US2761042A - Electrical resistance device and method of making it - Google Patents

Description

Aug. 28, 1956 B. B. SCOTT 2,

ELECTRICAL RESISTANCE DEVICE AND METHOD OF MAKING IT Filed July 15, 1955 TEFEI IN V EN TOR. BE/Vd/MV/A/ 3. 5 C 0 T United States Patent ELECTRICAL RESISTANCE DEVICE AND METHOD OF MAKING IT Benjamin B. Scott, Schenectady, N. Y., assignor to the United States of America as represented by the Secretary of the Air Force Application July 15, 1955, Serial No. 522,405

6 Claims. (Cl. 201-48) This invention relates to a resistance card having straight or curved elevated brush-contacting areas. These new resistance cards are designed for side-brushing and are useful in precision potentiometers incorporated in electrical analogue computers.

The new side-brush resistance-card potentiometer represents a major advantage in quality over conventional potentiometers of similar configuration and permits the manufacture of a product having quality comparable with that of the standard edge-brushed type. It provides the designer with considerable additional latitude in the type of potentiometer mounting and operating arrangement he may Wish to select for use. Also it has permitted the solution of hereto unsolved brushing problems on siderushed types of precision otentiometers.

It was found in earlier practice that the firm fastening of resistance-card wire-turns to cards for preventing movement or flexure under the influence of brush contact in passing produced thousand-fold increase in brushinglife operation expectancy. Institution and use of a thermo-setting resin-bonding process for firmly'attaching wire turns to cards on resistance card components of edge-brushed potentiometers was responsible for increasing average brushing life from 10,000 operations to 10,000,000 operations and has, of course, been standardized in our processing. However, in attempting to apply this bonding method to resistance cards intended for side-brushing it was found impossible to control the height of the bond between wires to the limits of about 10% to 25% found satisfactory for edge brushed type of potentiometers. Instead wire-turns would be found immersed in the bond or be completely unbonded or would be bonded to one another but not to the card supporting member.

The excess bond material between turns could not be removed by known techniques without substantially equal removal of metal from the wire which ruined the electrical quality of the resistance winding, without lowering the bond-height between turns sufficiently to provide brush contacting without electrical noise. Where turns were loose, the contact points of brush members were found to penetrate between the turns to the card surface and to cause winding breakage and card destruction in relatively few brushing operations. Where turns were bonded to one another but not to the card winding support it generally occurred that voids between wire-turns existed at the ends of these sections or were produced by brushingsurface cleaning operations so that destruction under relatively few brushing operations occurred similarly to the case for unbonded turns.

Investigation of the poor bonding properties of the sides of wound resistance cards disclosed the lack of flatness in the cards used as winding-forms produced nonuniform distribution of material and resulted in the unsatisfactory bonding conditions described above. Additional investigation disclosed that a thin elevating strip of insulating material approximately 0.005 thick attached to the card winding form before the winding operation or placed in position between the card and wire-turns after the winding operation in the location of the desired brushing surface produced the uniform contact with wire-turns needed for uniform distribution of bonded material between the wire-turns and using proper bond-curing techniques produced extremely uniform high-quality bonding of wireturns to cards which will readily clean for brush contacting in the area of the elevating strip. These finished flat resistance cards may be used flat or may be formed by conventional techniques into cylindrical shaped cards, which have elevating strip in the brush-contacting area on the inner or outer surfaces or both. 7

It is an object of this invention to provide a method for making a resistance card having elevated straight or curved brush-contacting surface on one or both sides of the card.

It is another object of this invention to provide a resistance card having an elevated straight or curved brusheontacting surface on at least one side of the card which card is usable in side-brush potentiometers.

It is a further object of this invention to provide a flat resistance card having elevated brush-contacting surface on one or both sides thereof which card can be shaped by conventional methods into a cylindrical card particularly useful for computer potentiometer.

These and other objects of the invention will become apparent as the detail description of the invention proceeds.

The invention will be more clearly understood from the following detailed description of the following specific examples thereof read in conjunction with the accompanying drawings wherein:

Fig. 1 is a partial elevational view of an embodiment of the invention at an uncompleted stage of development previous to the addition of the thermosetting resin but after the winding of the wire on the card;

Fig. 2 is a cross sectional view of the Fig. 1 embodiment taken along the lines 22 of Fig. 1;

Fig. 3 is a partial sectional view of the Fig. 1 embodiment of the invention incorporated in the curing fixture;

Pig. 4 is a cross sectional view of the embodiment of Fig. 1 in a completely finished condition;

Fig. 5 is a perspective view of the same embodiment of the invention as is shown in Fig. 1 but of the finished resistance card; and

Fig. 6 is an elevational view of another embodiment of my invention wherein an elevated curved brush-contacting area is incorporated rather than an elevated straight brush-contacting area.

Fig. 1 shows an embodiment of my invention wherein an elevated straight brush-contacting area is used. The resistance card of Fig. 1 has a base strip 11 to which is attached an elevational strip 12. These dielectric strips 11 and 12 are suitably made of varnish impregnated glass fabric material used in making conventional resistance cards. The strips may suitably be bonded together by a thermosetting resin. Suitable resins for bonding these strips are described in detail in U. S. 2,528,235. I Alternately elevated strip 12 could be formed as an integral and unitary part of base strip 11. A coil of resistance wire 13 is wrapped around strips 11 and 12 forming individual turns 14 thereon. Alternatively strip l zjneed not have been previously bonded to strip 11 but. may he slipped under the wire turns after winding the wire on card 11 since the later curing operations will bond thestrips and wire. The resistance wire used for these cards is resistance Wire conventionally used in making resistance cards. Normally this will be resistance wire which has been coated with an insulating coating of a suitable alkyd, polyvinyl formal resin, or a mixture thereof. vvFig. 1 shows the uncompleted resistance card, i. e., uncoinpleted to the extent that the themoplastic resin has not been added to bond the wire to the insulating strips and other finishing operations have not been performed on the card.

Fig. 2 is a cross sectional view taken along the lines 22 of Fig. 1. This view shows the relation of elevating strip 12 to base strip 11 and wire convolutions 14- prior to the addition of the thermoplastic resin to bond the Wire to the strips and prior to the curing operation.

The next step in the making of the resistance card is to add the flexible thermoplastic resin to the card suitably by dipping the card in a solvent solution of tie resin. The viscosity of the resin solution is adjusted by adjusting the concentration so excess resin solution runs off the card. This is a conventional procedure. The card is then placed with the elevation strip up and the excess resin is allowed to drain from the card. The card is allowed to air dry for a suificient time at ambient temperatures to cause the resin to take an initial set by the evaporation of some of the solvent. This air-drying step is a desirable step but not absolutely necessary and the next drying step at elevated temperature may be proceeded with directly after the dipping step.

The next step in forming the resistance card is to place the card having the air dried resin thereon in an oven at about 75 C. to remove the major portion of the solvent. Obviously this temperature might be somewhat lower or higher depending on the particular solvent used for the resin. The partially completed resistance card is now ready for the pressin operation.

Fig. 4 shows the oven dried resistance card placed in a curing fixture 15. This curing fixture is assembled by placing a cellophane or other suitable parting sheet 3.6 on a base pressing sheet 17 suitably made of metal as a support for the card. Next the resistance card is placed on the cellophane parting sheet 16, a second cellophane parting sheet 17 is placed over the resistance card, a silicone-rubber pressure-equalizing sheet 18 is placed on sheet 17, a third cellophane parting sheet 19 is placed on sheet 18 and finally a metal cover plate 20 is placed on sheet 19 to complete the assembling of curing fixture 15. This curing fixture is now ready for the pressing operation. It is placed in a hot press conventionally used for making the resistance cards of the prior art. The curing fixture containing the partially completed resistance card is pressed at about 50 pounds per square inch and at about 135 C. for about ten minutes to cause the wire convolutions 14 to be firmly bonded to the dielectric strips as the resin takes on a permanent set. These particular conditions as well as the drying conditions specified above are based on Permafil-Formvar thermosetting-resin bonding solution. This particular resin solution is not at all essential to the invention and many other resin solutions of similar nature would be satisfactory. Conditions of treatment would vary slightly depending on what resin solution was used. Suitable resins are described in detail in U. S. 2,528,235. In this pressing operation with the help of pressure equalizing sheet 18 convolntions 14 are pressed down as shown in Fig. 4, i. e., the wire is made to more closely conform to the shape of strips 11 and 12. That is, the wire turns are forced down into hollows 11a and 11]) causing this portion of the wire to be completely imbcdded in the bonding resin and so the wire is more rigidly attached to the strips. This is clearly shown in Fig. 4 which is a cross sectional view similar to Fig. 2 of the resistance card, except that Pig. 4 shows the card after hot-pressing and Fig. 2 before hot-pressing.

The last step in forming the resistance card consists of subjecting the wire turns directly over the brush-contacting area provided by strip 12 to a conventional treatment with a felt-wheel and jewelers rouge wire-cleaning technique for removal of the wire insulation and bonding material to give bare wires for electrical contact. Fig. 5 shows a perspective view of the completed resistance card.

Fig. 6 shows a finished resistance card which has a curved, circular in this case, brushing area elevation strip 21 on a base dielectric strip 22. In a conventional fashion coil 23 having wire turns 24 has been wound on the strips and the card has been treated in a manner similar to that used on the card having the straight brushing path to form a finished resistance card shown.

Although the invention has been described in terms of specified apparatus and method which is set forth in considerable detail, it should be understood that this is by way of illustration only and that the invention is not necessarily limited thereto, since alternate embodiments and operating techniques will become apparent to those sltilled in the art in View of the disclosure. For example, the fiat resistance cards described may be formed by conventional techniques into cylindrical resistance cards. If elevational brushing strips have been placed on both sides of the card, both internal and external brush-contacting is provided for the potentiometer in which the card is used. Normally brush-contacting would be provided for on either the inner or outer surface of the cylinder but not both surfaces. Accordingly, modifications are contemplated which may be made without departing from the spirit of the described invention or the scope of the appended claims.

What is claimed is:

1. A method of making a resistance card comprising step 1 of bonding a base strip of dielectric material to a second elevational strip of dielectric material shaped to conform to the brush-contacting area of the potentiometer for which the card is designed, step 2 of winding a coil of resistance wire around the product of step 1, step 3 of applying a flexible thermosetting resin to the product of step 2 and removing the excess resin, step 4 of drying ing the product of step 3 at a temperature and time sufficient to remove the major portion of the solvent from the resin but not sufiicient to completely set the resin, step 5 of assembling the product of step 4 in a curing fixture, step 6 of heating and pressing the fixture containing the product of step 4 in a hot-press at a temperature and for a time sufiicient to cause the resin to set and at a pressure sufficient to cause the wire to conform closely to the surface of said strips including the wire adjacent the edges of said second strip and to hold the elements of the product of step 4 in close association until the resin is set, step 7 of disassembling the curing fixture preferably while hot to prevent sticking difiiculties, and step 8 of removing insulation and resin from the brush contact surface of the wire to give the finished resistance card.

2. The method of claim 3 wherein a unitary strip of dielectric material having a raised brush-contacting area thereon is formed rather than using separate base and do vational strips and bonding them together.

3. A method of making a resistance card comprising step 1 of bonding a base strip of dielectric material to a second elevational strip of dielectric material shaped to conform to the brush-contacting area of the potentiometer for which the card is designed, step 2 of winding a coil of resistance wire around the product of step 1, step 3 of applying a flexible thermosetting resin to the product of step 2 and removing the excess resin, step 4 of airdrying at ambient temperatures the product of step 3 for a time sufficient to evaporate some of the solvent and give the resin an initial set, step 5 of drying the product of step 4 at a temperature sufficient to remove the major portion of the solvent from the resin but not sufficient to completely set the resin, step 6 of assembling the product of step 5 in a curing fixture, step 7 of heating and pressi the fixture containing the product of step 5 in a hot-press at a temperature and for a time sufficient to cause the resin to set and at a pressure sufficient to cause the wire to conform closely to the surface of said strips including the wire adjacent the edges of said second strip and to hold the elements of the product of step 5 in close association until the resin is set, step 8 of disassembling the curing fixture preferably while hot to prevent sticking difiiculties, and step 9 of removing insulation and resin from the brush contact surface of the wire to give the finished resistance card.

4. A method of making a resistance card comprising step 1 of bonding a base strip of dielectric material to a second elevational strip of dielectric material shaped to conform to the brush-contacting area of the potentiometer for which the card is designed; step 2 of winding a coil of resistance wire around the product of step 1; step 3 of applying a flexible thermosetting resin to the product of step 2 and removing the excess resin; step 4 of drying the product of step 3 at a temperature and time sufiicient to remove the major portion of the solvent from the resin but not sufficient to completely set the resin; step 5 of assembling the product of step 4 in a curing fixture by placing a cellophane parting sheet upon a base pressing plate, placing the product of step 4 on the cellophane parting sheet, placing a second cellophane parting sheet on top of the product of step 4 so it is sandwiched be tween cellophane parting sheets, placing a silicone-rubber pressure-equalizing sheet on the second cellophane parting sheet, placing a third cellophane parting sheet on the silicone-rubber pressure-equalizing sheet, and placing a metal cover pressing-plate on the third cellophane parting sheet to complete the assembling of the curing fixture; step 6 of heating and pressing the fixture containing the product of step 4 in a hot-press at a temperature and for a time sufficient to cause the resin to set and at a pressure sufiicient to cause the wire to conform closely to the sur face of said strips including the wire adjacent the edges of said second strip and to hold the elements of the product of step 4 in close association until the resin is set; step 7 of disassembling the curing fixture preferably while hot to prevent sticking difficulties; and step 8 of removing insulation and resin from the brush contact surface of the wire to give the finished resistance card.

5. A method of making a resistance card comprising step 1 of bonding a base strip of dielectric material to a second elevational strip of dielectric material shaped to conform to the brush-contacting area of the potentiometer for which the card is designed; step 2 of winding a coil of resistance wire around the product of step 1; step 3 of applying a flexible thermosetting resin to the product of step 2 and removing the excess resin; step 4 of airdrying at ambient temperatures the product of step 3 for a time sufficient to evaporate some of the solvent and give the resin an initial set; step 5 of drying the product of step 4 at a temperature and time sufficient to remove the major portion of the solvent from the resin but not sufficient to completely set the resin; step 6 of assembling the product of step 4 in a curing fixture by placing a cellophane parting sheet upon a base pressing plate, placing the product of step 5 on the cellophane parting sheet, placing a second cellophane parting sheet on top of the product of step 5 so it is sandwiched between cellophane parting sheets, placing a silicone-rubber pressure-equalizing sheet on the second cellophane parting sheet, placing a third cellophane parting sheet on the silicone-rubber pressure-equalizing sheet, and placing a metal cover pressing-plate on the third cellophane parting sheet to compiete the assembling of the curing fixture; step 7 of heating and pressing the fixture containing the product of step 5 in a hot-press at a temperature and for a time sutficient to cause the resin to set and at a pressure sufficient to cause the wire to conform closely to the surface of said strips including the wire adjacent the edges of said second strip and to hold the elements of the product of step 5 in close association until the resin is set, step 8 of disassembling the curing fixture preferably while hot to prevent sticking difficulties, and step 9 of removing insulation and resin from the brush contact surface of the Wire to give the finished resistance card.

6. A resistance card comprising a base strip of dielectric material having an elevational strip of dielectric material formed as a unitary part thereof, said elevational strip shaped to conform to the brushing path of the potentiometer for which the card is designed, a coil of resistance Wire wound around said strips, the turns of said coil being bonded to said strips by a flexible thermosetting resin and being shaped to conform closely to the surface of said strips including the wire adjacent the edge of said elevational strip, and the brush-contact surface of the Wire is bare for electrical contact.

References Cited in the file of this patent UNITED STATES PATENTS 2,557,790 Launey June 19, 1951

Claims (1)

1. A METHOD OF MAKING A RESISTANCE CARD COMPRISING STEP 1 OF BONDING A BASE STRIP OF DIELECTRIC MATERIAL TO A SECOND ELEVATIONAL STRIP OF DIELECTRIC MATERIAL SHAPED TO CONFORM TO THE BRUSH-CONTACTING AREA OF THE POTENTIOMETER FOR WHICH THE CARD IS DESIGNED, STPE 2 OF WINDING A COIL OF RESISTANCE WIRE AROUND THE PRODUCT OF STEP 1, STEPT 3 OF APPLYING A FLEXIBLE THERMOSETTING RESIN TO THE PRODUCT OF STEP 2 AND REMOVING THE EXCESS RESIN, STEP 4 OF DRYING ING THE PRODUCT OF STEP 3 AT A TEMPERATURE AND TIME SUFFICIENT TO REMOVE THE MAJOR PORTION OF THE SOLVENT FROM THE RESIN BUT NOT SUFFICIENT TO COMPLETELY SET THE RESIN, STEP 5 OF ASSEMBLING THE PRODUCT OF THE STEP 4 IN A CURING FIXTURE, STEP 6 OF HEATING AND PRESSING THE FIXTURE CONTAINING THE PRODUCT OF STEP 4 IN A HOT-PRESS AT A TEMPERATURE AND FOR A TIME SUFFICIENT TO CAUSE THE RESIN TO SET AND AT A PRESSURE SUFFICIENT TO CAUSE THE WIRE TO CONFORM CLOSELY TO THE SURFACE OF SAID STRIPS INCLUDING THE WIRE ADJACENT THE EDGES OF SAID SECOND STRIP AND TO HOLD THE ELEMENTS OF THE PRODUCT OF STEP 4 IN CLOSE ASSOCIATTION UNTIL THE RESIN IS SET, STEP 7 OF DISSEMBLING THE CURING FIXTURE PREFERABLY WHILE HOT TO PREVENT STICKING DIFFICULTIES, AND STEP 8 OF REMOVING INSULATION AND RESIN FROM THE BRUSH CONTACT SURFACE OF THE WIRE TO GIVE THE FINISHED RESISTANCE CARD.

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