Cam timepiece mechanism

Abstract

The clockwork mechanism (1) according to the invention comprises a cam (4) that is intended to be driven in rotation, a cam follower (2) and a return spring (9) arranged so as to keep the cam follower (2) pressed against the cam (4), the return spring (9) being arranged so as to work within a predetermined range of winding angles during each rotation of the cam (4). The stiffness of the return spring (9) is zero or negative in at least part of the predetermined range.

Description

[0001] The present invention relates to a cam-operated clock mechanism.

[0002] In watchmaking, mechanisms are known for the instantaneous operation of an indicator. These mechanisms include a spiral cam, sometimes called a snail cam, against which a rocker arm rests under the action of a return spring applied to the rocker arm. The return spring is a V-shaped, U-shaped, or spiral-shaped blade. With each rotation of the cam, the rocker arm slides from the lower to the upper part of the cam, progressively winding the return spring. The rocker arm then drops from the upper to the lower part. This sudden, instantaneous movement is used to actuate an indicator such as a hand associated with a graduated scale or a disc bearing markings and interacting with a window. Patent applications CH 702137 and EP 2241944, for example, describe such mechanisms for a minute counter.

[0003] These mechanisms have the drawback that the torque required to rotate the cam varies over time. Indeed, the resisting torque exerted by the rocker arm and its return spring increases during the rocker arm's movement from the lower to the upper part of the cam. This is due to the return spring's strength increasing linearly with its degree of winding, and also to the cam's spiral shape, which increases the lever arm of the force applied to the cam by the rocker arm.

[0004] This variation in torque increases energy consumption and affects the regularity of the oscillations of the watch's regulating organ, and therefore the accuracy of the measurement.

[0005] Similar problems arise with other types of cam-driven clockwork mechanisms, whether instantaneous or not, for example, retrograde display mechanisms including a snail cam cooperating with a rake, the instantaneous date display mechanism described in patent application EP 1746470 where the cam's periphery has a main spiral portion, a convex portion, and a concave portion, or potato-shaped cam mechanisms such as equation of time mechanisms or sunrise / sunset time displays. Document CH702804A2 shows a torque-modifying mechanism and, more specifically, a torque-reducing clockwork mechanism.

[0006] The present invention aims to mitigate these problems and to this end proposes a clockwork mechanism according to claim 1, namely a clockwork mechanism comprising a cam intended to be driven in rotation, a cam follower and a return spring arranged to keep the cam follower pressed against the cam, the return spring being arranged to work in a predetermined range of winding angles during each rotation of the cam, characterized in that the stiffness of the return spring is zero or negative in at least a part of the predetermined range.

[0007] Particular embodiments of the clockwork mechanism according to the invention are defined in the dependent claims.

[0008] The invention further provides a timepiece, such as a wristwatch or a pocket watch, comprising this clockwork mechanism.

[0009] Other features and advantages of the present invention will become apparent from the following detailed description, made with reference to the accompanying drawings, in which: there figure 1 is a planar view from below of a cam-operated clock mechanism according to an exemplary embodiment of the invention; the figure 2 is a top view of a component of the clockwork mechanism according to the invention, comprising a return spring; the figure 3 is a top-down plan view of a variant of said piece; the figure 4 is a schematic graphical representation of the elastic restoring moment exerted in the part illustrated in the figure 2 ; there figure 5 represents the coordinates of points defining a particular shape of an elastic arm constituting the return spring; the figure 6 is a graphical representation of the elastic restoring moment exerted in the part illustrated in the figure 2 by the return spring having the shape as shown in the figure 5 ; there figure 7 is a graphical representation of a normalized elastic restoring moment exerted in the part illustrated in the figure 2 by an elastic arm having the shape as shown in the figure 5 according to different variants of the elastic arm, namely such an arm with a constant cross-section (curve C1) and such an arm with a variable cross-section (other curves), the cross-section varying according to a first mode of variation; the figure 8 is a graphical representation of a normalized elastic restoring moment exerted in the part illustrated in the figure 2 by an elastic arm having the shape as shown in the figure 5 according to different variants of the elastic arm, namely such an arm with constant section (curve C1) and such an arm with variable section (other curves), the section varying according to a second mode of variation.

[0010] In the context of the present invention, the term "stiffness" means tangential stiffness.

[0011] To the figure 1 A clockwork mechanism 1 is shown according to an embodiment of the invention, mounted on a frame 1a. In this example, the mechanism 1 is an instantaneous minute counter mechanism for a chronograph. It comprises a rocker 2 pivoted at O ​​and having a feeler 3 cooperating with a snail cam 4 mounted on, and driven by, the chronograph shaft 5. This chronograph shaft 5 carries at its upper end the chronograph seconds indicator hand 6 and is rotationally fixed to the chronograph wheel 7 and the chronograph seconds reset mechanism 8. The rocker 2 is held against the periphery of the snail cam 4 by a rocker return spring 9 acting on the shaft 10 of a finger 11, this finger 11 itself acting on the rocker 2. The interaction between the finger 11 and the rocker 2 is of the bearing type.The finger 11 interacts with the wall of a recess 12 in the rocker 2 in a manner similar to a meshing mechanism, with virtually no friction. The rocker 2 and the finger 11 thus rotate in opposite directions.

[0012] A hook 13 is pivoted at P on the free end of the rocker 2 and is acted upon by a hook return spring 14, mounted on the rocker 2, tending to press the beak 15 of the hook 13 against the toothed teeth of a minute counter wheel 16. The shaft 17 of the minute counter wheel 16 carries a chronograph minute indicator 18, such as a hand (as shown) or a disc, displaying the chronograph minutes in conjunction with the chronograph dial. A chronograph minute reset heart 19 is rotationally fixed to the minute counter wheel 16. The minute counter wheel 16 is held in predetermined angular positions between its successive actuations by a jumper 20 on which a jumper return spring 21 acts.

[0013] In the illustrated example, the snail cam 4 has a slot 22 in its terminal part, in accordance with the teaching of patent application EP 2241944, but it could have a more classic shape, without this slot 22.

[0014] With each rotation of the snail cam 4, the rocker 2 and its return spring 9 are wound as the feeler 3 slides from the lower part B to the upper part H of the cam 4. Every minute, the feeler 3, and with it the entire rocker 2, falls from the upper part H to the lower part B of the snail cam 4 under the action of the rocker return spring 9. During this fall, the hook 13 advances the minute counter wheel 16 by one step to instantaneously change the value indicated by the chronograph minute indicator 18. Then, during the gradual rewinding of the rocker 2 by the snail cam 4, the hook 13 moves from the gap in the minute counter wheel 16 where it was during the fall to the previous gap against the action of its return spring 14, to advance the minute counter wheel 16 by another step during the next fall of the rocker 2.

[0015] According to the invention, the rocker return spring 9 is specially shaped to improve the consistency of the torque or moment of force that it exerts (indirectly) on the cam 4 and thus, on the one hand, improve the regularity of the oscillations of the chronograph regulating organ and therefore the accuracy of the measurement and, on the other hand, reduce energy consumption.

[0016] As shown to figures 1 And 2The rocker return spring 9 is in the form of an elastic arm or blade forming part of a component 23 further comprising a base 24 and a rotating element 25. The elastic arm 9 connects the base 24 to the rotating element 25, and only the elastic arm 9 deforms during the operation of the mechanism 1. The base 24 is fixed, for example by means of pins 26, to the frame 1a. The rotating element 25, intended to rotate about itself, is eccentric with respect to the base 24. The rotating element 25 is mounted on the axis 10 of the finger 11 and is rotationally fixed to this finger 11. In a variant of the invention, shown in the figure 3 , the rotating element 25 is the finger 11 itself, in other words the base 24, the elastic arm 9 and the finger 11 form the part 23.

[0017] Part 23 is typically a single piece. It is made, for example, of metal, alloy, silicon, plastic, mineral glass, or metallic glass. It can be produced by machining or LIGA technology, particularly when made of metal or alloy; by deep reactive ion etching (DRIE), particularly when made of silicon; by molding, particularly when made of plastic or metallic glass; or by laser cutting, particularly when made of mineral glass.

[0018] For the purpose of understanding the invention, the behavior of part 23 considered in isolation, that is, free from any interaction with the rest of the mechanism 1, is described below. figure 2 represents this isolated piece 23.

[0019] Due to the shape of its elastic arm 9, the part 23 has a preferred direction of rotation of its rotating element 25 relative to its base 24, this direction being defined as the one that allows, from a rest state of the isolated part 23 in which its elastic arm 9 is at rest, the greatest relative angular displacement of the rotating element 25 with respect to the base 24. This preferred direction of rotation is the counterclockwise direction to the figure 1 and the clockwise direction figure 2 .

[0020] Let θ be the angular position of the rotating element 25 of the isolated part 23 relative to the base 24, θ being equal to zero when the isolated part 23 is at rest, that is, when its elastic arm 9 is at rest, and increasing with the relative angular displacement of the rotating element 25 relative to the base 24 in the preferred direction of rotation of the isolated part 23; the figure 4 illustrates the evolution M(θ) of the elastic restoring moment exerted by the elastic arm 9 in the isolated part 23 as a function of the angular position θ of the rotating element 25 relative to the base 24.

[0021] In general, when the rotating element 25 is in the angular position in which θ = x°, we say that the part 23 is armed by x°.

[0022] As can be seen on the M(θ) curve of the figure 4 This moment of elastic recoil follows a three-phase evolution: For an angle θ between 0 and a first value θ₁, the elastic restoring moment increases rapidly with the angular position θ; beyond this first value θ₁, part 23 is in a substantially stable phase. Indeed, between this first value θ₁ and a second value θ₂, the elastic restoring moment is substantially constant with respect to the angular position θ. A "substantially constant" moment is defined as one that does not vary by more than 10%, preferably 5%, and preferably even 3%, it being understood that this percentage can be reduced further.More precisely, let M min and M max be the values ​​of the minimum and maximum moments exerted in the isolated part 23 over a given range [θ 1 , θ 2 ] of angular positions of the rotating element 25 relative to the base 24, the moment exerted in this isolated part 23 is substantially constant as soon as the inequality « (M max -M min ) / ((M max +M min ) / 2) ≤ 0.1 » is verified, more precisely, as soon as the inequality « (Mmax-Mmin) / ((Mmax+Mmin) / 2) ≤ y% , with y=10, preferably y=5, preferably even y=3 , is verified.In this essentially stable phase, the elastic restoring moment exerted by the elastic arm 9 in the isolated part 23 nevertheless reaches a local maximum for an angular position θa, then decreases in the range of angular positions between the values ​​θa and θb, where θa and θb are between θ1 and θ2; beyond the value θ2, the elastic restoring moment increases again until it reaches a limit value Mlimit, for an angular displacement θ = θ3. This limit value Mlimit depends on the properties of the material in which the part 23 is made and corresponds to the maximum stress that this part can withstand.

[0023] The isolated piece 23 exhibits a curve M(θ) of the type shown in the figure 4 differs from conventional elastic structures. Its properties are based on a sinuous shape of its elastic arm 9, which deforms in such a way as to generate a substantially constant elastic restoring moment (the curve M(θ) exhibits a plateau between θ1 and θ2) over a predetermined range of angular positions of its rotating element 25 relative to its base 24. Obtaining such an elastic arm 9 requires a specific and parameterized design. It can, for example, be obtained by topological optimization by applying the principles presented in the publication "Design of adjustable constant-force forceps for robot-assisted surgical manipulation," Chao-Chieh Lan et al., 2011 IEEE International Conference on Robotics and Automation, Shanghai International Conference Center, May 9-13, 2011, China.

[0024] The topological optimization discussed in the aforementioned article uses parametric polynomial curves such as Bézier curves to determine the geometric shape of the elastic arm.

[0025] Bézier curves are defined, together with a series of m=(n+1) control points (Q 0 , Q 1 , ... Q n ), by a set of points whose coordinates are given by sums of Bernstein polynomials weighted by the coordinates of said control points.

[0026] The geometric shape of the elastic arm 9 is a Bézier curve whose control points have been optimized to take into account, in particular, the dimensions of the part 23 to be designed as well as a constraint "(M max - M min ) / ((M max + M min ) / 2) ≤ 0.05". The inequality "(M max - M min ) / ((M max + M min ) / 2) ≤ 0.05" corresponds to a constant elastic restoring moment of 5% over an angular range.

[0027] In general, the elastic arm or rocker return spring 9 is designed, in particular by its shape, to exert, in the part 23, a substantially constant elastic restoring moment (consistency of 5%) over a range of angular positions of the rotating element 25 relative to the base 24 of at least 10°, preferably at least 15°, and even more preferably at least 20°.

[0028] More precisely, the geometric shape of the elastic arm 9 is defined by the set of points ∑ i = 0 n B i n t . Q i , with t ∈ [0, 1], where the B i n are the Bernstein polynomials given by the function B i t = m − 1 ! i ! m − 1 − i ! t i 1 − t m − i − 1 with t ∈ [0, 1], and where the Q i are the control points Q 0 to Q n. It corresponds to the graphical representation in an orthonormal coordinate system of the set of points defined by the pairs of coordinates (x ; y) defined respectively by the functions x(t) and y(t), t ∈ [0, 1], below: x t = ∑ i = 0 m − 1 Q ix B i t y t = ∑ i = 0 m − 1 Q iy B i t in which Q ix and Q iy are respectively the x and y coordinates of the control points Q i .

[0029] The formulas given above provide the coordinates of a Bézier curve of order m, that is, a Bézier curve based on m control points. For practical reasons, such a Bézier curve can be decomposed into a succession of Bézier curves of order less than m, in which case the geometric shape of the elastic arm is a succession of Bézier curves.

[0030] Using this principle, the applicant designed a specific part 23 with the following dimensions: Distance between the center of rotation of the rotating element 25 and the point of junction of the elastic arm 9 to the rotating element 25: 0.5 mm; Distance between the center of rotation of the rotating element 25 and the point of junction of the elastic arm 9 to the base 24: 2.5 mm; Distance between the two ends of the elastic arm 9: 2 mm; Curvilinear length of the elastic arm 9: 2.4 mm; Thickness (width) of the elastic arm 9: 25.6 µm; Height of the part 23: 0.3 mm.

[0031] As part of this design, seven control points Q0, Q1, Q2, Q3, Q4, Q5, and Q6 were used. The coordinates of these control points are shown in Table 1 below. Tableau 1 : Coordinates of control points Q0 to Q6. Variables Coordonnées x [mm] Coordonnées y [mm] Q 0 0,756625 0,653875 Q 1 1,87325 1,619 Q 2 2,8125 -0,59125 Q 3 3,4375 0,4535 Q 4 3,75 1,032875 Q 5 4,375 0 Q 6 5 0

[0032] With these seven control points it would have been possible to create a Bézier curve of order seven. However, according to the principle indicated above, the Bézier curve was decomposed into two segments, a first segment corresponding to a Bézier curve of order 4 based on the control points Q 0 to Q 3 and a second segment corresponding to a Bézier curve of order 4 based on the control points Q 3 to Q 6.

[0033] Using the coordinates of the control points Q0 to Q6 above in the aforementioned functions x(t) and y(t), the applicant obtained the coordinates of the points defining the geometric shape of the elastic arm 9. A number of these pairs of coordinates are given in Table 2 below. Tableau 2 : Coordinates of the points of passage of the optimized elastic arm. X [mm] Y [mm] 0,756625 0,653875 1,0861324 0,8545816 1,404044 0,903348 1,7094066 0,8387564 2,001267 0,699389 2,2786719 0,5238281 2,540668 0,350656 2,7863021 0,2184549 3,014621 0,165807 3,2246714 0,2312946 3,4155 0,4535 3,4155 0,4535 3,5242745 0,5815901 3,648736 0,628816 3,7871415 0,6110484 3,937748 0,544158 4,0988125 0,4440156 4,268592 0,326492 4,4453435 0,2074579 4,627324 0,102784 4,8127905 0,0283411 5 0

[0034] The graph of the figure 5 The diagram shows the external surface of the rotating element 25, the internal surface of the base 24, and the elastic arm 9 of the particular part 23 designed by the applicant. The geometry of the arm 9 is defined by a curve passing through the set of point coordinates defined in Table 2 above. This diagram is drawn in an orthonormal coordinate system.

[0035] There figure 6 represents the results of a simulation of the evolution of the elastic restoring moment of the particular part 23 thus produced as a function of the angular position θ of its rotating element 25 relative to its base 24.

[0036] The simulation performed considers a part 23 made of an amorphous alloy based on zirconium, titanium, nickel, copper, and beryllium, more precisely in a Vitreloy 1b type metallic glass, but any suitable material can be used. For example, materials such as other metallic glasses, other alloys such as Nivaflex® < 45 / 18 (a cobalt, nickel, and chromium alloy), nickel-phosphorus or CK101 (unalloyed structural steel), silicon, typically coated with silicon oxide, or plastic are also suitable. It is important to consider the ratio between the yield strength and the Young's modulus of the material when choosing the material constituting the elastic arm 9.

[0037] The analysis of the results presented at the figure 6 that a locally maximum elastic restoring moment, then decreasing and finally locally minimum, is obtained during a displacement of the rotating element 25 of the particular isolated part 23 studied relative to its base 24 from an angular position θ a = 17° to an angular position θ b = 28°, that is to say over a range of 11°.

[0038] The stiffness of part 23, more precisely of its elastic arm 9, is the derivative of the function M(θ) defined previously.

[0039] Over the range of angular positions [θ a , θ b ] the stiffness is zero at angular positions θ a and θ b and negative between these positions θ a and θ b . In the present invention, we are working within this range [θ a , θ b ] or at least partly within this range.

[0040] Within the mechanism 1, the part 23 is arranged so that, with each rotation of the snail cam 4 against the return action of the elastic arm or rocker return spring 9, the rotating element 25 moves within a predetermined range of angular positions relative to the base 24. This range is included within the range of positions [θ1, θ2] associated with the part 23 and includes at least a portion of the range of positions [θa, θb] in which the stiffness of the elastic arm 9 is zero or negative. Preferably, this predetermined range is included within, or constitutes, the range [θa, θb]. Even more preferably, this predetermined range is included within the range ]θa, θb[ where the stiffness is negative at each point.

[0041] To achieve such an arrangement, the rotating element 25 is angularly positioned during its mounting on the axis 10 of the finger 11 so that the rocker return spring 9 is cocked by θarm degrees when the feeler 3 of the rocker 2 is located on the lower part B of the snail cam 4, this value θarm being the lower limit of the aforementioned predetermined range. To facilitate this positioning operation, the rotating element 25 may have a reference mark 27 to be aligned, for example, with the finger 11. At the figure 1 Part 23 is shown in its rest position, before pre-cocking. The length of the predetermined range is defined by the difference in radius between the upper part H and the lower part B of the cam 4, the position of the rocker 2 and that of the finger 11. In the illustrated example, it is 3°.

[0042] Thanks to the at least partially zero or negative stiffness of the rocker return spring 9 within the predetermined range of angular positions that the rotating element 25 can assume during the operation of the mechanism 1, the average intensity of the force applied to the snail cam 4 by the rocker return spring 9 via the finger 11 and the rocker 2 over one rotation of the snail cam 4 can be reduced compared to a traditional rocker return spring, for the same force applied to the cam 4 when the feeler 3 is on the lower part B, thus reducing the energy required to rotate the snail cam 4. Traditional rocker return springs, whether V-shaped, U-shaped, or spiral, all exhibit linear behavior; their stiffness is positive and constant over their entire working range.

[0043] The negative stiffness of the rocker return spring 9 also makes it possible to compensate at least partially for the variation in the lever arm of the force applied to the cam 4 by the rocker 2 over one rotation of this cam, more precisely the increase in the lever arm of the force applied to the cam 4 during the movement of the rocker 2 from the lower part B to the upper part H. A smaller variation in the torque required to rotate the cam 4 and therefore better timing can thus be obtained.

[0044] It is possible to adjust the negative stiffness value by designing the rocker return spring or elastic arm 9 with a variable cross-section. figure 7 This shows different curves representing a normalized moment of force M(θ) exerted by the elastic arm 9 in the isolated part 23 for different variations in the cross-section of the elastic arm 9. The highest curve, designated by C1, corresponds to an elastic arm 9 of constant cross-section and thickness (width) 30 µm. The curves located below curve C1 correspond to an elastic arm 9 whose thickness increases linearly from the rotating element 25 to the base 24, the thickness at the point of junction with the base 24 being 30 µm for each curve, the thickness at the point of junction with the rotating element 25 being 29 µm for the first curve C2 under curve C1, 28 µm for the second curve C3 under curve C1, 27 µm for the third curve C4 under curve C1, and so on by decrement of 1 µm.It can be observed that, at least for the first few curves, the stiffness decreases (the torque decreases more) within the range of winding angles of interest where the stiffness is negative as the change in cross-section increases. It should also be noted that the length of the range of winding angles where the stiffness is negative increases. Therefore, a negative stiffness can be chosen that completely or almost completely compensates for the effect of the increased lever arm of the force applied to the snail cam 4 by the rocker 2 during its movement from the lower part B to the upper part H. Such compensation makes the energy consumed for the rotation of the cam 4 practically constant over time and thus minimizes disruption to the timing.

[0045] Other methods of varying the cross-section of the elastic arm 9 can be considered. The figure 8 shows different curves representing a normalized moment of force M(θ) exerted by the elastic arm 9 in the isolated part 23. The highest curve, designated by C1, corresponds to an elastic arm 9 of constant cross-section and thickness 30 µm. The curves below curve C1 correspond to an elastic arm 9 whose thickness increases linearly from the rotating element 25 to the middle of the elastic arm 9 and decreases linearly from the middle of the elastic arm 9 to the base 24, the thickness at the middle of the elastic arm 9 being 30 µm for each curve, the thickness at the point of junction with the rotating element 25 and at the point of junction with the base 24 being 29 µm for the first curve C2' under curve C1, 28 µm for the second curve C3' under curve C1, 27 µm for the third curve C4' under curve C1, and so on by decrement of 1 µm.We observe that this method of variation of the section of the elastic arm 9 also allows the negative stiffness to be adjusted to, for example, completely or almost completely compensate for the effect of the increase in the lever arm of the force applied to the snail cam 4 by the rocker 2 during its movement from the lower part B to the upper part H.

[0046] In general, when the elastic arm 19 has a variable cross-section, this cross-section typically varies in a strictly monotonic manner (increasing or decreasing continuously, but not necessarily linearly) over at least a continuous portion of the elastic arm representing 10%, preferably 20%, preferably 30%, preferably 40%, of the (curvilinear) length of the elastic arm. The variation in cross-section is further chosen to make the stiffness of the elastic arm 19 more negative over the range [θa, θb], or at least over the predetermined portion of the range that overlaps with the range [θa, θb], compared to an elastic arm of the same shape as the arm 19 but with a constant cross-section.

[0047] In some variations, the rocker return spring or elastic arm 9 may have a different shape from that illustrated in figures 1 And 2It can notably take a form such as that described in the article "Functional joint mechanisms with constant-torque outputs", Mechanism and Machine Theory 62 (2013) 166-181, Chia-Wen Hou et al.

[0048] It will be clear to those skilled in the art that, instead of consisting of a single elastic arm, the rocker return spring 9 could comprise several elastic arms connecting the base 24 to the rotating element 25, similar to the devices described in the two articles "Design of adjustable constant-force forceps for robot-assisted surgical manipulation" and "Functional joint mechanisms with constant-torque outputs" mentioned above. In the embodiment illustrated in the figure 1A single elastic arm 9 is sufficient since it does not have a guiding function—the rotating element 25 is guided by the axis 10 of the finger 11—but only performs an elastic return function. It should also be noted that making the rocker return spring 9 in the form of a single elastic arm offers the advantage of greater compactness. Generally, the choice of the number of elastic arms, their length, and their thickness determines the magnitude of the force produced. The inclination of the elastic arm(s) relative to the rotating element 25 (in the plane of part 23) can also be used to modify the magnitude of the force produced.

[0049] The present invention is not limited to a minute counter mechanism or a snail cam. Nor is it limited to an instantaneous action mechanism causing a jump-like movement of an indicator or other moving part. It can be applied to any clockwork mechanism comprising a cam that successively, once or several times per rotation, arms and (partially) disarms a rocker, a ratchet, or other cam follower. In the context of the present invention, a "cam follower" is understood to be a part that cooperates with the periphery of a cam, typically to read information, without having any function of holding the cam in predetermined positions during normal operation of the mechanism, unlike, for example, a jumper or a ratchet cooperating with a toothed wheel to position it.

[0050] The use of the intermediate finger 11 between the rocker return spring 9 and the rocker 2 allows, by adjusting the lever arms, for a reduction in the size of the mechanism 1 for a given return torque applied to the rocker 2. However, this finger 11 could be eliminated and the rocker return spring 9 could act more directly on the rocker 2; for example, the rotating element 25 could be mounted on the axis of the rocker 2. The rocker return spring 9 could also form a single piece with the rocker 2, or even be an integral part of the rocker and guide a rigid end acting as a cam follower in rotation relative to a base.

Claims

1. Timepiece mechanism (1) comprising a cam (4) intended to be driven in rotation, a cam follower (2) and a return spring (9) arranged to keep the cam follower (2) bearing against the cam (4), the return spring (9) being arranged to function within a predetermined range of winding angles during each rotational turn of the cam (4), characterised in that the stiffness of the return spring (9) is zero or negative in at least part of the predetermined range.

2. Timepiece mechanism (1) as claimed in claim 1, characterised in that the stiffness of the return spring (9) is zero or negative in substantially the whole of the predetermined range.

3. Timepiece mechanism (1) as claimed in claim 1 or 2, characterised in that the stiffness of the return spring (9) is negative in substantially the whole of the predetermined range.

4. Timepiece mechanism (1) as claimed in any one of claims 1 to 3, characterised in that the return spring (9) comprises at least one elastic arm.

5. Timepiece mechanism (1) as claimed in any one of claims 1 to 4, characterised in that the return spring (9) comprises a single elastic arm.

6. Timepiece mechanism (1) as claimed in claim 4 or 5, characterised in that the or each elastic arm is of a sinuous shape.

7. Timepiece mechanism (1) as claimed in any one of claims 4 to 6, characterised in that the geometric shape of the or of each elastic arm is a Bézier curve or a succession of Bézier curves.

8. Timepiece mechanism (1) as claimed in any one of claims 4 to 7, characterised in that the or each elastic arm has a variable cross-section, the variation of which is selected to render the stiffness of the return spring (9) more negative in said at least part of the predetermined range, preferably in substantially the whole of the predetermined range, with respect to an elastic arm of the same shape but with a constant cross-section.

9. Timepiece mechanism (1) as claimed in any one of claims 1 to 8, characterised in that the return spring (9) forms part of a single-piece part (23) further comprising a base (24) fixed to a frame (1a) of the timepiece mechanism (1) and a rotational element (25), the return spring (9) connecting the base (24) to the rotational element (25).

10. Timepiece mechanism (1) as claimed in claim 9, characterised in that the rotational element (25) is mounted on the rotational spindle (10) of a finger (11) arranged to cooperate with the cam follower (2).

11. Timepiece mechanism (1) as claimed in claim 9, characterised in that the rotational element (25) comprises a finger (11) arranged to cooperate with the cam follower (2).

12. Timepiece mechanism (1) as claimed in claim 9, characterised in that the rotational element (25) is mounted on an axis of rotation (0) of the cam follower (2).

13. Timepiece mechanism (1) as claimed in any one of claims 1 to 12, characterised in that it further comprises a mobile member (16, 17, 18) arranged to be driven by the cam follower (2).

14. Timepiece mechanism (1) as claimed in claim 13, characterised in that the mobile member (16, 17, 18) comprises an indicator (18).

15. Timepiece mechanism (1) as claimed in claim 13 or 14, characterised in that the cam (4) and the cam follower (2) are arranged to allow a jumping movement of the mobile member (16, 17, 18).

16. Timepiece mechanism (1) as claimed in any one of claims 1 to 15, characterised in that the cam (4) has a generally spiral shape or comprises a main part having a generally spiral shape.

17. Timepiece mechanism (1) as claimed in any one of claims 1 to 16, characterised in that the cam follower (2) comprises a lever or a rack.

18. Timepiece mechanism (1) as claimed in any one of claims 1 to 16, characterised in that the cam follower (2) comprises a lever and in that the timepiece mechanism (1) comprises a hook (13) pivoted on the lever and a wolf tooth wheel (16) arranged to be driven by the hook (13).

19. Timepiece comprising a timepiece mechanism (1) as claimed in any one of claims 1 to 18.

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