LOW INERTITUDE GEAR

Description

[0001] The present invention relates to a gear wheel for a watch mechanism. The present invention relates in particular to a gear wheel with low inertia.

[0002] In many applications, it is advantageous to reduce the inertia of a moving part in a watch movement, particularly a wheel, to limit the energy required to drive it and / or to enable its rapid movement. Low inertia, for example, allows a toothed wheel in a chronograph mechanism to start moving almost instantaneously when timing begins and to rotate rapidly during timing while consuming relatively little energy. Document CH 709 920 A2 shows a flat, openwork toothed wheel.

[0003] One solution for reducing the inertia of a watch movement's moving part is to choose a low-density material, thus minimizing the weight for a given size. A limitation of this solution is the mechanical strength of the chosen material, which must be sufficient to withstand the stresses it will encounter during use.

[0004] Another solution, which can be combined with the previous one, consists of creating openings, for example in the plate of a wheel, in order to reduce the amount of material constituting the moving part for given external dimensions. Some gears, for example, are thus made up of a rim on the outer circumference of which the teeth are formed, and arms connecting the rim to a hub designed to mount the wheel on an axle, the space between the arms, the rim, and the hub being at least partially openwork.

[0005] Although these techniques make it possible to offer gears with relatively low inertia for a given diameter, there is a need to further reduce the inertia of gears for certain applications, for example for gears that are part of gears rotating at very high speed and / or whose energy consumption must be minimal.

[0006] One aim of the invention is therefore to meet this need by providing gears with minimal inertia for a given diameter.

[0007] This purpose and other advantages are achieved by a gear wheel for a watch mechanism comprising a rim with teeth forming an external toothing of the gear wheel, the gear wheel comprising at least one openwork portion extending over a sector of the rim comprising a plurality of teeth and in which the rim is delimited by an inner periphery comprising notches at the base of the teeth.

[0008] The formation of notches in the inner periphery of the twill reduces the amount of material constituting the gear, thereby reducing the gear's inertia for a given diameter and external tooth profile. Since the notches are located at the base of the external teeth, they can be formed without reducing the minimum width of the twill at those points. The perforated portion extends over a sector of the twill comprising a plurality of teeth. In a relatively large perforated portion, the gear of the invention thus does not require any additional reinforcements or mechanical structures to support the teeth, thereby allowing for maximum weight reduction.

[0009] According to the invention, the inner periphery is a curve parallel to the profile of the outer teeth. The inner periphery thus follows the profile of the outer teeth, so that the width of the twill is substantially constant over the entire perforated portion. On the perforated portion, the twill is therefore contained between two parallel or offset curves.

[0010] The profile of the external teeth is, for example, an involute profile, and the teeth are preferably perforated, the contour of the tooth openings being preferably a curve parallel to the external profile of the teeth. The openings formed in the teeth further reduce the material constituting the gear.

[0011] The gear preferably includes a hub for mounting the gear on an axle and at least one connecting area linking the rim to the hub.

[0012] Preferably, the inner periphery including the notches delimits the serge over the entire circumference of the serge except for the portions directly connected to the bonding area(s).

[0013] The gear, for example, comprises three connecting zones in the form of three arms linking the rim to the hub. The arms are, for example, spokes of the gear.

[0014] The hub is, for example, configured to elastically clamp an axle.

[0015] In some forms of execution, the center of the hub is off-center relative to the geometric center of the gear.

[0016] The purpose mentioned above and other advantages are also achieved by a clockwork mechanism including such a toothed wheel.

[0017] The clock mechanism is, for example, a chronograph mechanism and the toothed wheel is a fraction-second counter wheel.

[0018] The invention will be better understood upon reading the following description, illustrated by the figures where: there figure 1 illustrates a gear according to one embodiment of the invention; the figure 2 illustrates a toothed wheel according to another embodiment of the invention.

[0019] According to the execution form illustrated in the figure 1 , the toothed wheel 1 of the invention comprises a serge 4 with, on its outer periphery, teeth 5 forming an external toothing 40 of the toothed wheel 1.

[0020] The gear 1 further comprises, for example, a hub 2 for mounting the gear 1 on a shaft 10. The hub 2 comprises a central opening for receiving the shaft 10. The central opening is, for example, bordered by elastically deformable arms arranged to grip the shaft 10 when it is inserted into the central opening. The hub 2 comprises, for example, three flexible arms 20 arranged to form a triangle inscribed in an annular support 21, and whose vertices are connected to the annular support 21. Other embodiments are, however, conceivable for the hub within the scope of the invention, the embodiment of the hub depending, for example, on the nature of the mounting of the gear on the shaft, the force with which the gear must be held on its shaft, the shape and dimensions of the shaft's cross-section, etc.In some embodiments, the gear does not have a hub and is, for example, mounted to rotate freely on a support, such as a clockwork mechanism plate. In other embodiments, the gear is part of a single unit comprising, for example, a shaft and / or a pinion.

[0021] According to the illustrated embodiment, with the exception of three connecting zones 3 linking the rim 4 to the hub 2, the gear 1 is completely open between the rim 4 and the hub 2, more precisely between an inner periphery 41 of the rim 4 and the annular support 21. The three connecting zones 3 are, for example, three arms linking the rim 4 to the annular support 21 of the hub 2. The three arms constitute, for example, three spokes of the gear 1. The three spokes are preferably evenly distributed around the circumference of the gear 1, with two adjacent spokes forming an angle of 120° with each other. The three connecting zones 3 are preferably connected to the annular support 21 of the hub 2, at a distance from the vertices of the triangle formed by the flexible arms 20, thus preventing the deformation of the flexible arms 20 during the insertion of the axle 10 from significantly impacting the position of the connecting zones 3 and therefore the geometry of the serge 4.Other forms of connecting areas, for example arms, are however conceivable within the framework of the invention, for example arms following an essentially helical line with respect to the axis 10 or a large single connecting area as described below in relation to the embodiment of the gear illustrated in the . figure 2 The number, shape, and dimensions of the contact areas are determined, for example, according to the stresses to which the gear will be subjected during its intended use. The number of contact areas is preferably minimized, however, in order to limit the amount of material constituting the gear of the invention while ensuring sufficient mechanical strength.

[0022] According to the invention, in the openwork portions of the gear 1, between the connecting zones 3, the band 4 comprises notches 6 at the base of the teeth 5 on its inner periphery 41. According to the invention, each openwork portion of the gear 1 extends over a sector of the band 4 comprising a plurality of teeth 5. Preferably, the profile of the notches 6 is similar to the profile of the teeth 5. According to the invention, the inner periphery 41 is a parallel curve, or an offset curve, of the profile of the outer teeth 40. In the openwork portions, the band 4 is therefore delimited by two parallel curves, the profile of the outer teeth 40 and the inner periphery 41, and thus forms a band of substantially constant width that follows the profile of the outer teeth 40.The offset between the outer teeth 40 and the inner periphery 41 is determined in order to give the serge 4 sufficient rigidity and / or mechanical strength for the application for which the gear 1 is intended.

[0023] Other embodiments are possible within the scope of the invention. In particular, the inner periphery with the notches may extend only over a part of the openwork portion(s) depending, for example, on the desired inertia of the gear and / or the balancing of the gear and the coincidence of the center of gravity with the center of the axis of rotation of the gear.

[0024] According to another form of execution illustrated in the figure 2 The gear 1 of the invention is a gear for a non-circular gear comprising a hub 2 offset with respect to the geometric center of the groove 4. Similar to the embodiment described above in relation to the figure 1 , hub 2 includes three flexible arms arranged in a triangle to grip an axle which would be inserted into the central opening of hub 2.

[0025] According to this embodiment, the rim 4 is connected to the hub by a single connecting zone 3, preferably located on the side of the hub 2 closest to the rim 4. The connecting zone 3 links, for example, two vertices of the triangle formed by the flexible arms of the hub 2 to the rim 4. The connecting zone 3 covers, for example, an area bounded by the rim 4 and a straight line passing through the center of the hub 2, the straight line passing through the center of the hub 2 preferably being perpendicular to the shortest radius between the hub 2 and the rim 4. The mass of the material forming the connecting zone 3 preferably compensates for the imbalance generated by the off-centering of the hub 2 with respect to the geometric center of the rim 4 by making the center of gravity of the gear 1 coincide with the center of the central opening of the hub 2.Preferably, the rest of the gear 1 is openwork, so that the space is empty between the hub 2 and the serge 4 outside the sector occupied by the connecting area 3. The gear 1 illustrated in the . figure 2 thus includes a single openwork portion which extends over a segment covering more than half of the circumference of the serge 4.

[0026] The rim 4 comprises teeth 5 on its outer periphery that form an outer tooth profile 40 of the gear 1. The profile of the outer tooth profile 40 is, for example, an involute profile. According to the invention, the inner periphery 41 of the rim 4, which delimits the rim 4 in the openwork portion of the gear 1, comprises notches 6 at the base of the teeth 5. The inner periphery 41 with the notches 6 is preferably a curve parallel to the profile of the outer tooth profile 40. Preferably, the teeth 5 at the base of which there are notches 6 are openwork, the outline of the openwork preferably corresponding to the extension of the offset curve forming the inner periphery 41 of the rim 4, as schematically illustrated by the dashed lines in the figure 2 .

[0027] The gear of the invention is a flat gear formed, for example, from a plate or wafer, or by the deposition of successive layers. The gear 1 of the invention is preferably made of silicon and / or silicon oxide, and produced by deep reactive ion etching (DRIE). Other materials and manufacturing processes are, however, conceivable for producing the gear 1 of the invention, such as, for example, machined steel, a nickel alloy formed by LIGA, or any other suitable material and / or manufacturing process.

[0028] According to the invention, the toothed wheel 1 is intended to be integrated into a watch movement, for example a chronograph movement, or any other watch movement requiring a toothed wheel with low inertia that can, for example, rotate at high speed while consuming a minimum of energy.

Claims

1. Flat toothed wheel (1) for a timepiece mechanism comprising a rim (4) with teeth (5) forming an outer toothing (40) of the toothed wheel (1), the toothed wheel (1) comprising at least one pierced portion extending over a sector of the rim (4) comprising a plurality of teeth (5) and wherein the rim (4) is delimited by an inner periphery (41) comprising notches (6) at the base of the teeth (5), characterised in that the inner periphery (41) is a curve offset from the outer toothing (40).

2. Toothed wheel (1) as claimed in any one of the preceding claims, wherein the profile of the outer toothing (40) is in the shape of an involute of a circle.

3. Toothed wheel (1) as claimed in the preceding claim, wherein the teeth (5) are pierced.

4. Toothed wheel (1) as claimed in any one of the preceding claims, further comprising a hub (2) for mounting the toothed wheel (1) on an axle and at least one connecting zone (3) connecting the rim (4) to the hub (2).

5. Toothed wheel (1) as claimed in the preceding claim, wherein the inner periphery (41) delimits the rim (4) over the entire circumference of the rim (4) with the exception of the portions directly connected to the at least one connecting zone (3).

6. Toothed wheel (1) as claimed in the preceding claim, comprising three connecting zones (3) in the form of an arm.

7. Toothed wheel (1) as claimed in any one of claims 4 to 6, wherein the hub (2) is configured to fit tightly around an axle in a resilient manner.

8. Toothed wheel (1) as claimed in any one of claims 4 to 7, wherein the centre of the hub is off-centre with respect to the geometric centre of the toothed wheel.

9. Timepiece mechanism comprising a toothed wheel (1) as claimed in any one of the preceding claims.

10. Timepiece mechanism as claimed in the preceding claim, wherein the timepiece mechanism is a chronograph mechanism and the toothed wheel (1) is a wheel for counting fractions of a second.