US2623144A - Variable resistor construction - Google Patents

Description

D 23, 1952 A R. w. sEARLE ET A1. 2,623,144

ARIABLE REsIsToR CONSTRUCTION I -Filed April 20, 1951 JOI Patented Dec. 23, i952 OFFICE VARIABLE RESISTOR CONSTRUCTION Raymond W. Searle, Waltham, and Lauren M.

Burgess, Lexington, Mass., assignors to Technology Instrument Corporation, Acton, Mass.

Application April 20, 1951, Serial No. 222,016

(C1. Ztl-48) 4 Claims.

The present invention relates to an improved variable resistor construction and the method of assembly thereof.

In the manufacture of precision variable resistors, generally called potentiometers, particularly suited for servo mechanisms, it is necessary to provide a construction which has stable characteristics not aiiected by changes in temperature and humidity. T.n the construction of such variable resistors substantially all parts thereof are coated with varnish and baked. Due to the temperature of the baking process certain precautions Inust be taken to insure retaining the resistance element in position.

It therefore is an object of the present invention to provide an improved variable resistor construction and method ci assembly.

A still further object of the present invention is to provide an improved variable resistor having means for retaining in position the variable resistance element during its manufacture and throughout the useful life of the resistor.

Other and further objects of the present -invention subsequently will become apparent by reference to the following description taken in conjunction with the accompanying drawing; wherein Figure l is an end view of a variable resistor,

with the cover removed, constructed in accordance with the present invention;

Figure 2 is a cross-sectional View oi a variable resistor shown in Figure 1 ras seen in the direction of the arrows along the line 2 2; and

Figure 3 is a perspective view of the retaining ring element employed in the resistor of Figures l and 2.

Referring to the drawing, the resistor construction illustrated employs a cylindrical casing II having a cylindrical Wall i2 and an end wail i3. The end wall i3 is provided with a boss ifi for supporting a bearing i5 for a shaft I6. The bearing I5 has a threaded end Il for receiving a mounting nut for holding the variable resistor in position on an apparatus. rlhe shaft it is provided vvith a groove into which is ntted a, split ring IS which engages the end of the bearing member IE. The shaft i@ at its other end carries a split sleeve I9 which is engaged by an arm 2l. The arm 2| at its lower end is provided with biiurcated portions 22, which may be drawn together by a stud bolt 23 passing through a suitable aperture in one of the bifuroated portions and engaging a threaded aperture in the other bifurcated portion 22. At its outer end the arm 2I has a threaded opening for receiving a screw 24 which engages a contact assembly Eil having an outwardly extending arm 25, which carries a precious alloy contact element 2l'. The precious alloy contact element 2l engages the turns of resistance wire on a resistance element. The contact assembly 2t also includes two similar brush arms 28, each provided with a precious alloy contact 2Q which engages a slip ring 3l. The slip ring ill is mounted on an insulated bushing 32, which'ts over the bearing boss ifi of the end wall i3 and the end portion of the bearing sleeve l5. A slip ring iii is suitably connected to the center terminal 32 of a plurality ci terminals carried by a terminal block 32 mounted on the outside of thecasing l I.

Mounted within the casing i l adjacent the inside of the cylindrical wall l2 is a resistance element 3d formed of a ilat strip of insulating material about which has been wound in spaced relation a resistance wire The ends of the resistance wire 5 are connected to electrical terminals 3e and Eil carried by the terminal block 33. In order to assure retaining the resistance element 3d in position during the assembly and during the useful life of the variable resistor, there is provided a retaining ring 38, which is shown in perspective in Figure S.

This split retaining ring 35% is positioned against the end wall i3 of the housing and against the resistance element til. The split ring 33 is expanded by a wedge t9 which engages the split of the ring, and which is provided with a central aperture l. The central aperture il receives a screw d2 which engages a threaded opening i3 in the end wall I3 of the housing H. The housing II is provided with a cover ed, which is provided with a vent or aperture i.

In order to assemble the device heretofore cle-- scribed so as to provide stable characteristics not affected materially by variations in temperature and humidity, the following method of manufacture is employed. All insulating material elements, such as the resistance element strip it, the insulated sleeve 32, and the retaining ring 32, together with its wedge Iii?, are baked for about one-half hour in an oven at appr Xiinately 212 lf". to drive ofi all possible moisture. lThereafter all exposed surfaces of these insulated elements are varnished. The entire inside of the housing I I is also varnished. These elements are then baked in a well ventilated oven for several hours at a temperature of 275 F.

Thereupon the slip ring bushing s2 is inserted into the varnished base, the slip ring 3l is placed in position, then insulating members are inserted to line the inside cylindrical Wall I2 and the back end wall I3. These insulating members are varnished into position and a suitable varnish is employed. The varnish is then applied to the exposed edges of the slip ring insulating bushing -both above and below the slip ring with a slight overlap of the varnish into the slip ring. This is then baked for several hours at a temperature of 275 F.

Previous to assembling other elements of the variable resistor, the resistance wire supporting strip 34 has been varnished and baked. Resistance Wire is applied thereto and suitably secured in position. The brush arm assembly is mounted on a dummy shaft and all exposed surfaces of the assembly are carefully painted with varnish, except for the precious metal contacts 2'I and 29. This is also baked at a temperature of 275 F. for approximately two hours. At the same time the inside of the cover 44 is varnished and likewise baked for a similar time at the same temperature. The terminals 32, 36 and 31 are pressed into the mounting plate 33 and varnished into position.

Thereafter, the resistance element strip 34 is varnished with exception of a buffed track for the brush arm. While the varnish is still wet, the resistance element is inserted into the housing II. The ring 38 and its Wedge 39 are again Varnished and then :placed into position within the casing II. Additional varnish is applied at the place where the ring 38 engages the resistance element 34. The terminal plate 33 is coated with the varnish and mounted in position. Varnish is now applied to the outside of the casing II, including the bushing I up to the point where the threads II begin. This is then baked for several hours at a temperature of 275 F.

Next, electrical connections are completed to the terminals of the resistance Wire 35 and to the slip ring 3l. The brush arm assembly is then installed on the end of the shaft I6. The exposed end of the shaft is varnished and varnish is extended over the brush arm 2l. Varnish is applied at the fitting edges on both the cover and the .base whereupon the cover is pressed into position. The entire outside surface of the cover and the edges of the housing II are varnished. This is then again :baked at a temperature of 275 F. for approximately two hours.

To illustrate the application of varnish to the various surfaces, double lines have been used in Figures 1 and 2.

While for the purpose of illustrating and describing the present invention a particular embodiment has been shown in the drawing, it is to be understood that the invention is not to be limited thereby, since such variations in construction and in assembly are contemplated as may be commensurate with the spirit and scope of the invention defined in the accompanying claims.

We claim as our invention:

1. In a variable resistor the combination comprising a cylindrical housing having a closed end provided with a bearing support for a shaft, a resistance element formed of a iiat strip of insulation wound with spaced resistance Wire, said element being located against the inside cylindrical wall of said casing, a split ring of insulation mounted against said housing end and against said resistance element, and means engaging the split of said ring for expanding said ring to retain said resistance element against the inside of the cylindrical Wall of said casing.

2. A variable resistor construction comprising a cylindrical housing having an end wall, a resistance element formed of a flat strip of insulation Wound with spaced resistance wire, said element being located against the inside cylindrical wall of said casing, a split ring of insulation mounted against, said housing end and against said resistance element, and means for expanding said ring to retain said resistance element against the inside cylindrical wall of said casing comprising a wedge engaging the split in said ring.

3. The variable resistor construction comprising a cylindrical housing having an end Wall containing a threaded aperture, a resistance element formed of a flat strip of insulation material wound with spaced resistance Wire, said element being positioned against the inside cylindrical wall of said casing, a split ring of insulation material mounted against said housing end and against said resistance element, and means for expanding said ring to retain said resistance element in position comprising a wedge engaging the split in said ring, and a screw passing through said wedge and the threaded aperture of said end Wall.

4. The method of manufacturing a variable resistor having a cylindrical housing, a cover, a resistance element mounted on an insulated strip, a brush assembly therefor, and an insulated retaining ring comprising the steps of baking said strip and said ring for about one-half hour at a temperature of at least 212 F., coating with varnish said resistance element, said retaining ring, said brush arm assembly and the inside of said housing and said cover, baking said coated elements for several hours at a temperature `of approximately 275 F., varnishing the inside cylindrical wall of said housing, inserting said resistance element against said wall, varnishing said retaining `ring and inserting it against said resistance element, baking said assembled housing, resistance element and retaining ring for several hours at approximately 275 F., inserting said brush assembly, varnishing the edges of said cover, applying said cover, varnishing the outside of said housing and cover, and baking the assembled variable resistor for several hours at approximately 275o F.

RAYMOND W. SEARLE. LAUREN M. BURGESS.

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

UNITED STATES PATENTS Number Name Date 1,876,921 Hall 1 Sept. 13, 1932 2,015,425I Gaubert Sept. 24, 1935 2,476,294 Hampton July 19, 1949

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