Patent Application
20100320661
This application claims priorities from European Patent Application No. 09163283.6, filed Jun. 19, 2009 and Swiss Patent Application No. 01343/09 filed Aug. 31, 2009, the entire disclosures of which are incorporated herein by reference.
The invention relates to a spring for a sprung balance and more particularly a spring of this type whose thermo-elastic coefficient is approximately zero and virtually insensitive to magnetic fields. The thermo-elastic coefficient (CTE) of a body represents the relative Young's modulus variation of said body as a function of temperature.
Seeking the lowest possible variation of rate for a mechanical timepiece movement is known. However, it is very difficult to achieve this particularly because of the sensitivity of the sprung balance assembly to variations in temperature and magnetic fields.
EP Patent No. 1 039 352 discloses an alloy balance spring made of a particular alloy, whose external surface has an oxide coating. The document discloses a balance spring, which is thermocompensated, i.e. its thermoelastic coefficient, also called the Young's modulus thermal coefficient, remains approximately close to zero, and whose sensitivity to magnetic fields is very low. However, the spring is very difficult to implement, which leads to a very high reject rate and cost price.
It is an object of the present invention to overcome all or part of the aforecited drawbacks by proposing a thermocompensated spring for a sprung balance whose implementation is simplified.
The invention therefore relates to a thermocompensated spring for a sprung balance comprising a section that includes a first metallic material, characterized in that at least one of the surfaces of the section has an external layer including a second metallic material, whose thermoelastic coefficient varies in the opposite direction to that of the first material.
Advantageously according to the invention, the spring is very simple and can thus use conventional materials, which avoids complex manufacturing steps.
According to other advantageous features of the invention:
The invention also concerns a timepiece including at least one spring that conforms with one of the preceding variants.
Finally, the invention relates to a method of manufacturing a spring for a timepiece that includes the following steps:
It is thus clear that the spring can be obtained with materials very simply, using well mastered mechanical steps that allow a very low reject rate.
According to other advantageous features of the invention:
An alternative consists in proposing a method of manufacturing a spring for a sprung balance that includes the following steps:
It is thus clear that the shape of the spring can be obtained with a very high level of precision using a first material, then, using well mastered steps of overmoulding and/or plating a second material, a very high quality thermocompensated spring can be obtained with a very low reject rate.
According to other advantageous features of the alternative:
Other features and advantages will appear clearly from the following description, given by way of non-limiting indication, with reference to the single FIGURE, which shows a flow chart of a method of manufacturing a spring according to the invention.
The invention relates to a thermocompensated spring for a timepiece regulating member of the sprung balance type. The spring according to the invention uses materials that can be worked using conventional metal shaping methods. Moreover, the materials used are ordinary and thus inexpensive.
According to the invention, the thermoelastic coefficient of the spring is made approximately zero, i.e. it has an approximately zero relative Young's modulus variation as a function of temperature. To achieve this end, two overlapping metallic materials are used, whose respective thermoelastic coefficients vary in opposite directions so that they compensate for each other. It is thus clear that if one of the materials has a positive thermoelastic coefficient, the second will have a negative thermoelastic coefficient.
It is of no import whether the material with a positive thermoelastic coefficient covers the other, or vice versa. It is only the thickness of the overlap or coating that has to be adapted in accordance with the thickness covered and, also in accordance with the type of balance so as to compensate overall for the sprung balance regulating member.
The overlap or coating of the materials can be partial or total. Thus, at least two parallel or adjacent surfaces of the section of the body of the first material can be provided with the second material. Likewise, the overlap or coating can be over all or part of the length of the body of the first material. It is also clear that each surface can also include a material that is specific thereto, i.e. it can comprise not merely one second material. Finally, at least one of the materials may be paramagnetic so as to make the spring virtually insensitive to magnetic fields.
Preferably, the spring uses the following pair of materials: stainless steel—FeMn alloy or stainless steel-invar type FeNi36 alloy. Which material covers which is not necessarily important. However, if one of the two materials is ferromagnetic, the core preferably uses said ferromagnetic material. Thus, for the above examples, the core preferably uses the FeMn alloy (antiferromagnetic) or FeNi36 (ferromagnetic) and the outer layer is steel (antiferromagnetic). It will also be noted that this configuration also limits any oxidation of said spring.
In order to calculate the respective thicknesses of the core (material 1) relative to the external layer (material 2), we calculate the desired corrected thermoelastic coefficient CTE′ and the desired corrected torque C′ for adaptation thereof to a balance in accordance with the following relations, where the expansion coefficients are deemed to be the same:
where:
Of course, the invention is not limited to the stainless steel—FeMn alloy or stainless steel—FeNi36 alloy pair. Thus other pairs could be envisaged. By way of example, the external layer can comprise stainless steel and/or chromium and/or nickel and/or iron. Likewise, by way of example, the spring core can comprise niobium, NbZr alloy, CrMn alloy, FeMn alloy, FeNi36 alloy or AuPd alloy.
The method 1 of manufacturing the above spring will now be explained with reference to
Formation step 11 generally includes phases 3, 5, 7 and 13, which are used for several embodiments. According to a first embodiment, the body is formed from a bar and a tube. In first phase 3 of method 1, the bar of a first material is formed. In a second phase 5 of method 1, which can be performed in parallel, prior to or after phase 3, the tube of a second material is formed. Then, in a third phase 7, the bar is fitted inside the tube. Preferably, the difference in section between the exterior of the bar and the hollow of the tube is as small as possible, so as to limit any relative movements.
Thus, in order to facilitate performance of phase 7, an optional phase 9 is preferably provided before phase 7. Phase 9 consists in performing a heat treatment of the bar and/or tube so as to guarantee the largest possible space, i.e. difference in section between the bar and tube. It is thus clear that heating to expand the tube and/or cooling to contract the bar can be envisaged. After phase 7, step 11 continues with the fourth phase 13 for securing the bar inside the tube. It is thus clear that this first embodiment involves covering the entire section of the core of the first material.
One could also envisage only covering some surfaces. Thus, in another embodiment, phases 3 and 5 are still performed to form first and second materials. In third and fourth phases 7, 13, the two materials are respectively brought together and then secured to each other. In a non-limiting manner, in this other embodiment, phase 13 can include cold and/or hot deformation but also bonding and/or welding.
Of course, other formation steps 11 are also possible, including more or fewer phases 3, 5, 7, 9 and 13. Thus, according to an alternative, phase 3 is used to form a body in a first material, and then in a second phase 5, a second material is overmoulded or plated. It is thus clear that phases 5, 7 and 13 can be performed at the same time, for example, by casting, galvanoplasty or physical or chemical vapour phase deposition.
The second step 15 is for decreasing the section of said spring to the desired section. By way of example, the largest section of the body can thus pass from 5 to 100 mm at the end of step 11 to a final dimension of between 10 μm and 1 mm.
It is also perfectly possible to envisage performing steps 11, 15 and 21 in a different order. Indeed, it is also possible to form only the first material in a first phase in accordance with steps 11, 15 and 21, and then to form the second material, as in the above alternative, in a phase 5 in which the first material is overmoulded and/or plated with a second material.
By way of example, the shape of the spring could be given to the first material, which can thus be obtained via a wire drawing-laminating-winding—heat setting treatment or by deep reactive ion etching a plate of the first material or by a LIGA type electroforming process (galvanic growth in a photosensitive resin mould). The first material can then be coated, as in the above alternative, by casting, galvanoplasty or by physical or chemical vapour deposition.
Preferably, for the first embodiment of the invention, phase 13 and step 15 are performed in a single plastic deformation process. Thus the deformation caused advantageously enables the bar and tube to be secured to each other, but also enables the section to be reduced to that of the future spring. Preferably, the deformation process is cold and/or hot, possibly with intermediate annealing phases to allow the material to be deformed to very small dimensions.
Thus, for all the embodiments, step 15 can include cold deformations, which may include wire drawing and/or drawing and/or forging and or lamination and/or embossing. Likewise, by way of example, hot deformations could also be envisaged and may include drawing and/or forging and/or lamination and/or embossing.
Preferably, after step 15, method 1 can also include an additional step 17 for giving the spring section its final shape. Thus, in step 17, the section of the body is changed into a polygonal section, such as, for example, a rectangular section. This step 17 is preferably performed by lamination so as to obtain very advantageous dimensional tolerances.
Method 1 continues with step 21, in which the body is wound to form the spring, for example, approximately in the shape of a spiral. According to the invention, step 21 can end method 1. However, other variants are possible.
Thus, a third optional step 23 can be provided after step 21 so as to raise the outer coil of the spring formed in step 21. This step 23 can thus form a Breguet type overcoil. According to the invention, optional step 23 can also end method 1.
However, after step 21 or 23, method 1 can also continue with step 31 and/or 33. Thus, as illustrated in
Nonetheless, step 33 can be replaced or preceded by step 31, as illustrated by a single line in
Of course, the present invention is not limited to the example illustrated, but is capable of various variants and alterations that will be clear to those skilled in the art. Thus, other hot and/or cold deformation steps could be envisaged.
| Number | Date | Country | Kind |
|---|---|---|---|
| 09163283.6 | Jun 2009 | EP | regional |
| 01343/09 | Aug 2009 | CH | national |
