Timepiece escapement with optimized draw

Abstract

A timepiece escapement comprising: an escapement wheel pivotably mounted about a first axis of rotation and intended to be driven by a power source; a lever pivotably mounted about a second axis of rotation, said lever comprising an entry pallet and an exit pallet, each pallet comprising a rest face arranged to alternately and sequentially lock the escapement wheel, the lever being suitable for transmitting pulses received from the escapement wheel to a regulating member arranged to oscillate, and for releasing the escapement wheel periodically under the control of said regulating member, wherein the rest face of the entry pallet is arranged such that, when it locks a tooth of the escapement wheel, a draw force is generated by the interaction between the rest face and the tooth to apply a torque that tends to retain the lever in the rest position. According to the invention, the rest face is shaped in such a way that the draw angle (Υ) at the contact point (C) of said face with a tooth of the escapement wheel is constant or decreases along the release stroke of the entry pallet.

Claims

1. An escapement for a timepiece comprising: an escapement wheel mounted in a pivotable manner about a first axis of rotation and intended to be driven by a power source; an anchor mounted in a pivotable manner about a second axis of rotation, said anchor comprising an entry pallet and an exit pallet, each pallet comprising a rest surface arranged to block said escapement wheel alternately and sequentially, the anchor being adapted to transmit impulsions received from the escapement wheel to a regulating member arranged to produce oscillations, and to release said escapement wheel periodically under the control of said regulating member, in which the rest surface of the entry pallet is arranged in such a way that, when it blocks a tooth of the escapement wheel, a draw force is generated by the interaction between said rest surface and said tooth in such a way as to apply a torque which strives to retain the anchor in its rest position, wherein said rest surface is configured in such a way that the draw angle at the point of contact of said surface with a tooth of the escapement wheel is constant or decreasing during unlocking of the entry pallet, wherein said rest surface is convex, and wherein said rest surface is configured according to the equation:
90−γ−α.sub.orientation=tan.sup.−1(R*cos(α)/Axe−R*cos(α)) in which: γ is the draw angle; α.sub.orientation is the angle between a tangent to the rest surface of the entry pallet at its point of contact with the escapement wheel and the center-to-center line between the anchor and the escapement wheel; α is the angle between a line joining said point of contact and the axis of rotation of the escapement wheel and said center-to-center line; R is the length of said line joining said point of contact and the axis of rotation of the escapement wheel; and Axe is the length of said center-to-center line.

2. The escapement as claimed in claim 1, in which γ is substantially constant.

3. The escapement as claimed in claim 1, in which γ reduces along at least a part of the unlocking path.

4. The escapement as claimed in claim 2, in which the value of γ lies in a range between 5° and 20°.

5. The escapement as claimed in claim 1, in which at least one of said pallets is integral with at least a part of the anchor.

6. A timepiece movement comprising an escapement as claimed in claim 1.

7. A timepiece comprising a movement as claimed in claim 6.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention will be more readily appreciated by reading of the following description of an embodiment, given by way of example and made with reference to the drawings, in which:

(2) FIG. 1 depicts a schematic plan view of an escapement according to the invention;

(3) FIG. 2 depicts a schematic plan view of the entry pallet of an escapement according to the invention, on an enlarged scale;

(4) FIG. 3 depicts the form of the rest surface of the entry pallet in FIG. 2 in comparison with straight lines;

(5) FIG. 4 depicts a useful generic geometric model for calculating the form of the rest surface of the entry pallet in FIG. 2;

(6) FIG. 5 depicts a simplified view of the geometry illustrated in FIG. 4;

(7) FIG. 6 depicts a view of one part of an escapement according to the prior art represented by document CH702689; and

(8) FIG. 7 depicts a graph illustrating the evolution of the draw angle in the unlocking path according to the prior art and according to the invention.

MODE(S) FOR THE INVENTION

(9) FIG. 1 depicts an escapement 1 according to the invention. This escapement 1 embodies the overall form of a Swiss anchor escapement, in which each pallet participates in providing an impulsion to the regulating member.

(10) As generally known, the escapement includes an escapement wheel 3, arranged to be driven by a power source, not depicted here. This power source may be for example a mainspring or an electric motor kinematically linked with the escapement wheel 3 by means of a going train (likewise not depicted).

(11) The escapement wheel 3 is mounted in a pivotable manner on an arbor (not depicted), of which the theoretical axis is indicated by the reference sign 5, and which corresponds to a first axis of rotation. In the variant depicted here, the teeth of the escapement wheel 7 each include a rest surface 7a, which interacts with the pallets when the escapement wheel 3 is blocked, and an impulsion surface. However, the invention is applicable to other forms of escapement wheel, for example with pointed teeth (English anchor escapement), or less conventional forms.

(12) The teeth 7 of the escapement wheel 3 interact in a manner known per se with an anchor 9, which pivots about a theoretical axis of rotation 11. In the variant depicted, this theoretical axis 11 coincides with an arbor (not depicted), although an anchor of the “suspended” type as described in document CH708113, or of any other appropriate type, is also possible. This axis 11 corresponds to a second axis of rotation.

(13) The overall form of the depicted anchor 9 is traditional. In this respect, it includes a rod 9a extending from the axis of rotation 11 and terminating in a fork 9c, which interacts with a regulating member (not depicted) in a known manner which need not be described here in detail. Furthermore, a pair of arms 9b extend to either side of the axis of rotation 11 in directions substantially perpendicular to the rod 9a, and are terminated by pallets 13, 15. It goes without saying that other less common shapes of anchor may also be utilized within the framework of the invention.

(14) Each of these pallets 13, 15 is arranged to block and to release the escapement wheel periodically, the latter being blocked by one of the pallets 13, 15, and then re-blocked by the other, in sequence.

(15) The pallet 13 depicted on the right in FIG. 1 is the entry pallet, situated upstream in relation to the direction of rotation of the escapement wheel 3 indicated by the arrow, and the pallet 15, situated downstream, is the exit pallet.

(16) In the variant depicted here, the pallets 13, 15 are integral with the anchor 9, although the invention is also applicable to pallets attached to the arms 9b in a conventional manner. Each pallet 13, 15 includes, as generally known, a rest surface 13a respectively 15a, and an impulsion surface 13b, 15b respectively. The rest surfaces 13a, 15a, serve to block the escapement wheel 3 during rest phases, and the impulsion surfaces 13b, 15b cooperate with the teeth 7 in order to transmit an impulsion to the anchor and thus to the regulating member. Even if already implicit in the foregoing, the rest surface 13a extends as far as a point where the contact between the pallet 13 and the tooth 7 no longer ensures blocking of the escapement wheel 3, at which point the contact between these elements begins to bring about the transmission of force between the tooth 7 of the escapement wheel 3 and the pallet 13. FIG. 6 clearly depicts this point of transition between these two surfaces.

(17) In a typical escapement of the kind just described, the rest surfaces 13a, 15a are typically planar, of which the angle is selected in such a way that, during the rest phases, the force F resulting from contact between the rest surface 13a, 15a comprises a component which tends to keep the pallet 13 or 15, as appropriate, engaged with the escapement wheel 3. This force F exerts a torque about the axis of rotation 11 of the anchor 9, which strives to keep it engaged with the toothing of the escapement wheel 3, that is to say which strives to cause the anchor to pivot in the anticlockwise direction (according to the orientation of FIG. 1) when the entry pallet 13 is engaged, and in the clockwise direction when the exit pallet 15 is engaged. It should be noted that friction alone, for example between a rest surface in the arc of a circle centered on the axis of rotation of the anchor and the escapement wheel, does not exert such a torque, since no force capable of generating a torque about the axis of rotation of the anchor 9 is exerted between the elements concerned when they are at rest and in the absence of a dynamic force seeking to displace the anchor. In other words, the draw force is applied statically.

(18) In a traditional escapement, however, the angle presented by the rest surface 13a of the entry pallet 13 in relation to a tooth 7 of the escapement wheel increases during the unlocking phase, which represents the part of the movement of the anchor between its initial position at rest and the moment at which the tooth 7 performs the transition from the rest surface 13a to the impulsion surface 13b of the pallet. This results from the fact that, when the anchor 9 pivots about its axis 11, this angle is modified according to the geometry of the anchor and of the entry pallet 13. In essence, the incline of the rest surface 13a becomes steeper with respect to the tooth 7. Consequently, the force and the torque that are necessary to overcome the draw increase during the unlocking phase of the entry pallet 13. This is detrimental to the efficiency and to the performance of the escapement and disturbs the oscillations of the regulating member, which impairs the isochronism.

(19) The same disadvantage does not occur at the exit pallet 15, since the angle that its rest surface 15a subtends with respect to the tooth 7 reduces during unlocking, since the exit pallet 15 is situated on the other side of the axis of rotation 11 of the anchor 9 with respect to the entry pallet 13.

(20) As a result, the invention relates to the form of the rest surface 13a of the entry pallet 13. Since the active surfaces 13a, 13b, 15a, 15b of the pallets do not need to be planar, the terminology “surface” is utilized in place of the usual formulation “plane of . . . ”.

(21) In particular, the rest surface 13a of the entry pallet 13 is formed in such a way that the angle that it subtends with respect to a tooth 7 of the escapement wheel 3 remains constant, or reduces during unlocking.

(22) FIG. 2 depicts, on an enlarged scale, the form of an entry pallet 13, of which the rest surface 13a exhibits a shape calculated in such a way that the angle of the draw force is constant when it is utilized in an escapement having the geometry of that in FIG. 1.

(23) In FIG. 2, the edge joining the rest surface 7a to the impulsion surface 7b of a tooth 7 of the escapement wheel 3 is depicted in two positions, one of which—on the right—being at the start of the unlocking phase, and the other—on the left—being just before the end of the unlocking phase and before of the transition to the impulsion phase. The rest surface 13a of the pallet 13 is convex and curved in such a way that the direction of the force F resulting from the contact between the tooth 7 and the pallet 13 remains oriented substantially in the same direction when the anchor 9 pivots during the unlocking phase.

(24) FIG. 3 depicts schematically the difference between this curvature and conventional rest planes 13c and impulsion planes 13d, depicted by straight lines.

(25) This figure clearly shows that, when the profile of the rest surface 13a crosses the line A, it departs from the line 13c representing a plane of rest B, which represents the start of the transition between the rest surface 13a and the impulsion surface 13b. At the line B, the profile joins an arc of a circle to which it is tangential, said arc of a circle likewise being tangential to the impulsion surface 13b, in order to make this transition continuous. In the variant depicted here, the impulsion surface 13b is straight and, as a result, represents a conventional impulsion plane. However, a curved impulsion surface 13b is also possible.

(26) FIGS. 4 and 5 depict schematically a geometric model which makes it possible to calculate the form of the rest surface 13a of the entry pallet independently of the form of the anchor. This model schematically represents the interaction between the escapement wheel 3 and the anchor 9. In FIG. 4, the anchor 9 and a number of teeth 7 are depicted schematically, whereas in FIG. 9 the model is depicted minimalistically.

(27) In this model, the center-to-center line O.sub.RO.sub.A between the axis of rotation 5 of the wheel 3 and the axis of rotation 11 of the anchor 9 serves as a reference for the geometry. The point of contact C between a tooth 7 of the escapement wheel 3 and the rest surface 13a of the entry pallet 13 traces the profile of said rest surface 13a during the unlocking phase, and may be expressed as Cartesian coordinates such as C=(X.sub.C, Y.sub.C). These coordinates X.sub.C, Y.sub.C are respectively perpendicular and parallel to the center-to-center line O.sub.RO.sub.A.

(28) According to the invention, the angle γ, which depicts the predetermined draw angle, is constant or decreasing during unlocking. This angle γ is measured between, on the one hand, the tangent T to the rest surface 13a of the entry pallet at its point of contact C with the escapement wheel 3, and, on the other hand, the normal to the line O.sub.AC joining the axis of rotation of the anchor and the point of contact between the rest surface 13b of the entry pallet 13 and a tooth 7 of the escapement wheel 3.

(29) After having selected γ (or its development) beforehand, as well as the geometry of the escapement wheel 3 and of the anchor 9, the geometry may be resolved as follows. It should be noted that, in the notation utilized below, a term such as “CF”, “O.sub.AF” or similar signifies the length of a straight line joining the points concerned.

(30) In the first place, it is observed that:
CF=R. sin(α)

(31) in which R is the distance between the axis of rotation O.sub.R of the wheel 3 and the point of contact C between the tooth 7 and the rest surface 13a, and α is the angle that O.sub.RC subtends with the center-to-center line O.sub.RO.sub.A. Subsequently,
O.sub.AF=Axe−R. cos(α)

(32) in which Axe is the length of the center-to-center line O.sub.RO.sub.A.

(33) For the sake of completeness, it should be noted that the angle θ that the line O.sub.AC subtends with the center-to-center line O.sub.RO.sub.A is expressed as
θ=tan.sup.−1(CF/O.sub.AF)

(34) The draw angle γ is measured between a line that is orthogonal to O.sub.AC and the tangent T of the rest surface 13a at the point C. Consequently
90°=γ+α.sub.orientation+tan.sup.−1(X.sub.C/Y.sub.C)

(35) in which α.sub.orientation is the angle that the tangent of the rest surface 13a of the tooth 7 subtends with respect to the center-to-center line O.sub.AO.sub.R. This equation may be reorganized as follows
90°=γ+α.sub.orientation=tan.sup.−1(X.sub.C/Y.sub.C)

(36) By redefining X.sub.C and Y.sub.C as polar coordinates, the following is obtained:
90−γ−α.sub.orientation=tan.sup.−1(R*sin(α)/Axe−R*cos(α)

(37) The profile of the rest surface 13 may thus be traced by calculating the value of α.sub.orientation at the point of contact C for each position of the anchor during the unlocking phase, while observing the above relationship. In the case where γ varies, a function representing this variation may be utilized in the calculation.

(38) In essence, α.sub.orientation may be calculated for a plurality of angular positions of the anchor, and the associated tangents may then be joined in a continuous manner in order to arrive at the desired profile.

(39) The value of γ may lie, for example, within a range of values between 5° and 20°, and preferably between 10° and 15°, and may reduce during at least a part of the unlocking phase, while remaining within the interior of this range.

(40) Furthermore, γ may also include a tolerance of +/−10%.

(41) The result of this draw is that the anchor 9 may be kept in engagement with a tooth 7 of the escapement wheel, and that the resistance to unlocking of the entry pallet 13 does not increase. Consequently, the oscillations of the regulating member are less disrupted during these unlockings.

(42) In essence, the graph in FIG. 7 illustrates a comparison between the evolution of the draw angle during the unlocking phase for a pallet of conventional planar form (such as that of document CH702689), shown as a dashed line, and for a pallet exhibiting a rest surface configured according to the invention, shown as a solid line. According to the results of this modeling, it is clear that the draw angle, and thus the resistance to unlocking, no longer increases in the case of a rest surface according to the invention.

(43) The anchor 9 and/or the escapement wheel 3 described above may be manufactured, for example, by micro-machining processes, such as LIGA, 3D printing, masking and engraving from a sheet of material, stereolithography or similar. Appropriate materials may be selected, for example, from among monocrystalline, polycrystalline or amorphous metals (such as steel, the nickel-phosphorus, brass or similar), non-metals such as silicon, its oxide, its nitride or its carbide, alumina in all its forms (for example ruby), diamond (including diamond-like carbon), these non-metallic materials being monocrystalline or polycrystalline. All of these materials may possibly be coated with another hard material and/or anti-friction material, such as diamond-like carbon, alumina or silicon oxide.

(44) Although the invention is described above in conjunction with specific embodiments, additional variants are also conceivable without departing from the scope of the invention as defined by the claims.