US2936516A - Method of making a dielectric core and resistor - Google Patents

Description

J. A. ADAIR May 17, 1960 METHOD OF MAKING A DIELECTRIC CORE AND RESISTOR Filed May 17. 1954 2 Sheets-Sheet l ELi l a. I

May 17, 1960 J. A. ADAIR 2,936,516

METHOD OF MAKING A DIELECTRIC CORE AND RESISTOR Filed May 17. 1954 2 Sheets-Sheet 2 United States Patent METHOD OF MAKING A DIELECTRIC CORE AND RESISTOR John A. Adair, Chicago, Ill.

Application May 17, 1954, Serial No. "430,375 13 Claims. (Cl. 29--155.68)

This invention relates to dielectric cores, to electrical elements having dielectric cores for use in electrical instruments, and to a method of making such cores and elements.

Dielectric cores for electrical elements have been made in several ways. In one method cores are formed from insulating fiberboard cut to desired lengths from strips.

An operation is then required to smooth the sharp edges so that the wire to be wound later upon the core is not so apt to be cut. Other cores are made from ceramic tubing. These methods are not adapted for use in making electrical elements in continuing strip form.

The primary object of the present invention is to provide a novel dielectric core of continuous length to be used in electrical elements and having a substantially uniform cross-section.

Another object is to provide a novel method of forming such a dielectric core of continuous length.

A further object is to provide a flattened dielectric core for electrical elements which is slightly elastic when flexed axially in a plane normal to the plane of the flattened core.

Another object is to provide a novel method of forming annularly shaped wire-wound resistors adapted for use as potentiometers or rheostats in connection with the manual controls of electrical circuits.

The invention is illustrated in a preferred embodiment in the accompanying drawings, in which:

Fig. 1 is a diagrammatic view of the preferred method of forming dielectric cores in continuous strip form for use in making wire-wound electrical elements;

Fig. 2, a broken perspective view of a portion of the fibrous material strip from which a dielectric core is made;

Fig. 3, a broken perspective view of a portion of the dielectric core on which several turns of a wire winding have been made to illustrate the form of the completed electrical element strip;

Fig. 4, a perspective view showing a cutter bar separating an electrical element length from the completed electrical element strip;

Fig. 5, a sectional view taken as indicated on line 5-5 of Fig. 4;

Fig. 6, a broken perspective view showing the preferred method of forming the electrical strip into a helical coil by using a mandrel;

Fig. 7, a perspective view of a single portion of the helical coil shown in Fig. 6 with terminals fixed to its opposite ends; and

Fig. 8, a broken perspective view of an electrical element formed from a length as shown in Fig. 4, said element coated with an insulating cement and having terminals secured to its ends.

Referring to the drawings, and particularly to Fig. 1, a flexible woven'fibrous strip of material 10, usually :purchased in continuous strip form upon a cylinder or spool 11, is pulled from the spool through various process stations by a plurality of feed rollers. For clarity 2,936,516 Patented May 17, 1960 2 only feed rollers 23 are shown. The strip 10 is treated with a stiffening agent and ultimately used as a dielectric core 9 for an electrical element.

The material of the strip 10 is preferably made from glass fibers woven to form a sleeve structure, but it is not intended that the invention be limited to the use of this sleeve formation. A so-called cable weave is preferred so that the sleeve can be stretched to a point where a uniform cross-section is attained, at which point additional tensioning forces will not materially alter the crosssection throughout the length of the tensioning sleeve. With such a weave the tensioning force need not be so accurately controlled; that is, a minimum tensioning force sufficient to fully stretch the sleeve can easily be maintained.

The strip 10 may first be passed through a radiant oven 13 to burn out organic materials, mainly starches and oils, inherent in the manufacture of the woven glass sleeving. Although higher temperatures may be used, the temperature in the oven is maintained between 550 F. and 750 F., the speed of the sleeve in passing through the oven 13 being varied in direct relation to the temperature. 7

During the application of the stiffening agent to the sleeve of material, and during subsequent drying and winding operations, both to be discussed later, the strip 10 is preferably maintained under a substantially uniform tensioning force, so that the cross-section of the strip 10 will be substantially uniform throughout its length as it is formed into a dielectric core. This tension is maintained by a feeder mechanism, generally designated 12, and a tensioning mechanism, generally designated 14.

As shown in Fig. l, the pulley 14a of the tensioning mechanism is normally pivoted in a downward direction by a weight 15 suspended on a lever arm 16. As the strip is pulled from the spool 11, some added tension is exerted on it to overcome the friction and inertia of the spool. The added tension moves the pulley 14a upwardly, actuating a micro switch (not shown) which in turn actuates the feeder mechanism 12. After the feeder mechanism pulls an additional length of material from the spool 11, the pulley 14a moves downwardly under gravity, breaking contact with the micro switch and turning off the feeder 12. In normal operation this cycle is repeated quite rapidly. The resulting intermittent feed prevents gross movement of the tensioning mechanism, and a substantially uniform tension is maintained in the strip 10 at all times.

From the tensioning mechanism 14 the fibrous strip 10 is passedover another pulley 14b and then into a container 17 filled with a stiffening agent 18. A pair of pulleys '19 and 20 guide the strip 10 between a pair of doctor blades 21 and 22 which are spring urged into contact with opposite faces of the strip 10 to wipe the excess solution. from thestrip before it leaves the container 17. t

The stiffening agent preferred is a solution of an alkyl aryl silicone. However, dihydrogen aluminum phosphate, sodium silicate, or a mixture of dihydrogen aluminum phosphate and aluminum phosphate have been found to serve as satisfactory stiffening agents. The purpose of the stiffening agent is to lock the fibers of the strip 10 in their tensioned position. In addition, it imparts high temperature resistant qualities to the core 9 and enablesthe core 9 to withstand temperatures in excess of those found in operating resistors, rheostats, potentiometers, etc.

over the surface 24. The oven 25 is generally maintained at temperatures varying from 250 F. to 500 F., depending upon the speed of the strip in passing through it and the degree of cure desired in the particular stiffening agent used. Upon leaving the oven 25, the strip 10 is shaped like a flattened sleeve having generally rounded marginal edge portions with the opposite inner surfaces of the sleeve normally adhering to each other to form a hardened laminate-like core. The strip 10 at this time is slightly elastic and may be axially flexed in a plane normal to the plane of the flattened surfaces of the strip.

The strip 10 is then passed into a conventional wire winding mechanism, generally designated 26. This mechanism operates in timed relation to the feed rollers 23 to determine the number of turns of wire per unit length of the strip 10. In the drawings, resistance wire 26a is shown wound upon the stiffened strip 10 to form a continuous-1ength electrical element strip 34. The turns of wire may be spaced to provide air insulation between each turn, or the wire may be'procured with an insulating coating. The carefully controlled cross-section of the strip 10, together with the properly spaced windings about the core, produces a resistance strip having a uniform resistance per unit length.

In the drawings only resistances are shown, but core- .wound inductances can also be formed by the method of this invention.

After the winding operation, the electrical resistance strip may be wound upon a cylinder, not shown, and removed to storage to await further use. However, if desired, the continuous strip 34 may be processed further to form individual electrical elements.

Fig. 6 illustrates one way in which the strip 34 may be prepared prior to being formed into a plurality of rounded resistance elements, such as element 33 shown in Fig. 7. This element 33 is particularly well adapted for use in a rheostat which functions as a variable control in electrical circuits. The strip 34 is passed from a winding mechanism 26 through a tank 31 containing a bath of a softening agent 30. The softening agent should be a plasticizer or solvent which will soften the stiffening agent and lend pliability to the core 9. In the prefered method, methylene dichloride which is non-inflammable is used for the silicone-softening agent. It evaporates rapidly, and so may be readily eliminated in the subsequent drying operation.

If the strip 34 is only partially cured in the radiant oven 25, further heating will initially render the core 9 of the strip 34 pliable without the use of a softening agent. The softened core may then be shaped as desired and subsequently be fully cured by further heating or merely be cooled to a set in its new formation.

From the tank 31 the strip 34, still in its tensioned condition, is tightly wound upon a mandrel 32 to form a helical coil 35.

When the mandrel is filled, the strip is cut, and the mandrel and the coiled resistant strip are taken away and dried. The drying may be done under ordinary strip 34 being fed from the winding mechanism 26 into acutter 27 which severs the strip into resistance elements 27a of the desired lengths. Terminals may be fixed to opposite ends of the element 27a to form a resistor immediately.

An element 27a may also be formed into a resistor of any shape desired. It is first immersed in a softening agent of the type previously described, and then bent to the desired form after which the softening agent is removed by drying. Fig. 8 illustrates a U-shaped resistor having terminals 28 secured to its opposite ends. A coating 29 of an insulating material is then applied to the preformed resistor. Preferably, but not by way of limitation, the resistor is coated by dipping or rolling it in a slip of insulating cement. The coated resistor is then heated, preferably at about 550 F. for from thirty-five minutes to an hour, to harden the insulating cement and to cure completely the stiffening agent in the core 9 at the same time.

It is also contemplated by this invention that the annular-type resistor 33 may be provided with an insulating material embracing one marginal edge portion, leaving the opposite marginal edge portion exposed for contact with a slidable element of the type used in volume controls for electrical circuits. Such a coating may be formed from insulating cement applied to each individual resistor 33. Preferably the coating is applied to the continuous strip 34 by folding another strip, treated like the strip 10 with a stiffening agent 18, about one marginal edge portion of the strip 34 as it leaves the winding mechanism 26. A subsequent heat treatment will cause the folded strip to adhere to the strip 34, and it then may be passed through the solution 30 and wound upon the mandrel 32. Individual elements similar to resistor 33 may then be prepared as previously described.

The foregoing detailed description is given for clearness of understanding only and no unnecessary limitations should be understood therefrom, for some modifications will be obvious to those skilled in the art.

I claim:

1. The method of forming a wire-wound electrical resistor, comprising: tensioning a length of fibrous material to obtain a substantially uniform cross section in said material throughout its length; impregnating said tensioned length of material with a high-temperature-resistant dielectric stiffening agent; drying said impregnated tensioned length so that said stiffening agent will lock the fibrous material of the length in said tensioned condition to form a resistor core; and uniformly winding a resistance wire of uniform resistance per unit length about said resistor core to form a length of resistor having a uniform resistance per unit length.

2. The method of forming an electrical resistor, comprising: fonning a resistor core length from a tensioned woven fibrous material, the core length being stiflened by a coating of a partially cured silicone resin; winding a. length of resistance wire about the stiffened core length to form a resistor element; heating said resistor element to soften the partially cured silicone resin; bending the resistor element to form a helical coil; and cooling said helical coil to harden the silicone resin and core length to form a helically-shaped electrical resistor.

3. The method of forming a plurality of substantially circular wire-wound electrical resistors, comprising: forming an electrical resistor strip having resistance wire coiled about a flat, laminate-like core formed from a tensioned woven glass fiber sleeve, said core being still?- ened by a coating of a partially cured silicone resin; applying a softening agent to said strip to render the core pliable; bending said strip to form a helical coil; drying said helical coil to evaporate said softening agent and again lock the fibers of the core in position; and cutting said helical coil longitudinally to form a plurality of ring-shaped resistor elements.

4. The method of forming a plurality of substantially circular wire-wound electrical resistors, comprising: forming an electrical resistor strip having resistance wire coiled about a flat, laminate-like core formed from a tensioned woven glass fiber sleeve, said core being stiffened by a coating of a partially cured silicone resin; heating said strip to soften the partially cured silicone resin; bending said heated strip to form a helical coil; cooling said helical coil to harden said silicone resin so that said core is again stifiened; and cutting said helical coil longitudinally to form a plurality of ringshaped resistor elements.

5. The method of forming individual resistors from a wire-wound electrical resistor strip, comprising: forming an electrical resistor strip having resistance wire coiled about a flat, laminate-like core formed from a tensioned woven glass fiber sleeve, said core being stiffened by a coating of a partially cured silicone resin; cutting the electrical resistor strip into the desired lengths; heating each of said lengths to soften the partially cured silicone resin and render each length pliable; forming each length into the desired resistor shape; and heating each of said lengths again to cure the silicone resin fully so that the silicone resin will lock the fibers of the core in the desired shape.

6. The method of forming individual resistors from a wire-wound electrical resistor strip, comprising: forming an electrical resistor strip having resistance wire coiled about a fiat, laminate-like core formed from a tensioned woven glass fiber sleeve, said core being stiffened by a coating of a partially cured silicone resin; cutting the electrical resistor strip into the desired lengths; applying a softening agent to each of said lengths to render each length pliable; forming each length into the desired resistor shape; and drying each of said lengths to evaporate the softening agent and permit the silicone resin to lock the fibers of the core in said desired resistor shape.

7. The method of claim 6 having the added steps of securing electrical terminals to opposite ends of each of the shaped resistor lengths, dipping each of said resistors into a slip of insulating cement, and heating each of said electrical resistors to give a permanent set to said cement and to cure fully the stifiening agent at the same time.

8. The method of forming a core for an electrical element, comprising: tensioning a length of woven fibrous material to obtain a substantially uniform crosssection throughout said length; impregnating said tensioned length of material with a high-temperature-resistant dielectric stiffening agent; and drying said impregnated tensioned length to permit said stiffening agent to lock the fibrous material of the length in said tensioned condition.

9. The method of forming a core for an electrical element, comprising: tensioning a sleeve of flexible fibrous woven material to form a substantially uniform cross section throughout the length of said sleeve; impregnating said tensioned sleeve with a high-temperature-resistant dielectric stiffening agent; applying pressure to the surface of the sleeve to fiatten said sleeve; and drying said impregnated, tensioned sleeve to permit said stiffening agent to lock the fibrous woven material of the flattened sleeve in said tensioned condition.

10. The method of claim 9 including the step of forcing the opposed inner surfaces of the flattened sleeve together during the drying operation so that said surfaces will adhere to each other when dry to form a laminatelike core.

11. The method of forming a core for electrical elements in a continuous strip, comprising: tensioning the leading portions of an advancing sleeve of flexible fibrous material in continuous strip form as said portions pass a predetermined point so that said leading portions have a substantially uniform cross section; impregnating said tensioned leading portions with a high-temperatureresistant dielectric stiffening agent; and drying said impregnated, tensioned leading portions so that said stiffening agent will lock the fibrous material of the leading portions in said tensioned condition.

12. The method of forming a core for an electrical element, comprising: passing a glass fiber woven sleeve through a first radiant oven to drive off organic compounds present in the glass fiber; pulling the sleeve taut until it assumes a substantially uniform flattened cross section throughout its length; passing the sleeve through a solution of a stiffening agent to impregnate the fibers of said sleeve with a high-temperature-resistant dielectric material; removing the excess of said stiffening agent from the exposed surface of said sleeve; and heating said sleeve to harden it into a laminate-like core, said stiffening agent locking the woven glass fiber sleeve in its tensioned position to maintain said substantially uniform cross-section.

13. The method of forming a core for an electrical element, comprising: passing a glass fiber woven sleeve through a first radiant oven to drive off organic compounds present in the glass fiber; pulling the sleeve taut until it assumes a substantially uniform flattened crosssection throughout its length; passing the sleeve through a solution of a stiffening agent to impregnate the fibers of said sleeve with a high-temperature-resistant dielectric material; removing the excess of said stiffening agent from the opposite flattened surfaces of said sleeve by passing it between a pair of opposed doctor blades; and heating said sleeve to harden it into a laminate-like core adapted to receive windings of wire, said stiffening agent locking the woven glass fiber sleeve in its tensioned posi tion to maintain said substantially uniform cross-section.

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