US2804529A

US2804529A - Wire wound element

Info

Publication number: US2804529A
Application number: US352470A
Authority: US
Inventors: Marlan E Bourns; Marvin E Harrison
Original assignee: Bourns Inc
Current assignee: Bourns Inc
Priority date: 1953-05-01
Filing date: 1953-05-01
Publication date: 1957-08-27
Anticipated expiration: 1974-08-27

Description

Aug. 27, 1957 BQURNS Er AL 2,804,529

WIRE WOUND ELEMENT Filed May 1, 1953 I N VEN TORS MARLAN E. BOURNS BY MARViN E. HARRISON WIRE WOUND ELEMENT Marlan E. Bourns and Marvin E. Harrison, Riverside, Calif.; said Harrison assignor to said Bourns The present invention relates to potentiometer elements, and more particularly to that type of precision potentiometer element made by winding resistance wire on a molded form or card.

In the making of wire wound resistance elements used in rheostats or potentiometers, it is customary to wind the wire on flat forms or cards produced by sawing strips from a sheet of plastic material and radiusing the edges of these strips to form rounded corners. This method has many limitations inasmuch as the form is necessarily rectangular when viewed from the side, with no steps in it. Also, it is difficult to produce because a sawing or grinding operation is involved. Furthermore, since it is important that extremely fine surface finish and parallelism be maintained in order that the finished element will be linear in electrical output, the tolerances involved often become difficult to achieve.

The primary object of our invention is to provide a new and improved card for wire wound resistance elements, which is easy and inexpensive to manufacture, and which can be produced to extremely tight tolerances of dimension and parallelism, as well as having a surface finish or smoothness that is adequate to permit the smooth winding of extremely fine wire, of the order of .005" to .0005, with no gaps or cross-overs caused by irregularities in the surface finish of the card itself.

It is a further object of our invention to produce a wire wound resistance element having a card which may be molded of fiber-glass-filled phenolic resin that will withstand very high temperatures. This material is not readily available in sheet form, and if obtained, is difficult to machine and to cut into strips. It is also very difiicult to machine a radius and a smooth finish on the edges of such a material after it has been cut, because of the glass fibers that are embedded therein.

It is a further object of our invention to provide a wire wound resistance element embodying a card molded of rubber or other flexible material, whereby the finished element may be bent in an are without damaging the element or card.

Another object of our invention is to provide a stop which is molded integrally with the element to prevent the wiper from traveling beyond the end of the element and lodging at this point, such that its return to the wiped element surface is impossible.

A further object is to provide a wire wound resistance element having resistance wire wound into a long cavity of such a depth that if the contact which rides on the wire-wound surface goes beyond the end of the actual winding, it will travel on the step molded into the card, and the junction between the end of the wound surface and the surface of the card itself will be smooth and will not cause the contact to hang up or stick at this juncture.

Another object of the present invention is to provide a wire wound resistance element having a cavity into which a shorting strip may be wrapped, so that the finished height of the shorting strip will be the same distance ates Patent ice above the remainder of the card as the diameter of the wire with which the element is wound, thereby providing a smooth juncture between the wire and the metal shorting strip. This permits the use of rather thick shorting strip material which is very durable and readily handled, rather than extremely thin material equal in thickness to the diameter of the wire, which would otherwise be required if the card had no step molded into it.

A further object of this invention is to provide an element which has metal shorting strips molded integral with the card itself. These strips project above the edge of the card such that the winding is flush with the shorting strip surface. These metal portions molded integral with the card may also be used as a termination means, since the element wire and also the electrical lead wire may both be spot welded or otherwise attached to them.

Still a further object of our invention is to provide a card having a cavity of predetermined length formed in one edge thereof, so that the wire with which the element is wound can readily be peeled bac to the required winding length without the necessity of making accurate measurements by means of a scale or special termination device.

The various features of novelty which characterize this invention are pointed out with particularity in the claim annexed to and forming a part of the specification. For a better understanding of the invention, reference may be had to the accompanying drawings, which illustrate certain preferred embodiments of the invention. In the drawings:

Figure 1 is a partially cut-away elevational view of a Wire wound resistance element embodying the principles of our invention;

Figure 2 is a similar view of a modified form of the invention;

Figure 3 is a transverse sectional view taken at 3'3 in Figure 2;

Figure 4 is an elevational view of a third form of the invention; and

Figure 5 is a cross-sectional view taken at 55 in Figure 4.

Figure 1 shows a molded plastic element card 11 having a cavity 12 into which fine resistance wire 13 has been wound. The card 11 may be made of any suitable plastic material, although we have found that excellent results are obtained with fiber-glass-filled phenolic resin, which is thermo setting and has satisfactory dielectric properties. Other thermo setting plastics, such as urea, melamine, and polyester resins may be used, either with or without fillers such as fiber glass, wood flour, and rubber; or if desired, certain of the thermo plastic resins such as polyethylene and polyamide may be used. The card 11 is an elongated, flat, relatively thin strip, with radiused top and bottom edges in the cavity 12. It may be produced by any suitable molding process, such as compression or transfer-compression molding, and comes from the mold in the desired shape with any necessary inserts embedded therein.

At the ends of the card, beyond the wire windings, the edges of the card are stepped up at 14 to provide surfaces fiush with the top surface of the wire, onto which the contact 15 may ride when it travels from the end of the wire wound surface 13 onto the remainder of the card. The contact 15 is a part of the variable resistor or potentiometer with which the resistance element is assembled, and is shown herein only for the purpose of illustrating the manner of operation. In the event the contact 15 travels beyond the normal excursion, it engages a limit stop pin 16, which is molded into the edge of the card as an insert. This prevents the contact from going beyond the end of the element and hanging up. Holes 17 are molded into the card adjacent the ends thereof for usein inserting termination rivets for fastening the element wire and the'lead wire."

When Winding the element shown in Figure 1, it is customary to wind nearly to the ends of the card, then peel back the resistancejwire," sincethe' beginning "and" ending of the winding is frequently of lo'werquality winding than the central portion". Peeling 'b'a'ck may easily be done, since the wire may merely bepeeled back until the cavity 12 is reached. This provides an easy and automatic means of stopping at the'desired place and thereby providing the required winding length.

Figures 2 and 3 show another embodiment of our invention. The card 18 has a recess 19 molded into one edge thereof beyond the end of the "wirewindings, and bent over the said recess is a shorting strip 20', which is secured in place by a terminal rivet '21. The bottom of the notch 19 is set down below the surface on which the wire 22 is wound, so that thetop surface 'of the shorting strip 20 is level with'the top surface of the wire 22. For example, if the wire 22 is .005" diameter, and the shorting strip 20 is .015" thick, the recess 19"would be .010" deep. Thus, the contact 23 may pass over the juncture between the wire and the shorting strip With' no irregularity being encountered. Anupwardly projecting limit stop 24 is molded integral with the card beyond the recess 19, to prevent the contact 23 from traveling beyond the end of the shorting strip 19. A

terminal rivet 25 is molded into the card 18 and is used surface of the plastic strip 26 on which the wire 33 is' wound, so that the juncture of the wire 33 with the shorting strips 27 will be smooth. Hence, thecontact 29 may travel beyond the end of the windingonto" the shorting strip with no irregularity or discontinuity. The element wire 33 is spot welded or otherwise attached to the shorting bar 27 at the point 30; likewise thelead wire 31 is similarly attached at the point 32. The shorting bars 27 are securely anchored into the plastic material by means of holes 28, which permit the plastic to flow through from both sides.

In still another aspect of the invention, we take advantage of the molding process of forming element cards to anchor the wire windings securely to the card. This is done by partially curing the thermo-setting resin under heat and pressure, to the point where the card is still slightly soft, and then removing the card from the mold. The partially cured card is wound with the wire windings, and then heated to complete the cure of the resin. As the wire is wound onto the soft card under tension, it tends to sink into the soft plastic, and thus becomes partially embedded therein. When the card is fully cured, the Wires are firmly adhered to it, and will not slip along the card under the thrust of the wiper contact.

While We have shown three illustrative embodiments of our invention, it will be understood that We do not Wish to be limited thereto, as various changes and modifications will occur to those skilled in the art, which come within the scope of the appended claim.

We claim:

A wire-wound electrical resistance element comprising a card of fiber-glass-filled phenolic resin molded into an elongated, relatively flat strip, a plurality of turns of resistance wire wound on said card, said card having a recess molded into one edge thereof beyond the end of said Wire, a shorting strip bent over said one edge of said card and seated within said recess, said shortingstrip being connected to the adjacent end of said wire and being of a thickness such that the outer edge of said shorting strip is flush with the outer edge of said wire turns, and a limit stop molded integrally with said card beyond the end of said shorting strip and projecting for a substantial'distance above said outer edge thereof.

References Cited in the file of this patent UNITED STATES PATENTS 822,332 Wood June 5, 1906 1,433,740 Nagel Oct. 31, 1922 2,307,626 Kelly Jan. 5, 1943 2,311,613 Slayter Feb. 16, 1943 2,332,376 Haberberger Oct. 19, 1943 2,346,598 Mucher Apr. 11, 1944 2,371,313 'Rast et al Mar. 13, 1945 2,408,093 Patterson Sept. 24, 1946 2,468,144 Van Alen Apr. 26, 1949 2,471,592 Thomson May 31, 1949 2,534,994 Scott et al. Dec. 19, 1950 2,566,911 Scott Sept. 4, 1951