US3808905A

US3808905A - Worm drive for electrically wound clock

Info

Publication number: US3808905A
Application number: US00293319A
Authority: US
Inventors: L Metzinger
Original assignee: Bunker Ramo Corp
Current assignee: Bunker Ramo Corp
Priority date: 1972-09-29
Filing date: 1972-09-29
Publication date: 1974-05-07
Anticipated expiration: 1991-05-07
Also published as: ES418558A1; JPS4973174A; FR2201494A1; CA985054A; GB1436006A; DE2340659A1

Abstract

For an electrically wound clock, a worm and worm gear drive, wherein the worm is a space-wound helical wire spring, frictionally fitted on a motor shaft, extending beyond the end of the shaft, and unsupported at its outboard end. Engagement with the worm gear takes place at a point beyond the end of the shaft.

Description

United States Patent [191 Metzinger [451 May 7,1974

[ WORM DRIVE FOR ELECTRICALLY WOUND CLOCK [75] Inventor: Leonard L. Metzinger, Delavan,

Wis.

[73] Assignee: Bunker Ramo Corporation, Oak

Brook, Ill.

[22] Filed: Sept. 29, 1972 [21] Appl. No.: 293,319

[52] US. Cl. 74/425 [51] Int. Cl. Fl6h 1/16 [58] Field of Search 74/424.5, 458, 425

[56] References Cited UNITED STATES PATENTS 3,238,804 3/1966 Goodykoontz 74/458 X 2,151,191 3/1939 Crane et al. 7'4/458 X 3,163,054 12/1964 Werner..... 74/458 X 3,049,936 8/1962 Schnell 74/425 X Primary Examiner-Leonard H. Gerin Attorney, Agent, or Firm-D. R. Bair; F. M. Arbuckle [5 7] ABSTRACT For an electrically wound clock, a worm and worm gear drive, wherein the worm is a space-wound helical wire spring, frictionally fitted on a motor shaft, extending beyond the end of the shaft, and unsupported at its outboard end. Engagement with the worm gear takes place at a point beyond the end of the shaft.

5 Claims, 2 Drawing Figures WORM DRIVE FOR ELECTRICALLY WOUND CLOCK BACKGROUND OF THE INVENTION ciency of the drive system is of great importance, be-

cause of its effect on battery life. A battery powered clock which is out of service frequently because the batteries are exhausted can be the cause of much user dissatisfaction. Hence it is important that the drive system waste a minimum of energy in friction.

Further, in a small, mass-marketed device for general public use, where price competition is strong, every effort must be madeto save fractions of pennies in the design and manufacture of the device.

In certain types of battery powered clocks, a tiny motor is used to wind the mainspring at uniform intervals, and a motor of suitably small size for use in a clock must run at rather high speed to have sufficient power output. This dictates the use of a worm and worm gear as a speed reduction means between the motor and the spring arbor. In prior art clocks it has been customary to use a molded or machined metal or plastic worm engaging the circumferential worm wheel teeth. In the present state of the art, the cut or molded worm must be held to close tolerances for diameter, pitch of teeth, smoothness, and concentricity or'run-out. If these pa rameters are not carefully maintained, or if, in the case of a molded part, there is flash or irregularity at a parting line, the worm and wheel combination. may generate a considerable amount of noise, with the objectionable results-which have been described. It has been found extremely difficult to control the dimensional properties and the parting line roughness of a molded plastic worm to an extent such that the noise made by the system in the periodic winding cycle would not be objectionable in a quiet room.

Worms using a spring to minimize operating noise have been known, as seen in US. Pat. Nos. 2,682,176 and 3,268,268, but they have required an arbor for backup support, which must be manufactured by an expensive type of machining process. They have also required bearings at each end of the worm, an undesir ably costly and complicated construction for a timpiece.

SUMMARY OF THE INVENTION This invention involves the provision of a worm consisting of a helical wire spring, tightly fitted onto the driving shaft, extending beyond the end of the shaft, unsupported at its outboard end, and engaging a worm gear at a point beyond the end of the shaft. in the small sizes involved in battery-operated clocks, the spring can be assembled to the shaft with the aid of simple fixtures, or even by hand, the inherent resilience of the spring serving to retain it on the driving shaft. No special machining or grooving of the driving shaft is required.

The pitch of the spring is made the same as that of the worm gear with which it meshes; this gives a quiet, long-wearing low friction drive, particularly suitable for service in a clock mechanism. The spring can be simply and economically fabricated on automatic spring coiling machines, with no initial investment such as the cost of molds for a plastic worm.

BRIEF DESCRIPTION OF THE DRAWING To facilitate further explanation of a construction having these advantages, the accompanying drawings are provided, wherein:

FIG. 1 is an elevational view of a drive construction embodying my invention, and

FIG. 2 is a cross sectional view taken on the line 22 of FIG. 1, and partly broken away, illustrating the feature of the construction by which the worm is retained on the driving shaft.

DESCRIPTION OF THE PREFERRED EMBODIMENT In the accompanying drawings, the motor 10 has an output shaft 12, over which is fitted a straight helical open wound coil spring 14, a substantial portion of which extends beyond the outer end 16 of the shaft 12. The portion 18 of the spring which thus extends beyond the shaft engages with the teeth 20 of a worm gear 22, which is mounted for rotation on a shaft 24, being part of the winding mechanism of a clock, not otherwise shown. As will be later explained, the spring is a friction fit on the shaft 12, and therefore when the motor is energized and the shaft 12 rotates, the spring is rotated about its axis, and serves asa worm to drive the worm wheel.

This construction has a number of advantages over the conventional worm gear drive. A suitable spring can be manufactured much more cheaply than a machined worm. Where a material such as bright music wire is used for the spring, a very smooth surface is presented, which contributes to quiet operation and has long-wearing characteristics.

The arrangement 'shown in the drawing, wherein the working portion of the spring extends beyond the end of the driving shaft as a cantilever, provides a degree of resilience which permits a considerable degree of manufacturing tolerance in the distance and alignment between the axis of the driving shaft 12 and the axis of the driven shaft 24.

The spring may be mounted on the driving shaft 12 with a minimum of preparation. No grooving, threading or special preparation of the shaft is required, except that a chamfer 26 may be provided to assure that there is no burr at the end of the shaft, and to aid in centering and leading the spring when it is pressed onto the shaft.

The spring as originally manufactured can be straight, i.e., of uniform diameter throughout. The internal diameter of the spring relative to the diameter of the shaft 12 must be controlled to provide a slight interference fit. That is, the coils of the spring which are slipped onto the shaft must experience some expansion in internal diameter beyond the normal internal diameter of the spring. This is illustrated, considerably exaggerated, in FIG. 2, which shows a cross section through the driving shaft 12 looking toward the spring. The numeral 28 designates a coil of the spring surrounding the shaft. A portion of the figure is broken away, to reveal another coil of the spring, such as 30, one of those in the portion 18 beyond the end of the shaft. Both figures of the drawing show, in exaggerated fashion, that the coil 30 is of smaller diameter than the coil 28. The spring 14 is thus retained on the shaft 12 by its own inherent resilience, which is adequate to keep it in place, meeting the torque requirements of a clock wind drive such as has been described, or of similar light duty applications.

When an open wound helical spring is compressed, its internal diameter increases. This effect may be sufficient, in most applications, to permit the spring to be mounted on the motor shaft by the application of an axial force, either in a simple jig, or by hand. It is compressed against the end of the shaft, and may simultaneously be twisted in a direction tending to unwind it. Both actions tend to cause the coils to increase somewhat in diameter, and they are thus enabled to slip onto the shaft. After the axial and twisting forces are removed, and coils tend to return to their normal uncompressed pitch and diameter, thus gripping the shaft.

While there are some teachings in the patent literature that a wire worm can advantageously be used having a pitch differing from that of the worm gear with which it is operated, that has been found not to be the case for a clock-wind drive. Experiments were made with a clock wind mechanism in which a 36-tooth 48 pitch worm gear was employed on the winding arbor. The correct pitch for such a gear is 0.0654 inch, or 1.66

mm. The wire worm was made of bright music wire,

Average Duration Worm pitch Motor current Wind cycle Ampere-hours millimeters milliamperes seconds per year L66 I08 1.] 2.l7 l.52 208 L7 6.48 [.78 I28 l.2 2.8l

The last column shows the computed values of ampere-hours per year required for the total number of winds in that time, for each of the worm-gear combinations listed. it is obvious that the nominal pitch of 1.66 mm. provides the conditions for the lowest battery consumption and that the spring should be sufficiently stiff to maintain this pitch throughout the range of torque encountered in the wind operation.

Observations of operating noise were also made. The frequency components of audible sound were somewhat different in the test of 1.52 mm. pitch, attributable to the fact that friction loading of the worm gearing slowed down the motor considerably, as shown in the table by the increased cycle time. Otherwise, however, there was no significant difference, and the sound levels for the various trials were about the same. They were noticeably lower, however, than the noise levels of previous similar devices using moulded plastic and solid metal worms.

The simplicity, economy, and operating advantages of the construction described will be noted by those skilled in the art. Some changes may be made in the application, or in the details of construction, without departing from the basic concept of the invention, which is defined in the following claims.

I claim:

1. Drive apparatus for winding a spring in an electrically-powered clock, operative to wind said spring in short wind cycles, said wind cycles being separated by relatively long non-operative periods, wherein power is transmitted from a rotatable shaft to a worm gear, and characterized by a worm comprising a helical wire spring secured to said shaft coaxially therewith by the gripping of several turns of said spring on said shaft, said spring extending beyond the end of said shaft, and engaging said worm gear at a point beyond the end of said shaft, said spring having a pitch the same as that of said worm gear, and being sufficiently rigid axially so that the pitch of the spring does not vary with the varying torque imposed on said spring by said worm gear during said wind cycle,and the number of teeth of said worm gear embraced between adjacent turns of the spring remains constant.

2. In a drive apparatus for an electric clock wherein power is transmitted from a rotatable shaft to a worm gear, a worm comprising an open-wound helical wire spring of normal internal diameter less than the diameter of said shaft, said spring having a portion of its length surrounding said shaft and retained thereon by its inherent elasticity, and having another portion of its length extending beyond the end of said shaft, said spring engaging the worm gear in driving relationship at a point beyond the end of said shaft.

3. Means for transmitting energy from an intermittently operated clock motor having an output shaft, to a worm gear, comprising a worm in the form of an open-wound helical wire spring of normal internal diameter less-than the diameter of said shaft, the spacing between turns of said spring being sufficient so that when the spring is compressed, its internal diameter increased enough to permit the turns so compressed to pass onto said shaft, the turns gripping the shaft when compression is released and thereby retaining the spring in driving engagement with the shaft, said spring when so retained having a portion including a plurality of turns extending beyond the end of the shaft, and said portion operably engaging the worm gear.

4. The invention in accordance with claim 3, wherein said spring is sufficiently rigid axially so that the pitch of the spring does not vary with the varying torque imposed on said motor throughout its intermittent operation, and the number of teeth of said worm gear embraced between adjacent turns of the spring remains constant.

5. Drive apparatus intermittently operable to wind a clock spring, comprising a motor having an output shaft, a worm in the form of an open-wound helical wire spring having one end secured to said shaft and another end extending beyond said shaft and operatively engaging a womi gear, whereby said spring is laterally resilient at the point of contact with said worm gear; but sufficiently rigid axially that the pitch of the spring does not vary with the amount of variation in load imoperation.

Claims

1. Drive apparatus for winding a spring in an electricallypowered clock, operative to wind said spring in short wind cycles, said wind cycles being separated by relatively long nonoperative periods, wherein power is transmitted from a rotatable shaft to a worm gear, and characterized by a worm comprising a helical wire spring secured to said shaft coaxially therewith by the gripping of several turns of said spring on said shaft, said spring extending beyond the end of said shaft, and engaging said worm gear at a point beyond the end of said shaft, said spring having a pitch the same as that of said worm gear, and being sufficiently rigid axially so that the pitch of the spring does not vary with the varying torque imposed on said spring by said worm gear during said wind cycle, and the number of teeth of said worm gear embraced between adjacent turns of the spring remains constant.

3. Means for transmitting energy from an intermittently operated clock motor having an output shaft, to a worm gear, comprising a worm in the form of an open-wound helical wire spring of normal internal diameter less than the diameter of said shaft, the spacing between turns of said spring being sufficient so that when the spring is compressed, its internal diameter increased enough to permit the turns so compressed to pass onto said shaft, the turns gripping the shaft when compression is released and thereby retaining the spring in driving engagement with the shaft, said spring when so retained having a portion including a plurality of turns extending beyond the end of the shaft, and said portion operably engaging the worm gear.

5. Drive apparatus intermittently operable to wind a clock spring, comprising a motor having an output shaft, a worm in the form of an open-wound helical wire spring having one end secured to said shaft and another end extending beyond said shaft and operatively engaging a worm gear, whereby said spring is laterally resilient at the point of contact with said worm gear; but sufficiently rigid axially that the pitch of the spring does not vary with the amount of variation in load imposed upon it throughout the intermittent winding operation.