[1]Patented Jan. 24, 1950 294959321 UNITED STATES PATENT OFFICE 2,495,321 VARIABLE RESISTOR Thomas B. Gibbs, Delavan, George W. Gilman, Janesville, and Gordon F. Laing, Delavan, Wis., assignors to The George W. Borg Corporation, Chicago, Ill., a corporation of Delaware Application January 20, 1945, Serial No. 573,680 12 Claims. (Cl. 201-56) 2 The present invention relates in general to variable resistors, or potentiometers, but more in particular to variable resistors in which the resistance element has the form of a cornpound helix ' and the object of the invention is to produce a 5 new and improved variable resistor of this character. According to a feature of the invention, the resistance elernent is of bare resistance wire helically wound on a core which itself has the form 1( of a helix, and the turns of the wire and the turns of the core are partially imbedded in the interior wall of a niolded casing which holds the turns in spaced relation. Accordii,ig to another feature of the invention, lr, a new and improved process of molding is provided, whereby the casing is molded in situ of a heat setting plastic material and whereby the material is prevented from flowing to the interior of the helix in the molding operation, thtis keep- 20 ing the resistance wire clean on the inside of the helix where it is engaged by the wiper or contact member of the resistor. The fo@regoing and other features of the inventioti will be described fully hereinafter, reference 25 being had to the accompanying drawings, in whichFig. 1 is a vertical section through a variable resistor or potentiometer embodying the invention; 30 Fig. 2 is a transverse section taken on the line 2-2, Fig. 1; Fig. 3 is a transverse section taken on the line 3-3, Fig. 1; Fig. 4 is a view, partly in section, which shows 35 the construction of the resistance elernent; and Fig. 5 is a sectional view of a suitable mold with which the process of molding the casing may be carried out. Referring to the drawings, the variable resistor 40 or potentiometer therein shown comprises a casng IC, having an end closure or cap I 1, a resistance element 12 in the form of a compound helix, .,,iid a rotatable slider or contact member 13 which is adapted for travel along the resistance element 45 to make contact with the successive turns thereof. The resistance element 12 is provided with terininals 14 and 15 and the slider or contact meinber 13 has a terminal 16. When the device is used as a potentiometer a source of electrical @50 p6tential is connected to the termi-nals 14 and 15 and a variable potential is delivered at the terminal 16 depending on the position of the slider 13, as is well knwvn. When used as a variable resistor connections are made to terminals r,.i 16 and 14, or to terminals 16 and 15, and the slider 13 is rotated in one direction or the other to vary the resistance between the connected terniinals in known manner. The resistance element 12 comprises a core 17, Fig. 4, having an insulating coating 18 of suitable insulating material such as Formvar or Formex, and a helically wound resistance wire 19. In the device shown, the core is of No. 14 copper wire, and the resistance wire has a diameter of 2.18 mils. The resistance wire is wound on the core in spaced turns, while the core is straight, and the ends of the winding are secured by soldering a few turns together as indicated at 20. This prevents the coils from unwinding. The insulated core with the resistance wire wound thereon is then formed into a helix by winding it on a suitable mandrel having a helical groove therein. In the winding operation the relatively soft copper core is given a permanent set and is removed from the form by unscreviing it in the manner of removing a nut from a bolt. As illustrated in Fig. I of the drawings the core has somewhat more than ten complete tiirns and the termilials 14 and 15 are 3600 degrees apart, measured in terms of the rotation of the slider or contact member 13. The number of turns in the core helix rnay be varied, however, as also the size of the core and the size of the resistance wire, in order to vary the ohmic resistance of the resistance element, or for other reasons. The total ohmic resistn@nce of the resistance element shown and described, is about 20,000 ohms between termiiials. A suitable nold for niaking the casing 10 is illustrated in Fig. 5 and compri@es the base plate 2 i, the core 2 3, the mold 2 2, t@he cylinder 2 5 and the piston 26. The mold is of the extrusion type and its construction conforms ger-ierally to the construction of similar irolds which are in common use. It @vill not be necessary therefore to give more than a brief description of the several parts. The core 23 has a cylindrical base 27 which is secured to the base plate 21 by means of screws such as 28. The mold 22 rests on the base plate 21 and has a cavity which conforrns to the outside dimensions of the casing 10. The mold is properly located on the base plate by means of dowel pins such as 29. The base 27 of the core 23 closes the mold cavity at the bottom and the rernainder of the core extends into the cavity where it defines the inside wall of the casing. It will be n6ted that the core includes a cylindrical section 30 which has a helical groove 31. The
[2]2,495,321 3 upper end of section 30 is an annular inclined plane 43 the ends of which are joined by the vertica'l abu4@ment 44. The reference character 24 indicates an insert which is slidable in the mold 22 and which is attached to the base 27 of the 5 core by means of dowel pins 32 and 33. This inse-.-@t provides a support for the terminals 14 and I 5. The cylinder 25 closes the mold cavity a't, the top and has one or more gates such as 34 which 10 lead @L-'orr@ the interior of the cylinder to the mold cavity. The cylinder has a centrally disposed recess at the bot@tlom for receiving the bearing meniber '.15, which is held in place by the spring pressed pi-,i 36. The piston 26 has a tapered slot 15 37 at the end. It will be understood that means is provided fo-.-- heating the mold, but since heating means suitable 'Lor thl-Ls p-arpose is well known, the showing thereof has beeii omitted in order to avoid go complicatiilg the drawings. The operatioii of rnolding a casin.- will, notv be described, it being assumed for this purpose that the several parts of the mold are disassembled. 25 As the first step in the operation a prepared resistan.-e element 12 in the form of a compound helix is takeii and is screwed onto the grooved section 3-3 of the core 23. The turns of the helix are slightly smaller in diameter than the sectio.,i 9 ,@1 30 and the core 17 of the helix has sufficient resiliency to riake the helix hu.- the core and follow the groove 3 i as it is screwed on. The groove has a depth equal to about one-half the diameter of the he,ix core I't and the turns of the groove are spaced apart so as to separate the turns of the helix and hold them in spaced relation as shown. The terminal 15, which may be already prepared, has a flexible metal strip 38 so'Ldered tO 4,) it at one end. The free end of this strip is now soldered to the resistance wire on the upper turn of the helix at a point about an inch or so from the end. The strip 38 overlaps a number of turns of the resistance wire and all these turns 43 are soldered together and to the strip. At this time the strip 38 is straight and together with the termi-nal @15 extends upward parallel to the axis of the core 23. The terminal 14 is arran-ed milar to termina, 15 and has ' ial , Si a flexible m@ stri-p 39 sim,,Ior to strip 33 which is soldered to the resistance wire a short distance from the end of '@he last turn o-f t'he helix. As previously rr@entioiaed th,3 helix has so7rewhat more than ten comp,'Lete turns, preferably about ten and a quarter turns. A suiluable guide may be used to faci-litate the correct angu.Lar location of the terminals and strips ivhen the latter are soldered in place. The terinina'@s may be in vertical alignment, as ipdicated iii Figs. 1 and 5, but if a full 3600 degree range is essential the terminals are spaced apart angularly by a few degrees, or enough to irsure that the short-circuited turns of resistance wire at one er-ld of the helix do not overlap those at the othe,, end. Fig. 2 illustrates this in the case of terminal 14, which is angularly displaced to the left, it being understood that terminal 15 is similarly displaced to the right. Having attached the terminals as described, the operator now applies a covering 48 to the grooved section 30 of the core and the helix 12 supported thereon. '@his covering consists preferably of several layers of a suitable textile fabric such as lit-ien whi-,h is tl-ioroughly coated and impregnated with a heat setting plastic material. 4 This material i-nay be the same kind of material that is to be used for the casing. Bakelite has been used with good ,esults. The covering may be applied by taking a strip of the coated and impre,@nated fabric cut on the bias, and wrapping it around the helix to form a cylindrical sleeve, '@hen applyiiig a few turns of silk thread and tioi,-,g it to hold the sleeve in place. Preferably, however, a @-iumber of such sleeves are prepared in advance by wrapping suitable strips of the coated and impregnated fabric on a mandrel a-qd tieing them with thread. With a supply of such sleeves on hand, the operator covers the helix by taking one of the prepared sleeves and slipping it into position around the section 30 of the co@-e, as shown in Fig. 5. The operator now bends the metal strips 38 and 39 around the ends of sleeve 40 so as to bring the terminals into approximately the Positions Iii Nvhich they are shown and then places the mold insert 24 in positio-Ti on the core 23. To eff-ect this operation th.- dowel pins 32 and 33 are start,@d into the corresponding holes in the co,,-e bas-- 27 and at the same time the terminals are started into the holes in the insert, after which the insert may be pressed into position against the core base. If the terminals 14 and 15 fail to -- r'ully c-iater the holes i-,i the insert they may be pressed in with the aid of a pair of pliers c)r other inst@,ument The operator -,low takes a couple of washers 4 1, made oi' the same material as the sleeve 40, ,tnd pl,,lces them in position on the end of the core as shown. As mentioned before, suitable heating means is provided for the rrold and parts thereof including the core 23. 'fhe mold 22 and cylinder 25 may be kept hot, but it -Ls convenient to cut off the supply of heat to the core 23 while the operations just described are being performed. The core will cool off someivhat, therefore, and will have to be re-heated. Ti-ie heating means may be a hot plate, or an electrical heating element embedded ip- the base plate 2.1, for instance. V@hatever the nieans einployed may be, the necessity @or heatilig the core may be taken advantage of for partially curing the sleeve 40 and the washers 41, which otherwise would reqiiire a separate operation. To attain this object a hood 5n is placed around the core while it is being reheated, and the heat beiiig confined, the temperature inside the hoocl is raised sufhclently to bring about the desired partial curing of the sleeve and wa. -liers, or rather of the Bakelite ma,terial with which they are impregnated. , ted core Ma,17 ijow be placed in position on the bed of the press and the mold 22 may be lowered iiito position as shown, being properly 619 lo . cated with respect to the core by the dowel pms such -as 29. The cylinder 25, with the bearing raember 3i) inserted therein, is then placed o--. the l@iold. The ey'@inder sho,,ild be so oriented with reference to the mold and core that the gate 'P4 is iiot directly above the terminal 15, to avoid 6 @-) 4 day-iger of breaking the strip 38 by the inflowing Bakelite material. A preheated cake 42 of Bakelite may now be placed, ii-i the cylinder 25, a--Fter which pressure -1 o is applied by r--c-a-ns of '@-he piston 26 in the usual manne,.. Tqie Bakelite is r-,dticed to a plastic eciidition a..,id fiows into the mold through the gates 34, the mold cavity and forming the desired casing. The heat and pressure sholild 75 be mai-ntained for a sufficient length of time, The hea
[3]Usually about 3 minutes, to fully cure or set the material. When the plastic material starts to flow into the mold cavity it first fills the space above the washers 4 1, but as soon as pressure begins to build up the washers are bent downward around their edges and the Plastic material flows down to f!II the rest of the niold cavity. 'nlese NA7ashers seal the end of the sleeve 40 and preveiit the entrance of the free flowing plastic material into the sleeve, As soon as the mold cavity becomes filled the pressure builds up to a high value and the heat and pressure softens the Bakelite material with which the washers and sleeve areimpregnated. The washers and the sleeve thus become bonded to the rest of the n-laterial and form a lining for the casing which ur@der the great pressure applied is caused to conform exactly to the core and to that port,'@on of the resistance element which is exposed outside of the groove 31. Looking at it another ivgy, the main turns of the resistance element and the individual turns of th,, resistance wire become embedded in the lined casil-ig, which fills the spaces between the turns and the groove 31 beyond the ends of the helix, whereby the resistance elerdeiit becomes firrt'lly fixed in the casing os it is formed. After the casing has become cured and set the raold may be removed from the press and taken apart. TI-ie cylinder 75, witill the -oiston '@16 -etained therein, is first separated fro,-n the mold 22. This may be accornplished b3r driving viedges between the cylinder and mold to break of-f the sprues in the gates 34, after which the cylindei: may be lifted off, the bearing member 35 i-emaining with the casing in which it is embedded. The sprues break where they have the smallest diameter, which is next to the casing. The core 23 n'lay now be removed froin the mold by use of a fixture comprising a plate similar to the plate 21 having thrce pit-is therein corresponding to the dowel pins 29 bi-it somewhat smaller i-n diameter. The mold is la,d on its side and the fixture is applied to the opeli end with the Dins in the holes OCCUDied by the dowel pins. A few blows with a hamrner will now drive the dowel pins and the core out of the mold. The insert 24 comes out with the core, to which it is attached by the dowel Pi_ns 32 and 33. Witli the core out of the n-iold the ir,.sert 2,S riiay be pulled off and the completed casing may then be iinscre,@ved from the core. When the p4.ston 26 is removed froin the cyl-inder 25 the residue of riaolding material, -Tiow formed into a Bakelite disc with attached sprues, comes out with $,he piston from which it can readily be detached by a blow with a hanir-qer. The apparatus is now ready for molding another casing. The process as described is a hon6 molding process, but it will be understood that in practice knoivn arrangemonts for rnanipulating the inold ai,id parts thereof vqith the aid of the press N@,Till be ernployed. Each mold should be provided with several cores so that while one core is in the rflold other cores rday be in course of preparation. Three cores may be used, for example, in ,i three stage process, coinprising an asse-mbly stage, a core heating and sleeve curing stage, and a nolding stn@ge. An essential fea'uure oi the process is the use of the sleeve 40 with the associated washers 41, which prevent the free flowing Bakelite from entering between the turns of the resi.stance wire into the groove 31 of the co-re and coating the turns of resistance wire on the inside of the helix, where they are engaged by the slider 13. ThO Bakelite impregnated fabric of which the sleeve is composed readily enters between the turns of the core wire 17 as far as the core of the mold, 6 but the spaces between the resistance wire turns in the groove 31 are very small and the fabric of the sleeve cannot enter them. These spaces are thus effectively sealed ofi' by the sleeve and entrance of the Bakelite into the groove 31 is 10 prevented. This result -is promoted by the partial curing of the sleeve and washers pr4@Or to the actual molding operation, Referring now to Fig. I again, after the casing 10 has been molded as described, it is placed in jr) a lathe aiid the bearing member 35 is turned down to the proper dimensions, the hole for the shaft 45 is drilled, and the threads are cut for the nut 46. The casing and resistance element are now ready to be assembled @7;ith the other parts to 20 complete the variable resistor or potentiometer. Tlle cap If, like the casing IC, is a Bakelite molding and is of the proper size to fit into the open end of the casing, as sho,47n in Figs. 1 and 3. The cap has a centrally disposed metal insert 47 .23 which is drilled and finished after the molding operation is completed. This operation is carried out by ony sui'Llable and known process and need not be described. The terminal 16, on the outside of the cap I 1, 30 is secured in place by means of a rivet 48. This rivet also serves to hold the contact spring 49, located on the -'-,nside of the cap, and conducti-vely connects the termi-,-).ai with the contact spring. TT-ie contact spr-lng is bifurcated and carries the 35 two col'itacts 50 and 5 1, as sho-ViTn in Fig. 3. Also shown in Fig. 3 is the stop 52 @vhich is formed integrally ivith the cap. The stop has the same height as the wall on which i@u is formed. The shaft 45 is rotp-table in the bearing member 4o 35 and has an axial bore at the left hand end to receive the cylindricol end 55 o:E the lead scre@v 56. At the other end the lead screw has a cylindrical section 57 which is located in the hole drilled in the meta! i-nsert 47 of the cap I 1. The lead screw is thus SUPDorted on the cap i I at one end and on the shaft 45 at the other end. The cylinder 58, preferably made of brass, is supported on the two discs 62 and 54, which are made of suitable insulating materip,,l such as 50 micarta. The disc 62 is rotatable on the part 57 of the lead screiv, while the disc 54 is rigidly fixed to the shaft 45. The lead screw 56 carries the nut 59 which includes a support 60 for the slider or contact spring 13. This support projects r) 5 through the longil'udinal slot 6 I in the cylinder 58. The shaft 45, lead screw 56, and cylinder 58 are assembled outside the casing 10. The disc 54 is first placed on the shaft against the flange 53 and is secured by stakii-ig. The cylinder 58 (10 is then placed in position on the disc 54 and the end of the cylinder is turned in by a spinning operation, which firmly secures the cylinder to the disc. The lead screw 56 with the nut 59 threa-ded thereon may now be introduced into 65 the cylinder, with the end section 55 inserted in the bearing opening in shaft 45. The disc 62 is then placed on the section 67 of the lead screw and is pressed into the end of cylinder 58. At thi-S point the cylinder and shaft should be tried i- 0 for end play on the lead screw, holding the disc 62 against the internal shoulder formed in the cylinder 58. The lead screw has shoulders formed at the opposite ends of the threaded section, the end sections 57 and 55 being of reduced 75 diameter, and one of these shoulders is adapted
[4]2,495,321 7 for engagement by the disc 62 and the other bY the end of shaft 45. There should be a smau amount of end play, sufficient to insure free rotation of the cylinder. To facilitate the 1-nanufacturinu operations P-nd to insure that another operatio@'will iiot have to be performed on the lepd screw to shorten the distance between the shoulders the parts are preferably so designed and proportioned that ordinarily there will be too much end play, Then dur, ,ng 'Lhe assernbling operation if too much end play is found the excess is taken 7Lip by means of one or more small viashers placed on the lead screw at one end. Havir-a tested for eid play and having made the necessary adjustrnent, if any was required, the operator inserts the disc 63 into the end of cylinder 53 on top of the disc 62 and then performs another spinnina operation which turns in the eiid of the cyl@@,nder and secures if, to the discs 63 and 62. The disc 63 is a contact member, adapted to be engaged by the contacts 50 and 51 on contact spring 49, and is preferably made of silver, or is silver plated. The contact spring 13 may now be mounted on the support 60 by means of a small screw as shown in l@lig. 1. At one end the spring 13 is provided vtith a contact 64, F,:g@ 2, aD-d at the other end it is formed into a loop 65 which extends through the slot GI in cylinder 5 8 and out again. T,Iie slot is necessarily made somewhat wider than the support to insure that the support will be freely movable, but the plpy which would otherwise result from this construction is taken up by the spring, which holds the support against one side of the slot. The nut 59 and support 60 are made of suitable insulating material and may be of molded Bakelite, for example. No filler should be used. The contact spring ! 3 is conductively connected to the cylinder by means of a flexible braided conductor 66 which has one end soldered to the contact sprin.- 13 and the other end soldered to the cyhnder 58. The conductor 66 should be a'Gtached to It'he cylinder at a point abo-Lit m,@d-way between the ends of the cyhnder and should be long enough to permit movement of the support 60 along the slot 61 as far as it ca-n go in both directions but not so long that it can touch the resi-Stance eleine--qt Ni7hei-i the support is cen@lrally located in the slot and the conductor is slack. Vvlien the shaft 45, lead screw 56, cylinder 58 and theassociated parts have been assembled as described, they are repdy for ossembly in the casing 10. r@or this purpose the casing i,-, supported in a fixtiire which holds i-t ii-, a vert@cal. position with the open end up. The shaft 45 is now rotated in a counter-clockwise dire,-tioii relative to lead screw 56 until the support 60 comes to tli@e end of the slot 61 in cylinder 58 and is then rotated approximately 90 degrees in a clockwise direction. The cylinder assembly is then brou.-ht to a position above the casing, with the sh-@ft 45 exterc,ling do@vnward, ai-id is so oriented relative to the casing that the contact 64 on contact spring 13 is in ali.-nment kvith tl-.ie tern-linal 14. Ftom this positioia the cylinder assembly is lowc-red in'tn- @'he casiiil,, the shq-ft 4r3 ciiLerii-ir_, its bearing in the bearing @-,qember 35, until the movement is arrested by the end of shaft 45 en.-aging a stop with which the fixture is provided. The operator now ;.Ilspects the contact 60 through the open end of the casing to rnake sure that it is properly centered on the en-d turn of the resistance element and adjusts the stop up or down if ne--essary. The cap II is now inserted in the end of the casing 10, with the lead screw projecting tl,.rough the hole in the metal insert 47. The cap should be inserted with the terminal 16 a few degrees past terminal 14 in a clockwise direction to insure that the @top 52 wiU clesr the support 60. The cap having been seated properly, the operator holds tlie lead screw with a screw driver and rotates tl-ae cap in a counterclockwise direction far enough to bring the terminal 16 into ali,@nment ;o with terminals 14 and I S. The lead screw is now soldered to the insert 47 and the casing with the parts assembled tberein is removed from the fixture. The next operation is the adjustment of the -,top 52. In order to carry out this adjustm=t an ohmmeter is connected between the terminals I 6 and 14, the support 6 0 is held against the stop by applying a counter-clockwise torque to the shq-ft 45, at-id ti-ie stop is adjusted by rotating the cap I 1 in one direction or the other until the conta-@t 64 is positioned on the last turn of the resistance wire which is soldered to the strip 38, -,olinting from the end of the resistance element. In this position of the contact the ohmmeter wiR read slibstantially zero and will show an increased reading immediately responsive to clockivise rotation of the shaft 45. When the adjustment is completed the cap is temporarily held in position by a strip of tape to avoid any danger of its rotati_rg relative to the casing. The device may now be inspected and tested and if no defects are found the holes are drilled for the drive screvvs such as 70 and the screws are inse@-ted. The cap I I is thus securely fixed :35 in the casing. The finished device may be mounted on a panel such as 7 1, as shown in Fig. 1. A knob may be mounted on the shaft 45 for rotating it, or the shaft may be rotated by gears, a flexible shaft, 40 etc. depending on the particular situation in which the device is employed. As mentioned before, the device is adapted to function either as a variable resistor or as a potentiometer. Assuming that it is to be used in the .15 former capacity, the necessary circuit connections are rliade to the terminals 16 and 14. The circuit through the variable resistor ex'@-ends from terminal 16 by way of rivet 48, spring 49 and contacts 50 aild 51 in parallel, disc 63, cylinder 58, conductor 66, slider or contact spring 13 and contact C,4, resistonce wire 19 of the resistance element, and strip 38 ]lo terminal 14. With the support 69 resting against the stop 52, which is the counterclockwise stop, only about one-half turn or less of the resistance wire is included in the circuit and the resistance between the terminals 4s substantially zero. When t-he shaft 45 is rotated in a clockwise direction, the cylinder 58 rotates the support So and nut 59 on the lead screw 56, causing the support 60 to travel to the right along the slot GI in the cylinder. The threads of the iead screw have the same pitch as the helical turns of the core of the resistance elenient, so that as the support C, 5 60 is rotated the contact 64 travels along the resistance element and engages success4@ve turns of the resistance %@iire. As the tur-ris are cut into the circuit one after the other the resistance between the terminals 16 aiid 14 is proportionptely in70 creased, as will be understood. 'Nhen the shaft has been rotated somewhat less than 360 degtees the support 60 has moved far enough to the right so that it clears the stop 52 i@Then it passes it. The rotation in a clockwise direction may continue for 3240 degrees more,
[5]9 or for a total of 3600 degrees, when the support 60 will engage the clockwise stop and the rotation will cease. The clockwise stop is not shown, btit it will be understood t-hat it is formed in the end of the casing IO by the abutment 44, Fig. 5. The 5 contact 64 will now be in engagement with a turn of the resistance wire which is adjacent the turns soldered to the strip 33 associated witli terminal 15 and the resistar-ce between terminals 16 and 14 will lie a maximum. It will be noted that clockwise rotation of the shaft 45 increases the resistgnee while counterclockwise rotation of the shaft decreases the resistance. For any particular angular setting of the shaft the resistance will always be the same ir, regardless of whether the setting is approached in a clockwise or counter-clockwise direction. This desirable result is due to the fact that there is no lost motion in the drive between the cylinder 58 and the support 60, the part 65 of the contact spring 13 being effective to hold the support against the side of the slot GI in the cylinder. The operation of the device as a pollentiometer will be understood from what has already been said and need not be explained in detail. 5 The invention having been described, that which is believed to be new and for which the protection of Letters Patent is desired will be pointed out in the appended claims. We
