sufficient lateral pressure on strip 20 as to firmly press fit the strip into groove 12a. The entire feature is then rotated about tho eixeurnferenee of annuli base m~:a~nl~.rr A ~;gn~f~~ant c:ircumfer~rc~tial force i.s exerted by the motion of roller member 32 so that the conductive strip aoo,ay be slightly elongated in order fio provide substantial abutment betvsreen the two ends of strip 20 when the entirety of strip 20 is in the groove 12a, At this point, as lmown in the art the second end of the strip may be fastened to the atwular base member I O through the opening therein provided in the groove.
Referring taow to Fig. Z, the presently prefom~d embodiment of the z~eventi.on addresses the above noted difficulties which may arise when a gap e~osts between the two ends of the strip 20. In order better to illustrate the advantages arisia,g from use of the inventive improvement, Fig. 2 is taken along line 2-2 in Fig. 1.
As shown in Fig. 2, grooves 12 are formed on the circumfercatial surface of annular base 10 between barriers 53. Conductive strips 20, which aze plated with silver 54, are wrapped into the grooves 12 formed between barriers 53. As above noted, the strips are rolled into position and fastened by means of threaded fasteners, shown at 55 in Fig. 2. In a modification of the prior art approach described with reference to Fig. 1, and in order to bypass gap 56 foTmcd between the ends of strip 2f1, a shunt 57 is provided in an access pocket 58 formed in base 10. blot shown are other threaded fastenez~s 55 which secure the strip 20 to base 10. Thus, threaded fasteners SS fasten strip 20 to base 10 and to shunt 57. Lead wires 59 are term~iz~.ated to the shunt-ring junction.
As hereinabove noted, accumulation ofmamafactuuring tolerances results in existence of gap 56 between the ends of strip 20, aaad a number of problems arise as a wnsequence.
One such consequence is accelerated wear of the brush material riding over the discontinuity. Such wear is dimin~zshed by chamfering the sharp edges of the ring ends, as shown in Fig. 2_ The current invention represents a significant improvement over the prior art by enabling precious metal plating over the entire ring surface and over the gap. The precious metal plating addresses both the drawbacks aSSOClaLea l7illtn B1C prCSGnGC v1 Q1G ~,a~r, as poll on i~c .~.canboolso aeeooiatod with the pzior art approach of filling a gap with solder.
It is noted that when solder is used, emf differences may arise because of the use of dissimilar conductive materials in the ring and in the gap. Such emf differences may adversely impact both the ring stnseture and its effectiveness for transmission of low lcvol signal voltages and currents.
Mol~over, because of diffcremces in thermal expansion eharaeterirtics ofthe gap filling material euW the riizg (or ring plating material), stresses occur within the gap and at the boundaries between the gap filling material and the ring. Such stresses particularly occur when the slip ring assembly is used in its operating environment, and more particularly when the environmental tcmperaturo is raised, Such stresses may result in cracking, wearing, and further deposit of debris along the ring suzface.
Applicants' novel approach to ovexcom~iaag the about problem includes the step of heating the gap to a predetezrniaaed temperature prior to filling the gap. Thus, when operating environment of the ring is at or below that temperature, the gap will be in compression, thereby eliminating cracking of the gap i~lliag materiale ac well as separation between the gap filling material and the ends of the slip zings.
Such therzaal manipulation thus overcomes a significant problem ofthe prior art, arising from thermal stressc~ occurring at or adjacent to the gap.
It should be noted, howesrcr, that heating of the gap for curing the filling material of the gap need not necessarily be to the maximum operating temperature ofthe slzp zing assembly. By careful thezzaal manipulation, use of an appropriate base ~matez~ial whose thermal expansion characteristics are closer to those ofthe xi~og and its plating material would x~sult in occurrence of iowox thermal 3ttr~eca during hvataag of the slip ring asseraably.
Thus, although heating to the maximal operating temperature will assure that, thtroughaut the operational temperature range of the assembly, the gap and its filler material will be in compression and thus that stress cracks will be eliminated, the amount of thermal stress occurring at temperatures above the curing tctnpcrature may be reduced by use of the proper base mattrial.
Accordingly, when an appropriate base anaterial is used which has thermal expansion characteristics that are closer to those of the ring, the gap rriaterial may 1~~ rnwi at a tPTnreTalllre bClOW the expected (oz zz~aaciznal) operating tempezature of the assezz~bly_ That is, considering the difference between the thermal expansion characteristics of the brio material and the same characteristics ofthe z~iuag or its plating material, when the environmental temperature exceeds the cure temperature. the level of stress developed within the gap $Iling material, or between the gap filling material and the ring, is less for utaterials for which the difference is smaller than for materials for which the dif~erenee is larger_ Thus, by using base materials having thermal characteristics which are a closer match for those of the ring or plating material, it becomes possible to reduce the thermal stress arising at raised temperatures. Aeeordiztgly, in accordance with the invention, cracks in the gap felling material may be eliminated or reduced even when curing takes place below the expected or maxiuoa~al operating temperature of the assembly.
Accordingly, the present i~avendan overcomes prior art problems arising fronts occurnences o:f gaps between the cads of the slip rings by filling the gap.
However, unlike the prior art, the present invrntian does not use solder to fill the gap_ Instead, the two ziaajor drawbacks of the wrapped ring are eliminated by using a gap filling matezial meeting the following criteria.
Fizst, Such a gap filling rnateriat must bond well to tl~e ri~,~ end.R_ Otherwise, one larger gap could turn into two smaller gape.
Moreover, the gap filling material moat not wear at a significantly di~ercnt raft thaw the surrounding rir~g material. Otherwise, the gap would develop into a physical disaentinuity during operation.
SriII furthtr, the gap filling material must not smear non-conductive debris onto the surrounding ring; otherwise, it could cause undesirable variations in coataot resistance.
Moreover, as above noted, although not a necessary cozlditioz~, it is ~5 ' helpful if the gap base materiel hss thermal expansion characteristics which match those of the surrounding ring.
In accordance with these criteria the presently preferred filler is a conductive epoxy, with sufficiently high loading (80% by weight) of silver powder, placed into the gap and cured at temperatures above the maximum use or storage temperature of the device, Although the presence of silver powder should be above approximately 70%, and preferably in the isnge of about 70'/0 to about 90%, the preferable value of loading is approxirr~ately 80% silver by weight.
The filled gap 56 is then shaped by machining or filing, and the section of the ring containing the gap is over-plated at 60 with the same plating as the remainder of the ring, The function of the conductive epoxy is to Sll the gap, tr, apt see a c»ifiahlR "atartAr" mrfs~ce felt electroplating. and t0 S11DD0lt the subsequent electroplated layer.
It is noted that commercially available conductive adhesives generally contain fillets that are quite conductive, but do u.ot unifozmoJ.y accept electroplate like wrought metals such as copper, brass oar s:dvez, and thus are unacceptable for the presently contemplated application. indeed, the conductive fillers used in such adhesives acre typically silver plated copper or other silver plated particles or ffa~krs, as opposed to the presently preferred formulation, which contains solid silver powder at the about levels of loading by weight_ Such loading is much higher than that commercially available in silver filled epoxies, and has been found to clcctroplatc uniformly and at a similar rate to the adjoining ring material.
A ~cvvnd oag~ifsoasat o.opoot of tho i~~'~tsoa is thv isov of tho tlaoairaal 26 ' expansion mismatch between the ring and epoxy base to exert compressive loads oa the gap. Without tensile forces on the gap, the filler material would not crack or debond. The wrapped ring formed in accordance with the prior art i~ constructed such that, as the temperature increases the gap tends to open slightly.
Therefore, is accord nee with the invention and the above disclosed considerations, the epoxy is filled into the gap and cured at temperatures equal to or above the maximum temperature at which the unit wall operate. Thus, the gap (gad the gap SIIer material) will t~lwaye be in compression. For eacample, fez CT sca~waez5 which vperabe at temperatunc ranges of -40°F to +1 SO°F, heating the gap to tcmpciatures in the range of 145°F to 165°F
have been successful. Hosting should not be implemcntcd to trmpcraturcs significantly above the normal tamp~ature range of the device, as the annular base is desxg~aed. for that range and, for temperatures significantly above the opcmting z~ange, the base expands and the rings may snap. It is expected that heating to teiaaper~-aturcs within. approximately t40° of the maximum tctnpcraturc will be GIrGGLlYG In lItljIIGII1G111JJ.lb' tllC 111yGJ1tlYC GOaGCpt.
It is indeed possible that failures of earlier experiments with solder filled gaps, which showed poor performance because of smearing, sad failures of filling materials using solders srld conductive plastics due to cracking during thermal cycling or to as inability to be successfully electroplated, could be eliminated by use of tlse inventive technique.
A further significant aspect of the present invention is the overQlating of the filled gap. This results in a uniform wear rate abound the perimeter of the ring with low electrical noise_ r.

The following provides ftuther desczi~ptxoz~ of the presently preferred specific epoxy eoznposxtion to implement the invention and the undczlying theoretical equations.
The conductive epoxy used was an nn-'house developed formulation consisting of 80% by weight silver powder and 20% by weight epoxy. The temporary internal experimental designation for this znatezial is NB561-FOObA. The formula is:
$0 parts by weight (pbw) Silver powder 14 phw Epuxy rrsiu 6 pbw Hardcaer The silver powder used was Aldrich 32,709-3 a 99+% silver povcrder with 5-8 micron average particle size. Although the preferred range of average patticlc size is about S-8 microns, the size may be is the range of 0.25 micron to 25 microns, or snore. That is, it is expected that silver powder or flake havi~ag average par~iclc size in the range of 0,25 to 25 microns will work well in accordance with the invention. The epoxy resin used was Ciba-Geigy AraIdite 502, a d="butyl phthalate plasticized diglycidal ether of bisphenol A
(DGrEBA) epoxy with an epoxy equivalent weight (EEV~ of approximately 230. The Hardener was Heakel Versamid 125, a polyamide with an amine equivalent weight (AHED of approximately 345, It is expected that other epoxy (such ss undiluted DGEBA) sad other hardener systems (such as other polyarnides, polyaaziues, and mercaptans) would work well. Furthermore, it is expected that conductive polymers based oa other polymers would also work well. Indeed, experiments wexe conducted with ND56X-I"006a3 whxcb, waa~ 8o9~u silver in 20~i'~ polyurcxhanc. In thc3c z e~cpez~ments, it was attempted to improve the ability of the gap filling material to Ilex and absorb the differential thermal expansions, but the material was not flexible enough to solve the problem (withoutthe change in cure temperature), Therefore, use of the material wens discontinued and experimentation continued with the NB561-F006A, with which more experimental history had been developed.
However. it is expected that polyurothaae and other polymer matrix materials will work as well as the abovo described epoxy matrix.
In connection with the above dcacribed aspect of the invention wherein the filling material and the gap is cured above the use range of temperatures, applicants provide the following cotnmeats. It is recognized that it is not required to provide a theoretical basis for an invention. However, it is believed that the following comments will aid in appreciation of the invention.
The current configuration uses an epoxy base and copper scrip rings. As the temperature changes the two materials expand (or contract) at dit~ereat rates. The predictive equation for the relative mismatch ofthe base relative to th~ rings (i.e. the change in gap size) is D = ~ D (CTEbase - CT~ing) fuse - Tassembly) Where: D is the change in the gap sue; D is the average diameter ofthe c~ppcr ring; CTS is the coefficient of the~nal expansion. (CTE) of the ring or epoxy base; T is the temperature during assembly (when the ring and base arc secured together) or use. In the current cvnfigutatiou, the ring is rapper with a CTE of approximately 18 x I0'6 /°C and the CTE of the epoxy base is 47 x 10-6 I°C_ 'Ihe dza~metez its approximately 38 inches, and the anticipated exposure t~perature range is -40°C tv +66°C (150°F).
Consequently, in the fret state, the gap could increase in size by up to 0.152" at +150°F_ At any temperature belovr the assembly tcmpctatutz d is negative and the gap tcztds to compress.
When the ring is continuous (i.c. the gap filled), O represents the amount that the rir~ and gap filling are stretched. (Ignoring the small positive xnlluenc:G of the CTE of the filling material). Since the cross-sccdonat area (normal to the tensile force) is oqual for the ring and gap filling, the deformation of the gap flung, and ring are 0 = ~flulng ~' ~Mr~
li ~lir~g~ ~Iling ~~ng~~ng~ Ering Orm~ S 0 ~ ~~ '~ Ur;r,~Lfilr~) ~EfiuI~Erlr,s)~
The stress on the filling - ring bond face is 6bond = Efllllng ~~Hing~~Hrig)~
Making an approximation (valid for large rings and rigid filling), a~ - E~~ (CTE~e - CTE~"g) (T"se - T~yembly~
Where: L is the length of the ring or filled gap; and E is the tensile modules of the ring or filled gap, Using the lengths of 3 8~ for the ring and O.I20" for the gap and modules of 17,000,000 psi for the ring and 1,000,000 psi for the filling, the stress on the bond if assembled at 22°C and subsequently exposed to 66°C would be in excess of 20,000 psi. This is well beyond the bond strength ofthis, or similar filling materials_ If a reasonable bond szrength of 2000 psi were used as the design limit, then this configuration could accon~.modate a D ofup to 0.014", This ntear~s that the cure texnpezatwre could actually be about 4°C lower than the upper use temperature. .

t~

It is believed that tlae cuttent pmcess works by raising the temperature of the base prior to filling the gap. This effectively increases the "assembly"
temperature and vastly reduces D, which in turn vastly decreases the bond stress (actually it is believed to decraase the azaximum tensile stress on the bond). The stress is primarily dependent on the CT'E a:sd temperatures. If different materials were used, with better CTE match, then more temperature tolerance can be allowed.
However, in addition to the foregoing, it is noted that advantageous results may be achieved by a specific stud securing method.
The assumption of free or ua-constrained expansion or contraction used is the above theory may be inapplicable to a specific configuration ofthe slip ring assembly, such as presently being implemented. In such a configuration, every ring is secured to the base by a plurality of studs. In such an assembly, at Ieast four studs secure the ring to base. This constraint causes the gap to be smaller than theory predicts. The studs are secured to the base or to tha shunt with nuts through clearance holes, and are torqued to specified levels. The fxicdon developed under the nut head must thus be overcome before the ring can slip relative to the base. To prevent any motion, the total friction forces from the studs and nuts must be F ~ E,;,~o (~~~ / L""9) ~;~a _ In the current configuration the cross-sectional arcs of the rings (A) is 0.1064 square inches for power rings and 0.013 6 square inches for signal rings.
The corresponding frictional forces required are approximately 2,300 pounds and 300 pounds_ It is estimated that in the current configuration the torque az~d nut size cozxabinati.o~xs provides up to 400-800 pounds of fricdon_ This is d adequate fvr smtall signal rings, but not enaugh to prevent the power rings from slippixlg.
To prevent the power rings from slipping the clearance between the stud and its hole in the base or shunt is filled with an epoxy. Therefore, in order to "slip" the thermal forces on the rung must not only overcome the bond strength of the filling and the frictional forces due to the stud-nut clamping, but also the stud must crush epoxy. The force required to crush the epoxy potting n~mt,r."mi ;n +hr r"rr~rat oonfagus~a~iosa would b. oftho order of?.,000 bo 4,000 pounds. This complementary securing method is required for large cross-sectional area rings, and is used on signal rings as "reduadaucy". The following table shows the relative forces contributed by tech of the ring-end to ring-end securing mechanisms. The potting around the stud is the dominant mechanism, Ring Force due Force due to Force due Force required bo to type flltin~ stud-nut frictionpotting ~mmr,~it<, ~,rPV~"t to rln~ Slid bond (Ib.) (Ib-) stud (Ib.) (Ib.) Signal 27 400 1,920 300 Power 212 800 4,000 2,300 Because of the process variations and the particular configuration of the power ring, slight temperature vari3tiono in the prooea9 oven has oa occasion caused the rings to slip after the filling had cured. This slight slip caused enough stress on the f lled gap to break the bond to the ring (resulting in crack). Potting the studs eliminates this problem. Another problem eIirninated d r by potting the studs is that if the ring slips and the gap opens pzior to filling, then, upon return to room temperature the ri»g (with an oversized gap) is loose on the base.
The foregoing description of the prcfcrnd embodiment of the itawention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed, since many modifications or variations thereof are possible in light VC aiaG dLV V G LG~~llil~.
Any such . modifications and variations are within the scope of the invention. The embodiments described herein were chosen and desen'bed in order best to explain the principles of the invention and its practical application, thereby to enable others skilled in the art to utilize the invention in various embodiments and with various modif canons as arc suited to the particular use contemplated therefor. It is intended that the scope of the invention be defined by the claims appended hereto, when iaterpretsd in aecordsunce wrath t'k~e full breadth to which they ar~ 1~gally oad ocysilobly entitaed_

Claims (10)

1. In a wrapped ring method for manufacturing an annular base for a slip ring assembly, wherein strips of conductive material are wrapped onto as insulating base, the improvement comprising heating at least ends of a strip of conductive material to a temperature level corresponding to a maximum operating temperature for the slip ring formed of the strip, whereby a gap between ends of a strip of conductive material is thermally expanded; and when the gap is thermally expanded, filling the gap with a rigid conductive material.

2. The method of claim 1, further comprising the step of plating over the filled gap with a conductive material to form a uniform layer of conductive material around an entire perimeter of a slip ring formed by the strip.

3. The method of claim 2, wherein said step of plating over comprises plating oven the filled gap with a same conductive material as used for plating a surface of the slip ring.

4. The method of claim 2 wherein said step of plating over comprises plating over the filled gap with a precious metal.

5. In a wrapped ring method for manufacturing an annular base for a slip ring assembly, wherein strips of conductive material are wrapped onto an insulating base, the improvement comprising heating at least ends of a strip of conductive material to a temperature level chosen to reduce tensile forces acting on a filler material added to a gap between ends of the strip of conductive material, wherein the gap is thermally expanded by the heating step, and filling the gap with a rigid conductive filler material.

6. The method of claim 5, wherein said step of heating comprises heating at least the ends of the strip to a temperature chosen in order to provide compressive forces on the filler material.

7. The method of claim 5, wherein said step of heating comprises heating at least the ends of the strip to a temperature determined in accordance with a maximum temperature to which the slip ring will be exposed in its subsequent utilization.

8. The method of claim 7, wherein said step of heating comprises heating at least the ends of the strip to a temperature determined in accordance with a relation between an amount of mismatch between thermal expansion characteristics of the strip of conductive material, the annular base and the filler material, on the one hand, and the maximum temperature to which the slip ring will be exposed in its subsequent utilization on the other hand.

9. The method of claim 5, wherein said step of filling the gap comprises filling the gap with a filling material which electroplates uniformly over the gap and which electroplates uniformly relative to an adjoining ring material.

10. The method of claim 5, wherein said step of filling the gap comprises filling the gap with a filling material which includes solid silver filler is the range of 70% to 90% by weight.