US3443375A - Friction mechanism for clock - Google Patents

Description

May 13, 1969 E. F. CIELASZYK FRICTION MECHANISM FOR CLOCK Filed Aug. 11, 1967 m 05 m. mm vC .WF 9

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United States Patent 3,443,375 FRICTION MECHANISM FOR CLOCK Edward F. Cielaszyk, Oglesby, Ill., assignor to General Time Corporation, Stamford, Conn., a corporation of Delaware Filed Aug. 11, 1967, SenNo. 660,041 Int. Cl. G041) 27/00 US. Cl. 5885.5 6 Claims ABSTRACT OF THE DISCLOSURE A clock friction mechanism for use in a clock having plastic gear wheels and interposed between the gear train and the minute hand shaft.

In clocks it is necessary to be able to manually set the hour and minute hands without rotating the gears in the gear train which normally drives the hands. This is accomplished by a friction connection, referred to in the art as a"friction between the minute hand shaft and gear which drives it, which gear is referred to as a centerwheel.

It is an object of the present invention to provide a clock friction mechanism intended particularly for use in clocks having plastic movements which has an easy, smooth setting action but which insures positive driving of the hands under normal running conditions. It is a related object to provide a friction mechanism which is long wearing and free of any tendency to gall or groove at the engaged surfaces. In this'connection it is an object to provide a friction mechanism which has predictable and uniformcharacteristics in quantity production runs in spite of the wide tolerances which are normally applicable to low cost plastic movements.

It is another object to provide a friction mechanism which is simpler and less expensive than conventional clock frictions and which is particularly well suited for use in low priced clocks, requiring only a small garter spring, in addition to the gear wheel and shaft normally present, and which, being highly compact, is suited for use in a wide variety of clock designs.

, it is a more specific object of the present invention to provide a clock friction which is easily assembled and which has a novel provision for automatically and positively locating the associated gear and shaft at the time of assembly and during the life of the clock.

Other objects and advantages of the invention will become apparent upon reference to the attached detailed description and to the drawings in which:

FIGURE 1 is a sectional-view, partially diagrammatic, taken through a clock mechanism employing the present invention;

FIG. 2 is an enlarged fragmentary section taken through the clock centerwheel; and

FIG. 3 is a fragmentary perspective showing the hub portion of the gear of FIG. 2.

While the invention has been described in connection with a preferred embodiment, it will be understood that I do not intend to be limited to the particular embodiment shown but intend on the contrary to cover such alternatives, modifications and equivalents as may be included within the spirit and scope of the invention.

Turning now to the drawing there is shown, in simplified sectional view, a clock mechanism mounted on frame plates 11, 12 and 13. The clock drive train, generally indicated at 15, is conventional having a balance 16, an escapement pawl 17, and an escapement wheel 18. The pinion 19 of the latter is driven by a gear 20 having a pinion 21. This pinion is, in turn, driven by a gear 22 having a pinion 23.

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Torque is supplied to the drive train by means of a spring 25, the outer end of which is coupled to a barrel 26 connected to a winding stem 27. The inner end of the spring is connected to an output gear 28.

Drivingly connected between the spring output gear 28 and pinion 23 is a centerwheel 30 having a pinion 31 and integral hub 32. Telescoped within the centerwheel 30 is a minute hand shaft 35. Mounted on the forward end of the minute hand shaft is a minute hand 36. The knob 37, at the rear end of the shaft, is provided for setting the hand.

For driving the associated hour hand, a pinion 40 on the minute hand shaft engages a reduction gear 41 having a pinion 42 which meshes with a further reduction gear 43. The latter has an integral quill 44, on the forward end of which is mounted the hour hand 46. An alarm set gear 50, which is set by pinion 51 is connected to an outer quill 52 mounting an alarm setting hand 56.

In accordance with the present invention the centerwheel 30, its associated pinion 31, and its axially extending hub 32, are integrally molded of plastic material, and the cylindrical hub is separated, by axially extending slots, into segmented jaws which embrace the shaft, the jaws being encircled by a loop of garter spring to force the jaws into intimate engagement with the shaft. Further in accordance with the invention the shaft has a conically tapered section which extends substantially the length of the jaws and which terminates, at its narrow end, in a shoulder thereby insuring that the centerwheel, although rotatable on the shaft, is maintained in a precise axial position.

Turning to the drawing, the hub 32 in the present embodiment is formed into three jaws 61, 62, and 63 which are defined by equally spaced slots 64, 65, 66. Encircling the jaws is a garter spring 70 having ends 71, 72. The garter spring, which may be formedof spring steel or bronze, has a normal inner diameter which is less than the hub diameter and is of sufficient strength, readily determined by one skilled in the art, to apply a force upon the jaws which is directed radially inward for generating the desired friction. To facilitate installation of the spring on the hub the tip portion of each of the jaws is chamfered.

For the purpose of locating the centerwheel axially with respect to the shaft and simultaneously improving the smoothness of the setting action, the shaft is formed with a conically tapered section 80 which begins at a line or region 81 located at the base portion of the jaws and which tapers down to define a relatively abrupt shoulder 82 alined with the tips of the jaws. Prior to installation of and compression by the garter spring, the hub has a sectional profile as indicated by the dot-dash lines a.

When the centerwheel is installed on the shaft 35 the tips of the jaws, being inwardly biased, snap into position adjacent the shoulder. There is no tendency for the centerwheel to move away from this position since the jaws are constantly urged in the down hill or bottoming direction .by the force of the spring; thus the conflicting requirements of slippage and accurate axial positioning are both achieved.

The invention may be practiced using any one of a number of plastics having adequate strength, dimensional stability, surface hardness, and elastic modulus. An acetal plastic is found to combine the desired qualities in high measure, one example of an acetal plastic being Delrin manufactured by E. I. du Pont de Nemours of Wilmington, Del.

Such material has an unlubricated coefiicient of friction, against steel, within the range of 0.1 to 0.3. This relatively low coefficient, which is equal for the static and dynamic states, combined with relatively high per unit pressure, is considered to contribute to the unusual smoothness of the setting action. It is found that by using more than one convolution of spring there is a tendency for the spring to resist retrograde, or expanding, movement so that spring wire of relatively light gauge may be employed to develop appreciable squeezing pressures. However, it is found that where more than two convolutions are employed the non-retrograde feature tends to be accentuated to the extent that the spring locks-up on the hub so that the jaws find it impossible to yield in the face of minor dimensional irregularities of the hub and shaft. Consequently in carrying out the invention it is proposed that between one and two convolutions be used, preferably one and a half as shown.

'In accordance with one of the more detailed aspects of the invention the transverse sectional area taken through the base of each of the jaws is less than the sectional area at the tip portions of the jaws, with the result that the jaws, under the urging of the spring force, tend to bend bodily about the base, thereby to follow more closely the surface of the conical section of the shaft. This is brought out in FIG. 2 where it will be noted that the radial thickness of each of the jaws is somewhat greater at the mid portion and approaching the end than it is at the base.

It is found that the above friction provides a setting action, which is predictable and consistent in quantity production manufacture in spite of the relatively wide tolerances applicable to low cost components. Life tests indicate that using a conventional shaft material, for example, steel, with a centerwheel made of Delrin or any one of a number of the durable plastics currently available, there is no tendency toward galling or grooving, even after the setting of the clock hundreds of times more frequently than would be encountered in normal usage. The invention is ideally suited for use in clocks having a so-called plastic movement in which substantially all of the gears in the timing train are formed of plastic in lieu of the brass and steel used in the past. It will be apparent that the frictional connection has been very simply achieved with only a single added part, the spring, the other elements being integral with the gear and shaft respectively. Since the construction in addition to possessing the features usually sought in a friction, is inherently compact, the invention is applicable to a wide range of clock designs, including clocks of premium quality.

The centerwheel is easily molded since the center opening is cylindrical, notwithstanding the fact that it is deformed, upon installation, into conical shape.

Although the term conical has been applied to the surface 80, it will be understood that the side walls need not be perfectly straight provided that a circular crosssection is maintained and provided that a shoulder is defined at the narrow end.

While a coeflicient of friction, both static and dynamic, within the range of 0.1 to 0.3 is preferred, it is found that satisfactory operation is achieved where the coefficient in the static and dynamic states is approximately equal and Within the range of 0.1 to 0.5.

I claim as my invention:

1. In a clock the combination comprising a driving train having a centerwheel and source of driving torque, a minute hand shaft telescoped through the centerwheel, said centerwheel being formed of molded plastic and having an integral axially projecting hub, said hub being separated by axial slots into segmental jaws embracing the shaft, said shaft having a conically tapered section extending the length of the jaws and terminating in a shoulder, and a garter spring encircling the jaws for pressing the same against the conically tapered section of the shaft with the tips of the jaws positioned adjacent said shoulder for locating the center wheel in predetermined axial position on said shaft while permitting relative slippage upon exceeding of predetermined torque.

2. The combination as claimed in claim 1 in which the jaws each have a base portion at the end of the slots in which the transverse cross section is less than the cross section at the tip portion for bodily bending of each of the jaws about the base portion into conformity with the conically tapered section of the shaft.

3. In a clock the combination comprising a driving train having a centerwheel and source of driving torque, a minute hand shaft telescoped through the centerwheel, said centerwheel being formed of molded plastic and having an integral axially projecting hub, said hub being separated by axial slots into segmental jaws embracing the shaft, and a garter spring encircling the jaws for pressing the same into intimate engagement with said shaft while permitting relative slippage upon exceeding of predetermined torque.

4. The combination as claimed in claim 3 in which the garter spring is formed of a helical loop of spring metal having between one and two convolutions and pre-sprung to a diameter less than the diameter of the hub.

5. In a gear mechanism the combination comprising a driving train having a drive member and source of driving torque, a shaft telescoped through the drive member, said drive member being formed of molded plastic and having an integral, axially projecting hub of cylindrical shape, said hub being separated by equally spaced axial slots into segmental jaws embracing the shaft, said shaft having a conically tapered section extending the length of the jaws and terminating in a shoulder, and a garter spring encircling the jaws for pressin the same against the conically tapered section of the shaft with the tips of the jaws positioned adjacent said shoulder for maintaining the drive member in predetermined axial position on said shaft while permitting relative slippage upon exceeding of predetermined torque.

6. The combination as claimed in claim 5 in which the plastic material has an unlubricated coefiicient of friction against the material of the shaft which is approximately equal in the static and dynamic states and which lies in the range of 0.1 to 0.5.

References Cited UNITED STATES PATENTS 1,265,598 5/1918 Borresen 58-59 3,057,647 10/1962 Wood 64-11 X 3,213,645 10/1965 Pease 64-30 RICHARD B. WILKINSON, Primary Examiner,

G. H. MILLER, JR., Assistant Examiner.

US. Cl. X.R. 64--30

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