The present invention concerns a micro-mechanical part comprising a shaped aperture for facilitating assembly on a shaft for a moving part, or on a stud for a fixed part, and limiting or removing the risk of the part breaking when it is driven onto the shaft or stud, particularly when said part is made of a brittle material.
In the field of micro-mechanics, which will hereafter be more specifically illustrated by the field of horology, the driving in technique is very widely used, for example for fixing a wheel onto a shaft. When the material forming the part has a plastic region, which is the case of metals and alloys, the tolerances necessary for the shaft and bore can be calculated so as to obtain a tight fit without any risk of breaking the part, or deforming it. When the material does not have any, or very little, plastic region, which is the case of glass, quartz or silicon, there is a high risk of the part being broken during assembly.
These materials are used more and more frequently in horology, particularly because of their lack of sensitivity to magnetic fields, their very low thermal expansion coefficient and their density, which is much lower than that of metals or alloys. Moreover, modern machining techniques can achieve complex shapes with a high level of precision.
If a push fit is made to prevent stresses in the brittle material, there is then a risk of the part becoming detached or a moving element not being driven by the shaft. In order to overcome this drawback, one could employ the bonding technique that has long been used for securing a balance-spring onto a collet, as disclosed for example in FR Patent No. 1 447 142. U.S. Pat. No. 3,906,714 discloses an embodiment wherein the dot of adhesive both secures the balance-spring to a ring forming the collet and said ring to the balance staff.
The use of an adhesive has, however, the drawback of requiring additional machining steps to provide recesses for the adhesive, and additional step during assembly. Further, the phenomenon of aging can lead to a certain play over time.
In EP Patent No. 1 331 528, which relates to an escapement mechanism pallet for a timepiece movement, for preventing the risk of breakage when the male part of a dart is mounted in the shaped aperture formed in a fork, it is proposed fitting resilient tongues to the aperture. Moreover, in certain embodiments such as that shown in
The solutions envisaged in this document are not completely satisfactory since they do not enable the male part to be precisely centred in the aperture.
It is thus an object of the present invention to overcome the drawbacks of the aforecited prior art by providing a micro-mechanical part, in particular a horological part, made of a brittle material that can be assembled by being driven onto a shaft or a stud without any risk of breaking.
The invention therefore concerns a micro-mechanical part made from a plate made of brittle material comprising an aperture into which a shaft or a stud will be driven. A “brittle” material means a material with no plastic deformation region, such as glass, quartz or silicon.
The invention is characterized in that the aperture is a “shaped aperture”, i.e. an aperture not having a perfectly circular contour, said aperture alternatively including rigidifying and positioning zones and resilient deformation zones for gripping or tightening around the shaft. The resilient deformation zones are formed of portions of plate having a recess on either side joining the aperture and the end of which penetrates said aperture. These plate portions have the shape of a tongue that abuts the shaft tangentially when the latter is driven in. The rigidifying and positioning zones and the tongues are arranged alternately around the shaft, each tongue being separated from the adjacent rigidifying zones by a recess, and the rigidifying and positioning zones are distributed in a substantially regular manner around the aperture.
According to other aspects of the invention:
each rigidifying and positioning zone comprises at least one shoulder provided to come into contact with the shaft, the shoulders are distributed in a substantially regular manner around the aperture in order to centre the shaft in the aperture;
each rigidifying and positioning zone forms a shoulder, which is delimited by the two recesses framing said rigidifying and positioning zone;
each tongue describes an overall curve of determined profile, and the two adjacent recesses are formed by two elongated slots of the same general profile as the curve of said tongue;
the brittle material is selected from among glass, quartz and silicon.
The invention also proposes an arrangement for immobilising a micro-mechanical part comprising an aperture by driving the same onto a support block including a positioning stud. The micro-mechanical part is made with any of the preceding features.
The invention further proposes an arrangement for driving a micro-mechanical part that is mobile in continuous or alternate rotation onto a shaft. The micro-mechanical part is made in accordance with any of the preceding claims.
According to variants of this arrangement:
the micro-mechanical part forms part of a timepiece movement selected from among an escape wheel, a star wheel, a toothed wheel, a collet, a lever and a pallet;
the shaft and the aperture also have contours providing an anti-rotational effect;
the shape of the contour of the shaft and the aperture is oblong or triangular;
the contact zones of the shaft and the aperture are rough or provided with flutes;
the micro-mechanical part comprises at least one weld point or one dot of adhesive securing the micro-mechanical part to the shaft.
Other features and advantages of the present invention will appear in the following description of various embodiments, given by way of non-limiting illustration with reference to the annexed drawings, in which:
A first embodiment will be described with reference to
Plate 1 is formed of a brittle material, i.e. a material that has no plastic region within normal use temperatures, such as glass, quartz or silicon. Plate 1 can simply form a construction element, for example a bottom plate, a bridge or a dial of a timepiece. It may also have a functional role, carrying a printed circuit board or a Micro-Electro-Mechanical Systems (MEMS) that has to be secured to block 11. In order to avoid breaking the part during a driving-in assembly, aperture 2 is a shaped aperture shown in a larger scale top view in
In order to show that rigidifying zones 8 do not exert any tightening function on shaft 5, the space 9 between said zones 8 and shaft 5 has been greatly exaggerated. Rigidifying zones 8 are used for centring the escape wheel in relation to shaft 5. As can be seen in
Resilient deformation zones 10 are obtained by making recesses 13, 15 in plate 1, which open into the central aperture and delimit in this example a tongue 12 whose end 14 extends beyond the theoretic contour of shaft 5 and thus performs a tightening or gripping function when shaft 5 is set in place by driving in.
Of course, the number of rigidifying zones 8 and the number of tongues 12 could be greater than the number shown.
It is of course possible to imagine any other non-circular contour able to provide an anti-rotational effect, without departing from the scope of the present invention.
It is also possible, in any of the embodiments that have just been described to provide the ends 14 of resilient deformation zones 10 and shaft 5 with surface roughness, for example flutes, to further reduce the risk of the part rotating on the shaft.
The examples given in the preceding description concern parts that rotate continuously, but it is clear that those skilled in the art could adapt the same principle to parts having an alternate movement, such as a lever, a pivoting part, a collet, a pallet or an escape wheel.
Depending upon the application for which the micro-mechanical part is intended, it is possible to finalise assembly of the part on its shaft with a bonding or welding step, which provides a more rigid attachment, if this is necessary. The adhesive or weld completes fixing by the resilient tongues. In such case, fixing by the tongues constitutes an intermediate fixing step guaranteeing precise centring of the shaft in the aperture, with correction of any play, and the bonding or welding step constitutes a final fixing step.
Number | Date | Country | Kind |
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06004074.8 | Feb 2006 | EP | regional |
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/EP07/51775 | 2/23/2007 | WO | 00 | 11/25/2008 |