Electrical resistance devices

July 17, 1962

3,045,200

P. J. HINES ETAL

ELECTRICAL RESISTANCE DEVICES

Filed March 16, 1959

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United States Patent Office

3,045,200

Patented July 17, 1962

3,045,200

ELECTRICAL RESISTANCE DEVICES

Paul J. Hines, Waltham, Mass., and Earl M. Worth, North Windham, Maine, assignors to Raytheon Company, a corporation of Delaware

Filed Mar. 16, 1959, Ser. No. 799,646

Claims. (CI. 338—174)

This invention relates generally to electrical resistance devices and, more particularly, to potentiometers capable of being easily assembled and disassembled, said potentiometers having long-wearing and low-noise characteristics.

In the design of potentiometers, especially those of substantially small size which may be used in airborne systems, it is desirable to provide units which have long life and which have substantially stable resistances over wide ranges of temperature and moisture conditions. Moreover, it is desirable to obtain resistive elements in the potentiometers which have low noise characteristics and which maintain substantially linear resistance characteristics over the usable range of the unit. In addition, such potentiometers should be capable of being easily assembled so as to reduce construction difficulties and, hence, reduce manufacturing costs.

Prior art devices of this type have utilized resistance elements fabricated from the newer conductive plastic materials in order to obtain potentiometers having infinite resolution, a high reliability of operation and long life. However, even these potentiometers, as well as those using more conventional resistance materials, have some undesirable disadvantages. For example, many prior art potentiometers, especially those where small size is a necessity, are relatively difficult to assemble correctly without a great deal of skill and time being required. Moreover, the disassembly of such potentiometers usually involves such difficulties that the resistance track or other elements in the potentiometer are easily damaged. In the assembly of such prior art potentiometers, it is not always possible to assure that the commutator wiper arm is actually making a good contact with the commutator. In addition, the susceptibility to change in electrical or mechanical characteristics due to moisture conditions has been an undesirable feature of such prior art devices. In many instances, expansion of the materials used in prior art potentiometers has caused cracking of the potentiometer cases.

This invention, however, provides a potentiometer which is easily assembled and disassembled without damage to the potentiometer elements. In the invention, this is accomplished by providing a rotor-commutator subassembly unit which is itself easily assembled prior to the final assembly of the entire unit. This subassembly unit may be easily attached to the potentiometer housing by means of a retaining washer readily accessible for that purpose. The commutator wiper arm of the invention is constructed in a V-shaped form and secured to the wiper terminal mounted on the potentiometer end plate prior to securing the end plate to the housing. The rotorcommutator subassembly is constructed so as to have rounded or beveled edges to provide an inclined surface. Thus, during final assembly, the V-shaped wiper arm is allowed to slide easily over this inclined surface so that the V-shaped legs of the wiper arm straddle the commutator in an appropriate manner to maintain continuous contact with the commutator during rotation of the potentiometer shaft. Thus, there is a high degree of assurance that, when the end plate containing the resistance element and the commutator wiper arm is secured to the housing containing the rotor-commutator subassembly, the commutator will be brought into contact with and continue to remain in contact with the V-shaped wiper arm.

Other advantages in the potentiometer of the invention over the prior art devices will become apparent to those in the art in the more detailed description which follows. The potentiometer of the invention can be most easily described with the help of the drawing in which:

FIG. 1 shows a particular embodiment of the potentiometer of the invention; and

FIG. 2 shows a portion of another particular embodiment of the invention utilizing an alternative method for contacting the resistance track during rotation of the potentiometer shaft.

In FIG. 1, there is shown a partially cut-away, exploded view of a particular embodiment of the potentiometer of the invention. The potentiometer is shown in three sections: a housing unit 11, a rotor-commutator subassembly unit 24, and an end-plate unit 40. The potentiometer comprises a housing shell 19 having an opening 12 at one side through which extends a shaft 13 shown as part of rotor-commutator subassembly unit 24. Shaft 13 is supported by bearing 14 which is held within a recess 15 of housing 11 by means of screws (not shown) inserted within holes 17 placed in washer 16.

End plate 40 is made of a suitable nonconductive plastic material which has a low coefficient of expansion in the presence of moisture or heat. End plate 40 is sometimes familiarly known to those skilled in the art as the resistance card. It has a raised circular track 18 mounted concentric with shaft 13. Track 18 has a first section 41 made of a suitable resistance material. For many applications, it is desirable to utilize a conductive plastic material as the resistance material of section 41 of track 18. Besides the advantage of long life and reliability obtained from using such conductive plastics, it has been found that wider flexibility with regard to the ranges of resistance values desired is possible with these materials. A pair of terminals 20 are electrically connected to conductive plastic section 41 of track 18 and extend through end plate 40 so that external connections can be easily made. Conductive plastic section 41 comprises substantially the entire length of track 18. A second section 42 of track 18 is located in the region comprising the shorter path between terminals 20. Section 42 is made of a nonconductive plastic material having a low coefficient of friction.

A bearing 22 supports the inner end of shaft 13 in an opening 23 of end plate 40. Inner end 21 of shaft 13 extends through opening 23 and has a slot 43 into which a screw driver may be inserted to adjust the mechanical zero of the potentiometer when the potentiometer is mounted for use in conjunction with other equipment.

Rotor-commutator subassembly 24 is mounted on shaft 13 in the general vicinity of resistance track 18. Subassembly 24 is composed of an insulating element 25 mounted directly on shaft 13 near its inner end. A rotor 26 has a cylindrical portion 27 mounted so as to enclose insulating element 25. A commutator ring 28 is press fit over cylindrical portion 27 of rotor 26. The exposed ends of rotor portion 27 and fluorocarbon insulating element 25 and the outer edges 45 of commutator 28 are rounded for easier assembly, as described in subsequent paragraphs. A contact wiper arm 29 is secured to rotor 26 by means of a pair of rivets only one of which (rivet 39) is shown on the drawing, the other being hidden behind the rotor, commutator and insulating element subassembly. Contact wiper 29 includes a spring element 31 having a contact tip 32 which is tensioned against resistance track 18 so that tip 32 moves along track 18 as shaft 13 is rotated. The low coefficient of friction of conductive plastic section 41 and of nonconductive plastic

3,045,200 the invention is not to be construed to be limited to the specific embodiment described herein and shown in the drawing except as defined by the appended claims. The edges of the commutator, the rotor and the insulating 5 element need not be necessarily rounded as shown, but other methods may be devised by those in the art to eliminate the sharp edges and, thus, provide an inclined surface for the V-shaped wiper arm. For example, the edges of these elements may be beveled so that, during 10 the assembling process, the wiper arm may easily slide over the beveled portion and come to rest in a straddled position about the commutator.