FIELD OF THE INVENTION
The present invention relates to a vertical clutch device for a timepiece, more particularly for a chronograph.
BACKGROUND OF THE INVENTION
Disengaging devices are used in the field of watchmaking and in particular for chronographs. In a chronograph, the chronograph wheel that carries the chronograph hand is connected to the seconds wheel via a clutch. The clutch can occupy a clutched position, corresponding to the chronograph operating position, where the chronograph wheel is driven by the seconds wheel, and a disengaged position, corresponding to the chronograph stop position, where the chronograph wheel is not driven by the seconds wheel. The operation of a vertical clutch device 1 within a partially shown chronograph mechanism 8 is illustrated in FIGS. 1a and 1b for the disengaged position and the clutched position, respectively. The clutch device generally comprises on the same axis a first wheel 3, a second wheel 2 and a clutch disc 4. The first wheel 3 is the driving element which rotates continuously and which is in engaged with the seconds wheel 9. The second wheel 2 is engaged with the chronograph wheel 10. The clutch disc 4 cooperates with a pair of clamps 5, the opening and closing of which is controlled by a column wheel (not shown). Closing the clamps 5 raises the clutch disc 4 against the action of a spring 6 as shown schematically in FIG. 1a. In this disengaged position, the clutch disc 4 is not in contact with the first wheel 3 with the corollary that the second wheel 2 is not driven. When opening the clamps 5, the clutch disc 4 is pressed against the first wheel 3 under the action of the spring 6 (FIG. 1b). In this clutched position, the first wheel 3 drives the second wheel 2 by friction. So that the friction is sufficient, the clutched force Fe must be high, that is to say that a significant pre-stress must be applied to the spring.
According to the prior art, the springs are made of standard materials such as steel which have elastic behaviour over a few tenths of a percent before entering the plastic range. In operation, the spring must work within its elastic range to avoid any irreversible deformation. In this elastic range, the spring has a linear behaviour with a return force proportional to the displacement. FIG. 2 typically shows the force-displacement curve in the elastic range. The clutched force (Fe) is fixed by the pre-stress applied (displacement p) on the spring and the disengaged force (Fd) is fixed by the displacement (d) required to separate the clutch disc from the first wheel. In practice, the spring works at the limit of its elastic capacities because it is subjected to a significant pre-stress with a risk of plastic deformation during displacement when disengaged. In addition to the risk of inducing an irreversible deformation of the spring, these large deformations cause premature fatigue of the spring. Moreover, the behaviour of the spring being linear in the elastic range, any increase in the clutched force leads to an increase in the disengaged force which will have to be provided by the clamps.
In the illustrated example, starting with a sufficient clutched force Fe so that the clutch does not slip, namely 0.67 N in the example, the remoteness of the clutch disc from the first wheel by a distance d, equal to 0.1 mm in the example, requires a significant force Fd of 1.5 N to counter the spring return force. Typically, the disengaged force Fd is thus more than two times greater than the clutched force Fe.
SUMMARY OF THE INVENTION
The object of the present invention is to provide a clutch device providing a maximised clutched force for a disengaged force which, in turn, is minimised. In other words, the object of the invention is to reduce the ratio between the disengaged force and the clutched force.
To this end, the present invention provides a clutch device comprising a spring made of a shape memory alloy used at room temperature for its superelasticity properties. The spring made of a shape memory alloy has a nonlinear behaviour in the elastic range with a stress which peaks at an almost constant value over a wide range of deformation. These superelasticity properties and nonlinear behaviour allow the disengaged force and the clutched force to be easily adjusted according to the required operating conditions. Thus, a significant pre-stress can be applied to the spring without risk of entering the plastic range when the mechanism is disengaged. As a corollary, the spring is no longer biased to the limit of its elastic capacities unlike the spring of the prior art, which allows premature fatigue of the spring in use to be avoided. Moreover, the disengaged force can be minimised by biasing the spring in the range where the stress, and therefore the force, peaks at an almost constant value.
According to the invention, the spring may be dimensioned to increase the clutched force while maintaining an equivalent disengaged force or conversely be dimensioned to reduce the disengaged force while maintaining an equivalent clutched force. Advantageously, the ratio between the disengaged force and the clutched force is comprised between 1.1 and 2.0.
BRIEF DESCRIPTION OF THE FIGURES
Other features and advantages of the invention will become apparent upon reading the detailed description which follows, with reference to the appended drawings.
FIGS. 1a and 1b schematically illustrate the operation of a clutch device with the latter in the disengaged position in FIG. 1a and in the clutched position in FIG. 1b. These figures relate to the prior art but they are also applicable for a clutch device according to the invention.
FIG. 2 shows the force-displacement curve for a standard alloy used in a clutch device according to the prior art.
FIG. 3 shows the typical tensile (stress-deformation) curve of a shape memory alloy.
FIG. 4 shows the tensile curve of a shape memory Ni—Ti alloy used in the clutch device according to the invention.
FIG. 5a shows the geometry of the spring, according to a variant of the invention, used in the clutch device according to the invention. FIG. 5b shows using a plan view the respective dimensions of the second wheel, of the sleeve of the chronograph axis and the spring.
FIG. 6 shows the force-displacement curve for the spring having the mechanical properties of FIG. 4 and the geometry of FIGS. 5a and 5b.
FIG. 7 shows a watch provided with a chronograph mechanism according to the invention.
DESCRIPTION OF THE INVENTION
The invention relates to a clutch device comprising a spring made of a shape memory alloy. It relates more specifically to a clutch device intended to equip a chronograph mechanism 8 with a timepiece 11 (FIG. 7).
According to the invention, the superelasticity properties of the shape memory alloy are utilised to reduce the difference between the clutched force and the disengaged force. FIG. 3 illustrates the superelastic behaviour of a shape memory alloy which has at room temperature an austenitic structure which transforms into martensite under the application of a stress a, which allows the material to be deformed in a reversible manner by several percent. The tensile curve first has a linear elastic behaviour up to a critical stress where the martensitic transformation induces a superelastic behaviour with increasing deformation under an almost constant stress. This is the plateau seen in FIG. 3. As soon as the stress is released, the reverse transformation from martensite to austenite takes place and the alloy returns to its original dimension. Thus, a spring made of this material allows a stress to be obtained, and therefore a force, according to the displacement which is not proportional but peaks at a certain value on the plateau of the curve unlike a conventional material such as steel.
Preferably, the shape memory alloy according to the invention is a copper-based alloy or a nickel and titanium-based alloy. The copper-based alloy is one of the alloys having, for a total percentage of 100% and a percentage of possible impurities less than or equal to 0.5%, the following composition by weight:
- Cu between 64.5 and 85%, Zn between 9.5 and 25% and Al between 4.5 and 10%,
- Cu between 79.5 and 84%, Al between 12.5 and 14% and Ni between 2.5 and 6%,
- Cu between 87 and 88%, Al between 11 and 12% and Be between 0.3 and 0.7%.
The nickel and titanium-based alloy consists of nickel with a percentage by weight comprised between 52.5 and 63% and titanium with a percentage by weight comprised between 36.5 and 47%, for a total percentage of 100% and a percentage of possible impurities less than or equal to 0.5%.
This alloy has at room temperature, in the absence of stresses, an austenitic microstructure.
Preferably, the spring 6 includes a central annular part 6a and several tabs 6b starting from said central annular part 6a as illustrated in FIG. 5a. For example, the number of tabs can be 3. Typically, the thickness of the spring is comprised between 0.05 and 0.4 mm. Preferably, the tabs 6b are inclined relative to the plane defined by the central annular part 6a as schematically shown in FIGS. 1a and 1b. Depending on the level of pre-stress applied to the tabs in the clutched position (FIG. 1b), the latter are more or less inclined relative to the plane of the annular part.
The spring 6 is arranged within the clutch device 1 as previously described with reference to FIGS. 1a and 1b with the clutch disc 4, the first wheel 3 and the second wheel 2.
Starting from the stress-deformation curve of the shape-memory alloy material, the dimensioning of the spring, namely the number of tabs, the active length of each tab and the section of the tabs will define the corresponding force-displacement curve of the spring made of this material as schematically shown in FIG. 6 for the dotted curve. In use, the spring is dimensioned to work with a disengaged force Fd which is on the upper bearing of the hysteresis and with a clutched force Fe which is on the lower bearing of the hysteresis. Note that the shape of the hysteresis can vary depending on the shade selected for the shape memory alloy. Thus, the force on the upper bearing and the lower bearing can be more or less constant depending on the selected shade.
The spring operates in a pre-stressed mode with the deformation of the spring, and advantageously of the tabs of the spring, which defines the clutched force Fe on the lower bearing. The clutched force can thus be adjusted according to the pre-stress applied on the spring. As the material is superelastic, a significant pre-stress can be applied without the risk of plastically deforming the spring. Furthermore, the disengaged force Fd can be adjusted according to the minimum displacement d required to avoid any contact between the clutch disc and the first wheel.
According to the invention, the ratio between the disengaged force and the clutched force is minimised and comprised between 1.1 and 2.0, preferably between 1.3 and 1.6. Expressed in absolute value, the vertical force Fd is comprised between 1 and 3 N and the vertical force Fe is comprised between 0.5 and 2 N, with Fd greater than Fe, for a vertical displacement d between the clutched position and the disengaged position comprised between 0.05 and 0.3 mm. Another way to define the nonlinear superelastic behaviour of the spring in use is to characterise it according to its stiffness which is not constant during deformation. Thus, referring to FIG. 6, the slope of the straight line connecting the origin of the axes X-Y to the point (Fe, p) is greater than the slope of the straight line connecting the origin of the axes X-Y to the point (Fd, p+d). In other words, the angle α2 is greater than the angle α1.
Finally, the present invention is illustrated using an example and FIGS. 4 to 6. FIG. 4 shows the mechanical properties of the shape memory nickel and titanium-based alloy with the above composition. FIG. 6 shows the corresponding force-displacement curve for a spring made of this alloy and having the dimensions given in FIG. 5a. This spring has a thickness of 0.2 mm and includes three tabs with a length of 0.85 mm for a width of 0.06 mm. After insertion between the sleeve 7 of the chronograph axis and the second wheel 2, the active length of each tab is approximately 0.5 mm (FIG. 5b).
To be comparable to the operating conditions in FIG. 2, a disengaged force Fd of 1.5 N was selected with the same disengagement stroke d of 0.1 mm. For these values Fd and d, the clutched force Fe could be maximised at 1.05 N, corresponding to a pre-stress distance p of 0.15 mm, compared to 0.67 N for a steel, which ensures that the clutch does not slip. Thus, it was possible to advantageously increase the clutched force without increasing the disengaged force while maintaining the same disengagement stroke. Consequently, the ratio of disengaged force to clutched force amounts to 1.4 compared to 2.2 for steel.
With a steel having a linear behaviour according to FIG. 2, increasing the clutched force up to 1.05 N would have required a significant pre-stress p on the spring with the corollary of a disengaged force clearly greater than 1.5 N, which would have led to a plastic deformation of the spring.
With reference to the curve in FIG. 6, it is also considerable to apply a pre-stress displacement p less than 0.15 mm, which for the same displacement d during disengagement, leads to a disengagement force less than 1.5 N.
LEGEND
- (1) Vertical clutch device
- (2) Second mobile also called second wheel
- (3) First mobile also called first wheel
- (4) Clutch disc
- (5) Clamp
- (6) Spring
- a. Central annular part
- b. Tab
- (7) Sleeve of the chronograph axis
- (8) Chronograph mechanism
- (9) Seconds wheel
- (10) Chronograph wheel
- (11) Watch or timepiece
- (12) Vertical axis
- (13) Jewel
- (14) Central axis
- Fe: clutched force
- Fd: disengaged force
- d: disengagement distance
- p: displacement for pre-stressing the spring
Patent Application
20210271205
