US2176090A - Vernier rheostat - Google Patents

Description

Oct 1939- w.-G. MOALLISTER ET AL 2,176,090

VERNIER RHEOSTAT Filed June 8, 1936 INVENTORS VVaZier M AZZL'S fer 0. Sfewar Michael BY QM m 1 EM ATTORN EYS Patented Get. 17, 1939 UNITED STATES VERNIER RHEOSTAT Walter G. McAllister and Oscar Stewart Michael,

Foxboro, Mass, assignors to The Foxboro Company, Foxboro, Mass,

chusetts a corporation of Massa- Application June s, 1936, Serial No. 84,032

6 Claims.

This invention relates to rheostats, and more inparticular to those for use in controlling and recording instruments.

An object of this invention is to provide a rheostat having a low-resistance element and a highresistance element, both of which may be manipulated for current-controlling purposes.

Another object is to provide a rheostat in which the electrical circuit includes only two movable contacts, the other connections in the circuit being fixed.

The above and other objects will be apparent from the detailed description and drawing wherein the prefeired embodiment of this invention is illustrated.

In the drawing:

Figure 1 is a top view with the operating knob removed so as to show the base member and certain elements mounted thereon;

Figure 2 is a sectional view on the line 22 of Figure 1;

Figure 3 is a sectional view on the line 3-3 of Figure 2;

Figure 4 is a diagrammatic view of the device for the purpose of explaining its operation.

This embodiment comprises, broadly, a base member of insulating material whereupon a cupshaped shell made of any suitable material is mounted as an enclosing and dust-tight cover.

The base member acts as a mounting for the resistance elements, binding posts and operating 'shaft. One end of the operating shaft may, for

convenience, extend through the bottom of the shell. Secured to the end of the shaft where it extends above the base member is a usual operating knob 8.

Referring to Figure 2, base I provides mounting means for the rotatable elements, which comprise the shaft 5, surrounded by and fixed to rotatable sleeve 4 which is centrally journaled in the base I. The upper end of sleeve 4 has a collar 6 wherein is mounted a screw 1 to prevent the shaft from rotating relatively to the sleeve. Sleeve 4 is reduced on its lower end to provide a shoulder 9, upon which is fixed a disc I0. Thread ed upon the end of sleeve 4, the nut II retains disc I!) in place. Surrounding sleeve 4, below collar 6, is a spring washer I2 which is arranged to exert pressure downwardly upon the upper surface of base I and upwardly upon the lower surface of collar 6.

The base I, extending into shell 2, is shapedbearing I4 and extended collar I5. Due to the tendency of spring washer I2 to bias rotatable sleeve 4 upwardly, disc II] is held tightly against the lower fiat end of the extended collar I5. Mounted between annular bearing l4 and disc Ill, and journaled upon extended collar I5, is upper disc I6. U per disc I6 is provided on its under side with hub I! which retains the two discs in spaced relationship.

As best shown in Figures 2 and 3, mounted upon upper disc I6 and extending to points near lower disc I!) are two pins I8 and I8. In a like manner, mounted upon the lower disc Ill and extending to a point between pins I8 and I8 is similar pin I9. These three pins are mounted in line upon an arc of a circle which has its center at the center of rotation so that pin i9 may move freely to any point between pins I8 and i8, but may not be moved beyond these pins.

As best shown in Figure 1, spaced from the edge of base I and extending therethrough are binding posts 20 and 2| to which connecting wires (not shown) may be attached. On the under side of base I (see Figures 2 and 3), and attached to binding posts 20 and 2!, are segment- shaped brackets 22 and 23 which provide mounting means for fixed spring brushes 24 and 25, respectively.

Brushes 24 and 25 are made of thin, arc-shaped strips of resilient metal; and extend from brackets 22 and 23 in opposite directions to points which, in the present embodiment, are spaced radially approximately apart. Upon the free ends of brushes 24 and 25 are contact portions 24 and 25', respectively.

Mounted in a groove about the periphery of the lower disc I0 is low-resistance element 26. This resistance element is formed of resistance Wire coiled upon and insulated from a Wire of larger diameter. Mounted in a groove about the periphery of the upper disc is high-resistance element 21. The structure of element 27 is similar to the structure of element 26, but diifers in that the resistance wire is of higher resistance. As best shown in. Figures 3 and 4, these resistance elements are each attached at one end to a flexible connecting wire 28 which is coiled about hub I1 and is of a length sufiicient to allow limitedrelative movement between the two resistance elements.

Contact portions 24 and 25 are held under tension against the peripheries of resistance e1ements 26 and 21, respectively, due to distortion of the resilient spring portions of the brush elements. As best shown in Figure 1, integral with 55 the top of base I, and adjacent collar 6, are stop members 29 which provide two limits of adjustment. Threaded into a hole in the side of collar 6, in a position so as to be engageable with stops 29, is screw 30. During manipulation of the rheostat, the engagement of screw 30 with stops 29 prevents rotation of the elements beyond either of these two limits of adjustment. Shell 2 is retained in place by screws 3|, and is provided at the bottom (see Figure 2) with holes 32 which may be used to secure the rheostat in place.

The connection between the two resistance elements is such that rotation of the two elements in a counter-clockwise direction (Figure 4) will increase the portions of both of them in the circuit.-

The operation of the device is better understood by reference to the diagrammatic showing of Figure 4, wherein the various elements have been given reference characters corresponding to the reference characters in the device described above. In Figure 4, pin l9 corresponds to pin l9 and is attached to a segment of disc IIJ. Assuming the rheostat to be in the position shown in Figure 4 and it to be desirable to increase the resistance in the circuit, the shaft 5 will be turned in the counter-clockwise direction. The first movement will merely turn the low-resistance element 26, and the change in resistance will be very small. Upon further movement in the same direction, pin l9 engages pin I8; and high-re sistance element 21 is thus picked up and carried along with the low-resistance element. This turning increases the resistance rapidly. When slightly more than the desired resistance is in the circuit, rotation is stopped. Backward or clockwise turning of the shaft will now move the low-resistance element, and the high-resistance element will remain stationary. In this way the resistance can be slowly decreased to the desired value. If turning the shaft in the clockwise direction is continued, pin I9 will engage pin l8, picking up and carrying with it the high resistance element, with the result that the resistance will be decreased rapidly. Due to this construction, rapid change in resistance may be obtained and, at the same time, slow and accurate adjustment can be made with the use of a single knob and shaft.

In one embodiment of this invention, a highresistance element 21 was chosen with a resistance of five times the resistance of the low-resistance element 26. Thus, the amount of change of resistance for a given rotation is one-fifth as great in the low-resistance element as in the high-resistance element. Pins l8, l8 and I9 are so spaced that low-resistance'element 26 may move approximately 45 free from high-resistance element 21. This movement of lowresistance element 26 of 45 causes the same change in total resistance in the circuit as would occur during a movement of high-resistance element 21 approximately 9. This has been found to give efficient, accurate adjustment of the resistance.

It is to be understood that the positions shown for pins I8 and I8, and the ratio assigned to the resistance values of the two resistance elements, are illustrative only; and that changes therein may be made for obvious reasons.

With the construction described, it is possible, if desired, to place resistance element 26 on sleeve 4 in such a position with respect to stop screw 30 that when shaft 5 is turned to the position of least resistance there still remains a predetermined desired amount of the resistance 21 in the circuit in which the rheostat is connected.

This rheostat is useful where contact resistance and insulation difficulties must be avoided, as there are only two open connections in the circuit, and the entire device, except the shaft and the electrical circuit, would normally bemade from an insulating material.

As various embodiments might be made of this invention, and as various changes might be made in the construction herein described, all without departing from the scope of the invention, it is to be understood that all matter herein set forth or shown in the accompanying drawing is to be interpreted as illustrative and not in a limiting sense.

We claim:

1. In a rheostat, a pair of resistance elements rotatably mounted together and electrically con nected by a direct flexible connection, a first resistance element of said pair being mounted upon and fixed to a shaft and a second resistance element of said pair being free to rotate independently of said shaft, fixed contacts to bear against said resistance elements at points radially spaced approximately from each other, and interengaging means upon said first and second elements to move said second element so that said elements may be moved together or so that said first element may be given a predetermined .movement while said second element remains stationary.

2. In rheostat construction, the combination of, a rotatable shaft structure, a first disk member carrying a resistance unit fixedly mounted upon said shaft structure, a second disk member carrying a resistance unit mounted to rotate about said shaft structure, means electrically connecting said resistance units together, means holding said disks in definite parallel rotatable relationship, a pair of abutment portions upon one of said disk members, and a single abutment portion upon the other of said disk members, said abutment portions being in circumferential align- -ment with respect to said shaft structure, said single abutment portion being between said pair of abutment portions and positioned so that when one of said disks is rotated with respect to the other disk said single abutment Willcontact one of said pair of abutment portions.

3. In rheostat construction, the combination of, an adjusting means, a first res stance unit fixedly mounted upon said adjusting means, a second resistance unit mounted adjacent said first resistance unit, means electrically connecting said resistance units together, fixed contacts to bear against said resistance units, means holdng said resistance units in definite relationship so that they may be moved along adjacent paths, a pair of projecting portions upon one of said resistance units, a single projecting portion upon the other of said resistance units, said single projecting portion extending between said pair of projecting portions so that when one of said disks is rotated with respect to the other disk said single abutment will contact one of said pair of projecting portions.

4. In apparatus of the class described, the com bination of, a rotatable shaft structure, a first frame structure carrying a first resistance unit fixedly mounted upon said shaft structure, a second frame structure carrying a second resistance unit mounted to rotate about said shaft structure, flexible means electrically connecting said resistance units together, means holding said frame constructions in definite parallel rotatable relationship, fixed contacts to bear against said resistance units at points radially spaced approximately degrees from each other and so related to said resistance units as to have constantly changing electrical connections with said resistance units as said frame constructions are rotated, interengaging means upon said first and second frame constructions -to move said second frame construction whereby limited free movement may be imparted to said first frame construction without movement of said second frame construction, and said second frame construction is moved with said first frame construction when said first frame construction is moved in one direction beyond that limit of said free movement, the electrical circuit being from one of said fixed contacts, through a variable portion of one of said resistance units, said flexible means, and a variable portion ofthe other of said resistance units to the other of said fixed contacts, in the order named.

5. In apparatus of the class described, the combination of, a shaft structure, a first disc member carrying a resistance unit and mounted upon said shaft structure, a second disc member carrying a second resistance unit mounted to rotate about said shaft structure, a frame construction rigidly'carrying a pair of contact members, one of said pair of contact members contacting said first resistance unit and the other of said'pair of contact members contacting said second resistance unit, means-electrically connecting said resistance units together, means holding said discs in definite parallel rotatable relationship, a pair of abutment portions upon one of said disc members, and abutment means upon the other of said disc members, said abutment means and said abutment portions being in circumferential alignment with respect to said shaft structure, said abutment means being between said pair of abutment portions and positioned so that when one of said discs is rotated with respect 3 to the other of said discs said abutment means will contact one of said pair of abutment portions and the adjustment of said other of said discs will not be disturbed except when said abutment means and one of said abutment portions .are in engaging position.

6. In apparatus of the class described,the combination of, a first variable resistance unit, a second variable resistance unit, each of said units comprising a disc carrying a resistance element and a cooperating contact member electrically engaging said resistance element, and mounting means holding'said resistance units in definite parallel relationship and including an adjusting means to impart relative movement between each of said resistance elements and its cooperating contact member to adjust the value of the effective resistance of each of said units, said adjusting means comprising a direct mechanical connection to said first unit and a mechanical connection between said first unit and said second unit to permit the direct adjustment of said first unit and the adjustment of said second unit through said first unit, said mechanical connection between said first unit and said second unit comprising a first abutment means upon one of said units having two abutment surfaces and a second abutment means upon the other of said units, said second abutment means and said abutment surfaces of said first abutment means being in circumferential alignment with respect to the axis of said units, and said second abutment means being positioned between said abutment surfaces, in the sense that when adjusting movement of an amount dependent upon the relative adjustment of the two units is imparted to said first unit said second abutment means will be moved into contact with one of said abutment surfaces to thereby impart concurrent movement to said second unit.

WALTER G. McALLISTER. 0. STEWART MICHAEL.

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