Drive mechanism for a timepiece calendar date display

A calendar date display comprises a ten-day indicator (2) and a unit indicator (1) driven by a program drive (8), which is provided with a calendar date display (9) whose toothing is engaged with the beak (15) of a lever (16) in such a way that a wheel is moved one step forward by one date number change. Between each calendar number change, the beak (15) is engaged between the first and second teeth (11, 12) of the wheel (9). At the time of the calendar number change, the beak (15) is disengaged from said first and second teeth (11, 12) in such a way that it is engaged between the second and third (12, 13) teeth of the wheel (9), thereby moving said wheel one step forward.

This is a national Phase Application in the United States of International Patent Application No. PCT/EP2006/069305 filed Dec. 5, 2006, which claims priority on European Patent Application No. 05111919.6, filed Dec. 9, 2005. The entire disclosures of the above patent applications are hereby incorporated by reference.

FIELD OF THE INVENTION

The present invention relates to a drive mechanism for a day of the month display for a timepiece comprising a first indicator and a second indicator on which Figures are affixed respectively indicating the units and tens of said day of the month, these indicators being driven by a program wheel set. The latter bears a day of the month wheel comprising thirty one teeth, this wheel being in turn driven by the beak of a lever so as to be moved one step forward by one calendar number change.

BACKGROUND OF THE INVENTION

The mechanism described briefly in the paragraph above is well-known in the art. This mechanism is fitted to a timepiece, called a large date timepiece where the day of month appears through a large aperture arranged in the timepiece dial. The date then appears in larger dimensions than if it were printed on a single ring making one revolution less and moving by approximately 11.5 degrees per day.

The first and second indicators respectively indicating the units and tens of the day of the month are driven by a program wheel set which is described in document WO 01/77756. This wheel set is arranged to drive the first indicator at the end of every day by one step except at the end of the thirty first day when it is not driven, and the second indicator by one step at the end of the ninth, nineteenth, twenty-ninth and thirty first days of the month.

This program wheel set carries a day of the month wheel having thirty one teeth which is driven in a known manner by the beak of a lever controlled in turn either by the drive mechanism of the timepiece, or by a manually actuated pusher when a need to correct of the date is felt. In both cases, the change of date takes place when the beak of the lever—controlled manually or by the driving mechanism—penetrates between two teeth of the day of the month wheel and causes the latter to advance by one step.

Once the function is completed and due to the effect of a return spring acting on the lever, the beak is extracted from the wheel toothing.

This constructive arrangement has a major disadvantage, that of the risk of one of the indicators racing. Indeed, as the correction of the date is carried out by means of a pusher and as the force exerted manually on this pusher is not constant but depends on the strength with which the latter is pushed, the day of the month wheel and with it the program wheel set which is connected thereto can drive either the tens indicator, or the units indicator, as a result of said indicators, own inertia beyond the desired swing angle of a single step, consequently a progression of two units or only one unit can occur. For the tens indicator this danger exists if a correction is made the ninth, nineteenth, twenty-ninth and thirty-first day of the month and for the units indicator if a correction is made the thirtieth day of the month. This necessarily results in a loss of synchronization between the two indicators, and this synchronization can be restored only by opening the timepiece to restore order.

To avoid the disadvantages described in the paragraph above, at least at the time of the passage from the twenty-ninth to the thirtieth then from the thirtieth to the first day of the next month, document WO 01/77756 cited above implements a ratchet and two pins which prevent from one or other of the indicators racing. This system is relatively complicated and requires additional parts thus increasing the price of the timepiece.

SUMMARY OF THE INVENTION

The present invention prevents from one or other of the indicators racing whatever the date of the change of day of the month and without it being necessary to add any additional parts to the timepiece. In order to do so, day of the month display the drive mechanism, and in addition to answering the description of the first paragraph of this description the drive mechanism is characterized in that between each change of day of the month the beak of the lever is engaged between first and second teeth of the day of the month wheel and in that at the time of the change of day of the month said beak is initially disengaged from said first and second teeth to fall then between second and third teeth of said wheel and thereby drives said wheel forward by one step.

DETAILED DESCRIPTION OF THE ILLUSTRATIVE EMBODIMENTS

The day of the month display drive mechanism is illustrated in the Figures enumerated above. This day of the month display comprises two indicators. The first indicator1displays the units3of the day of the month and the second indicator2displays the tens4of said day of the month. Indicators1and2are mounted coaxially to each other, that of the tens being mounted inside that of the units. The date appears through a large aperture arranged in the dial of the timepiece, indicators1and2being driven by a program wheel set P.

The program wheel set8drives first indicator1of the units3by one step at the end of every day except at the end of the thirty-first day when it is not driven. ThusFIGS. 3 and 4show that unit indicator1is provided with a star25having ten branches26meshing with a wheel27having thirty teeth28, the latter being secured to the program wheel set P. Wheel27has a space29with no teeth so that indicator1of units3is not driven when the date passes from the 31stof the month that is ending to the first of the following month, the number1remaining immobile and thus being used for both dates.

Program wheel set8is also arranged to drive second indicator2of tens4by one step at the end of the ninth, nineteenth, twenty ninth and thirty first days of the month. Thus, the Figures show that indicator2of tens4is provided with a pinion30having four branches31meshing with one of the four projecting parts32fixed to program wheel set8, these projecting parts being arranged on wheel set8to drive indicator2of tens4forward by one step at the end of the days referred to above.

The Figures also show that program wheel set8carries a day of the month wheel9having thirty one teeth10regularly distributed around wheel9. This day of the month wheel9is driven in turn by the beak15of a lever16. In the embodiment shown in the drawings, a return spring17acts on lever16, the latter pivoting about an axis18.

FIG. 1shows the driving mechanism in its configuration between each change of day of the month. It is the rest position according to the original idea which makes present the invention, beak15of lever16is then engaged between first and second teeth11and12of the day of the month wheel9. This arrangement has a first obvious advantage, which is to block the mechanism and thereby make it insensitive to possible shocks exerted on the timepiece, which could inadvertently rotate the day of the month wheel between the change of date periods. This arrangement thus deviates from that generally adopted in the prior art which consists in releasing the beak of the toothing of wheel9, the insensitivity to shocks being assured only by one jumper spring34acting on the toothing. The engagement of beak15in the toothing of the day of the month wheel is ensured by the force of a return spring17which presses on lever16. As shown inFIG. 1, the displayed date is the 24th.

FIG. 2shows the driving mechanism in its configuration at the beginning of a change of day of the month due to an action exerted on a correction pusher. In this Figure, the pusher itself is not shown, but a stem21which is associated thereto. This stem is biased by a spring35.FIG. 2shows that beak15of lever16is disengaged manually from the day of the month wheel9to correct the date. This release is carried out by means of stem21of the pusher. Stem21cooperates with a driving pin22mounted on lever16and makes this lever rotate clockwise when stem21moves in the direction of the arrow B via manual action exerted on the pusher associated with this stem. At this moments beak15of lever16is thus disengaged from first and second teeth11and12and is presented opposite second and third teeth12and13of the day of the month wheel9. As shown inFIG. 2, the displayed date is still the 24th.

FIG. 3shows the driving mechanism in its configuration at the beginning of a change of day of the month due to an action exerted by the timepiece movement. This is a change of day of the month such as it occurs at the end of each day. Here beak15of lever16is disengaged from the day of the month wheel9by a cam19driven by the timepiece movement, this cam19making a revolution in twenty-four hours while turning in the direction of arrow A and cooperates with a finger20with which lever16is provided. As in the preceding case, the beak of the lever is disengaged from the first and second teeth11and12and is presented opposite the second and third teeth12and13of the day of the month wheel9. As shown inFIG. 3, the displayed date is still the 24th.

FIG. 3still shows that cam19has a first rising sector23with a small inclination. As cam19is rotating in the direction of arrow A, finger20of lever16starts by climbing this first sector23, thereby rotating lever16clockwise to release its beak15from the toothing of the day of the month wheel9. As shown inFIG. 3, the release is maximum when the top of finger20reaches the end of the first sector23.

FIG. 4shows the driving mechanism in its configuration at the end of the change of day of the month.FIG. 4shows that beak15has fallen between the second and third teeth12and13of the day of the month wheel9, which has made this wheel rotate clockwise and indicator1of the units3advance by one step, said indicator having then passed from the4to the5, the displayed date having passed from the 24thto the 25th. The change of day of the month was caused either manually by releasing the pressure exerted on the pusher, or mechanically by the timepiece movement at the end of a day.

FIG. 4shows that stem21has returned to its rest position in the direction of the arrow C by releasing the manual pressure exerted on its pusher, this return being effected by the return spring35. At this moment, biased by spring17, lever16rotates in an anti-clockwise direction and its beak15lodges between second and third teeth12and13of wheel9, thereby rotating this wheel by one step as described above.

Thus the change of day of the month by the correction pusher takes place not when this pusher is pressed, but when this pusher is released. This is the core of the invention i.e. the actuation force exerted on the day of the month wheel to drive it forward by one step only results from return spring17and no longer from the manual force exerted on the pusher. This manual force is not controllable and therefore not constant, whereas the force of a spring is perfectly constant and adjustable. As explained in the introduction of this description, the manual force exerted on the pusher can cause one or other of the indicators to race during a date correction and desynchronize one indicator with respect to the other, which means that the timepiece will have to be opened to restore this synchronization. According to the present invention, indicator racing is not possible since the force exerted by the return spring on the lever to achieve the change the day of the month is perfectly controllable and adjustable to prevent any possible racing.

FIG. 3also shows that cam19has a second descending sector24arranged substantially radially with respect to the axis of rotation25of the cam. The second sector24has an empty space in which finger20falls as soon as the cam continues to rotate in the direction of arrow A, i.e. at the moment of the day of the month change. The drop of finger20into the empty space of the cam causes lever16to rotate instantaneously in an anti-clockwise direction and causes beak15to penetrate between second and third teeth12and13, thereby causing the instantaneous forward movement of the day of the month wheel and therefore, the instantaneous change of one or other of the indicators. The instantaneous passage of the day of the month at midnight has not been proposed yet, to the knowledge of the applicant, in large date display timepieces comprising trailing or semi-instantaneous mechanisms which require several tens of minutes to pass from one date to the other.