Patent Grant
9411314
The invention relates to a collet. The invention also relates to an integrated single-blade or double-blade hairspring-non-split collet assembly which is intended to be driven onto a balance staff, notably to an integrated assembly including a collet according to the invention. Another aspect of the invention also deals with an integrated hairspring-collet assembly comprising at least two stages and to a method of manufacturing such an assembly.
One of the critical points in using a hairspring in a high-precision clock movement is the reliability of the attachments (in settings) of the hairspring to the balance staff and to the balance bridge. In particular, the attachment of the hairspring to the balance staff is usually performed using a collet, which originally was a small split cylinder intended to be driven onto the balance staff and drilled laterally to receive the interior end of the actual hairspring proper. The development of micromanufacturing techniques, such as DRIE methods for silicon, quartz and diamond or UV-liga methods for Ni and NiP, have opened up options regarding the shapes and geometries used.
Silicon is a very advantageous material from which to make clock springs and micromanufacturing techniques allow the collet to be produced such that it is integral and manufactured as one with the hairspring. One potential problem is that silicon does not have a plastic deformation domain. The collet may thus soon break if the stresses exceed the maximum permissible stress and/or the elastic limit of the material. It is therefore necessary to be sure to dimension the collet both to hold the hairspring on the balance staff when the oscillator is operating (minimal tightening torque) and also so that the collet can be assembled with staffs the diameters of which may fluctuate, all this without breaking or suffering plastic deformation if the diameter of the balance staff remains within a given tolerance band.
Thus, there are various documents that disclose collet geometries.
European patent application published under no. EP 1 826 634 proposes, in its FIG. 4 in conjunction with line 34 of column 3, a collet comprising elastic zones consisting of curved arms. That document does not indicate where the hairspring is to be fixed.
European patent applications published under numbers EP 1 513 029 and EP 2 003 523 propose collets having a triangular opening. The hairspring is fixed in place at an attachment point (reference 3 in the figures of both documents) located at one of the vertexes of the triangles. The collet is formed of an external stiffening structure to which are attached flexible arms which deform to accommodate the balance staff.
European patent application published under no. EP 1 655 642 describes in its FIG. 10D a hairspring of a hairspring resonator having a collet the opening of which is circular. In this case, the balance is attached using rounded arms.
Also, patent application WO2011026275 discloses a hairspring-collet assembly with a collet having a bore provided with four circular bearing parts to receive the balance staff. The bearing parts are delimited by longitudinal grooves made in the bore of the collet.
The geometries described in these documents are not entirely satisfactory which means that many hairsprings (made of silicon, diamond, quartz, etc.) mounted on movements are equipped with a conventional collet which is then driven onto and/or bonded to the balance staff.
It is an object of the invention to propose new collet geometries that are fully satisfactory, i.e. that make it possible to obtain the highest possible clamping torque on the balance staff and the lowest possible stress within the material. In addition, these collets need to be as well balanced as possible in order not to generate any imbalance, as this would impair the time keeping properties of the hairspring.
Such an object is achieved by means of an integrated single-blade or double-blade hairspring-non-split collet assembly, in which:
These features have the notable effect of preventing the point of attachment of the hairspring from moving significantly with respect to the points of contact with (of bearing against) the balance staff after the latter has been driven in. It then follows that the positioning of the hairspring and of its insetting point can be defined with precision.
Another aspect of the invention relates to an integrated single-blade or double-blade hairspring-collet assembly, it being possible for this collet to be split or non-split. This assembly has the particular feature of having at least two levels (or stages or parts), the hairspring being located on a different level from the level on which the bearing surfaces of the collet for the balance staff lie. This feature is particularly advantageous because it allows the retaining torque that holds the collet on the balance staff to be best optimized without requiring an increase in bulkiness in the plane of the hairspring. According to another aspect of the invention, this feature allows the point of attachment of the hairspring to be brought closer to the balance staff without being limited by the periphery of the collet.
The invention also relates to a method of manufacturing an integrated hairspring-split or non-split collet assembly, in which method the hairspring is produced on a different level from the level on which the bearing surfaces of the collet for the balance staff lie.
A collet according to the invention is defined as a collet comprising a bore intended to receive a balance staff, at least a first part and a second part, the first and second parts being separated by a plane perpendicular to the axis of the bore, an element for attaching the collet to a hairspring being exclusively located on the first part and an element for connecting the collet to the balance staff being essentially, or even exclusively, located on the second part.
Various embodiments of collets are defined as follows:
An integrated assembly according to the invention is defined as an integrated hairspring-collet assembly comprising:
Various embodiments of assemblies are defined as follows:
A method of manufacturing an assembly is defined as a method of manufacturing an integrated assembly as above, in which the hairspring is produced on a different part to the part on which the bearing surfaces via which the collet bears against the balance staff lie.
One way of carrying out the method of manufacturing an assembly is defined as the method of manufacture as above, in which the starting material used is an SOI wafer the layer of SiO2 of which has a thickness greater than 3 microns.
A method of manufacturing a collet is defined as a method of manufacturing a collet as above, in which an element for attaching the collet to a hairspring is produced on a different part than the part on which an element for connecting the collet to the balance staff lies.
A method of carrying out the method of manufacturing a collet is defined as the method of manufacture as above, in which the starting material used is an SOI wafer the layer of SiO2 of which has a thickness greater than 3microns.
An integrated assembly according to the invention is defined as an integrated hairspring-collet assembly made of a material that has no plastic deformation domain, in which:
Various embodiments of assemblies are defined as follows:
An oscillator according to the invention is defined as an oscillator comprising an integrated assembly as above and a balance staff of circular cross section.
A timepiece movement or a timepiece according to the invention is defined as a timepiece movement or a timepiece comprising an integrated assembly as above or comprising an oscillator as above, or comprising a collet as above.
Other features and advantages of the invention will now be described in detail in the following description which is given with reference to the attached figures which schematically depict:
The invention applies both to assemblies having a single-blade hairspring and those having a double-blade hairspring. However, it is the latter that it suits the best.
What is meant by a “double-blade hairspring” is a hairspring comprising two blades wound in the same direction, but with a 180° offset, as described in patent application EP 2 151 722 A1. The respective internal ends of these blades are secured to the collet and their respective points of attachment are positioned symmetrically on opposite sides of the periphery of the collet.
The “attachment point” or “insetting point” for the attachment or insetting of the hairspring is generally well defined in the case of a hairspring assembled on a collet made from a different material from the hairspring. In the case of an integrated collet-hairspring assembly for which the hairspring and the collet are manufactured as one, produced for example using a micromanufacturing technique from a silicon or “silicon-on-insulator” wafer, the insetting point may be defined as the point at which the local rigidity along the neutral axis reaches a value that is 10× higher than the rigidity of the blade of the hairspring. In the case of a hairspring of variable blade thickness, the minimum value of local rigidity along the blade will be considered. The local rigidity is equivalent to the flexural rigidity, determined when the blade is flexed or when the hairspring is in operation, over a portion of given length, for example 1 μm. The corresponding insetting points 10, 11 are indicated by way of example in the collet-hairspring assemblies of
The collets according to the invention are dimensioned both to keep the hairspring on the balance staff when the oscillator is in operation and also to be able to be assembled with staffs which have a certain spread on their diameter (no breaking or plastic deformation on the driving-in of a staff of a diameter falling within a given tolerance band). These collets normally have at least 2, and preferably 4, bearing surfaces for the balance staff.
According to the invention, the precise shape of the connecting parts is not crucial provided they are able to deform elastically, notably in bending, when a balance staff is being driven in. Under normal conditions of use of the collet, the receiving parts are therefore parts which are rigid or nondeformable and the connecting parts are therefore parts that are deformable, notably deformable in bending or flexible. The flexibility of these parts stems from the fact that they are thinner than the receiving parts. The deformable parts have smaller cross-sectional areas than the non-deformable parts. This thinning is performed, according to the invention, by making the deformable parts not as wide as the receiving parts. What is meant here by “width” is the thickness measured in the plane of the collet or, in other words, the distance between the contour of the collet and the contour of its central opening (for example, the minimum width e or e′ or the width mid-way along the rigid receiving parts b or b′ in
The junctions between the receiving parts and the connecting parts generally lie more or less at the base of a bearing surface (see hereinbelow and, by way of examples,
As can be seen in
The V-shape of the pairs of rigid arms has the effect of wedging the balance staff better than a single bearing point could. The important thing in fact is for the collet-staff insetting to be as firm as possible so that the points of contact between the collet and the balance staff do not move under the effect of the torque developed by the hairspring when the movement is in operation, i.e. during oscillations of the hairspring once the hairspring-collet assembly has been driven onto or assembled with a balance staff. Having a geometry with two receiving parts facing one another (notably 180° from one another) and each comprising a pair of bearing surfaces allows a vice-like action held by the flexible connecting parts. Under the effect of their elastic deformation, the connecting parts apply elastic return actions returning the receiving parts towards one another and each into contact with the balance staff. Nevertheless, it is also conceivable (but less favorable) to use a single bearing point, such as for example a contact surface that is planar, convex or concave with a radius of curvature greater than the radius intended for the balance staff.
In
However, in this last instance, the positive radius of curvature is strictly greater than 0.51 times the diameter dmax of the largest circle that can be drawn inside the contour of the central opening (when the collet is not deformed, notably when it is not mounted on the balance staff), which circle is also referred to as the “inscribed circle” in the remainder of the description. For preference, the positive radius of curvature is greater than 0.62 times the diameter dmax, making it possible to define a single point of contact between the bearing part and the balance staff. A radius of curvature greater than 0.75 times, or even than 1 times, the diameter dmax of the inscribed circle is also suitable. In the case of a balance staff of circular cross section, the diameter of the staff is slightly greater than dmax, for example comprised within a tolerance band of between 1.01 and 1.02 dmax.
It is important to plan for there to be no flexible part between the points of collet/balance staff contact and the point of attachment of the hairspring, so that the distance between the insetting point or attachment point and the bearing surfaces varies as little as possible and in particular does not vary substantially following the driving-in of the staff.
The collet 1 has order 2 rotational symmetry and has two axes of reflection symmetry, one formed by the bisector of the angle α, the other being perpendicular to the latter and located at equal distance from the intersection of the arms. It may be considered that it comprises two rigid balance staff receiving parts connected by two flexible connecting parts, as can be seen in
The symmetry of the geometry of the collet of
Thus, the geometry makes it possible precisely to define the bearing points, of which there are four in the case of
That much is confirmed by the numerical simulations reported in
The collet is thus formed of two rigid balance staff receiving parts 17, 18 symbolized in black in
Another embodiment of the invention is depicted in
The collet according to the invention is particularly suited to fixing a double-blade hairspring to a balance staff. Specifically, most known collets of the prior art do not deform symmetrically with respect to the attachment points. With a collet like the one depicted in
Second Aspect of the Invention
Another aspect of the invention relates to a collet having at least two levels or stages or parts. The hairspring attachment or anchor point (or attachment points in the case of a two-blade hairspring) is therefore located on a different level from the level on which most, or even the entirety, of the bearing surfaces lie. This is applied in particular to an integrated hairspring-collet assembly.
What happens is that the inventors have discovered that it was possible to maximize the torque withstand of the collet, while minimizing its bulkiness, by lengthening the collet in the plane perpendicular to the hairspring. That allows the function of attaching the hairspring to the staff via the collet (first level, in the plane of the hairspring) to be dissociated from the function of holding onto the staff, notably of holding the collet on the staff (first and second level, and preferably exclusively on the second level, outside of the plane of the hairspring), while at the same time distributing the elastic stress in as balanced a way as possible along the flexible parts.
An integrated hairspring-collet assembly corresponding to that of
As may be seen from those figures, the flanges are not perfectly superposed; there is an offset of a few microns between the first and the second layer.
It is obvious that such an integrated hairspring-collet assembly produced on two levels can also be applied to other types of collets, notably to split collets, and to other types of hairspring, notably to single-blade hairsprings.
Method of Manufacture
The collet or the hairspring-collet assembly can be manufactured using known methods, such as the method covered by patent application no. EP 1 655 642. The collet or the hairspring-collet assembly according to the second aspect of the invention can be manufactured using known methods, such as those covered by patent applications no. EP 1 835 339 or EP 2 104 007.
The main steps in a method of manufacturing a collet or an integrated hairspring-collet assembly produced on two levels, stages or parts are depicted in
The starting substrate used is a wafer of the “SOI” (silicon-on-insulator) type, made up of two parts of monocrystalline Si separated by a thin layer of silicon oxide SiO2 (
Various steps in addition to the methods explained hereinabove may be provided, for example (and nonlimitingly):
According to an advantageous alternative form of the second aspect of the invention, the collet has at least two levels, and the point of attachment or of insetting of the hairspring (or the points of attachment in the case of a two-blade hairspring) is located on a different level from the level at which the bearing surfaces lie and at a distance from the center of the collet that is less than the distance between the center of the collet and its contour or periphery.
As illustrated in
For preference, use is made of an SOI wafer from which to produce such a collet or integrated collet-hairspring assembly including such a collet, the first and second part being made of silicon and separated by a layer of silicon oxide. Specifically, the use of SOI wafers in which the internal layer of SiO2 separating the two layers of Si is thick, or even very thick (for example 2 to 3 microns as usually but preferably with a thickness greater than 5 or even than microns) makes it possible to produce a flexible collet superposing the turns as depicted in
Thus, the element that attaches the hairspring to the collet or the insetting point 10, 11 lies at a distance D1 from the axis of the bore 107 that is less than half the diameter D2 of a cylinder inside which the second part can be inscribed, notably at a distance D1 less than or equal to the mean of half the diameter D2 and half the diameter of the inscribed circle dmax. This is the case for the hairspring-collet assembly of
A collet as described above may in particular be included in an integrated hairspring-collet assembly.
The fact of bringing the attachment point closer to the balance staff allows a considerable improvement in the time keeping properties. In addition, this type of approach is not restricted to a two-blade hairspring but is also perfectly suited to a single-blade hairspring and is not restricted to a closed-contour collet but is also suitable for a split collet. Any combination of collet and hairspring can be obtained in this way, the effect being a hairspring-collet assembly with markedly improved time keeping properties.
Simulations
Finite element simulations were carried out on two integrated double-blade hairspring-two-part non-split collet assemblies of the kind depicted in
These two similar assemblies A and B are depicted in
The hairspring layer height (first part) is 150 microns and the layer height of the level bearing the bearing surfaces (second part) is 500 microns.
The balance staffs have a toleranced diameter comprised between 0.5 and 0.506 mm, with a nominal value of 0.503 mm.
The graph of
It is found for each of the assemblies that the retaining torque is higher than the demanded minimum torque, even for small diameters below the minimum tolerance.
The graph of
It is found, for each of the assemblies according to the invention, that the maximum stresses are well below the maximum permitted value. The advantage of the collet of
For the assembly according to the prior art however, the stress very soon exceeds the maximum permissible value. It can therefore be seen this type of collet is not suited to a driven push-fit assembly. This is because such a contour geometry does not provide both adequate retention and deformation of the collet without breaking following the driven push-fitting of the balance staff. In addition, the inscribed diameter is only 0.2 of a micron smaller than the lower limit of the tolerance so that the stresses are below the maximum permissible limit for the bottom limit of the tolerance, thus requiring extremely close manufacturing tolerances.
The same behavior is predicted for other collets of the prior art, as depicted in FIG. 10D of document EP 1 655 642. The increase in stress with staff diameter is not as steep as it is in the case of the collet of
This example illustrates the advantage of a closed contour collet with rigid receiving parts connected by flexible connecting parts. This difference in rigidity can be estimated to a first approximation using the small deformation beam theory: for a beam, the rigidity k of an element of width e, of thickness h and of length L is proportional to e3×h/L3. By making the approximation that the width e is constant along the parts, the ratio between the rigidity of an receiving part, kr, and of a connecting part, kf, is therefore equal to kr/kf=(er3×hr×Lf3)/(ef3×hf×Lf3)=(er3×Lf3)/(ef3×Lr3), if the thickness is the same. Reducing the mean width of the connecting parts by comparison with the receiving parts and maximizing the length of these same connecting parts thus allows a very significant reduction in the rigidity of the connecting parts. For preference, a ratio kr/kf higher than 10, more preferably higher than 50, more preferably still higher than 100, will be chosen.
Given that the rigidity is dependent on the cube of the width, the difference in width between the rigid receiving parts and the flexible connecting parts is preferable for obtaining a lower rigidity on the connecting parts than on the receiving parts.
There are various possible ways of obtaining a lower rigidity: thus, the mean width of the connecting parts may preferably be smaller than the mean width of the receiving parts, more preferably smaller by a factor of two than the mean width of the receiving parts.
Alternatively, or in combination, the two connecting parts have a minimum width and/or a width at mid distance from the receiving parts that is/are smaller than the maximum width of the receiving parts.
The minimum width e of the connecting parts is then preferably less than 0.5×a, more preferably equal to or less than 0.3×a where a is the maximum width of the receiving parts.
Alternatively, or in combination, the width at the middle of the connecting parts, at mid distance from the receiving parts, is preferably less than 0.7×a, more preferably equal to or less than 0.5×a.
The thickness of the receiving parts and of the connecting parts can also be varied, notably by making the connecting parts thinner by comparison with the receiving parts, but it is more favorable to vary the width than the thickness in order to vary the rigidity.
Of course, a person skilled in the art will know to adapt the dimensions of the collet to suit the circumstances, according to the thickness of the hairspring, the space at his disposal, while taking care to ensure sufficient torque withstand and to keep the stresses well below the maximum permissible stress in order to remain in the elastic deformation domain.
The benefit of at least two levels for an integrated hairspring/collet assembly can be explained as follows. For a hairspring/collet assembly with just one layer, the height is determined by the dimensions of the hairspring, amongst other things by the torque required and the size (diameter). The height of the collet, and therefore of the arms bearing the bearing surfaces and the flexible parts, will necessarily be dictated by the height of the hairspring and there will be no freedom to adjust this. For a single layer assembly 150 microns in height, the retaining torque values are lower, by a factor of 500/150 in relation to a multilayer assembly equipped with a hairspring of the same height (150 microns), because it is held over 150 microns rather than over 500 microns. As a result, these retaining torque values will be below the minimum value (broken line in
It is also possible to conceive of having the bearing parts also borne by the level comprising the hairspring, and this in the example mentioned hereinabove would make it possible to increase the retaining torque values to a factor of 650/150 by comparison with an assembly having just one level. However, the tolerances on the manufacturing method make creating continuous surfaces over two levels a very tricky matter. It is therefore preferable to separate the functions of attaching the hairspring and of connecting the collet to the balance shaft between two distinct levels and not have to provide bearing parts on the level that has the element or elements for attaching the collet to the hairspring.
Thus, one way of increasing the retaining torque of a single layer or single stage collet is to increase the torque developed by the flexible parts without increasing the stress, and this entails a larger collet diameter. The consequence of this is that the point of attachment of the blades of the hairspring needs to be further away from the balance staff, impairing time keeping properties.
It is evident from the foregoing that an integrated hairspring/collet assembly having at least two levels, for example two stages of silicon separated by a layer of silicon oxide, offers the possibility of maximizing the retaining torque while optimizing size, i.e. while avoiding increasing the diameter of the collet. A collet in which the second part 103 extends, along the axis of the bore 107, over a length greater than one times the thickness E of the hairspring, or even greater than 3 times the thickness E of the hairspring, is therefore particularly well suited notably to forming an integrated hairspring-collet assembly.
The two-stage integrated hairspring/collet assembly of
The thermal compensation of the hairspring of the single-blade or double-blade hairspring-collet assembly is afforded by known means. It is possible for example to use a layer of material at the surface of the turns which compensates for the first thermal coefficient of the Young's modulus of the base material. In the case of a hairspring made of Si, a suitable material for the layer is SiO2.
For preference, in the various alternative forms and embodiments, each connecting part is mainly loaded in bending, once the integrated assembly has been mounted on the balance staff.
What is meant by “mainly loaded in bending” is that, in each connecting part, it is possible to identify a neutral axis directed substantially in a direction in which the connecting part extends and separating a zone that is loaded in tension from a zone that is loaded in compression.
For preference, in the various alternative forms and embodiments, each connecting part has a portion distant from the balance staff by at least 0.5 times the radius of the balance staff, or even by at least 0.9 times the radius of the balance staff, once the assembly has been mounted on the balance staff.
For preference, in the various alternative forms and embodiments, the receiving parts and the connecting parts form an element able continuously to surround the balance staff, i.e. able without topological interruption to surround the balance staff. They thus form a closed collet, as opposed to a split collet.
In this document, “nondeformable part” or “rigid part” means a part that suffers no or substantially no deformation during operation or during the mounting of the integrated assembly on the balance staff or a part the deformation of which is not required and/or plays no part during operation or during fitting of the integrated assembly.
In this document, a “deformable part” means a part that deforms elastically during operation or during mounting of the integrated assembly on the balance staff or a part the elastic deformation of which is sought after or performs a function during operation or when mounting the integrated assembly.
According to one aspect of the invention, the integrated hairspring-collet assembly comprises:
These various parts are preferably included within a collet.
| Number | Date | Country | Kind |
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| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2012/069372 | 10/1/2012 | WO | 00 |
| Publishing Document | Publishing Date | Country | Kind |
|---|---|---|---|
| WO2013/045706 | 4/4/2013 | WO | A |
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| 20150023140 A1 | Jan 2015 | US |
