EP2802942B1 - Timepiece having a plurality of balances - Google Patents

Description

The present invention relates to a timepiece, typically a wristwatch, with several pendulums.

There exist in the state of the art timepieces with several pendulums using a physical principle that watchmakers call the "resonance". According to this principle, the pendulums influence each other to oscillate synchronously, generally in phase opposition. Watchmakers such as Antide Janvier, Louis-Abraham Breguet, François-Paul Journe and Antoine Preziuso have proposed timepieces that operate on this principle. Such pieces are undeniably beautiful achievements both aesthetically and technically. The synchronous oscillations of the rockers confer indeed a certain harmony as well as a protection against the external vibrations.

These resonance timepieces, however, have a disadvantage: they must be set very precisely so that the resonance effect can occur. According to François-Paul Journe, the balance difference of the pendulums must not exceed a cumulative value of 5 s / d in the six standard positions of the watch.

In general, it seems accepted that the resonance is a difficult phenomenon to master and that it requires great dexterity on the part of the watchmaker, especially if the number of pendulums is high.

Some prior art documents, for example patent applications WO 2005/111742 , WO 2011/058157 and WO 2012/062659 and the patent CH 695.196 , describe timepieces with multiple pendulums, without specifying whether or not the principle of resonance is used. These parts include a differential that average the steps of the different pendulums, so that obtaining a resonance is not mandatory for their operation. However, during their operation, depending on external conditions such as temperature, a resonance can occur and cause the rockers to synchronize at a frequency different from their initial oscillation frequency. In such a case, where the accuracy of the timepiece is obtained by averaging the steps of independent rockers, the resonance is a disturbance likely to significantly deteriorate walking.

The present invention aims to remedy these disadvantages, or at least to mitigate them, and proposes for this purpose a timepiece comprising a box, a movement and a display device housed in the box, the movement comprising a frame and , mounted in the frame, motor means, a plurality of regulating members each comprising a rocker arranged to oscillate about an axis of rotation, connecting means connecting the motor means to the regulating members and at least one differential to average the steps of the regulating members, the regulating members being each carried by a support rigidly fixed to the frame or forming part of said frame, said timepiece defining an axis perpendicular to its mean plane and passing through its center of gravity, characterized in that the ratio d / D of the distance d between the centers of gravity of the nearest balances on the diameter D of each balance is at least 2.5 and in that the ratio J / Jb of the moment of inertia J of the timepiece with respect to said axis thereof to the moment of inertia Jb of each balance relative to said axis of rotation thereof is at least 2x10 ⁴ .

The present invention is a completely different approach than those proposed so far, in that the multi-pendulum timepiece is specifically designed to minimize or even exclude the resonance between the pendulums.

The present applicant has identified three types of coupling that can lead to the appearance of a resonance in a timepiece with several pendulums.

The first coupling is an air coupling occurring by friction of the air between the rockers. When two rockers cause air in the same meaning, they can interact with each other. This coupling is rendered negligible in the present invention by the ratio d / D greater than or equal to 2.5, implying that the rockers are sufficiently far apart from each other (or from each other).

The second coupling is an elastic coupling between the external attachment points of the spirals associated with the rockers, coupling induced by the microdeals of the mechanical connection connecting these attachment points. This coupling is rendered negligible in the present invention by the fact that the regulating members are each carried by a support rigidly fixed to the frame of the movement or forming part of said frame, implying that the mechanical connection (supports, plate and / or bridges of the frame) between the outer attachment points of the spirals is rigid.

Finally, the third coupling is an inertial coupling between the kinetic moment vectors of the pendulums and the kinetic moment vector of the timepiece as a whole (considered as a solid). This coupling tends to oscillate together the solid timepiece and each of the pendulums. If we model with Lagrange equations the dynamic behavior of such a coupled system, we can see that the greater the J / Jb ratio, the greater the inertial coupling between the pendulums and the solid piece. clock is weak. Thus, with a J / Jb ratio of at least 2x10 ⁴ , the inertial coupling can be significantly reduced.

Each of the three couplings above causes a change in the oscillation frequency of the rockers relative to their natural natural frequency. Indeed, by coupling, the oscillating system is extended and it is the eigenfrequencies of this extended system that determine the oscillation frequency of the pendulums.

Ce phénomène est souvent traduit par la formule : « les couplages écartent les fréquences propres ». En pratique, le mode (vecteur propre associé à la fréquence propre) selon lequel l'ensemble couplé va osciller dépend des couples extérieurs d'entretien à savoir, dans le cas de balanciers, du couple appliqué par le ou les échappements. Généralement, l'un des modes correspond à des déplacements angulaires en phase des oscillateurs, tandis que l'autre mode correspond à des déplacements angulaires en opposition de phase.Mathematical models of the dynamic behavior of coupled systems are proposed in the literature, for example in the book Yves Rocard's "Dynamique générale des vibrations", third revised edition and augmented, Masson et Cie, 1960, chapter VIII , or in that of Jules Haag and Raymond Chaléat "Problems of the general theory of oscillations and chronometry", Gauthier-Villars, Eyrolles, 1960 . These models show that the eigenfrequencies of a coupled system are the solutions of a system of differential equations and that, in the case of two oscillators having respective natural eigenfrequencies f1 and f2 in the absence of coupling, the frequencies f1 'and f2' of the coupled set satisfy the inequality: $$\left|\mathrm{f}\mathrm{2}\mathrm{\text{'}}-\mathrm{f}\mathrm{1}\mathrm{\text{'}}\right|>\left|\mathrm{f}\mathrm{2}-\mathrm{f}\mathrm{1}\right|\mathrm{.}$$

This phenomenon is often translated by the formula: "the couplings separate the eigenfrequencies". In practice, the mode (eigenvector associated with the natural frequency) according to which the coupled assembly will oscillate depends on the external maintenance couples, namely, in the case of pendulums, the torque applied by the exhaust or exhausts. Generally, one of the modes corresponds to angular displacements in phase of the oscillators, while the other mode corresponds to angular displacements in opposition of phase.

où n est le nombre de balanciers. Les marches étant moyennées, les erreurs de marche le sont aussi. Par rapport aux pièces d'horlogerie à plusieurs balanciers et à différentiel non spécialement conçues pour minimiser les couplages, la pièce d'horlogerie selon l'invention est plus précise car non sujette aux risques de changements d'états résonant / non résonant.The present invention therefore seeks to minimize or even eliminate the couplings, so that each pendulum oscillates at a frequency as close as possible to its natural natural frequency, on the basis of which the movement is designed. Compared to a timepiece operating on the principle of resonance, the timepiece according to the invention is more easily reproducible and less sensitive to the accuracy of the settings, while being very accurate in its measurement of time. In fact, the differential (s) perform (s) an averaging of the respective steps M _i of the pendulums, the step M of the timepiece can be expressed as follows: $$\mathrm{M}=\frac{\mathrm{1}}{\mathrm{not}}{\displaystyle \underset{\mathrm{1}}{\overset{\mathrm{not}}{<figure-callout\; id="1"\; label="\Sigma "\; filenames="imgf0002.png,imgf0003.png"\; state="\{\{state\}\}">\Sigma </figure-callout>}}}{\mathrm{M}}_{\mathrm{i}}$$

where n is the number of pendulums. The steps being averaged, so are the operating errors. Compared to timepieces with multiple balance wheels and differential not specifically designed to minimize the coupling, the timepiece according to the invention is more accurate because not subject to the risk of resonant / non-resonant state changes.

Preferably, in the present invention, the ratio d / D is at least 2.65 and the ratio J / Jb is at least 6 × 10 ⁴ , more preferably at least 10 ⁵ .

The differential (s) that includes the timepiece according to the invention is (are) typically part of said connecting means.

The rockers can be regularly distributed in a circumferential direction around the geometric axis of the movement.

Preferably, the rockers are inclined relative to said mean plane of the timepiece. Tilting the rockers makes the air coupling between said rockers even weaker. In exemplary embodiments, at least two of the rockers have their axes of rotation which are substantially orthogonal to each other. In a particular embodiment, the rockers are each inclined by approximately 45 ° with respect to said mean plane and such that their axes substantially intersect the geometric axis of the movement, and are all inclined in the same direction with respect to said axis geometric.

Advantageously, the regulating organs are four in number. In this case, the connecting means may comprise a center mobile arranged to be driven by the motor means, first and second average mobiles arranged to be driven by the center mobile, first and second second movers arranged to be driven by the first average mobile and third and fourth second mobiles arranged to be driven by the second average mobile, the first to fourth second mobiles being arranged to respectively drive the regulating members.

The center wheel may comprise a first differential gear and the first and second average wheels may respectively comprise second and third differential gears for averaging the steps of the regulating members.

• la figure 1 est une vue de dessus d'une pièce d'horlogerie selon l'invention ;
• la figure 2 est une vue de profil de la pièce d'horlogerie selon l'invention ;
• la figure 3 est une vue plane, prise depuis le côté cadran (vue de dessus), du mouvement équipant la pièce d'horlogerie selon l'invention, avec la platine du mouvement montrée de manière schématique ;
• la figure 4 est une vue de profil montrant un décalage en hauteur entre deux organes régulateurs faisant partie dudit mouvement ;
• la figure 5 est une vue en coupe selon l'axe 3 heures - 9 heures d'une partie dudit mouvement ;
• la figure 6 est une vue en coupe, prise suivant une ligne brisée, dudit mouvement ;
• la figure 7 est une vue en perspective, prise depuis le côté fond, d'une partie dudit mouvement ;
• la figure 8 est une vue en perspective d'une partie d'un pont de rouage dudit mouvement.

Other features and advantages of the present invention will appear on reading the following detailed description given with reference to the accompanying drawings in which:

• the figure 1 is a top view of a timepiece according to the invention;
• the figure 2 is a side view of the timepiece according to the invention;
• the figure 3 is a plan view, taken from the dial side (top view) of the movement equipping the timepiece according to the invention, with the platen of the movement shown schematically;
• the figure 4 is a profile view showing a height offset between two regulating members forming part of said movement;
• the figure 5 is a sectional view along the axis 3 hours - 9 hours of a portion of said movement;
• the figure 6 is a sectional view, taken along a broken line, of said movement;
• the figure 7 is a perspective view, taken from the bottom side, of a part of said movement;
• the figure 8 is a perspective view of a part of a gear wheel of said movement.

With reference to Figures 1 and 2 , a timepiece according to a preferred embodiment of the invention is in the form of a wristwatch comprising a BT box housing a MV movement and a display device AF. The average plane of the timepiece is designated P. The axis of the timepiece is designated A. This axis A passes through the center of gravity of the timepiece, is perpendicular to the average plane P and is parallel and substantially coincident with the geometric axis of the movement and the box, designated by A ', this axis A' corresponding to the axis of central indicator hands AF1, AF2 of the display device AF.

The BT box comprises, in a conventional manner, a middle part BT1, a BT2 bottom, a BT3 bezel and a BT4 glass, the middle part defining BT5 horns for attachment to a bracelet. The display device AF also conventionally comprises said central indicator hands AF1, AF2 and an AF3 dial, and may furthermore comprise other indicator members such as AF4 off-center hands. As shown in figure 3 , the movement MV comprises, mounted in a frame, a mobile center 1, two barrels 2a, 2b located on either side of the mobile center 1, two mobile average 3a, 3b located on either side of the mobile center 1, four mobile seconds 4a, 4b, 4c, 4d. and four regulating members 5a, 5b, 5c, 5d. The frame of the movement MV comprises a plate 6 and bridges, in particular a first axle 7a receiving pivots of the shaft of the mobile of average 3a and second mobiles 4a, 4b, a second axle 7b receiving pivots of the shafts of the mobile of average 3b and second mobiles 4c, 4d, a center bridge 7 'and two barrel bridges 7 ", 7"' (cf. figure 6 ). Each regulating member 5a to 5d comprises an escapement 8a to 8d, a balance 9a to 9d and a hairspring 10a to 10d, the hairspring being mounted on the same shaft as the balance by a ferrule, in the usual manner. Each exhaust 8a to 8d typically comprises an escape wheel, comprising a wheel and an escape pinion, an anchor and a double plate mounted on the balance shaft. Differentials, which will be described later, allow the indicator hands AF1, AF2 to display a time corresponding to the average of the times measured by the four organs. regulators 5a to 5d, thus giving the movement a high accuracy of operation.

In the example illustrated, the regulating members 5a to 5d are inclined relative to the plane of the plate 6, which plane is parallel to the plane P, in a manner that advantageously compensates for the effects of gravity on the movement's progress. depending on the positions of the timepiece. More specifically, each regulating member 5a to 5d is disposed in a plane inclined by 45 ° with respect to the plane of the plate 6. In other words, the axis of rotation 12a to 12d of each balance 9a to 9d forms an angle 45 ° with the plane of the plate 6. In top view of the movement ( figure 3 ), the pendulums 9a to 9d form the ends of a cross whose center is at the center of the movement and whose two branches are perpendicular. The rockers diametrically opposed to each other 9a, 9c have their axes 12a, 12c which intersect the axis A 'of the movement at the same point 14 ( figure 6 ). The other two rockers opposite each other 9b, 9d have their axes 12b, 12d which intersect the axis A 'of the movement at the same point 15 which is typically on the same side of the plate 6 as the point 14 but distinct from the latter since the regulating members 5a, 5c are at a raised position relative to the regulating members 5b, 5d, as shown in FIGS. figures 4 and 6 in order to allow the second wheels of the second wheels 4a, 4b and the second wheels of the second wheels 4c, 4d to overlap (cf. figure 3 ).

In this way, the angle between them the axes 12a, 12c of the pendulums 9a, 9c is 90 °. Similarly, the angle between them the axes 12b, 12d of the pendulums 9b, 9d is 90 °. In addition, the axes 12a, 12c of the pair of pendulums 9a, 9c are not parallel to any of the axes 12b, 12d-of the other pair of pendulums 9b, 9d, thus providing coverage of all directions of space . The orthogonality between the axes of the rockers of the same pair 9a, 9c or 9b, 9d makes it possible to effectively compensate the effects of gravity on these rockers and to particularly well cover the possible positions of the movement between the horizontal and the vertical ( flat-hung). With this orthogonality, the average amplitude of oscillation of the rockers of a given pair remains substantially constant between the different angular positions of the movement in the diametral plane containing the axes of these rockers.

Moreover, thanks to the inclination of 45 ° of the rockers 9a to 9d, they are never in their most unfavorable position in terms of sensitivity to gravity, namely the vertical position, when the movement is in motion. 'one of its reference positions, namely the horizontal' dial up 'and' dial down 'and vertical' 3 o'clock up ',' 6 o'clock up ',' 9 o'clock up 'and' 12 o'clock positions up ". The difference in path between the reference positions of the movement is therefore small.

Although the arrangement of the rockers as described above is particularly advantageous, the rockers could, alternatively, be inclined differently or not be inclined, that is to say be arranged parallel to the plate 6 and the plane P. In the illustrated example, the rockers are regularly distributed in a circumferential direction around the axis A 'of the movement. Alternatively, however, they could be distributed unevenly.

The structure of the movement according to the invention will now be described in more detail.

As shown in figures 5 and 6 , the center wheel 1 comprises, around a center shaft 20, a roadway 21 frictionally mounted on the shaft 20 and bearing the minute hand AF2, a wheel of hours 22 free to rotate around the shaft 20 and carrying the hour hand AF1, a differential gear 23 and a center pinion 24 integral with the shaft 20. The center pinion 24 meshes with the two barrels 2a, 2b (whose associated ratchets have not been represented) which, thus arranged in parallel, add their torques to drive the center shaft 20. The hour wheel 22 and the roadway 21 mesh respectively with the pinion and the wheel of a timer wheel 25 (cf. figure 1 ). The timer wheel is connected to the time setting rod 26 of the movement by comprises, in addition to the shaft 20 which forms the input, a first output wheel 28 free to rotate relative to the shaft 20, a pinion 29 integral with the wheel 28, a second output wheel 30 free to rotate by relative to the shaft 20, a central pinion 31 integral with the shaft 20, and a satellite mobile comprising a pinion 32 which meshes with the central pinion 31 and a wheel 33 which is integral with the pinion 32 and which meshes with the pinion 29 , the pivots of this satellite mobile being respectively mounted in the second output wheel 30 and in a bridge 34 fixed to the wheel 30.

The two average mobiles 3a, 3b each comprise, around an average shaft 35a, 35b, a pinion of average 36a, 36b integral with the shaft 35a, 35b and a differential gear 37a, 37b. The average gear 36a meshes with, and is driven by, the first output gear 28 of the differential gear 23 and the average gear 36b meshes with, and is driven by, the second output gear 30 of the differential gear 23. The differential gear 37a, 37b of each mobile of average 3a, 3b is of the same type as the differential gear 23 of the center mobile 1. The average shaft 35a, 35b is the input. One, 38a, of the output wheels of the differential gear 37a meshes with, and drives, a pinion (not shown) of the second wheel 4a, while the other wheel 39a meshes with, and drives, a pinion 40b of the mobile of the second 4b. Similarly, one, 38b, of the output wheels of the differential gear 37b meshes with, and drives, a pinion 40c of the second gear 4c, while the other output gear 39b meshes with, and drives, a pinion (not shown) of the mobile of the second 4d. The wheels of the second movable 4a to 4d meshing by means of bevel gears with the exhaust gears of the regulating members 5a to 5d.

As they are arranged, on the mobiles of center 1 and average 3a, 3b, the differential gears 23, 37a, 37b are closer to the barrels 2a, 2b, that is to say where the torque is Most important. This provision allows compensate for the disadvantages of a differential gear that are its weight and inertia.

The structure as described above has the advantage of a small footprint because the four regulating members 5a to 5d are driven by the same motor member, constituted by the two barrels 2a, 2b, and a single mobile. average is used for two regulating bodies. The use of two barrels in parallel makes it possible to increase the torque necessary for driving the regulating members. It also allows, by the arrangement of these barrels 2a, 2b on either side of the mobile center 1, to balance the transmitted torque and to reduce the air pressure on the pivots of the mobile center 1. Alternatively, , drive members connected by the differentials could separate separately the regulating members 5a to 5d. In another variant, a single barrel could be used to drive the four regulating members 5a to 5d.

Each regulating member 5a to 5d is carried by a support or frame 41 (cf. figures 4 and 7 ) comprising an exhaust holder 42, an exhaust bridge 43 and a balance bridge 44 fixed to the exhaust door 42. The pivots of the balance shaft rotate in bearings 45, 46 (cf. figures 4 and 6 ), preferably anti-shock, respectively equipping the exhaust door 42 and the balance bridge 44. The escape wheel and the anchor are mounted between the exhaust port 42 and the exhaust bridge 43. The spiral is mounted between the pendulum and the balance bridge 44. The outer end of the balance spring is fixed to a pin 10 'itself attached to the balance bridge 44. The supports 41 of the regulating members 5a to 5d are fixed rigidly to the frame of the movement, by free by means of screws, against bearing surfaces 47 of the cog axles 7a, 7b which are inclined by 45 ° so as to obtain the inclination of 45 ° of the rockers 9a to 9d with respect to the plate 6, while allowing said platen to remain flat. These inclined surfaces 47 are represented at figure 8 and, schematically, at the figure 6 . In the exemplary embodiment shown, the supports 41 of the regulating members 5a, 5b are fixed on the first gearbridge 7a, while the supports 41 of the regulating members 5c, 5d are fixed on the second gearbridge 7b.

Mechanically, the external attachment points of the spirals 10a to 10d, at the level of the studs 10 ', are thus connected to each other via a rigid structure comprising the supports 41, the axles 7a, 7b and 7b the plate 6. The elastic couplings between these attachment points can therefore be considered negligible.

Moreover, as is apparent on the figures 1 and 3 , the distance between the pendulums 9a to 9d is large. Specifically, in the present invention, the ratio d / D of the distance d between the centers of gravity of the rockers on the diameter D of each beam (defined as the outer diameter of the beam of the balance) is greater than or equal to 2.5 preferably greater than or equal to 2.65. In the case where the rockers are not evenly distributed around the axis A ', this ratio d / D greater than or equal to 2.5 concerns the two rockers closest to each other. In an exemplary embodiment, the distance d and the diameter D are respectively about 22.64 mm and about 8.5 mm. With such a spacing of the rockers, air couplings can also be considered negligible. These air couplings are further diminished by the fact that the rockers are inclined.

Finally, to make the inertial couplings negligible, the moment of inertia J of the timepiece BT, MV, AF with respect to the axis A and the moment of inertia Jb of each balance 9a to 9d relative to its axis of rotation 12a to 12d are such that their J / Jb ratio is greater than or equal to 2x10 ⁴ , preferably greater than or equal to 6x10 ⁴ , more preferably greater than or equal to 10 ⁵ . To obtain such values, it is possible to play on the mass of the timepiece, for example by producing the BT box in a sufficiently heavy material and with sufficiently thick walls, and / or on its dimensions, namely its diameter and his height. In an exemplary embodiment, each pendulum has a moment of inertia with respect to its axis of rotation of about 8x10 ^-10 kg.m ² while the timepiece as a whole has a moment of inertia with respect to its axis A of about 8 , 6x10 ^-5 kg.m ² .

Thanks to these characteristics, the resonance effects are avoided and the pendulums are allowed to oscillate independently of each other at their natural natural frequency.

It will also be noted that the high number of balance wheels, four in the illustrated example, further reduces the risk of appearance of a resonance.

The present invention has been described above by way of example only. It goes without saying that modifications could be made without departing from the scope of the claimed invention. For example, the regulating members could be in the form of vortices each carried by a support rigidly fixed to the frame of the movement or forming part of said frame.

Claims (12)

1. Timepiece comprising a case (BT), a movement (MV) and a display device (AF) which are housed in the case (BT), the movement (MV) comprising a frame (6, 7a, 7b, 7') and, mounted in the frame, drive means (2a, 2b), a plurality of regulator members (5a-5d) each comprising a balance (9a-9d) arranged to oscillate about an axis of rotation (12a-12d), linking means (1, 3a, 3b, 4a-4d) connecting the drive means (2a, 2b) to the regulator members (5a-5d) and at least one differential (23, 37a, 37b) to average the rates of the regulator members, the regulator members (5a-5d) each being carried by a support (41) rigidly fixed to the frame (6, 7a, 7b, 7') or forming part of said frame, said timepiece defining an axis (A) perpendicular to its middle plane (P) and passing through its centre of gravity, characterised in that the ratio d/D of the distance d between the centres of gravity of the closest balances (9a-9d) to the diameter D of each balance (9a-9d) is at least 2.5 and in that the ratio J/Jb of the moment of inertia J of the timepiece with respect to said axis (A) thereof to the moment of inertia Jb of each balance (9a-9d) with respect to said axis of rotation (12a-12d) thereof is at least 2 x 10⁴.
2. Timepiece as claimed in claim 1, characterised in that the ratio d/D is at least 2.65.
3. Timepiece as claimed in claim 1 or 2, characterised in that the ratio J/Jb is at least 6 x 10⁴.
4. Timepiece as claimed in claim 3, characterised in that the ratio J/Jb is at least 10⁵.
5. Timepiece as claimed in any one of claims 1 to 4, characterised in that said at least one differential (23, 37a, 37b) forms part of said linking means (1, 3a, 3b, 4a-4d).
6. Timepiece as claimed in any one of claims 1 to 5, characterised in that the balances (9a-9d) are distributed evenly in a circumferential direction about the geometric axis (A') of the movement (MV).
7. Timepiece as claimed in any one of claims 1 to 6, characterised in that the balances (9a-9d) are inclined with respect to said middle plane (P).
8. Timepiece as claimed in claim 7, characterised in that at least two (9a, 9c; 9b, 9d) of the balances (9a-9d) have their axes of rotation (12a, 12c; 12b, 12d) substantially orthogonal to one another.
9. Timepiece as claimed in claim 7 or 8, characterised in that the balances (9a-9d) are each inclined by about 45° with respect to said middle plane (P) and such that their axes (12a-12d) substantially bisect the geometric axis (A') of the movement and are all inclined in the same direction with respect to said geometric axis (A').
10. Timepiece as claimed in any one of claims 1 to 9, characterised in that the regulator members (5a-5d) are four in number.
11. Timepiece as claimed in claim 10, characterised in that the linking means (1, 3a, 3b, 4a-4d) comprise a centre wheel assembly (1) arranged to be driven by the drive means (2a, 2b), first and second intermediate wheel assemblies (3a, 3b) arranged to be driven by the centre wheel assembly (1), first and second seconds wheel assemblies (4a, 4b) arranged to be driven by the first intermediate wheel assembly (3a) and third and fourth seconds wheel assemblies (4c, 4d) arranged to be driven by the second intermediate wheel assembly (3b), the first to the fourth seconds wheel assemblies (4a-4d) being arranged to drive the regulator members (5a-5d) respectively.
12. Timepiece as claimed in claim 11, characterised in that the centre wheel assembly (1) comprises a first differential gear (23) and the first and second intermediate wheel assemblies (3a, 3b) respectively comprise second and third differential gears (37a, 37b) to average the rates of the regulator members (5a-5d).

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