Driving mechanism of a member of timepiece moving intermittently

A crown ring for a date indicator is moved once during each revolution of a continuously rotating plate or driving member, which plate has a cam-shaped member of magnetic material that rotates with it and cooperates with a magnet in order to substantially eliminate the variations in torque required to rotate the plate. To this end, the profile of the cam-shaped member is such that during the portion of each revolution of the plate when the crown is moved, the surface of the cam comes rapidly closer to the magnet increasing the pull of the magnet on it, and during the remainder of each revolution, it recedes. The magnet, therefore, helps to rotate the plate when the crown is being moved in order to offset the torque required to move the crown, while placing only a slight drag on the plate during the balance of each revolution of the plate.

BACKGROUND OF THE INVENTION 
The present invention relates to a driving mechanism for a member of 
timepiece moving intermittently, especially of a date or days indicator 
which moves one step each 24 hours, and comprises a driving member which 
rotates continuously and which is provided with means acting once each 
revolution, at least indirectly on a driven member, i.e. the 
intermittently moving member. 
Mechanisms of this type, especially date mechanisms of the so-called 
dragging type (namely those in which the intermittently driven member 
rotates at least for a portion of each of its steps at the speed of the 
driving member) have the advantage of having good efficiency in that the 
energy which is transmitted by the gearing of the movement to the 
mechanism driving the intermittently moving member is very close to that 
required for driving this member. Consequently, there is less loss of 
energy as compared to mechanisms of the so-called instantaneous type in 
which the member moving intermittently effects its movements in sudden 
steps, thereby giving rise to a loss of energy due to the shock each time 
the intermittent member moves. 
However, in the case of the mechanisms of the dragging type, if the 
efficiency is good, the distribution of the consumption of energy is bad 
since the driving member travels through a large portion of its stroke 
without furnishing any driving energy to the member moving intermittently 
and must furnish energy for this purpose only during a short portion of 
its stroke during which it is necessary to apply a relatively high torque 
to this member. Consequently, the movement has to be arranged in such a 
way as to be able to furnish such high peaks of torque. 
In mechanisms of the instantaneous type, this inconvenience does not exist 
because the energy is generally accumulated by a spring during the entire 
revolution of the driving member, this stored energy being suddenly 
released when the intermittently moving member is driven. 
The purpose of the present invention is to better distribute the 
consumption of energy in the case of the mechanisms of the dragging type, 
thereby permitting reduction in the maximum torque which has to be 
furnished to the driving mechanism of the intermittently moving member. 
SUMMARY OF THE INVENTION 
To this end, the driving mechanism according to the invention is 
characterized by the fact that the driving member is rigid with a 
cam-shaped member made of a soft magnetic material with which cooperates a 
stationary magnet, the profile of the said cam being such that during the 
portion of each revolution of the driving member when the driven member is 
being moved, the distance between the surface of the cam and the magnet 
decreases as the driving member rotates, while during the remainder of 
each revolution of the driving member, the distance between the surface of 
the cam and the magnet increases, whereby the reluctance of the magnetic 
circuit constituted by the cam and the magnet varies in such a way that 
the torque applied to the driving member by the gearing of the movement of 
the timepiece is substantially constant throughout, whether or not the 
driven member is actually being moved. 
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
The drawing shows, by way of example, one embodiment of the invention. 
FIG. 1 is a diagrammatic plan view of a driving member of the dragging type 
for a date indicator, and 
FIG. 2 is a diagram of the torque as a function of time.

The driving mechanism represented in FIG. 1 comprises a plate 1 rigid with 
a wheel, not represented, making one revolution per twenty-four hours, 
driven in the direction of the arrow 2 by the hour wheel of the movement, 
designated by 3. This plate 1 is provided with a finger 1a which 
cooperates on each revolution, with the inner toothing, designated by 4a, 
of a crown 4 carrying the indication of the dates. The step-by-step 
movements of the crown 4 are controlled at each half step by the finger 
1a, and are ended by a jumper diagrammatically represented at 5. 
The plate 1 making one revolution per twenty-four hours is rigid with a cam 
6, made of a soft magnetic material, situated at the level of a magnetic 
bar 7 carried by the frame of the movement. This bar could be mounted on 
the frame in such a way that its position be adjustable. The profile and 
the angular position of the cam 6 on the plate 1 are such that, when the 
finger 1a of the plate is meshing with one of the teeth 4a of the inner 
toothing of the crown 4, the distance between the said cam and the magnet 
7 is decreasing as the plate 1 is rotating, and when the finger 1a is not 
meshing with the toothing of the crown 4, the distance between the cam and 
the magnet is increasing as the plate rotates. In the first case, the 
portion of cam 6 extending from A to B (counterclockwise about the axis of 
rotation) is situated opposite the magnet, this portion representing only 
a short part of the whole periphery of the cam, while in the second case, 
the balance of the surface of the cam from B to A is situated opposite the 
magnet. 
Owing to this arrangement, the reluctance of the magnetic circuit 
constituted by the cam 6 and the magnet varies continuously during the 
rotation of the cam, permitting energy to be stored and released. Cam 6 is 
thus attracted by the magnet 7, which applies thereon a force when the 
finger 1a cooperates with the date crown 4 for driving it, while on the 
contrary, cam 6 must tear itself from the magnet when the plate 1 effects 
its portion of rotation during which it does not drive the crown 4. 
The profile of the cam is determined in such a way that the energy which is 
necessary to rotate the plate 1 that is furnished by the gearing of the 
movement is substantially constant, whether or not the plate is acting on 
the date crown 4. 
Calculation shows that, for good distribution of the torque, it is 
necessary that the derivative of the square of the distance D separating 
the magnet 7 from the cam 6, with respect to the angle .alpha. at the 
center of the cam 6, measured with respect to any point of origin, should 
be constant. In other words, one must have the following relation: 
EQU dD.sup.2 /d.alpha. = constant 
In the diagram of FIG. 2, the torque C is indicated along the ordinates of 
the time T along the abscissa. The three curves a, b, and c are, 
respectively, the torque necessary for driving the date crown, the torque 
applied to the cam 6 by the magnet 7, and the torque necessary for driving 
the plate 1 which is furnished by the gearing of the movement. 
As can be seen, the torque necessary for driving the date crown (curve a) 
is zero most of the time and shows a relatively high peak which extends 
only for a very short time. Constructively, the torque necessary for 
driving the cam 6 (curve b) is low most of the time and shows a peak, 
extending for a short time, in opposite phase to the peak of the torque of 
curve a. Hence, the torque which has to be applied to the plate 1, 
represented by the curve c, is substantially constant, being slightly 
higher than the torque necessary for driving the cam 6, represented by the 
curve b, in the period of time when the plate 1 does not drive the date 
crown, the difference being due to the torque necessary for driving the 
plate itself, independently of the presence of the cam 6. 
It is to be noted that, in the case of an electric watch, one could, owing 
to the present arrangement, eliminate the friction device acting on the 
seconds axis for preventing the "flapping" of the second hand. As a matter 
of fact, the torque of the driving mechanism for the date indicator is 
exerted permanently on the gearing of the movement and prevents any 
flapping of this gearing and, consequently of the second hand.