US3771094A

US3771094A - Adjustable electrical resistor having a helical coil of resistance material in threaded, biased engagement with a rotatable internal contact member

Info

Publication number: US3771094A
Application number: US00212686A
Authority: US
Inventors: S Johnston
Original assignee: Bunker Ramo Corp
Current assignee: Amphenol Corp; Bunker Ramo Corp
Priority date: 1971-12-27
Filing date: 1971-12-27
Publication date: 1973-11-06
Anticipated expiration: 1990-11-06

Abstract

A helical resistance element of wire sufficiently stiff to make the element self-supporting is traversed by a contact member threadedly engaging the inside of the helix at circumferentially spaced points. Contact is maintained by the fact that the member is a tight fit in the element, actually expanding it somewhat from its normal diameter. The contact member is mounted on a conductive shaft, for which a low friction, high pressure and high conductivity bearing of silver graphite material is provided at one end of the helix, external connection to the contact member being made through the bearing and the shaft. Assembly is done by temporarily expanding the end turns of the coil to permit insertion of the associated parts, which are retained by the spring contraction of the coil when released.

Description

United States Patent 11 1 Johnston 14 1 Nov. 6, 1973 1 ADJUSTABLE ELECTRICAL RESISTOR 2,899,661 8/1959 Gruer 338/202 HAVNG A HELICAL COIL 0F RESISTANCE 2,680,896 6/1954 Groce 308/D1G. 10 8221;: :122? 2188 ENGAGEMENT WITH A ROTATABLE 3:427:548 2/1969 DetWell61'.... 338/143 INTERNAL CONTACT MEMBER 1,438,753 12/1922 Douglass 338/202 Inventor: Samuel A. Johnston, Fontana, Wis.

Bunker Ramo Corporation, Oak Brook, 111.

Filed: Dec. 27, 1971 Appl. No.: 212,686

Related US. Application Data Continuation-impart of Ser. No. 108,503, Jan. 21, 1971, abandoned.

Assignee:

US. Cl 338/143, 338/180, 338/202, 338/304 Int. Cl [1016 5/02 Field of Search 308/DIG. 10, 73; 200/166 C; 310/90; 338/136, 143, 147-149, 160, 162, 171,202, 68, 75, 98, 118,148,157, 176, 180, 296, 304, 315, 330

References Cited UNITED STATES PATENTS Mairs 338/143 Primary Examiner-Robert K. Schaefer Assistant Examiner-Gerald P. Tolin AttorneyFrederick M. Arbuckle [57 ABSTRACT A helical resistance element of wire sufficiently stiff to make the element self-supporting is traversed by a contact member threadedly engaging the inside of the helix at circumferentially spaced points. Contact is maintained by the fact that the member is a tight fit in the element, actually expanding it somewhat from itsnormal diameter. The contact member is mounted on a conductive shaft, for which a low friction, high pressure and high conductivity bearing of silver graphite material is provided at one end of the helix, external connection to the contact member being made through the bearing and the shaft. Assembly is done by temporarily expanding the end turns of the coil to permit insertion of the associated parts, which are retained by the spring contraction of the coil when released.

12 Claims, 7 Drawing Figures PATENTED NW 6 I975 mm NN ln'ventor Samuel AJ'ohnston ADJUSTABLE ELECTRICAL RESISTOR HAVING A HELICAL COIL OF RESISTANCE MATERIAL IN THREADED, BIASED ENGAGEMENT WITH A ROTATABLE INTERNAL CONTACT MEMBER CROSS-REFERENCE TO RELATED APPLICATION This application is a continuation-in-part of US. Pat. application Ser. No. 108,503, filed Jan. 21, 1971, now abandoned.

BACKGROUND OF THE INVENTION Many applications in the electrical and electronic fields require a variable resistance to permit adjustment of a circuit, usually to bring its operating characteristics within certain desired parameters. Such requirements may arise in many types of control, logic, and computer circuitry, for example. The proliferation of such circuits in present day technology, and the accompanying need for large quantities of variable resistances, have brought forth many devices for the purpose, but there is always a demand for lower cost, less bulk, greater stability, simpler mounting, and, for a unit of given space needs, greater power dissipation capability.

SUMMARY OF THE INVENTION The present invention responds to all those demands.

In a preferred embodiment, the present invention provides the resistance element as a helically wound coil of solid wire. A conductive contact member threadedly engages the inside of the helical coil, its electrical contact therewith being maintained by the fact that it is a tight fit within the coil, the contact pressure therefore being maintained by the spring character of the coil itself. Rotation of the contact member, and hence its movement along the length of the resistance element, is made possible by mounting it on a conductive shaft supported by a conductive bearing disposed at one end of the coil, external electrical connection to the. contact member being made through the bearing and the shaft. The bearing is of predetermined design and of specially chosen self-lubricating material of high electrical conductivity so as to permit obtaining high pressure, low friction and low resistance contact with the shaft while also providing for ease and reliability in shaft adjustment. At the ends of the helix, extensions of the wire from which it is wound serve both as terminals and as supports, the wire being of such size and stiffness that the helix maintains its shape when supported by the extensions.

There is thus achieved in the preferred embodiment disclosed a variable resistance involving a minimum number of parts, and quite simple to assemble, so that the prime object of low cost is attained. Further, there is no housing, enclosure, or accessory mounting hardware required, and these facts point to the attainment of several more of the desired objects less space is needed, and the mounting is simple. Since ambient air can circulate'freely around the open helix, power dissipation characteristics are good, and by the avoidance of hotspots, the resistance value tends to be stable.

' BRIEF DESCRIPTION OF THE DRAWING Details of the arrangement by which these objects are attained are more fully set forth in the following description and-illustrated in the accompanying drawing, in which: 1

FIG. 1 is a side elevational view, partly in section, of a preferred embodiment of the invention;

FIG. 2 is an elevational view of one end of the device, on the same scale as FIG. 1;

FIG. 3 is a side elevational view of a pair of blocks which form a support bearing;

FIG. 4 is an end elevational view of the blocks of FIG.

FIG. 5 is an end elevational view of a contact member;

FIG. 6 is a side elevational view of the contact member of FIG. 5;

FIG. 7 is an elevational view, similar to FIG. 2, but on a somewhat enlarged scale, to illustrate the action of the contact in spreading a coil of the resistance element.

DESCRIPTION OF THE PREFERRED EMBODIMENT lnthe drawing, the reference numeral 10 is used to indicate generally a resistance element. It is a coil wound from resistance wire, primarily in the shape of a single layer open helix ll, i.e., one with some space left between adjacent turns of the wire. At the ends of the helix, the wire is brought out to form the

terminals

12 and 14, by which the resistance element can be mounted. In the substrate 16 of a printed circuit, for example, the wires can be thrust through holes 18 in the substrate and soldered in place by well-known means. The terminal wires can be swedged as at 20, to form an enlargement which will abut against the substrate, to hold the resistance element at a desired elevation above the substrate.

A contact member 22 is formed with peripheral grooves 24 in a helical pattem,-such as to engage the inside surface of the wire helix 1 l. The contact member is formed of suitable electrically conductive material, such as carbon, graphite or powdered metal, and can be molded by well-known techniques. Because of unavoidable imperfections in the helix, such as eccentricity, out-of-roundness, or variations in pitch, the groove 24 is preferably not continuous, but the contact member is relieved at several places around its circumference, as at 26, so that it does not contact the wire at these places. This leaves groove 24 in discrete sections of limited circumferential extent. Three such sections, equally spaced circumferentially, are preferred.

The contact member is mounted on an operating shaft 30, which has a portion of reduced diameter 31 on which the contact member is retained by swedging a rivet-like head as at 33. The shaft is formed of a conductive material, and the attachment between the shaft and the contact member must provide for good electrical continuity. Soldering or welding can be used where the members are metallic, and conductive adhesives can be used when the contact member is of carbon or graphite. The shaft is provided, at its end opposite the contact member, with a slot 32 permitting its rotation by a screwdriver or similar tool.

At the end of the helix through which the operating shaft passes, a pair of

bearing blocks

34 and 36 are supported in the last turn of the coil. The bearing-blocks are formed, in effect, as the two parts of a spool split along a diameter. To facilitate assembly within the coil, it is advantageous to form the parts as somewhat less than half circles, as indicated by the space across the diameter in FIG. 4, where the outline of the parts is a constituted when the

blocks

34 and 36 are located in the relation shown in FIG. 4, one end flange of the spool, designated at 38 and 40, is substantially continuous, while the other end flange has a

helical groove

42, 44, formed therein, of pitch and diameter to fit the inside of the helix 1 l, and leading into the circular groove 45, 47. The final turn of the resistance element is formed with about a half turn which is a plane circular form, i.e., that portion of the coil between the crosssection at 46 in FIG. 1 and the terminal 12 is circular, not helical. For assembly of the

parts

34 and 36 within the coil, it may be necessary to spring the last turn of the coil temporarily to a larger diameter, by holding the main portion 11 of the helix in a suitable fixture, suggested as shaped clamp blocks 48 shown in dotted lines in FIG. 2, and moving the terminal 12 in the direction of the arrow 51 in that figure.

The

blocks

34 and 36 are provided with notches 52 in order to assure positive high pressure contact in limited areas only between the blocks and the shaft 30, which is shown dotted in FIG. 4.

The end of the resistance element opposite the bearing blocks is continued in the helical form to the point where the terminal 14 diverges from the helix. This permits the contact member to be assembled into the resistance element by threading it into the coil at that end. The contact member must be a close fit in the helix,

and the bearing blocks must be a pressure fit on the v shaft. Therefore, for assembly it may be desirable to use the technique previously described, i.e., to hold the main portion 11 of the helix in clamp blocks 48. The terminal 12 can then be moved in the direction 51 (FIG. 2) to open up the left end turn of the coil (as seen in FIG. 1) thus increasing the available diameter for the

blocks

34 and 36, and permitting the slotted end of the shaft to be inserted between them. Similarly, the right end turn of the coil (as seen in FIG. 1) can be opened up by moving the terminal 14 in the direction 50 (FIG. 2) to permit the contact member to be threaded into the coil.

After the terminal 12 is released, and the coil seeks to return to its normal diameter, the constriction upon the

blocks

34 and 36 will cause them to have a firm pressure engagement with the shaft 30.

After the terminal 14 is released, there will be a similar pressure engagement between the helix 11 and the contact member 22. Wherever the contact member may be traversed along the helix, this pressure engagement will be maintained, because the contact actually expands the helix somewhat at the point where it is located. This expansion is illustrated (exaggerated) by the dotted lines 56 in FIG. 7.

After the contact member has been assembled within the helix, the latter may be swedged as at 54, adjacent to the terminal 14, to provide a stop which will prevent the contact member from being inadvertently run out of the helix in subsequent adjustments.

It will be observed that the preferred embodiment of this invention provides a rheostat with only five parts,

of relatively simple construction, fulfilling the needs,

previously described. Contact pressure, both on the contact member and on the bearing block, is maintaincd without the need for any spring element other than the resistance coil itself.

Some changes may be made in the form and arrangements of the invention, yet still be within the intent and 4. purport of the following claims. For example, although a housing, enclosure or other accessory mounting or terminal structure are not required in the preferred embodiment of the invention disclosed herein, such additional structure may be provided where desirable or appropriate in a particular application.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A rheostat comprising a self-supporting helical coil of resistance wire, portions of said wire diverging from said coil at the ends thereof and forming supporting terminals, a contact member threadedly engaging the inside of said helical coil in a manner so that the position of said contact member in said coil may be varied by threaded movement of said contact member relative to said coil along the length thereof, said contact member engaging the coil at a plurality of discretely separated areas within one turn thereof, an operating shaft for said contact member extending axially of said coil and being operative to rotate said contact member to cause the position of said contact member in said coil to be varied as a result of threaded movement of said contact member relative to said coil, and a conductive bearing for said shaft retained in one end of said coil, said conductive bearing including a pair of conductive blocks of self-lubricating silver graphite material having axially extending notches for providing pressure engagement between said blocks and said shaft at a plurality of spaced high pressure contact areas around said shaft wherein said conductive blocks are held in pressure contact with said shaft by the diametral contraction of said coil as a spring upon said blocks.

2. A rheostat in accordance with claim 1, wherein said portions of said wire are swedged to provide stop means whereby said helical coil will be located at a predetermined minimum distance above a mounting surface. i 3. A rheostat in accordance with claim 1, wherein said helical coil comprises a self-supporting single layer coil, wound in spaced helical form with the exception of a segment of less than one turn at one end of said coil, which segment is in plane circular form, said bearing being retained in the end of said coil having said segment of plane circular form in a manner so as to provide said diametral contraction.

4. A rheostat in accordance with claim 3, wherein said blocks are placed together along a diameter to form a spool having a plane, circular circumferential groove adapted to fit into said coil with said segment thereof engaging said groove.

5. A rheostat in accordance with claim 4, wherein the wire of said coil at the opposite end from said segment is deformed to form a stop engageable with said contact member, preventing overtravel of said contact memher.

6. A rheostat in accordance with claim 3, wherein said blocks are placed together along a diameter to form a spool having plane circular circumferential groove, and having a helical groove leading into said plane circular groove, said plane circular groove being adapted to fit into said coil with said segment thereof engaging said plane circular groove, and said helical groove being adapted to engage a helical portion of said coil adjacent to said segment.

7. A variable resistance comprising a helical coil of wire of sufficient size and stiffness so as to be selfsupportable, a conductive contact member biased against the inside of said coil and including position varying means threadedly engaging, the inside of said coil so as to provide for varying the axial position of said contact member in said coil by threaded relative movement therebetwcen, an operating shaft coupled to said position varying means for varying the axial position of said contact member, said shaft extending axially of said coil and being operative to rotate said contact member to cause the position of said contact member in said coil to be varied as'a result of threaded movement of said contact member relative to said coil, a conductive bearing for said shaft secured at one end of said coil, said conductive bearing comprising a pair of oppositely disposed bearing blocks of highly conductive self-lubricating material biased against said shaft and held in pressure contact therewith, said conductive bearing including a pair of conductive blocks of selflubricating silver graphite material having axially extending notches for providing pressure engagement between said blocks and said shaft at a plurality of spaced high pressure contact areas around said shaft, and means for providing external connection to said variable resistance such that the resistance provided thereby is dependent upon the axial position of said wherein said pair of bearing blocks are formed as the two parts of a spool split along a diameter and having opposed notches shaped to provide high pressure contact of said blocks with said shaft.

11. The invention in accordance with claim 7, wherein said blocks have opposing notches shaped to provide high pressure engagement of said blocks with said shaft in a limited plurality of areas.

12. The invention in accordance with claim 11, wherein said notches are of triangular shape so that said blocks engage said shaft at four discretely separated areas.

Claims

2. A rheostat in accordance with claim 1, wherein said portions of said wire are swedged to provide stop means whereby said helical coil will be located at a predetermined minimum distance above a mounting surface.

3. A rheostat in accordance with claim 1, wherein said helical coil comprises a self-supporting single layer coil, wound in spaced helical form with the exception of a segment of less than one turn at one end of said coil, which segment is in plane circular form, said bearing being retained in the end of said coil having said segment of plane circular form in a manner so as to provide said diametral contraction.

5. A rheostat in accordance with claim 4, wherein the wire of said coil at the opposite end from said segment is deformed to form a stop engageable with said contact member, preventing overtravel of said contact member.

7. A variable resistance comprising a helical coil of wire of sufficient size and stiffness so as to be self-supportable, a conductive contact member biased against the inside of said coil and including position varying means threadedly engaging the inside of said coil so as to provide for varying the axial position of said contact member in said coil by threaded relative movement therebetween, an operating shaft coupled to said position varying means for varying the axial position of said contact member, said shaft extending axially of said coil and being operative to rotate said contact member to cause the position of said contact member in said coil to be varied as a result of threaded movement of said contact member relative to said coil, a conductive bearing for said shaft secured at one end of said coil, said conductive bearing comprising a pair of oppositely disposed bearing blocks of highly conductive self-lubricating material biased against said shaft and held in pressure contact therewith, said conductive bearing including a pair of conductive blocks of self-lubricating silver graphite material having axially extending notches for providing pressure engagement between said blocks and said shaft at a plurality of spaced high Pressure contact areas around said shaft, and means for providing external connection to said variable resistance such that the resistance provided thereby is dependent upon the axial position of said contact member in said coil, and such that external connection to said contact member is made through said bearing and said shaft, wherein said conductive blocks are held in pressure contact with said shaft by the diametral contraction of said coil as a spring upon said blocks.

8. The invention in accordance with claim 7, wherein silver constitutes the major proportion of said silver graphite material.

9. The invention in accordance with claim 8, wherein said silver graphite material is 93 percent silver.

10. The invention in accordance with claim 7, wherein said pair of bearing blocks are formed as the two parts of a spool split along a diameter and having opposed notches shaped to provide high pressure contact of said blocks with said shaft.