Patent Application
20170336761
This application claims priority from European Patent Application No. 16170213.9 filed on May 18, 2016, the entire disclosure of which is hereby incorporated herein by reference.
The present invention concerns anti-shock devices for timepieces. Such anti-shock devices are generally associated with bearings which guide in rotation pivoting elements of the timepiece movement, in particular balances. They are also called shock absorbing devices, shock dampers or shock absorbers. The invention more particularly concerns the damping of axial shocks to which pivoting elements are subjected and the mechanical stresses brought to bear on the pivots during such axial shocks.
A usual timepiece anti-shock device includes a resilient member which bears on or which exerts a pressure against at least one endstone of the bearing comprising the anti-shock device, the endstone forming a stop for the pivot inserted into the bearing in the direction of the axis of rotation of the pivoting element concerned. This anti-shock device is arranged to be able to generate, through the endstone, a restoring force on the pivot in question, when the pivot presses against the endstone in the event of a shock. It is understood that “endstone” means any structure, made of any suitable material, which defines an axial bearing surface for the pivot.
Such anti-shock devices generally include mechanical springs which are dimensioned empirically, according to practical rules, such as that of the best compromise between mechanical stability in operation and elastic resistance to mechanical deformations. Indeed, it is desirable to have a relatively stiff shock absorber which does not cause axial movements of the pivoting element with each small shock, while ensuring the shock absorber function for violent shocks causing high axial (positive or negative) accelerations for the pivoting element which could damage its pivots.
In particular, conventional anti-shock devices for the sprung balance, “parachutes” and lyre-springs, are dimensioned such that they are only actuated upon relatively high shock accelerations (between 200 g and 500 g, g being the earth's acceleration), as a result of the prestress of the spring forming these “parachutes” and lyre-springs, which defines a threshold value. Beyond this threshold value, it is provided that the spring deforms and absorbs part of the shock energy. However, because of the low mechanical shock absorption of the metallic strips used, most of the energy is restored to the balance. Local deformation of the balance pivot is thus highly probable, even for relatively low shocks. Such a deformation, which has a considerable impact on the chronometric precision of the watch, is generally ignored because the certified chronometer standard for the chronometric stability of a watch following a one metre shock is not strict (difference of 60 seconds/day).
It is an object of the present invention to provide a timepiece movement equipped with at least one effective anti-shock device which provides a solution to the problem of damage to the pivots if shocks occur, even in the event of strong shocks.
To this end, the present invention concerns a timepiece movement as defined in claim 1.
As a result of the features of the invention, which will be described in detail hereinafter, the anti-shock device presents less resistance for relatively strong shocks while ensuring good stability for smaller shocks. Indeed, the stiffness of the anti-shock device according to the invention means it no longer behaves like a mechanical spring that produces a restoring force substantially proportional to the axial displacement of the endstone. On the contrary, it exerts a relatively high force when the displacement is zero, which then diminishes over at least the initial part of the shock damping travel that can be made by the endstone.
In a main embodiment, the first and second magnets and the highly magnetically permeable element are aligned in a direction substantially parallel to the axis of rotation of the pivoting element, the first and second magnets having opposite polarities in this direction.
In a preferred variant, the highly magnetically permeable element is fixed to the first magnet.
The invention will be described below with reference to the annexed drawings, given by way of non-limiting example, and in which:
With reference to
Generally, anti-shock device 30 includes a resilient member 32 that exerts a force on an endstone 36, which forms a stop for pivot 26 in the direction of the axis of rotation of the pivoting element. This anti-shock device is arranged to be able to generate, through the endstone, a restoring force on pivot 26 when the pivot presses against the endstone in the event of a shock. According to the invention, the anti-shock device further includes a magnetic system 40 comprising two magnets 42, 44, and a highly magnetically permeable element 46 arranged between these two magnets and secured to one of them. These two magnets are respectively secured to a support 48 of the anti-shock device and to resilient member 32, in order to present between them a relative movement over a certain relative distance D (referenced in
In a remarkable and very advantageous manner, as will be explained below with reference to
Particular variants of the first embodiment, all represented in
The highly magnetically permeable element 46 is fixed to magnet 42 integral with support 48;
The highly magnetically permeable element consists of a plate having a central axis that is substantially coincident with the axis of magnetization of magnet 42;
When the resilient member is in its rest position, the two magnets 42, 44 and the highly magnetically permeable magnet 46 are aligned in a direction substantially parallel to the axis of rotation 50 of pivoting element 24;
Magnets 42 and 44 have opposite polarities along their direction of alignment.
In particular, according to the variant shown in
Referring to
In particular, element 6 is bonded to the first magnet, as shown in
Highly magnetically permeable element 6 has a central axis 10 which is substantially coincident with the axis of magnetisation of first magnet 4 and also with the axis of magnetisation of second magnet 8. The respective directions of magnetisation of magnets 4 and 8 are opposite. These first and second magnets thus have opposite polarities and they are capable of being subjected between them to a relative motion over a certain relative distance D. In the example represented in
The two magnets 4 and 8 are arranged in magnetic repulsion such that, in the absence of highly magnetically permeable element 6, a force of repulsion tends to moves these two magnets away from each other.
However, surprisingly, the arrangement between these two magnets of element 6 reverses the direction of the magnetic force between the first and second parts of the magnetic system when they are at a short distance from each other, such that an overall force of magnetic attraction is then produced between these two parts.
The overall magnetic force is a continuous function of the distance between the components and it has a zero value at distance Dinv. Thus, when the distance between magnet 8 and element 6 is greater than a distance Dinv, this magnet is subjected to an overall force of magnetic repulsion which tends to move element 6 away. However, when the distance between element 6 and movable magnet 8 is less than distance Dinv, magnet 8 is subjected to an overall force of magnetic attraction which tends to move it closer to element 6 and, if there is no resistance, to place it in contact with element 6 and then hold it in this position. This is a characteristic function of magnetic system 52 which is put to good use in the anti-shock device according to the invention. The reversal distance Dinv, is determined by the geometry of the three magnetic components forming the magnetic system and by their magnetic properties.
Below will be described in more detail the anti-shock device 30 according to the first embodiment and its behaviour resulting from the incorporation, according to the invention, of magnetic system 40. Resilient member 32 is formed by a flat spring having a first end 56 and a second end 58, the first end being fixed to support 48 by means of a screw 60 and the second end carrying second magnet 44. According to an advantageous variant, endstone 36 is located, in projection into a general plane of the flat spring, between the first and second ends. Bearing 28 comprises a base 62, fixedly arranged inside an opening in support 48. In a conventional manner, this base has at its centre a hole into which pivot 26 passes. Pivoting element 24, which is the balance staff here (not represented), has a bearing surface 70 which limits the displacement of the element along axis 50 in a conventional manner, this bearing surface moving into abutment against a surface defined by the base at the periphery of the hole. Bearing 28 also includes a setting 64 into which endstone 36 is inserted. In the variant represented, this is a magnetic bearing. Thus, the setting also carries a magnet 66 and a closing jewel 68. This setting is also part of the anti-shock device. It is arranged inside a housing formed by base 62 and a closing plate 72 fixed to support 48, in order to be subjected to an axial movement at least on a distance corresponding to the maximum displacement to which pivot 26 can be subjected in the event of a shock when bearing surface 70 moves into abutment against the base. A short pipe 74 is fixed to flat spring 32 on the side of its end 58 in order to rest against the setting or the closing jewel. The anti-shock device acts on the assembly integral with the endstone via this pipe. It will be noted that the invention is not limited to a magnetic bearing. Thus, in another variant, there is a conventional bearing with a setting incorporating a jewel hole and an endstone, with the latter able to have a flat surface facing the pivot.
The magnetic system and the resilient element are arranged such that, in a rest position of the anti-shock device, the endstone, or a setting to which it is fixed, is held resting against the bearing support or against a base of the bearing while the force exerted by the pivot concerned against the endstone is less than a limit value, the latter preferably being higher than the gravitational force acting on the pivoting element, notably the sprung balance. In a particular variant, the resilient element is prestressed in the rest position of the anti-shock device, so that the endstone remains immobile over a larger range of values of the force exerted by the mobile element subjected to an axial acceleration in the event of a shock.
The anti-shock device according to the invention exhibits remarkable behaviour, as represented by curve 78. The force exerted on the pivot resting against the endstone, at least for a displacement distance of the endstone less than DPinv, is maximum for the distance at rest DPR of the anti-shock device. As soon as the force applied by the pivot to the endstone rises above the maximum value for the rest position of the anti-shock device, the endstone moves away from its rest position and then the total force which is exerted against pivot 26 decreases relatively rapidly, which immediately ensures a relatively large movement of the endstone and good shock damping to the stop position. In the example given in
The dependency of the total force on the pivot according to the axial displacement of the balance and to the corresponding displacement of the anti-shock device allows for the following operation (for a variant with a balance having a weight of around 40 mg and an element made of ferromagnetic material between the two magnets of the magnetic system):
1) For an acceleration shock of less than 400 g, the anti-shock device remains immobile as a result of the force of magnetic attraction and the prestress of the spring which are summed.
2) For a shock that exceeds 400 g, in particular of 1,000 g, the movable magnet carried by the spring detaches from the ferromagnetic element and the magnetic force rapidly decreases and is then reversed, in that case resisting the elastic force applied by the spring. Once the axial movement activation threshold force of the anti-shock device has been exceeded, the total resulting force decreases at least over most of the possible displacement for the pivot, since the deformation of the anti-shock device immediately becomes very significant and allows the balance to very quickly reach a mechanical stop. This allows the kinetic energy of the balance to be absorbed while limiting the force applied to the pivot over the entire shock damping travel.
Once the shock has ended, the anti-shock device can return to its initial position, since the total force remains positive (restoring force) and exceeds the friction forces. The magnetic force reversal, which occurs when the movable magnet moves sufficiently close to the ferromagnetic element, simultaneously ensures the complete absence of mechanical hysteresis and the recentring of the bearing after a shock.
The following advantages result from the features of the anti-shock device according to the invention:
The anti-shock device operates like a true shock absorber (unlike conventional anti-shock devices);
Possibility of dimensioning the anti-shock device by optimising the prestress (and thereby operation for small shocks where bearing stability is desired) and the damping response for large shocks;
After a large shock, repositioning of the anti-shock device in its given rest position and recentring of the setting (defining the axis of rotation of the balance) ensured by the force of magnetic attraction;
The force to which the balance pivot is subjected in the event of a large shock is reduced, as the maximum force is preferably the total force of the anti-shock device occurring in its rest position.
Referring to
The two magnetic systems are respectively associated with two structures 92 and 94, which are respectively fixed to the two branches 89 and 90 substantially in their median area. These two structures respectively carry two magnets 44A and 44B each forming the movable magnet of the respective magnetic system. Thus, the two branches are respectively associated with the first and second magnetic systems and each carry, via structures 92 and 94, a movable magnet 44A, respectively 44B, which cooperates with a fixed magnet 42A, respectively 42B. Each magnetic system further includes a highly magnetically permeable element 46A, respectively 46B, which is integral with the fixed magnet of the respective magnetic system.
It will be noted that each of branches 89, 90 of the lyre-spring, is axially retained, in a conventional manner, at its two ends by angularly projecting portions of an upper ring of base 62A of the bearing. Thus, it is in the median area of these branches that the lyre-spring, when stressed, is subjected to a maximum elastic deformation. It will also be noted that each branch presses substantially in the middle thereof onto the endstone. Preferably, but in a non-limiting manner, the two structures 92 and 94 are integral with the lyre-spring and have a greater stiffness than that of the respective branches, in particular through a greater thickness, as represented in the Figures. However, in another variant, the structures have the same thickness as the lyre-spring branches, to facilitate fabrication, but have larger cross-sections. However, in another variant, the stiffness of the movable magnet carrier structures is not greater than that of the branches, as the movable magnets make longer travels, in the event of large shocks, than the endstone.
The arrangement of the two magnetic systems associated in a symmetrical manner respectively with the two resilient lyre-spring branches is advantageous, since this arrangement results in the same pressure from each branch on the endstone, or more generally, on the movable bearing assembly 96, for the same elastic deformation of the two branches. Uniform behaviour of the anti-shock device, and in particular endstone 36A, is thus maintained in a general plane perpendicular to the axis of rotation of the balance in the event of axial shocks.
Finally, the stiffness of the lyre-spring and the dimensioning of the two magnetic systems are provided so that the total resulting force applied by the anti-shock device remains a restoring force higher than the friction forces, to ensure, after a shock producing a force higher than the maximum force occurring in the static situation on movable bearing assembly 96, the return of the anti-shock device to its initial position and proper recentring of this movable assembly (a crucial property for ensuring good chronometry of the timepiece movement).
It will be noted that, advantageously in the second embodiment, the two bearings of a sprung balance are equipped with a shock absorber device of the type described above.
| Number | Date | Country | Kind |
|---|---|---|---|
| 16170213.9 | May 2016 | EP | regional |
