US2868934A - Precision resistance devices - Google Patents

Description

Jan. 13, 1959 L. E. ASKE 2,868,934

PRECISION RESISTANCE DEVICES Filed April 22, 1954 IN V EN TOR. LEONARD E. ASKE ATTORNEY United PRECISION RESISTANCE DE'VHCES Application April 22, 1954, Serial No. 424,934

3 Claims. (Cl. 201-63) This invention in general pertains to an improved precision type characterized resistance device and also to a method of making the same.

Characterized resistance devices are broadly old in the art, there being many types of these devices as well as several methods of making the same. A characterized resistance device in general is a resistance device wherein a nonlinear change in resistance is effected per unit change in position of the wiper means cooperating with the resistance. This nonlinearity between resistance change and wiper movement can be achieved by, in one type of device, the use of a resistance winding support having a nonlinear cross-section so that successive turns of resistance wire thereon will have shorter or longer lengths with the end result being that as the wiper is moved from one turn of wire to the adajacent turns, it successively picks ofi more or less resistance depending upon the direction of movement, the amount of resistance change being indirectly dependent upon the change in cross-sectional area of the resistance winding support. Another method of obtaining a characterized resistance device is to have a winding support or bobbin of uniform cross-sectional area such as a cylinder and winding thereupon a first section of resistance wire of a certain size for a given number of turns, then soldering or splicing or by other means securing a different size of resistance wire onto the end of the first resistance wire and then winding a certain given number of turns of the second resistance wire on the support. Additional re sistance sections may be added with still different sizes of wire as desired until the entire device is made.

There are numerous disadvantages to the prior art methods of manufacturing characterized resistance devices as well as disadvantages in the finished resistance devices per se. Disadvantages of the nonlinear Winding support method are that the winding support per se is expensive to manufacture and elaborate winding machines are required. One disadvantage of the second method is that it is very tedious and time consuming and delicate inasmuch as each resistance section has to be Wound, then the winding operation interrupted while a second wire is soldered to the first wire, this type of assembly being time consuming and also requiring very highly trained and skilled workers. Further, it is relatively quite difiicult to solder or splice the adjacent wire sizes together because they are usually very small in diameter and difiicult to handle. Another problem is the fact that it is difiicult to accurately control the length of each section of the resistance device inasmuch as the soldering often shunts out some of the turns of Wire. Another difiiculty is that the difference in diameter of one section of the resistance device as compared to the adjacent sections of resistance produces a hump or step therebetween which step is objectionable in many applications in that the wiper or slider device cooperating with the resistance device will tend to jump or skip several turns of wire at the step and hence produce inaccuracies Patented Jan. 13, 1959 "ice in the system in which the resistance device is being used. Still another disadvantage of this method is the fact that if any one of the resistance sections becomes damaged requiring its replacement, it is generally necessary to discard the entire device resulting in a total loss. Another difficulty is that Nichrome type wire which has desirable resistance characteristics is generally unsuitable for soldered connection type resistance devices because it is inherently difiicult to solder.

It is with the above disadvantage of the prior art resistance devices and methods of manufacturing in mind that I have devised an improved precision characterized resistance device and method of making same. My precision characterized resistance device in the preferred embodiment is made up of a plurality of prewound re sistance sections, each comprising a washer-like member having metalized portions on each end face thereof and with resistance wire wound on its outer periphery, the ends of the wire being bent over so as to abut against the metalized ends. The size of each of the washers would be preselected in accordance to the diameter of wire to be wound thereupon so that the summation of the outer radial dimension of the washer together with the radial dimension of the wire to be wound thereupon would produce a common dimension. Thus, when a plurality of washers, each of which has wound thereupon a resistance wire of a diameter different from the others, are placed side by side in axial abutting relationship, their outer peripheral surfaces are all at a common level and there is no hump or step between adjacent resistance sections. Hence, there will be no tendency for a wiper member cooperating therewith to jump or skip several turns of wire as it moves from one resistance section to another. Further, there is no need for a separate soldering operation in joining or electrically connecting one resistance section to another inasmuch as each washer member has a conductive coating on its end so that when adjacent sections are axially abutted against one another, they are electrically connected automatically, which connection is maintained throughout the life of the device by maintaining an axial pressure upon the whole assembly.

A more complete description of the precision characterized resistance device per so as well as the method for making the same will be set forth below but it will be obvious that a broad object of this invention is to provide an improved precision characterized resistance device and to provide an improved method of making the same.

Another object of the invention is to eliminate the need of wire splicing, soldering and masking by the coil winding operator in order to characterize a resistance device.

A further object of the invention is to accurately control the length of each section of a resistance device.

Still another object of the invention is to accurately control the outside diameter of the bobbin or support member of each wire wound resistance section so that in changing from a small wire size to a larger wire size, the exposed contacting surface of the'entire resistance device will be at a uniform level so as to promote smooth wiper contact.

Another object of this invention is to utilize a type of construction that allows the use of resistance wire or" the nickel chromium or Nichrome type which can be used due to the elimination of soldering operations.

A further object of the invention is to eliminate the use of windings for a dead spot on a resistance device and thus provide for more accurate control of the dead spot length.

A further object of the invention is to provide a method of manufacturing precision characterized resistance devices whereby great latitude in design is permissive due to the universal sectional assembly method.

Still another object of the invention is to provide a resistance device wherein a damaged resistance section may be replaced without discarding the other resistance sections.

Another object of the invention is to provide a characterized resistance device wherein machine winding technique may be utilized to the fullest so that uneven tension on the wire produced by hand winding operation is eliminated as well as finer control and spacing of the turns of wire on each resistance section.

The above and still further objects and advantages of the invention will be made more clear in the following specification and drawings in which:

Figure l is a side elevational view of a potentiometer device utilizing in part an improved precision characterized resistance member;

Figure 2 is a cross-section view of the resistance member shown in Figure 1 taken along section line 22;

Figure 3 is a side view of an individual winding section or bobbin member;

Figure 4 is an end View of the bobbin shown in Figure 3;

Figure 5 is an illustration of a plurality of winding sections or bobbins and spaces between winding sections prior to the winding operation;

Figure 6 is a showing of the same assembly shown in Figure 5 with the assembled winding sections or bobbins and spacers being wire covered;

Figure 7 is a side view of an individual resistance section after it is wire wound and before the ends of the wire are bent over;

Figure 8 is an axial end view of the individual resistance section shown in Figure 7 with the ends of the wire bent over so as to abut against the conductive film which is disposed on the axial end of the disc; and

Figure 9 is a cross-section view of a modified resist ance device.

In Figure l is shown a typical application of the improved precision characterized resistance device where in a characterized resistor it) is supported by a screw member 12 which has a headed portion 14- extending up through an aperture 16 in an upper support plate A curved snap washer 2th positioned between the top of support plate 18 and the underside of head member 14 on screw 12 holds a shoulder portion 22 of screw 12 up against the underside of support plate 13 but allows rotation of screw 12 with respect to support plate 18.

A lower support plate 24 having a boss member as with a conical recess 28 therein provides a bearing support for the lower cone-shaped end 39 of screw member Also mounted on lower support plate 24 is a pressure responsive bellows member 32 and a bracket 3%, the latter of which serves as the pivotal support for a wiper carrying arm as. Bellows member 32 is operatively connected by means of a link 38 to an intermediate point of arm 36, which latter member carries on its unpivoted extremity 4d a pair of wiper members 42 which are adapted to bear against the resistance member 19.

Referring to Figures l and 2, it is seen that resistance device lb comprises a bobbin or support member which may be fashioned out of any suitable rigid material, metallic or nonmetallic, such as aluminum or Bakelite. Support member 44 has a lower headed portion 46 as well as an inner bore 48, the lower portion of which is threaded as at 5% so as to cooperate with the threaded portion on screw member 12.

A pin member 52 is secured in a suitable aperture in the headed portion of support member and extends parallel to the longitudinal axis of support member 44. A hole 54 in the lower support plate 24- serves as a guideway for guide pin 52. Thus, when screw member 12 is rotated with respect to support member 13,

rotation of resistance member 10 is precluded by guidc pin 52 being engaged in hole 54 and hence the elevation of resistance member 1d varies in accordance to rotation of so: v. member 12. Resistance member 10 may thus be adjusted with respect to wipers 4-2.

Positioned intermediate on support member 4 iare a plurality of resistance sections 56, 6d and 62. Each of the resistance sections comprises a washer-like member 6 shown in greater detail in Figures 3 and 4, said washers being made [1"..21 of some nonconductive material such as p astic or out of a conductive material with an in re exterior. Cine material that has proved very satisractory for this member is Pyrex glass. Each washer member as has an outer periphery 66 and an inner bore of bore 68 being such that the inwill fit snugly on support member .4.

In the preferred embodiment shown, the resistance sections comprise a washer-like member 64 as a winding form. However, it is to be well understood that the teaching of the invention may be applied to other shapes of winding forms and thus the invention should not be limited only to washer-like elements. A further point to be kept in mind is that while the abutting surfaces of the adjacent resistance sections are shown to be normal to the longitudinal axis of the device, other arrangements will occur to those skilled in the art and the invention should be so construed.

Prior to the winding operation, each washer 64 has a metalized portion or conductive film 7t} placed on each end face thereof. Various metalizing procedures may be used to place the conductive film '70 on the end faces of discs 64*. One metalizing procedure that has been found to be successful is an embossed gold leaf transfer process. However, other metalizing processes may be used and the invention should not be limited to any one specific type of procedure. While conductive film 7i) is shown on Figure 3 to have a considerable axial extent, it is in practice only of miscroscopic thickness, the thickness in the drawing being exaggerated in order that its presence be noted.

it is to be noted that the conductive film 70 on the end faces of washer members 64 does not extend all the way to the inner periphery 68 of the washers. The reason for this is that it support member 44 is metallic or electrically conductive, it will not short out adjacent resistance sections. Obviously, if support member 4-4 were nonconductive, then conductive film '70 could cover the entire end face of each washer 64.

in the preliminary design of a resistance device, it will be calculated as to what axial length and what wire size is desired for each resistance section. With this preliminary data in mind, the axial lengths of each washer member 6 is determined as well as the outer diameter 66 of each washer. That is, the axial length of each washer 64 should coincide with the desired axial length of the particular resistance section. Further, the outer diameter 66 of the specific washer 64 should be such that when it is covered with successive turns of wire, the outer surface of the wire should be at a common level away from the center of the bobbin 01' support member 44. To explain further, if a resistance wire of relatively large dimension is to be used in a particular resistance section, the outer diameter 66 of that particular washer member 64 should be proportionately smaller as compared to an adjacent I resistance section which, for this illustration, would have a resistance wire thereon of much smaller diameter. Thus, by having the outer diameter of the individual washer members 64 inversely proportioned in accordance to the size of wire thereupon, the outer surface of the wound sections is at a common level and no steps or humps are produced between adjacent resistance sections and consequently a smooth surface will be presented to the wiper members 432.

After the individual washer members 64 have their conductive film 7d thereon, all of the washers 64 which have a common outer diameter and which are to receive the same size resistance wire thereupon are placed on a mandrel member 72 shown in Figures 5 and 6. Mandrel member 72 has a headed portion 74 at one end thereof and on the opposite end has threads 76. A stripper washer 78 is first placed on the mandrel member 72 and abuts against headed portion 74. Then the individual washer members 64 are placed on the mandrel, being interspaced by additional washer members or spacers 88. These spacer washers 80 are preferably made out of a material commercially available under the trade-name Teflon. While washer members 80 may be of any suitable material, Teflon is one that has been found to be very satisfactory. Teflon is made by the E. l. du Pont Company and is a polytetrafluorethylene. Another fiuorinated hydrocarbon material which may be used is known as fluorothene or Kel-F, a trifluoromonochloroethylene substance made by the M. W. Kellogg Company which has characteristics generally similar to those of the aforementioned Teflon. The fluorinated hydrocarbon materials above suggested are most desirable for this particular purpose because of an inherent slipperiness and also the additional characteristic which is that most adhesives such as glue and varnish will not adhere to it. Obviously, other materials having the properties mentioned to the extent necessary for the particular application in mind may be used.

After the washer members 64 and spacer washer members 80 are placed on mandrel member 72 in interspersed relationship, the entire assembly is clamped together by a nut member 82 screwed down over threads 76 on the one end of mandrel member 72. It is to be noted that the outer diameter of the spacer washer members 80 is the same as the outer diameter of washer members 64.

The assembled washers 64 with spacer washers 8d are then given a coating of varnish and allowed to dry somewhat so that the varnish becomes tacky. Obviously, other suitable adhesives may be used in place of varnish for this purpose. I have found that a convenient method of giving the assembled washers a coating of varnish is to dip the assembled sections in a vat of varnish having a viscosity of 60 to 70 centipoiseswith a drying time in air from 8 to 24- hours. I have found thatwithdrawing the mandrel from the varnish vat at a rate of 2 inches per minute willgive a desirable coat of varnish of a thickness from .0004 to .0005 inch.

Due to the special slippery properties of the Teflon washers 80, the varnish does not adhere thereto but only to the exposed peripheral surfaces 66 of the washers 64.

After the dipped assembly of washers has dried a said cient length of time, the mandrel is placed on a winding machine and an enamel or otherwise insulated wire of the desired size and resistivity is wound thereon using a conventional machine winding process producing a finished result as shown in Figure 6. It will be noted that the winding machine winds over the spacer washers 80 as well as the washer 64. During the winding process, the wire is embedded into the thin coating of varnish on each of the washers 64 but naturally does not adhere to the washers 80 inasmuch as they did not have a coating of varnish thereupon. Thus, after the winding operation, each individual washer member 64 has its resistance wire attached thereto. The bonding of the wire to the washer 64 is further improved by bakingthe entire mandrel as sembly which. accelerates the drying of. the varnish. I have found that baking the assemblies at a temperature of 265 F. for a period of time of two hours is satisfactory.

During the baking process, the wire is bonded to the peripheral surface of the washer 64 from the underside of the wire. Sufficient adhesion is acquired during the baking process so as to make it unnecessary to coat the outer surface of the wire with a special adhesive as is sometimes done in winding conventional resistance devices.

After the baking process, the individual wire wound Washers 64 may be removed from the mandrel, this being done by removing nut 82 and exerting axial pressure against the side of stripper washer 78 which forces washers 64 and 80 off the threaded end of mandrel 72. The individual resistance sections are separated from the Teflon spacer washers 80 by clipping off and discarding the excess coiled wire Wound on the Teflon sections. Inasmuch as the axial length of the Teflon washers 88 is relatively short with respect to the axial length of the washers 64, very little resistance wire is wasted. The wire should be cut so that short unbonded wire ends 84 and 84 are left on each end of a resistance section as shown in Figure 7. The next step is to remove the enamel or other insulative coating from the free ends 84 and 84 and then bend ends 84 and 84 in against the such as du Ponts #4922 conductive silver coating may be employed; a small amount of the coating being sufficient to hold ends 84 and 84 in place.

It will be understood that each of the resistance sec tions 56, 58, 60 and 62 assembled on support member 44 are independently wound in the preliminary winding process. That is, all of the resistance sections 56 for a plurality of resistance devices would be wound at the same time and so on for the resistance sections 58, 60 and 62.

In the actual assembling of the resistance devices, an insulative spacer member 88 having a headed portion 90 is first slipped down over the shank of support member 44 until the headed surface 90 abuts against the headed portion 46 on support member 44. The axial portion of insulative spacer member 88 serves to space a metallic spacer member 92 away from support member M:- as is clearly shown in Figure 2. Positioned between the bottom axial end of metal spacer member 92 and headed portion 90 of the insulative spacer 88 is a conductive terminal member 94 having a suitable tab 95 thereon for connecting external circuit means (not shown) to one end of the resistance device. Next the plurality of resistance sections 62, 60, 58, and 56 are slipped over the shank of bobbin member 44 and are placed in axial abutting relationship with one another. Each of the resistance sections is carefully oriented so that the ends 34 and 8 5 on adjacent resistance sections do not abut against one another. Thus the adjacent resistance sections are electrically connected to one another inasmuch as their conductive portions 70 with the ends 84 and 84 are touching one another with the only gap between adjacent sections being equal to the diameter of the largest wire present. Further, they are electrically connected without the necessity of any separate soldering operation. An additional metal spacer member 96 is abutted against the top portion of resistance section 56 and is electrically insulated from bobbin member 44 by another insulative spacer member 98 which may be identical to spacer member 88, the former also having a headed portion 100 thereon. Another terminal member 102 is positioned between the top end of metal spacer 96 and the headed portion 100 on insulative spacer 98 and serves to connect the other end of the resistance device to external circuit means, not shown. A washer member 104 abuts against the outboard axial end of insulative spacer member 98 and a curved snap ring 106 fitting in a notch 108 on bobbin member 44 serves to maintain the entire assembly in a tight relationship which assures electrical continuity from terminal member 102 through metallic spacer member 96, resistive portions 56, 58, 60 and 62, metallic spacer 92, and to terminal member 94.

If it is desired to have a tapped resistive device then an additional terminal member 110 similar to terminal members 102 and 94 may be disposed between adjacent 7, resistance sections such as is shown in Figure 2 between resistance sections 56 and 58. This, of course, would be an optional feature depending upon the application desired.

Another optional feature of this invention is shown in Figure 9 wherein a modified construction of a resistance device is disclosed generally similar to- Figure 2 but having a deadspot between two resistance sections 60 and 62'. T he deadspot is formed by a conductive spacer 1% generally similar to spacer members 92 and 96 and p0 tioned with respect to support member 3 by a sleeve member 122 of insulative material. Using this method of construction a deadspot between adjacent resistance sections is provided, the axial extent of the deadspot, of course, being equal to the axial length of the spacer rncmher 129. I have found this to be a much simpler and more accurate method of providing deadspots in a resistance device than prior art methods such as shorting out a plurality of turns of the resistance wire with a conductive coating such as solder or the like.

After the resistance device has been assembled and secured in place by bowed snap ring 196, an axial portion of the winding is exposed by a hurling operation as at 112 shown in Figure l. This butting operation removes the enamel or other similar insulation on the exposed periphery of the resistance wire and thus allows wiper members to make electrical contact with the individ ual turns of wire.

It will be appreciated that displacement of the bellows member or any other type of device used to actuate wipers 4-2 will result in rotation of arm member 36 about its pivot point so as to vary the vertical elevation of wiper members 4-2. Wiper members 42 would obviously be connected to external circuit means, not shown, so that changes in electrical resistance in accordance to displacement of the actuating device could be used for some useful purpose.

While 1 have shown and described a specific embodiment of this invention, further modifications and improvements will occur to those skilled in the art. I desire it to be understood therefore, that this invention is not limited to the particular form shown and I intend in the appended claims to cover all modifications which do not depart from the spirit and scope of this invention.

What I claim is:

1. A precision resistance device of the class described comprising a support, a plurality of insulative washers mounted coaxially on said support so that the end faces of adjacent washers abut one another, a conductive film on said end faces of said washers, resistance wire wound about substantially the entire outer periphery of said washers, the sum of the outer radius of one of said washers, plus the radial thickness of the resistance Wire wound on said one of said washers being equal to the respective sums of the outer radius of the other of said washers plus the radial thickness of the resistance Wire wound on said other of said washers, said resistance wire having ends abutting said conductive films on said end faces, metal spacer means abutting the end faces of said plurality of discs, terminal means abutting said spacer means, means insulating said spacer means and said terminal means from said support, and means holding said washers, spacer means, and terminal means tightly together.

2. A precision resistance device of the class described comprising a support, a plurality of forms with faces thereon mounted on said support so that said faces of adjacent forms abut one another, conductive means on said faces of said forms, resistance means found on substantially the entire periphery of said forms, the sum of the cross-sectional dimension of one of said forms plus the thickness of the resistance means wound on said one of said forms being equal to the respective sums of the cross-sectional dimension of the other of said forms plus the radial thickness of the resistance means wound on said other of said forms, means connecting said resistance means to said conductive means on said faces, terminal means contacting said plurality of forms, and means holding said forms and said terminal means in assembled relationship.

3. A precision resistance device of the class described comprising a support, a plurality of insulative washers mounted coaxially on said support so that the end faces of adjacent washers abut one another, a conductive film on said end faces of said Washers, resistance wire having different diameters wound about substantially the entire outer periphery of said washers, the sum of the outer radius of one of said Washers, plus the radial thickness of the resistance wire wound on said one of said washers being equal to the respective sums of the outer radius of the other of said washers plus the radial thickness of the resistance wire wound on said other of said washers, said resistance wire having ends abutting said conductive films on said end faces, metal spacer means abutting the end faces of said plurality of discs, terminal. means abutting said spacer means, means insulating said spacer means and said terminal means from said support, and means holding said Washers, spacer means, and terminal means tightly together.

References fired in the file of this patent UNITED STAi ES PATENTS

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