EP2141555B1 - Coupled resonators for timepiece - Google Patents

Description

The present invention relates to a timepiece resonator resulting from the coupling of a first low frequency resonator with a second resonator at a higher frequency.

A resonator as defined above has been described in the document EP 1 843 227 A1 . In this presentation, the first low-frequency resonator is a balance-spring and the second high-frequency resonator is a tuning fork. A branch of the tuning fork is directly connected to the outer turn of the hairspring to form the coupling between the two resonators. The purpose of this arrangement is to stabilize the frequency of operation of the timepiece and make this frequency more independent of external stresses to ultimately improve the running accuracy of the same room. In the arrangement described the natural frequency of the first resonator is a few hertz, that of the second resonator being of the order of kHz. The idea is to enslave a first resonator very sensitive to external disturbances to a second resonator which, because of its high operating frequency, is much less sensitive to said disturbances. This enslavement will result in an improvement in the performance of the first resonator as for its impact resistance for example, this first resonator cooperating with a conventional exhaust system.

The embodiment that has just been described, however, uses two resonators very different from each other which we can think that the coupling and adjustment will present difficulties, certainly not insurmountable, but still large enough in view of the low inertia of the high frequency resonator and therefore its ability to influence the operation of the first low frequency resonator.

As a result of what has just been described, it is possible to regulate the operation of a first low-frequency resonator using a balance spring by means of a second resonator with a higher frequency also using a balance-spring it will be possible to stabilize up to a certain point the operating frequency of the timepiece and this by implementing resonators that have no secret to the skilled person.

The hourly alternation numbers of 18000, 21600 and 28800, corresponding to oscillation frequencies of 2.5, 3 and 4 Hz, are commonly used in watchmaking for the sprung balance resonator. However, watches are known with oscillating balance springs oscillating at higher frequencies, the purpose being to allow the watch to achieve better chronometric performance to wear.

As the book shows "Exhaust and Stepper Engines" by Charles Huguenin et al (FET, Neuchatel 1974, pages 137 to 148 ) multiplying the frequency by two decreases the effect of a lack of equilibrium on the diurnal walk of four times. Thus an increase in the oscillation frequency of the balance has the double advantage of increasing the regulating power of the resonator and making the operation of the watch less sensitive to changes in positions.

These advantages must however be paid for by an increase in the number of teeth of the escape wheel. Indeed, the conventional escape wheel generally has 15 teeth for frequencies of the balance-spring resonator of 2.5 to 3 Hz. This number has been widely accepted as such, because it takes into account the manufacturing problems of the escape wheel and a judicious distribution of the ratios and the number of teeth of the wheels and gears of the finishing gear of the watch. With higher frequencies of the resonator between 4 and 10Hz, gear ratios become too great, but this disadvantage disappears if the number of teeth of the escape wheel is increased. The number of 21 teeth is cited for a frequency oscillation of 5Hz, this change however bringing a reduction in safety such as rest and falls that require special care during reassembly. On the other hand and generally, it is well known that the performance of a Swiss lever escapement drops sharply above 4 or 5 Hz.

Thus, to benefit from the advantages of a high-frequency resonator, it will be coupled with a low-frequency resonator, the latter being controlled by a conventional escapement without increasing the number of teeth of the escape wheel and with the well-known safety devices that accompany this type of exhaust.

This arrangement is represented in the block diagram of the figure 1 . In this figure, the first low frequency resonator 2,41 is constituted by a sprung balance driven by an escapement and a finishing train 70, this wheel being actuated by a cylinder 71. Derived from the gear train 70, there is the display 72 hour materialized by needles for example. The second higher frequency resonator is represented by block 3,42. The coupling between the two resonators is represented by the double-headed block 8.46.

We also know of the document FR 563 314 a mechanically oscillating system for controlling the operation of time measuring apparatus.

The document CH 276,465 discloses a resonator system having a first resonator having a first mass of inertia associated with a first spring, and a second resonator having a second mass of inertia associated with a second spring, the second spring connecting the first and second masses of inertia for coupling the first and second resonators.

The present invention relates to a resonator according to claim 1 of the patent.

• la figure 1 est un schéma bloc illustrant le résonateur de l'invention et son implication dans une pièce d'horlogerie,
• la figure 2 est un schéma équivalent montrant comment sont arrangés et couplés les deux résonateurs selon un premier mode de réalisation de l'invention,
• la figure 3 est une vue en plan du premier mode de réalisation d'un résonateur résultant du couplage de résonateurs composés chacun d'un balancier-spiral,
• la figure 4 est une coupe selon la ligne IV-IV de la figure 3,
• les figures 5 et 6 sont des vues en perspective du résonateur montré en plan et en coupe sur les figures 3 et 4.
• la figure 7 est un graphique montrant la fréquence d'oscillation propre de chacun des résonateurs lorsqu'on fait varier le couple du ressort spiral reliant les deux résonateurs,
• la figure 8 est un graphique montrant l'effet stabilisant résultant du couplage des premier et second résonateurs sur des perturbations affectant soit le couple du ressort spiral du premier résonateur, soit la masse d'inertie du balancier dudit premier résonateur lorsqu'on fait varier le couple du ressort spiral reliant les deux résonateurs,
• la figure 9 est un schéma équivalent montrant comment sont arrangés et couplés les deux résonateurs selon un exemple de résonateur ne faisait pas partie de l'object revendiqué
• la figure 10 est une vue en plan du premier exemple de résonateur résonateur résultant du couplage de résonateurs composés chacun d'un balancier-spiral,
• la figure 11 est une coupe selon la ligne XI-XI de la figure 10,
• les figures 12 et 13 sont des vues en perspective du résonateur montré en plan et en coupe sur les figures 10 et 11,
• la figure 14 est un graphique montrant la fréquence d'oscillation propre de chacun des résonateurs lorsqu'on fait varier le couple du ressort spiral du premier résonateur, et
• la figure 15 est un graphique montrant l'effet stabilisant résultant du couplage des premier et second résonateurs sur des perturbations affectant soit le ressort spiral du premier résonateur, soit la masse d'inertie du balancier dudit premier résonateur lorsqu'on fait varier le couple du ressort spiral de ce même premier résonateur.

The invention will now be explained in detail below by means of drawings in which:

• the figure 1 is a block diagram illustrating the resonator of the invention and its implication in a timepiece,
• the figure 2 is an equivalent diagram showing how the two resonators are arranged and coupled according to a first embodiment of the invention,
• the figure 3 is a plan view of the first embodiment of a resonator resulting from the coupling of resonators each composed of a balance-spring,
• the figure 4 is a section along line IV-IV of the figure 3 ,
• the Figures 5 and 6 are perspective views of the resonator shown in plan and in section on the Figures 3 and 4 .
• the figure 7 is a graph showing the own oscillation frequency of each of the resonators when the torque of the spiral spring connecting the two resonators is varied,
• the figure 8 is a graph showing the stabilizing effect resulting from the coupling of the first and second resonators on disturbances affecting either the pair of the spiral spring of the first resonator or the mass of inertia of the balance of said first resonator when the spring torque is varied. spiral connecting the two resonators,
• the figure 9 is an equivalent diagram showing how are arranged and coupled the two resonators according to an example of resonator was not part of the claimed object
• the figure 10 is a plan view of the first resonator resonator example resulting from the coupling of resonators each composed of a balance-spring,
• the figure 11 is a section along line XI-XI of the figure 10 ,
• the Figures 12 and 13 are perspective views of the resonator shown in plan and in section on the Figures 10 and 11 ,
• the figure 14 is a graph showing the own oscillation frequency of each of the resonators when the spring coil torque of the first resonator is varied, and
• the figure 15 is a graph showing the stabilizing effect resulting from the coupling of the first and second resonators on disturbances affecting either the spiral spring of the first resonator or the mass of inertia of the balance of said first resonator when the torque of the spiral spring of this same first resonator.

First embodiment of the invention

The resonator 1 executed according to the first embodiment of the invention can be assimilated to the equivalent diagram of the figure 2 . This resonator 1 results from the coupling of a first resonator 2 with a second resonator 3. The first resonator 2 comprises a first mass of inertia 4 (here illustrated by a square mass) associated with a first spring 5 (illustrated here by a spring helical, one end of which is attached to the square mass and whose other end is attached to a fixed part 73 of the timepiece, for example to the plate). The second resonator 3 comprises a second mass of inertia 6 (here illustrated by a square mass) associated with a second spring 7 (illustrated here by a spring helical whose one end is attached to the square weight and the other end is attached to a fixed portion 74 of the timepiece, for example to a bridge). A third spring 8 (here represented by a helical spring) is disposed between the first (4) and second (6) masses of inertia for coupling said first (2) and second (3) resonators.

The Figures 3 to 6 illustrate a practical construction of the first embodiment of the invention. Here the first and second masses of inertia are respectively constituted by first and second balances 4 and 6, and the first, second and third springs are respectively first, second and third spiral springs 5, 7 and 8.

It can also be seen that, according to a preferred embodiment of the invention, the first and second resonators 2 and 3 are arranged coaxially inside the timepiece between a plate 11 and a bridge 17. The invention However, it is not limited to this arrangement, the two resonators being able to be arranged, for example, side by side in the timepiece.

More particularly and as it appears on the figure 4 , the first resonator 2 essentially comprises a first rocker 4 with which is associated a first spiral spring 5. This first resonator 2 is mounted on a first shaft 9 which pivots at its first end in a bearing 10 fixed in a plate 11 and its second end in a bearing 12 fixed in an intermediate bridge 13. The outer and inner turns of the first spiral spring 5 are respectively fixed on a peak 23 carried by the plate 11 and on an inner attachment point 28 fixed on the first shaft 9.

The second resonator 3 essentially comprises a second rocker 6 with which is associated a second spiral spring 7. This second resonator 3 is mounted on a second shaft 14 which pivots at its first end in a bearing 15 fixed in the intermediate bridge 13 and its second end in a bearing 16 fixed in a bridge 17. The turns outer and inner of the second spiral spring 7 are respectively fixed on a stud 25 carried by the bridge 17 and on an inner attachment point 26 fixed on the second shaft 14.

The examination of Figures 3 to 6 shows again that the first resonator 2 comprises a balance 4 having a larger diameter than the balance 6 of the resonator 3, which indicates that the frequency of the first resonator is lower than the frequency of the second resonator, provided of course that the torque developed by each of the spiral springs is substantially the same. Under these conditions, it will be understood that the escape mechanism will be linked to the first resonator, the one that is enslaved to the second to improve its resistance to disturbances. The figure 4 shows that the first shaft 9 which is attached to the first resonator 2 carries a plate 18 and a plate pin 19, the latter cooperating for example with an anchor which in turn cooperates with an escape wheel.

It remains to describe the coupling existing between the resonators 2 and 3. This coupling is achieved by means of a third spiral spring 8. figures 4 and 5 show that this spiral spring 8 comprises two windings 20 and 21 arranged in series and mounted on either side of the intermediate bridge 13. In this way, the inner turn of the first winding 20 is fixed to an internal point of attachment. 27 fixed on the second shaft 14 while the inner coil of the second winding 21 is fixed to an inner attachment point 22 fixed on the first shaft 9, the outer turns of said windings being connected to each other by a ribbon 75.

The invention is not limited to what has just been said. Indeed the third spiral spring may have only one winding. In this case, and without it being necessary to show it by a drawing, the inner turn of this single winding is fixed to an attachment point 27 fixed on the second shaft 14 while the outer turn is fixed to a peak carried by the first pendulum 4.

We will now briefly show the advantage of coupling two resonators oscillating one low frequency and the other higher frequency in order to make more stable oscillating resonator low frequency.

A mechanical resonator composed of a mass and a spring is characterized by the weight of its mass m and the constant of its spring k which are expressed, in the case of the equivalent diagram of the figure 2 and in orders of magnitude relating to watchmaking, in milligram (mg) and micronewton per meter (μN / m), respectively. In the present case the mass m is a balance characterized by its mass of inertia expressed in milligram square centimeter (mg · cm ² ), and the constant k is relative to a spiral spring which is characterized by its unit torque expressed in micronewtonmeter by radian (μN · m / rad). That being so, the frequency of a resonator is written: $$f=\frac{1}{2\pi }\sqrt{\frac{k}{m}}$$

To take an example taken from a current clock gauge on the market, we have ak = 1 · 10 ^-6 Nm / rad and m = 16 · 10 ^-10 kg · m ² , which results in the frequency f = 4 Hz.

relation dans laquelle ω₁ est la pulsation normale du premier résonateur seul, ω_1p la pulsation perturbée du premier résonateur seul, Ω₁ la pulsation normale du système couplé et Ω_1p la pulsation perturbée du système couplé. On comprendra que si le facteur de stabilisation S est égal à deux, la pièce d'horlogerie est deux fois plus précise avec un système de résonateurs couplés qu'avec le premier résonateur seulement. Par exemple, une pièce d'horlogerie ayant une avance de dix secondes par jour n'en n'aura plus que cinq pour la même période.The central question is whether the presence of the second higher frequency resonator stabilizes the frequency of the first low frequency resonator. This effect is taken into account by the stabilization factor S defined by: $$S=\frac{{\mathrm{<figure-callout\; id="1"\; label="\omega "\; filenames="imgf0003.png,imgf0005.png"\; state="\{\{state\}\}">\omega </figure-callout>}}_{1p}-{\mathrm{<figure-callout\; id="1"\; label="\omega "\; filenames="imgf0003.png,imgf0005.png"\; state="\{\{state\}\}">\omega </figure-callout>}}_1}{{\mathrm{<figure-callout\; id="1"\; label="\omega "\; filenames="imgf0003.png,imgf0005.png"\; state="\{\{state\}\}">\omega </figure-callout>}}_1}\frac{{\mathrm{<figure-callout\; id="1"\; label="\Omega "\; filenames="imgf0003.png,imgf0005.png"\; state="\{\{state\}\}">\Omega </figure-callout>}}_1}{{\mathrm{<figure-callout\; id="1"\; label="\Omega "\; filenames="imgf0003.png,imgf0005.png"\; state="\{\{state\}\}">\Omega </figure-callout>}}_{1p}-{\mathrm{<figure-callout\; id="1"\; label="\Omega "\; filenames="imgf0003.png,imgf0005.png"\; state="\{\{state\}\}">\Omega </figure-callout>}}_1}$$

in which ω ₁ is the normal pulsation of the first resonator alone, ω _1p the disturbed pulsation of the first resonator alone, Ω ₁ the normal pulsation of the coupled system and Ω _1p the disturbed pulsation of the coupled system. It will be understood that if the stabilization factor S is equal to two, the timepiece is twice as accurate with a system of coupled resonators as with the first resonator only. For example, a timepiece with a lead of ten seconds a day will have only five for the same period.

• Résonateur 1 : m₁=21 mg·cm², k₁=1µN·m/rad d'où f₁ =3,47Hz
• Résonateur 2 : m₂=21 mg·cm², k₂=5µN·m/rad d'où f₂=7,75Hz

ces résonateurs étant couplés par un ressort spiral de constante k_c.We will now take a practical example using first and second resonators having the following characteristics:

• Resonator 1: m ₁ = 21 mg · cm ² , k ₁ = 1μN · m / rad hence f ₁ = 3.47 Hz
• Resonator 2: m ₂ = 21 mg · cm ² , k ₂ = 5 μN · m / rad hence f ₂ = 7.75 Hz

these resonators being coupled by a spiral spring constant k _c .

If we refer to Figures 2 and 4 , The low frequency resonator 1 bears the reference 2, m ₁ being the rocker arm 4, k ₁ being the constant of the spiral spring 5. The resonator 2 higher frequency carries the reference 3, m ₂ being the balance wheel 6, k ₂ being the constant of the spiral spring 7. It will be noted however that in this practical example the rockers are of the same dimensions, which is not the case of the rockers of the figure 4 the second resonator having a higher natural frequency due to its higher spring constant.

Analytical calculations made it possible to establish the graphs of Figures 7 and 8 on the basis of the practical data set out above.

The figure 7 is a graph showing the evolution of the natural frequencies f ₁ and f ₂ of the coupled resonator system as a function of the constant k _c of the spiral spring which couples the two resonators.

The figure 8 is a graph showing the evolution of the stabilization factor S as a function of the constant k _c of the spiral spring 8 which couples the two resonators.

The curve S _m shows the stabilizing effect resulting from the coupling of the first and second resonators on disturbances affecting the mass of inertia of the balance of the first low frequency resonator when the constant k _c is varied. This effect is not very pronounced, which is relatively unimportant, the mass of inertia of the balance being little influenced by external disturbances.

The curve S _k shows the stabilizing effect that results from the coupling of the first and second resonators on disturbances affecting the torque of the spiral spring of the first resonator, ie that caused by the exhaust system. It can be seen that for a value of k _c of 1 μNm / rad, the stabilization factor is not far from reaching 2, which is positive, since it is mainly on the spiral spring that the disturbances due to inter alia spring position, shock and temperature variations.

Example of embodiment of a resonator not forming part of the claimed object.

One can assimilate the resonator 40 executed according to this example to the equivalent diagram of the figure 9 . The resonator 40 results from the coupling of a first resonator 41 with a second resonator 42. The first resonator 41 comprises a first mass of inertia 43 (here illustrated by a square mass) associated with a first spring 44 (illustrated here by a spring helical, one end of which is attached to the square mass and whose other end is attached to a fixed part 73 of the timepiece, for example to the plate). The second resonator 42 comprises a second mass of inertia 45 (here illustrated by a square mass) associated with a second spring 46 (here illustrated by a helical spring whose one end is attached to the square mass 43 and whose other end is attached to the square mass 45). This second spiral spring 46 thus connects the first (43) and second (45) masses of inertia to couple said first (41) and second (42) resonators. In fact the spring 46 plays a dual role here: that of forming the second resonator 42 and that of coupling the first and second resonators 41 and 42.

If in the first mode represented by the figure 2 , we remove the third spring 7 and its attachment to a fixed point 74, we find the equivalent diagram of the figure 9 which illustrates this example and which will be explained now in detail with the help of Figures 10 to 13 .

The Figures 10 to 13 illustrate a practical construction of the exemplary embodiment of a resonator. Here, as has been said with respect to the first embodiment of the invention, the first and second masses of inertia are constituted respectively by first and second balances 43 and 45, and the first and second springs are respectively first and second spiral springs 44 and 46.

We also see that the first balance 43 has a circular cage inside which is confined the second resonator 42 at higher frequency, said circular cage 43 forming with the first spiral spring 44 the first resonator 41 low frequency.

As the cup of the figure 11 , the circular cage 43 forming the first balance is equipped with a first cheek 47 carrying a first pivoting pin 48 in a bearing 49 fixed in a plate 50. This first pin 48 carries a plate 51 and a plateau pin 52, the latter cooperating for example with an anchor which cooperates in turn with an escape wheel. The circular cage 43 is also equipped with a second cheek 53 carrying a second pin 54 pivoting in a bearing 55 fixed in a bridge 56. The bridge 56 is equipped with a pin 57 to which is fixed the outer turn of the first spiral spring 44 , the inner turn of said first spiral spring 44 being fixed to an internal attachment point 58 fixed on the second pin 54. The circular cage or balance 43 and the spiral spring 44 form the first resonator 41 at low frequency, the one of which it s is to improve performance.

The figure 11 shows again that the second balance 45 and spiral spring 46 constituting the second resonator 42 - and which is confined in the cage 43 - are supported by a shaft 59 pivoting at its first end in a bearing 60 fixed in the first cheek 47 of the cage 43 and at its second end in a bearing 61 fixed in the second cheek 53 of the cage. In addition, the outer and inner turns of the second spiral spring 46 are respectively fixed to a peg 62 carried by the second cheek 53 of the cage 43 and to an inner attachment point 63 fixed on the shaft 59.

The examination of Figures 10 to 12 shows that the first resonator 41 comprises a rocker or cage 43 having a larger diameter than the rocker 45 of the second resonator 42, which indicates that the frequency of the first resonator is lower than the frequency of the second resonator, the torque developed by each spiral springs being equal elsewhere. It will therefore be understood that the escape mechanism will be linked to the first resonator, the one that is to enslave the second to improve its resistance to disturbances.

The advantages of coupling two oscillating resonators, one at low frequency and the other at higher frequency, in order to improve oscillating resonator performance at low frequency. We will not return here to the developed theory that also applies to the second embodiment just described.

• résonateur 1 : m₁ = 20mg·cm², k₁=variable
• résonateur 2 : m₂ = 6,4 mg·cm ² k_c=0,4µN·m/rad, k₂=0

However, we will take a practical example:

• resonator 1: m ₁ = 20mg · cm ² , k ₁ = variable
• resonator 2: m ₂ = 6.4 mg · cm ² k _c = 0.4μN · m / rad, k ₂ = 0

If we refer to Figures 9 and 11 , The low frequency resonator 1 is referenced 41, m ₁ being the balance, or the cage 43, k ₁ being the constant of the spiral spring 44 and the resonator 2 at a higher frequency has the reference 42, m ₂ being the balance 45 , k _c being the constant of the spiral spring 46, k _c being also the spiral spring which couples the two resonators.

On the basis of the practical data set out above, analytical calculations have made it possible to Figures 14 and 15 . The chosen variable is no longer k _c as in the first execution mode but k ₁ which seemed to be the most decisive parameter.

The figure 14 is a graph showing the evolution of the natural frequencies f ₁ and f ₂ of the coupled resonator system as a function of the constant k ₁ of the spiral spring 44 constituting the first resonator 41.

The figure 15 is a graph showing the evolution of the stabilization factor - which has been defined above with respect to the first embodiment - as a function of the constant k ₁ of the spiral spring 44 affecting the first resonator 41.

The curve S _m shows the stabilizing effect resulting from the coupling of the first and second resonators 41 and 42 on disturbances affecting the mass of inertia of the balance of the first low frequency resonator 41 when the constant k ₁ of the spiral spring is varied. 44. This effect is much more pronounced than that observed with respect to the embodiment.

The curve S _k shows the stabilizing effect which results from the coupling of the first and second resonators 41 and 42 on disturbances affecting the torque of the spiral spring 44 of the first resonator 41. It can be seen that for a value of k ₁ of 2 μN · m / rad, the stabilization factor S is of the order of 2.5.

conclusions

The foregoing has shown that the performances of a first low-frequency spiral balance resonator, the latter being of the order of 2 to 6 Hz, can be improved if it is coupled to a second resonator with a balance-spring balance. high frequency, the latter being of the order of 10Hz. The first low frequency resonator is more sensitive to certain disturbances due for example to wearing, or to shocks than the second resonator to more high frequency. One can imagine to compensate the thermal variation and / or the isochronism defect of the first resonator by those of the second. Moreover, the first resonator cooperates easily with a usual exhaust system whereas this is not the case of the second resonator. It is therefore logical to couple the two resonators in question to benefit from both the good adaptation of the first to the exhaust system and the good insensitivity of the second to the disturbances mentioned above.

Claims (5)

1. Resonator (1) for a timepiece resulting from coupling a first, low frequency resonator (2) with a second, higher frequency resonator (3), wherein the first resonator (2) has a first inertia mass (4) associated with a first spring (5), wherein the second resonator (3) has a second inertia mass (6) associated with a second spring (7), wherein a third spring (8) is arranged between the first (4) and second (6) inertia masses to couple said first (2) and second (3) resonators, said resonator being characterized in that the first and second inertia masses are respectively formed by first (4) and second (6) balances and in that the first, second and third springs are respectively first (5), second (7) and third (8) balance springs.
2. Resonator according to claim 1, characterized in that the first (2) and second (3) resonators are arranged coaxially inside the timepiece.
3. Resonator according to claim 2, characterized in that the first resonator (2) is mounted on a first arbour (9) arranged so as to pivot at the first end thereof in a bearing (10) fixed in a bottom plate (11) and at the second end thereof in a bearing (12) fixed in an intermediate bridge (13), the outer and inner coils of the first balance spring (5) of said first resonator (2) being arranged so as to be respectively fixed to a balance spring stud (23) carried by the bottom plate (11) and on an inner point of attachment (28) fixed to said first arbour (9) and in that the second resonator (3) is mounted on a second arbour (14) arranged so as to pivot at the first end thereof in a bearing (15) fixed in said intermediate bridge (13) and at the second end thereof in a bearing (16) fixed in a bridge (17), the outer and inner coils of the second balance spring of said second resonator (3) being arranged so as to be respectively fixed one a balance spring stud (25), carried by the bridge (17) and on an inner point of attachment (26) fixed to said second arbour (14).
4. Resonator according to claim 3, characterized in that the first arbour (9) carries a roller (9) and an impulse pin (19), said impulse pin being arranged so as to cooperate with an escape mechanism.
5. Resonator according to claim 3, characterized in that the third balance spring (8) includes two windings (20, 21) arranged in series and arranged so as to be mounted on either side of the intermediate bridge (13), the inner coil of the first winding (20) being secured to an inner point of attachment (27) fixed to the second arbour (14) and the inner coil of the second winding (21) being secured to an inner point of attachment (22) fixed to the first arbour (9).

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