Patent Application
20180107162
This application claims priority from European Patent Application No. 16193768.5 filed on Oct. 13 2016; the entire disclosure of which is incorporated herein by reference.
The invention relates to a balance-spring intended to be elastically fixed by a washer. More precisely, the invention relates to a collet devised to fix a balance-spring to a staff using the elastic clamping of a washer.
EP Patent 2916177 in the name of the Applicant discloses such a collet without recourse to adhesive or soldering. Said collet advantageously makes it possible to avoid shifting the point of attachment when the collet is received on the staff, to position the centre of gravity on the geometric centre of the strip and also to limit material breakage caused by the washer clamping the surface of the collet.
However, on applying the Von Mises criterion, it emerged that the collet is subjected to large variations in stress along its polygonal-shaped strip. Indeed, high contact stresses are generated at the points of contact with the staff, accompanied by bending stresses at the vertices of said polygon.
It is an object of the invention to overcome all or part of the aforementioned drawbacks by proposing a collet that maintains the advantages of EP Patent Application 2916177 while generating fewer contact and bending stresses in the strip and, in particular, at the contact points and vertices of said polygon.
To this end, the invention relates to a balance-spring comprising a spiral spring formed of a strip wound on itself in several coils and a collet including a strip extending substantially in the form of a triangle, the strip comprising, at each of the vertices of said triangle, a bulge extending radially towards the inner coil of the spring, the point of attachment between the spring and the collet being located on one of the bulges of said triangle which is symmetrical with respect to the staff passing through the centre of the collet and said point of attachment, characterized in that the strip, between each of the vertices of said triangle, is formed of first and second areas of substantially constant widths and of a third area, between the first and second areas, which has a thicker width compared to those of the first and second areas and which is arranged to be fitted onto a staff in order to shift the elastic deformation stresses on the first and second two areas.
Surprisingly, by shifting the areas of elasticity between the points of contact between the staff and said polygon vertices of the collet, it was possible to reduce the contact stresses by 70% and, the bending stresses by 40% compared to those of the collet of
In accordance with other advantageous variants of the invention:
Moreover, the invention relates to a resonator for a timepiece, characterized in that it includes a staff on which there is fitted a balance and a balance-spring according to any of the preceding variants.
Other features and advantages will appear clearly from the following description, given by way of non-limiting illustration, with reference to the annexed drawings, in which:
The invention relates to a balance-spring intended to be elastically fixed by a washer. More precisely, the invention relates to a collet devised to fix a balance-spring to a staff using the elastic clamping of a washer.
This assembly system is described in EP Patent Applications 2860591 and 2860592 in the name of the Applicant and incorporated by reference in the present description. This assembly system offers a great deal of freedom as to the materials used. It is thus particularly applicable for a part whose material has no usable plastic domain, i.e. with a very limited plastic domain, with a member comprising a different type of material.
In the field of horology, this type of assembly allowing a great deal of freedom as to the materials used must henceforth be proposed due to the increasing share of materials having a very restricted plastic domain such as those of silicon-based materials such as doped or undoped single crystal (or polycrystalline) silicon, silicon oxide, such as quartz or silica, single crystal or polycrystalline corundum or more generally alumina, silicon nitride and silicon carbide.
An example of this type of assembly is presented in
The collet presented in FIG. 9 of EP Patent 2916177 includes a strip 67, extending in the form of a triangle, devised for this type of assembly described in EP Patent Applications 2860591 and 2860592. Between each of the vertices of said triangle, strip 67 has a non-constant width. More precisely, strip 67, between each of the vertices of said triangle, changes from a maximum width at the clamping points gradually decreasing towards the vertices on two of the arms and a third arm with a fourth bulge allowing the centre of gravity to be positioned on the geometrical centre of the strip.
On applying the Von Mises criterion, it emerged that collet 65 of EP Patent Application 2916177 is subjected to large variations in stress along its strip 67 for a gripping of 6 μm in diameter. Indeed, the contact stresses at the points of contact with the staff are comprised between 300 and 450 MPa and the bending stresses are between 500 and 600 MPa at the vertices of the triangle, whereas they are virtually zero between said contact points and said vertices. Although these maximum values are not in themselves disadvantageous for most of the materials used in horology, the collet is nonetheless subjected to large variations in stress along its strip. This is why the present Application proposes to limit the variations in stress along the strip, and particularly at the contact points.
Surprisingly, it was found that simply by adapting the geometry of the collet arms, it is possible to drastically decrease the contact stress applied to the points of contact with the staff and also to limit the maximum bending stresses observed along the strip while still enjoying the advantages of EP Application 2916177 and maintaining the minimum retaining clamping torque on the staff.
According to the invention, balance-spring 1, 21, 41 thus includes a spiral spring 3 comprising a strip wound on itself in several coils and a collet 5, 25, 45 including a strip 7, 27, 47 extending substantially in the form of a triangle. Strip 7, 27, 47 includes, at each of the vertices of said triangle, a bulge 2, 4, 8, 22, 24, 28, 42, 44, 48 extending radially towards the inner coil SI of spring 3. Finally, the point of attachment between spring 3 and collet 5, 25, 45 is located on one of the bulges 2, 22, 42 of said triangle, which is symmetrical with respect to the axis passing through the centre C of the collet and said point of attachment. Centre C may be defined as the centre of the circle inscribed within the opening of collet 5, 25, 45.
It is proposed to adapt the geometry of the arms of collet 5, 25, 45, i.e. parts of strip 7, 27, 47 between the vertices of the triangle, by shifting the areas of elasticity, i.e. the flexible areas, between the points of contact P1, P2, P3 between staff A and the vertices of said triangle of collet 5, 25, 45.
Thus, advantageously according to the invention, between each of the vertices of said triangle, strip 7, 27, 47 is formed of first and second areas Z1x, Z2x of substantially constant width and of a third area Z3x, Z4x, Z5x, between first and second areas Z1x, Z2x, which has a thickened width compared to those of first and second areas Z1x, Z2x, and which is arranged to be fitted on a staff A allowing the elastic deformation forces on first and second areas Z1x, Z2x of strip 7, 27, 47 to be shifted.
On applying Von Mises criterion again, it emerged, surprisingly, that the collet 5, 25, 45 of the present Application is subjected to lower variations in stress along its strip 7, 27, 47 compared to those of EP Patent Application 2916177. Indeed, for an identical desired gripping of 6 μm, the contact stresses at the points of contact P1, P2, P3 with staff A are all around 100 MPa and the bending stresses do not exceed 400 MPa along strip 7, 27, 47.
It is perhaps most surprising that the contact stresses are also reduced at contact point P2 although the fourth bulge 6, 26, 46 could already be considered to be a thickened portion of strip 7, 27, 47 on one of the arms of collet 5, 25, 45. It therefore seems that the geometry of the three areas Z1x, Z2x, Z3x, Z4x, Z5x, on each of the arms must be substantially identical and respect a certain shape for the bending and contact stresses to be better balanced and lower within collet 5, 25, 45.
To obtain these results, first and second areas Z1x, Z2x were kept with preferably identical lengths L13, L23, L1x, L2x, heights and widths W13, W23, W1x, W2x. Further, it became clear that a length of the third area Z3x, Z4x, Z5x comprised between 20% and 50% of the total length of the three areas Z2x, Z3x, Z4x, Z5x, and a width of third area Z3x, Z4x, Z5x representing between 140% and 300% of the width W1x, W2x of the first and second areas Z1x, Z2x make it possible to maintain the above advantages.
Three example applications of the invention are presented below, maintaining the height and with variations in the length and width of the third area Z3x, Z4x, Z5x. Thus, according to a first embodiment presented in
The first and second areas Z1x, Z2x of each arm of strip 47 form right rectangular prisms that are all identical and offer an identical elastic character to each other. A length of the third area Z51, Z52, Z53 substantially equal to 20% of the total length of the three areas Z51, Z52, Z53, and a width of third area Z51, Z52, Z53 substantially equal to 143% of the width W1x, W2x of the first and second areas Z1x, Z2x are applied in the first embodiment of
Finally, it is seen that a fourth bulge 46 extending radially towards the inner coil is present on third area Z52 of the arm opposite to bulge 42 in order to position the centre of gravity of collet 45 at centre C of collet 45.
According to a second embodiment presented in
The first and second areas Z1x, Z2x of each arm of strip 27 form right rectangular prisms that are all identical and offer an identical elastic character to each other. A length of third area Z41, Z42, Z43 substantially equal to 42% of the total length of third areas Z1x, Z2x, Z4x, and a width of third area Z41, Z42, Z43 substantially equal to 267% of the width W1x, W2x of the first and second areas Z1x, Z2x are applied in the second embodiment of
Finally, it is seen that a fourth bulge 26 extending radially towards the inner coil is present in the extension of third area Z42 of the arm opposite to bulge 22 in order to position the centre of gravity of collet 25 at centre C of collet 25. Further, it can be seen that, proportionally, the fourth bulge 26 of the second embodiment is more voluminous than that 46 of the first embodiment.
According to a third embodiment presented in
The first and second areas Z1x, Z2x of each arm of strip 7 form right rectangular prisms that are all identical and offer an identical elastic character to each other. A length of the third area Z31, Z32, Z33 substantially equal to 38% of the total length of the three areas Z31, Z32, Z33, and a width of third area Z51, Z52, Z53 substantially equal to 167% of the width W1x, W2x of the first and second areas Z1x, Z2x are applied in the third embodiment of
Finally, it is seen that a fourth bulge 6 extending radially towards inner coil SI is present in the extension of third area Z32 of the arm opposite to bulge 2 in order to position the centre of gravity of collet 5 at centre C of collet 5.
Of course, the present invention is not limited to the illustrated example but is capable of various variants and modifications that will appear to those skilled in the art. In particular, the three embodiments can be combined with each other without departing from the scope of the invention. Thus, by way of example, the surfaces on point of contact P1, P2, P3 with staff A could be switched between the first and third embodiments. It is thus clear that the surfaces on contact point P1, P2, P3 may be plane, convex or concave without departing from the scope of the invention.
Further, collet 5, 25, 45 according to the invention is not limited to a triangular-shaped strip 7, 27, 47. The invention may also be applied to a different type of polygon.
Balance-spring 1, 21, 41 may also be formed using several materials. Thus, in a non-limiting manner, the base material, such as for example silicon, could receive a complete or partial coating composed of a temperature compensating layer and/or a layer that is resistant and impermeable to moisture so as to make balance-spring 1, 21, 41 less sensitive to climatic conditions.
Finally, in order to improve the chronometry of the resonator in which it is incorporated, balance-spring 1, 21, 41 may also include an inner coil SI having a Grossman type curve and a partially thickened outer coil.
| Number | Date | Country | Kind |
|---|---|---|---|
| 16193768.5 | Oct 2016 | EP | regional |
