US2458314A - Variable electrical resistor - Google Patents

Description

Jan. 4, 1949. L. STRAUB ETAL 2,458,314

VARIABLE ELECTRICAL RESISTOR Filed Jan. 11, 194s s Shets-Sheet 1 v V an-l 06E/YE 4, 575 9 5 Inventors PET/10 E-c. ZEN V57 Attorney Jan. 4, 1949. E. STRAUB ET AL 2,458,314

VARIABLE ELECTRICAL RESISTOR Filed Jan. 11, 1946 5 Shets-Sheet 2 504mm? .4. STRH gI E/IC DPS Mar/leg Jan. 4, 1949. E. sTRAUB EI 'AL 2,458,314

VARIABLE ELECTRICAL RESISTOR Filed Jan. ll, 1946 3 Sheets-Sheet 3 Patented Jan. 4, 1949 VARIABLE ELECTRICAL RESISTOR Eugene Leopold Straub, Wembley Hill, and

Arthur Edward Charles Bennett, Kenton, England, assignors to Dubilier Condenser Company (1925) Limited, London England, a company of Great Britain Application January 11, 1946, Serial No. 640,490 In Great Britain January 18, 1945 3 Claims. 1

This invention relates to variable electrical resistors and is more particularly concerned with small resistors, such as are commonly employed as volume controls in wireless receivers.

When such resistors are called upon to withstand frequent relatively large temperature cycles, and the combination of high temperatures and high humidity which are found in tropical climates, it is essential for them to be protected against the ingress of moisture and otherwise to be so constructed that the moving contacts and the cooperatin resistance element will not be exposed to moisture. The main object of the present invention is to provide a simple and relatively cheap construction of hermetically sealed variable resistor which permits a standard resistor suitable for temperate climates to be used and does not entail special construction of the complete device.

The improved resistor according to the invention is hermetically sealed for operation under tropical conditions by enclosure in a metal casing having one closed end through which extends an adjusting spindle for the resistor whilst the other end of the casing is closed by a sealing disc of rubber or water-resisting rubber-like material, through which extend leads to the resistor, this end of the casing being hermetically sealed by resiliently pressing into the outer surface of the sealing disc both the edge of the easing and the flanges or rims of tubular rivets or eyelets fixed to the leads, so that the sealing disc is compressed and deformed to provide waterprooi' joints between the rubber and the metal pressed into it. Preferably the casing is cupshaped and may, for example, be of aluminium, or other non-brittle metal.

This form of sealing is particularly applicable to so-called miniature construction and in the improved resistor according to the present in vention, which enables the dimensions to be very considerably reduced as compared with normal construction, the resistor element is supported by the casing sealing disc whilst the adjusting spindle, carrying a moving contact for engaging the e ement, is carried by a bearing on the closed end of the casing so that the casing itself, which is cup-shaped, acts as the mechanical supporting structure of the resistor.

The invention may be carried into practice in various ways, but two convenient practical arrangements, each according to the invention, are shown by way of example and as applied to a volume control in the accompanying drawings, in which Figure 1 shows one construction in side elevation,

Figure 2 is an exploded longitudinal section on the line IIII of Figure 4, but on a larger scale,

Figure 3 is a section on the line III-III of Figure 2,

Figure 4 is a section on the line IVIV of Figure 2,

Figure 5 illustrates the resistor in end elevation from right to left of Figure 1,

Figure 6 is a diagrammatic representation of the electrical connections within the resistor casing,

Figure 7 is a fragmentary section through a modified form of scaling for the electric leads where they extend through the end discs,

Figure 8 illustrates a modified arrangement of the sealing rings between the adjusting spindle and its bearing,

Figure 9 is a View, similar to that of Figure 1, of a modified construction of resistor,

Figure 10 shows the resistor in end elevation from right to left of Figure 9,

Figure 11 is a view, similar to that of Figure 2, of the modified construction shown in Figures 9 and 10 but on a larger scale,

Figures 12, 13 and 14 are sections on the lines XII-XII, XIII-XIII and XIV-XIV of Figure 11 respectively,

Figure 15 is a diagrammatic representation of the electrical connections within the casing of the resistor shown in Figures 9 to 14 inclusive, and

Figures 16 and 1? illustrate two modified forms of tubular rivet which may be employed with either of the constructions above described.

In the construction shown in Figures 1 to 6 the resistor itself is of the usual type consisting of a disc A of rigid insulating material having on one surface a resistance element B consisting of a fiat strip of conducting resistor material, such for example as a carbon compound, shaped as an annulus but with a break to provide two ends B1, B2 which are spaced from one another and are respectively connected through rivets B3, B4 to two terminal tags B5, B6 lying in recesses B7 in the disc A. A moving contact C, consisting of several springy fingers or wires, is resiliently pressed into contact with the strip B and is supported on an insulating contact arm C1 carried at one end of a spindle D extending through a bore in the centre of the disc A. A main bearing the recess or indentation F2 on that side thereof remote from the strip B. The adjusting spindle D projects beyond the end of the bearing E and, when assembled, has an operating knob D1 secured thereto. A sliding contact is provided between a metallic annulus C2, carried-'by'the contact arm C1 and connected to the spring contact C through a metal strip anchored to the arm C1 by a rivet C3, and a bifurcated springy contact C4 connected through a rivet C5 to a third terminal tag Cs lying in one of the recesses B7 of the disc .A. The-spindle 3D is held in position in the bearing -E, soithat there is appropriate pressure between the contacts "B, C and C2, C4, by a C washer D2 which clips into an annular groove D3 in the spindle D and thus forms a flange-like abutment bearingagainst the outer end of the bearing boss E. The outside of the bearing boss E is screwthreaded to receive a collar E1 wh ch is screwed on to the bearing'boss E so as firmly to clamp the resistor unit within the metal casing F.

In order, in accordance with the invention, to make such resistor trop c-proof, it is enclosed inthemetal casing'F which is substantially cupshaped and has a holeF1 in'its closed end through which projects the spindle D and a part of the bearing boss E. Athin annular ring'G1 'of 'insulating material, for example of fibre, elephantide or similar material, "and of a diameter slightly less than the internal diameter of the casing F, is inserted in the casing'before the resistor'is assembled in the casing F and'serves to prevent metallic contact between the casing F and any of the terminal tags B5, B6, C6 on the main resistor disc A.

The jo nt-between theedge of the hole'F1 in-the end of the "casing "F and the bearing boss 'E is hermetically sealed. When'the-metal of the casing F and the boss E can readily be-soldered, this seal can be effected by running solder around the ed e of theholeFl after the resistor isin posit on. When, however, as isusual andpreferable, the casing F is of aluminium, andthus cannot eas ly be soldered, the hermetic seal is provided by compressing, between'the insulating disc G1 and the inside of the end of the casing F, 'a'washer Gof rubberora water-resistingrubber-like material, such'for exampleasthe synthetic-rubber neoprene. The rubber washer G is inserted on to'the bearing boss'E a'fterthe-insulating disc G1, and the *casing F is countersunkto form'a recess F2 around 'the inside of the hole F1 so that the washer G is trapped in the annular recess thereby "preventing or reducing outwardradial 'expansion of the washer when it is axially compressed. The washer G is compressed, so as to render the 'jointmoisture-proor and water-proof, "by the'collar E1 which, when tightened, draws the resistor towards the end. of the casing F'so that "the "washer G is compressed between the disc A an'dthe sai'den'd df'the casing. Theradial inward expansion of the washer G 'as a "result of th s compression will cause the washer G 'to "tighten 'on"to"the"boss E, thereby assisting in'the sealing 'against ingress *of moisture.

An additional indentation- Fe is provided adjacent to 'the outer rim of the casing F and projecting outwardly to a greater extent than The relative depths of the two indentations are preferably such' that -when the collar E1-is clamped'up tightly'its outer face is approximately level with the outer indentation F6. "The outer end surface of the casing F of the complete resistor-can thus be pressed against the surface of the :panel idndicated at "1) on which it is mounted. A nut E2 on the threaded boss E enables the user of the component to clamp it on to the panel T.

The disc A of the resistor unit itself with the interposed insulating washer G1, bears against the annular surface Fzbeing clamped thereto by the collar E1. The insulating washer G1 is preferably arranged to fit over the outside of the resilient washer G which thus bears directly against the more moisture impervious moulded disc A.

:disc Hrbeing engaged by an annular indentation or shoulder F4 formednear the rim of thecasing Flby'spinning or other. suitable shaping operation. The hermetic seal is obtained by spinning over the riinFs of the casing F so that it presses into and deforms the outer surface of the rubber, or

rubber-like, sealing disc H without perforating it. The leads J, J1, J2 to the three terminal tags B5, B6, Ce also extend through spaced holes in the discs H, H1. :throughthe two discs I-I, H1, the edges of the Three tubular rivets K extend outertfianges of the rivets being pressed into the outer surface of the rubberdisc H so as to deform it .and thus form a water and moisture-proof joint a's in the case of the joint F5 at the rim of the casing F. The leads J, J1, J2 extend axially through the rivets K and are sealed therein by solder K1 which also closes the bores of the rivets K. Alternatively, each lead J3 (Figure 7) may have dishedmetal eyelets or washers K2 soldered thereto after assembly, the edge of each outer washer'Kz being-resiliently pressed into the outer surface of the disc H to-effect the requisite seal. In order that the bearing for the spindle D "may also act as asealing gland, the spindle is provided with'an annular groove D4 which, prior 'to assembly, is filled with a water-resistant oil -or-grease which serves also as a lubricant. addition, the spindle D has formed therein two annular retaining grooves D5 on opposite sides respectively of the groove D4. Each groove D5 has "fitted therein a ring or washer D6 of water- 'res'istant rubber-like material, such, for example,

as neoprene-or'hycar-or, the internal diameter of the ring D6 being less than the diameter at the bottomofthe associated groove D5 whilst the external diameter of the ring is slightly greater than the internal diameter of the boss E. The ringsDs'arepreferably of a'synthetic rubber such for example as neoprene, or any other suitable rubber-like material possessing adequate oil resistance which will prevent deterioration by the oil or grease-spreading over the surface of the spindle 'and'its hearing from the groove D4. Thus, :on assembly, the resilient rings De grip the spindle and are radially compressed between the spindle D and the bore of the bearing boss E and, though permitting free rotation of the spindle D, assist the action of the oil channel D4 to prevent the ingress of moisture. Instead of the retaining grooves being formed in the spindle D, grooves D7 (Figure 8) may be formed in the boss E in addition to the oil channel D4 in the spindle D, the external diameter of each resilient ring De being greater than the diameter at the bottom of the associated groove D7 whilst the internal diameter of the ring is slightly less than the diameter of the spindle D.

In order to limit the angular movement of the spindle D, and therefore the travel of the moving contact C over the resistor element B, an abutment A1 formed integral with the disc A cooperates with a stud A2 carried by the spindle D.

The construction above described particularly lends itself for mounting within the containers or casings of radio sets or other apparatus, which casings are sealed against ingress of moisture. To this end a ring of neoprene or other suitable material may be arranged in the recess F7 (Figures 2 and 3) so that when the nut E2 outside the casing is tightened the said ring is compressed and held against lateral expansion by the recess F1. This ring thereby seals the opening in the casing through which the boss E extends.

In the modified construction shown in Figures 9 to 15, the dimensions may be very considerably reduced as compared with normal constructions of resistor or volume control, the resistor element L being supported by the rigid insulating backing disc M associated with the rubber, or rubber-like, sealing disc N. The adjusting spindle O, carrying the moving contact 01 for bearing against the strip resistor element L, is journalled in a bearing boss P formed integral with the closed end wall R1 of the metal casing R. Thus, the casing R itself, which is cup-shaped as shown in Figure 11, acts as the mechanical supporting structure for the resistor. With this construction the casing R and the boss P may be formed in two parts of brass, soldered together to form a hermetic seal, in which case they may conveniently be tin plated or cadmium plated to increase their resistance to corrosion by damp atmospheres. Alternatively, these two parts may be formed integrally together as for example by an extrusion process, in which case they may for example be constructed of hard aluminium, or aluminium a1- loy. As clearly shown in Figure 12, the strip-type resistor element L, formed as an annulus with a break in it, lies on, or in a recess in, the inner face of the rigid disc M which backs the rubberlike sealing disc N. The three leads Q1, Q2, Q extend into three tubular rivets Q3, Q4, Q5 respectively, each rivet. extending through the two discs M and N. The two rivets Q3, Q4 are electrically connected respectively to the two ends of the resistor element L and secure the resistor ele- M ment to the rigid disc M (Figures 12 and 15), whilst the third rivet Q5 is electrically connected to a bifurcated springy contact member whose two arms S carry contacts S1 both of which bear against a metal plate 02 lying on, or in a recess in, the outer surface of the insulating disc 03 within which the adjusting spindle O is moulded. The moving contact 01 is constituted by a tongue struck upwards from the plate 02, as shown in Figure 13, the plate being secured to the disc 03 by integral tongues 01 on the plate 02 extending through notches O5 in the disc 03 and being clinched over behind the disc, as shown in Figure 14. A stop R2 formed within the casing R cooperates with a rib 06 formed on the disc or block 03 so as to limit the angular rotation of the spindle O and therefore the movement of the contact 01 over the resistor element L.

The moving contact 01 and the contacts S,

vS1 may be of any suitabe form to provide a good and durable rubbing contact with the resistor element L and the plate 02 respectively, but preferably these contacts are of stiff springy metal which may be plated with silver, nickel or other non-rusting metal, a cup or pimple being pressed out at or near the ends of each contact arm to provide a curved profile for engaging the cooperating surface L or 02.

Alternatively the contact 01 may be duplicate so as to provde a dual point of contact with the element L, a second tongue being struck upwards from the plate 02 for this purpose so that the second contact lies closely adjacent to the first one. According to a further arrangement a multiple wire contact shimilar to that shown in Figure 2 and secured to the plate 02 may be employed. Such multiple contacts are preferably arranged so that their points of contact with the resistor element L are made substantially on a radial line.

The seal at one end of the casing is eifected as described with reference to Figure 2, that is to say by pressing the outer flanges of the rivets Q3, Q4, Q5 into the outer surface of the rubberlike sealing disc N and then soldering the leads to close the outer ends of the rivets as indicated at Q6. At the other end of the casing R, the boss P is externally screw-threaded to receive the clamping nut T1 by which the resistor is locked to the panel T, whilst the adjusting spindie is held in position by a C washer T2, as above described. Alternatively, the spindle 0 may be provided with a collar or the equivalent at its inner end which engages the inside of the casing R (or the insulating block 03 may serve this purpose).

It will be appreciated that the dimensions of the various parts are such that when the end discs M and N are fitted on and the edge of the casing R spun over to effect hermetic sealing, the moving contact 01 bears against the element L with the appropriate pressure whilst the contacts S1 press against the plate 02.

The bearing of the spindle O is sealed by means of the oil groove V and the two retaining grooves V1 containing the compressible rings V2, as described with reference to Figure 2, through the arrangement of Figure 8 may be employed.

Instead of the tubular rivets K or Q3, Q4, Q5 described above each rivet may be formed with a tubular socket W (Figure 16) at its inner end for riveting over and soldering on to the associated lead within the casing, an intermediate integral rib W1 being turned or pressed on the rivet for clinching over into the outer surface of the resilient sealing disc H or N. The outer end of the rivet has either a solder tag W2, as shown in Figure 16, or a screwthreaded stub W3 (which may itself have a solder tag) for receiving a terminal nut We as shown in Figure 17.

It has been found from comprehensive tests that the improved volume control according to the invention is not only capable of withstanding normal tropical tests but can in fact withstand a temperature of C. at 100% relative humidity for prolonged periods without any moisture penetrating the casing. It will also withstand continuous immersion in water for substantial periods without leakage into this casing.

Moreover, the construction of Figures 9 to 15 has a minimum of parts which are so arranged as to facilitate midget construction. Thus, for example, with this construction a tropic-proof r'netically sealed.

volume control having the characteristics normally required for wireless receivers may have a casing less thanthree-quarters of an inch in diameter and less than half an inch long. A further -advantage of the construction, in addition to the very great reduction in size as compared with normal volume controls, is the improved heat dissipation obtained by the use of -.the metal casing as the main mechanical supporting structure.

It will be understood that the constructions above described are given by way of example only and that details'may be made, within the scope of the invention, to suit the form and use of the resistor. Thus, for example, whilst the invention has been described as applied to a volume control of the circular type with a flat strip composition element, the resistor body may be ofvany other convenient Shape and the element may be disposed in any other convenient manner, for example edgewise, and may consist of a conducting composition or may be wire- -wound.- It is also to be understood that where practicable any suitable materials other "than those specifically mentioned may be used.

What we claim as our invention and desire to secure by Letters Patent is:

l; A variable electrical resistor comprising a cup-like aluminium casing open at one end and having anapertured end wall at the other end, an axially bored and externally screwthreaded bearin boss extending through said aperture, an

insulating body secured to the boss within the casing, a rotary adjusting spindle within the bearing boss and extending through the insulating body into the casing, an annular groove, for containing sealing oil o'r' grease, formed in the part of the spindle lying within'the bearing, two supplementary annular grooves formed in the spindle on opposite sides of said oil groove respectively, rings of resilient rnaterial located in ing, the rubber washer being compressed between the base of said recess and'the insulating washer when the clamping nut is tightened to draw the insulating body towards the said casing end wall,

a resilient sealing disc closing the said open end of the casing, a rigid backing disc of insuating material arranged against the inner face of said sealing disc, the rim at the open end of the casmg being spun into a bead housing the edge portions of the sealing and backing discs with the edge of said rim pressed into the outer surface of the sealing disc without perforating the disc, thereby hermetically sealing the joint between thecasing and the discs, three tubular rivets extending through apertures in the discs, and three electric leads extending respectively into the rivets and electrically connected to the moving contact operated by the spindle and to the two ends of the resistor element, the outermost flanges of the rivets being pressed into the outer surface ofthe sealing disc whilst the leads'are soldered in the rivets whose bores are thus her- 8 "2'. A variable"electri'c'al resistor comprising a cup-like aluminium casing open at one end-arid having an apertured end wall at the other end, an axially bored and: externally screwthreaded bearing boss formed integral with the casing and projecting "axially :from the said end wall thereof,

a rotary adjusting spindle within the bearing boss andextending'into the casing, an annular groove, for containing sealing oil or grease, formed in thepart'of the spindle lying within the bearing, two supplementary annular grooves formedin the spindle on opposite sides respectively of said oil groove, resilient rings locatedin said supplementary grooves and compressed between the spindle and the bearing bore, an insulating body carried at the inner end of the spindle within the'casing, a'metal end plate carried'on the inner face of the *saidinsulating body, a moving contactcons'tithted-by "an integral tongue struck-up fromsaid endplate, a resilient sealing disc closing the open end of the casing, a rigid insulating backing disc arranged onthe inner face of the sealing disc, the rim at the open end of the -casin -being spun into a bead housing the edges of the sealing and backing discs with the edge of said rim pressed into the outer surface of the sealing disc without perforating the disc, thereby hermetically sealing the 'joint between the casing and the sealing disc, a strip resistor element carried on the inner face of'th'e backing disc, three tubular rivets extending through apertures in the discs, three electric leads extending into the tubular rivets respectively and soldered therein so as to seal the bores of the rivets whose outer flanges or heads are pressed into theoutersurfa'ce of the sealing disc withoutperforatin the disc, thereby hermetiresiliently against the metal end plate of the spindlewhilstthe moving contact operated by the spindlebears resiliently against the resistor element.

3. A variable electrical resistor comprising a metal casing open at (One end and having an :apertured end wall at the other end, an external,-

ly-screwt'hreaded and axially bored bearing boss extending through'the hole in said end wallfia rotary adjusting spindle-extending through the bore of said boss, at least one grease retaining annular groove being formed between the spindle and the said boss whilst at least one supplementary annular groove is formed between the spindle and the boss, a radia ly compressible sealing washer in said supplementary groove, an insulating block carried by the bearing boss within the casing, a moving contact carried by the spindle within the casing, a resistance element carriedby said block and engaged by said moving contact, an axially'compressible resilient washer embracing said bearing boss and located in a recess formed in the end wall of the casing, a locking nut engaging said external thread onth'e bearing-boss-outside the end wall of the casing, said nut causing the said resilient washer to be axially compressed in said recess between the end wall of the casing and said insulating block whereby the compressible washer seals the joint between the insulating block and the end wall of the casing, a sealing disc of resilient moisture- 75 resistin material aclo'sing the said open :end of the casing, a rigid insulating backing disc arranged against the inner surface of the sealing disc, the rim of the casing being spun into a bead forming a shoulder abutting said backing disc whilst the said rim is spun inwards and pressed inwards into the outer surface of the sealing disc without perforatin the disc, thereby sealing the joint between the casing and the sealing disc, three axially bored rivets in alignment with apertures extending through the sealing and backing discs, three electric leads extending into the said rivets whose rims are bent over and pressed into the outer surface of the sealing disc thereby sealing the joints between the rivets and the sealing disc, means Within the casing connecting two of said leads to opposite ends respectively of the resistance element, and means Within the casing connecting the third lead to said moving contact.

EUGENE LEOPOLD STRAUB.

ARTHUR EDWARD CHARLES BENNETT.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,160,142 Hathorn May 30, 1939 2,358,991 Miller Sept. 26, 1944 FOREIGN PATENTS Number Country Date 473,402 Great Britain Oct. 12, 1937

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