US3576511A

US3576511A - Potentiometer worm screw retention

Info

Publication number: US3576511A
Application number: US869575A
Authority: US
Inventors: Robert D Michik
Original assignee: Bourns Inc
Current assignee: Bourns Inc
Priority date: 1969-10-27
Filing date: 1969-10-27
Publication date: 1971-04-27
Anticipated expiration: 1988-04-27

Abstract

Simple inexpensive means for preventing undesired axial translation of the worm screw and attendant operational changes in a worm screw adjusted resistor irrespective of loose mechanical dimensional tolerances, consisting of providing on the screw a beveled annular conical cam surface and a complementary beveled surface on the interior of the resistor housing and a seat for the interior end of the worm screw, the beveled surfaces coacting to force the end of the screw against the seat irrespective of dimensional tolerance variations.

Description

United States Patent [72] Inventor Robert D. Michik Walnut, Calif. [21] Appl. No. 869,575 [22] Filed Oct. 27, 1969 [45] Patented Apr. 27, 1971 [73] Assignee Bourns, Inc.

[54] POTENTIOMETER WORM SCREW RETENTION 6 Claims, 8 Drawing Figs.

[52] US. Cl 338/162 [51] H0lc 9/00 [50] Field of Search 338/(Digest 1), 180, 181, 162l75 [56] References Cited UNITED STATES PATENTS 3,399,367 8/1968 Tumbusch 338/175 3,522,573 8/l970 Michik 338/171 Primary Examiner-Laramie E. Askin Assistant Examiner-D. A. Tone Attorney-Fritz B. Peterson ABSTRACT: Simple inexpensive means for preventing undesired axial translation of the worm screw and attendant operational changes in a worm screw adjusted resistor irrespective of loose mechanical dimensional tolerances,- consisting of providing on the screw a beveled annular conical cam surface and a complementary beveled surface on the interior of the resistor housing and a seat for the interior end of the worm screw, the beveled surfaces coacting to force the end of the screw against the seat irrespective of dimensional tolerance variations.

POTEN'I'IOMETER WORM SCREW RETENTION BRIEF SUMMARY OF THE INVENTION a. Background of the Invention In potentiometers and like variable resistors in which a rotatable wiper or contact is traversed along the extent of an arcuate resistance element by rotation of a worm wheel to which the contact is connected, and which worm wheel is adjustedby rotation of a worm screw, difficulty has been experienced in providing simple and inexpensive but effective means for retaining the worm screw in operating position and preventing undesired axial translation of the worm screw. Any such axial movement or end play permits some uncontrolled rotation of the worm wheel and contact with consequent undesired and uncontrolled change of the resistance setting of the resistor.

Many sorts of devices and arrangements have been tried and used in efforts to retain the worm screw in place and prevent such axial translation. For example, as shown in U.S. Pat. No. 3,399,367, a keeper blade 65 engages in a circumferential groove in the screw and is restrained against some movements. As shown in U.S. Pat. No. 3,208,024, a pin 44 engages in a similar circumferential groove for the same pur-- 'pose. In U.S. Pat. No. 3,150,343, a retainer screw 42 has a reduced end which engages in a circumferential groove in the screw. In US. Pat. No. 3,108,245, an annular seal 48 and a sleeve 49 are similarly utilized. In U.S. Pat. No. 2,887,554, an annular flange on the worm screw engages in semiannular mating recesses formed in half shell members of the resistor housing. In each and every one of the noted types of constructions, looseness and axial translation of the worm screw can occur when the manufactured interfitting parts are within allowed dimensional tolerance limits but one is at or near a positive tolerance limit and the other is at or near a negative tolerance limit. All manufactured parts which are required to fit other parts are made with dimensional variations, since it is physically impossible to manufacture parts to exact dimensions. Thus in the case of a pin or a key in an annular groove as above noted, if the groove is of maximum permitted width and' the pin of minimum diameter or width, end play of the shaft or screw is permitted. Only in the very exceptional case wherein both parts are exactly correct dimensionally, or are both at maximum, or both at minimum, tolerance limits, will end play or axial translation be prevented.

An additional undesirable feature associated with most if not all of the prior-art solutions to the problem of eliminating possible end play of the wormshaft is that of the cost or ex pense involved in (l) producing the pin, key or like part, and (2) assembling the parts.

b. The Present Invention, Brief Summary The present invention eliminates the part exemplified by the pin, key, ring or similar retainer device employed in prior-an variable resistors, and eliminates the labor involved in assembling such part, in addition to providing means which positively avoids entirely any possibility of undesired worm screw end pl'ay despite liberal permitted dimensional tolerance limits, by providing a steeply tapered or annular bevel cam surface or portion of the shaft, a cooperating arcuate complementarily tapered or beveled cam surface fixed in the resistor housing, a worm wheel having at least a slightly flexible or resilient rim portion, and a thrust seat or bearing on an inner wall of the housing against which the inner end of the worm screw is forcibly urged by the cam action of the coactin'g bevel cam surfaces of the housing and worm screw. Dimensional variations are accommodated by the bevel surface on the worm screw traveling along the fixed cam surface in the housing only to the extent permitted by engagement of the end of the worm screw with the seat or bearing, the worm screw assuming the required one of a plurality of possible transverse-of-axis positions and pressing against the worm wheel with more or less than a median value of force. Such variation of lateral posi-' tioning of the axis of the worm screw is permitted, without any possibility f backlash, by the resilience of the worm wheel and the resilience of the material of which the housing body is formed. Only the rim portion of the wonn wheel need be of yielding character, but the entire wheel may be of stiff elastic material, such as, for example, nylon.

The preceding brief summary description of the present invention makes it evident that it is a broad general object of the invention to provide improvements in worm screw adjusted resistors. Another and more specific object is to provide inexpensive means for retaining in place and preventing undesirable axial translation of the worm screw in a resistor of the noted type. Another object is to improve the operating characteristics of a worm screw adjusted resistor while making the structure and assembly thereof less expensive. Other objects and advantages of the invention are hereinafter set out or made evident in the appended claims and the following detailed description of a presently preferred exemplary physical embodiment of the invention illustrated in the accompanying drawings.

DESCRIPTION OF THE DRAWINGS In the drawings;

FIG. 1 is a pictorial view of an exemplary worm screw adjusted variable resistor of the potentiometer class incorporating the invention, the drawing being to no specific scale;

FIG. 2 is a plan view of the resistor of FIG. 1, with portions broken away to reveal internal details, the drawing being to a different scale;

FIG. 3 is a pictorial view of the body of the housing of the resistor of FIG. 1, to a different scale;

FIG. 4 is a plan view of the resistor housing body, at no particular scale;

FIG. 5 is a sectional view taken as indicated by broken line and directors 5-5 in FIG. 4;

FIGS. 6 and 7 are fragmentary sectional views taken as indicated by indicators 6-6 and 7-7, respectively, in FIG. 5; and

FIG. 8 is an enlarged plan view of the worm screw shown in place in FIG. 2.

DETAILED DESCRIPTION As used herein, the term resistor is intended to mean a variable resistor of the type commonly referred to as a potentiometer. 1 I

Referring first to FIGS. 1 and 2, the potentiometer indicated generally by number 10 comprises a boxlike housing composed principally of a body 12 preferably of molded or cast insulation and a cover 14 which is a ceramic plate that fits in a recess in the body and is held against exit by cement or a sealant potting compound 16. Terminal wires or leads L1, L2 and L3 extend from resistive and conductive films or coats (not shown) on the interior face of the cover to the exterior of the housing through a mass 16' of the potting compound disposed in a depression or recess formed in the body, whereby conventional electrical connections are provided for the potentiometer. As indicated in FIGS. 3 and 4, a circular gudgeon or bearing 12a upstanding from the floor of the body is provided to journal a worm wheel 18 (FIG. 2) which comprises resilient means so as to be capable of yielding radially under the influence of appliedforce. For example, the worm wheel may have a resilient rim portion, or may be formed of resilient synthetic polymer. The worm wheel carries a conventional contact (not shown) which connects the noted resistive and conductive films in known manner.

The body is preferably of open-top boxlike form, and is provided with a bore 12b (FIG. 3) for reception of a worm screw 20 (FIG. 8) that is threaded for driving engagement with the worm wheel 18. Further the housing is provided with an elongate abutment which provides an elongate arcuate seat 12d which partially encircles the worm screw and limits upward and lateral outward movement of the threaded portion 20: of the worm screw. The upper portion of the abutment 12c serves to restrain the worm screw against upward movement when the latter is in operating relationship with the worm wheel; and the upper surface of the abutment serves, in conjunction with a generally arcuate shelf 12f formed in the recess of the body 12, to position and support the active face of the ceramic cover 14.

Adjacent a lower corner of the recess formed in body 12 of the potentiometer housing there is provided a shaped abutment or protuberance 12e which presents first and second fixed bevel cam surfaces 12m and l2n, respectively. As indicated in FIGS. 3, 4 and 7, the abutment 12s is formed with a radius, that is, it is curved; and hence the stationary bevel cam surfaces 12m and l2n are curved. As is indicated in FIG. 7, the fixed cam surfaces are disposed laterally of the axis A of the bore 12b. The latter bore is coaxial with the arcuate surface 12d of the elongate abutment l2c. Further, the interior wall of body 12 of the housing is formed to provide a flat face or bearing 12:: against which the circular inner end face 20x of the worm screw is urged in a manner presently described.

During assembly of the potentiometer, the worm screw is inserted into bore 12b and pressed inward until the endmost annular bevel cam surface 20a (FIG. 8) thereon engages the cam surface 12m of the abutment ll2e in the housing body. Continued inward movement of the worm screw under applied force results in the inner head 20h of the worm screw being deflected laterally by the interaction of the noted cam surfaces, followed by the outer annular'periphery of head 20h snapping past the ridge of abutment 12e and sliding movement of cam surface 20b of the worm screw downwardly along the slope of cam surface l2n of the abutment. Thereafter the worm wheel 18 is pressed onto bearing 12a on the floor of the housing body, with the contact (not shown) extending upwardly. As the worm wheel is pressed in place it resiliently presses against the worm screw, forcing the latter toward seat 12d and causing cam surface 12n to act on cam surface 20b of the worm screw inner head and thereby move or translate the worm screw axially until bearing surface 20.x at the inner end of the worm screw engages bearing surface 12x on the wall of the body. Thereafter the ceramic cover plate 14, bearing on its inner or lower face an arcuate resistance element and return conductor, is pressed into the recess in the body, and cemented or potted in place with the periphery of the lower face resting on shelf 12f and upper faces of

abutments

12c and 12a of body 12. As the ceramic cover is pressed into position, the resistive and conductive return coatings thereon are brought into conductive contact with the rotatable contact carried or driven by the worm wheel, in a known procedure.

The dimensions of the parts including the worm wheel, worm screw, and cam and bearing surfaces, are such that with maximum permissible separation of bearing surface 12:: and

cam surface 12a, and with minimum permissible separation of end face 20x of the worm screw from cam surface 20b, the worm wheel will still apply force in the direction of the arrow in FIG. 2, against the worm screw, and will have caused movement of the threaded portion of the worm screw nearly into contact with seat 12d, and movement of the unthreaded neck 2011 (FIG. 8) of the worm screw nearly into contact with the ridge 12r separating cam surfaces 12m and Mn on abutment 12c, all as depicted in FIG. 2. When the opposite extreme of permissible dimensional tolerances exists, that is, when minimum permissible separation of surface 12x from cam surface l2n exists concurrently with maximum permissible separation of end face 20x from cam surface 20b, as may occur due to manufacturing operations and variations, the worm wheel merely becomes slightly deformed at its area of engagement with the worm screw and exerts a somewhat greater lateral force on the worm screw in the direction of the arrow in FIG. 2. In all cases, the threaded portion of the worm screw is partially within the confines of the arcuate seat 12d, and is precluded from significant upward displacement by the overhanging ledge of the elongate abutment 12c, and end face 20x of the worm screw is firmly held in contact with bearing 12x. Thus the worm screw is positively retained in place with the

surfaces

20x and 12x constantly urged into contact by the axial component of the force acting on cam surface 20b, and preventing axial translation of the worm screw. This complete elimination of possibility of undesirable axial translation of the worm screw is attained without the cost of a separate retaining member and the attendant expense of usual assembly of such a member into the potentiometer structure, and irrespective of dimensional variations among the components.

The preceding detailed description of the preferred embodiment of the invention indicates full attainment of the noted objectives. While in the interest of brevity of disclosure only one specific embodiment of the invention has been described, it is evident that changes may be made within the true spirit and scope of the invention and will occur to those skilled in the art in view of the present disclosure. Accordingly, the scope of the invention is defined by the appended claims.

Iclaim:

1. A worm gear adjusted potentiometer comprising:

first means, including a housing comprising a body having a bore therein;

second means, including worm' gearing comprising a worm wheel journaled in said body and a worm screw rotatable in said bore and engaging said worm wheel for driving the latter incident to rotation of the worm screw, said worm screw comprising an annular bevel cam surface and said worm wheel comprising resilient means permitting radial yielding and being adapted to resiliently apply force to said worm screw transversely of the axis of the worm screw;

said first means presenting a bearing surface to absorb axial inward thrust of said worm screw and further presenting a fixed bevel cam surface engaging said annular bevel cam surface on said worm screw and effective in conjunction with said force to urge said worm screw axially inwardly into engagement with said bearing surface, whereby occurrence of axial translation of said worm screw in said body is prevented during adjustment of said potentiometer;

2. A potentiometer as defined in claim I, in which said bearing surface is a portion of a smooth interior wall of said body, and in which said worm screw comprises at the inner end thereof adjacent said annular bevel cam surface a flat circular end face for hearing engagement with said bearing surface.

3. A potentiometer as defined in claim 1, in which said fixed bevel cam surface is a surface on an abutment which is an integral portion of said body.

4. A potentiometer as defined in claim 3, in which said abutment comprises a second fixed bevel cam surface separated from said first-mentioned fixed bevel cam surface by a ridge comprised in said abutment.

5. A potentiometer as defined in claim 4, in which said worm screw comprises an outer head portion accessible from the exterior of said body and further comprises adjacent the inner end thereof a second annular bevel cam surface adapted for cooperation with said second fixed bevel cam surface on said body whereby to facilitate assembly of said worm screw in said body.

6. A potentiometer as defined in claim 5, in which said body is an open-top boxlike molded structure and comprises an upstanding circular bearing for said worm wheel, said circular bearing being integral with the floor of said body.

Claims

1. A worm gear adjusted potentiometer comprising: first means, including a housing comprising a body having a bore therein; second means, including worm gearing comprising a worm wheel journaled in said body and a worm screw rotatable in said bore and engaging said worm wheel for driving the latter incident to rotation of the worm screw, said worm screw comprising an annular bevel cam surface and said worm wheel comprising resilient means permitting radial yielding and being adapted to resiliently apply force to said worm screw transversely of the axis of the worm screw; said first means presenting a bearing surface to absorb axial inward thrust of said worm screw and further presenting a fixed bevel cam surface engaging said annular bevel cam surface on said worm screw and effective in conjunction with said force to urge said worm screw axially inwardly into engagement with said bearing surface, whereby occurrence of axial translation of said worm screw in said body is prevented during adjustment of said potentiometer.

2. A potentiometer as defined in claim 1, in which said bearing surface is a portion of a smooth interior wall of said body, and in which said worm screw comprises at the inner end thereof adjacent said annular bevel cam surface a flat circular end face for bearing engagement with said bearing surface.