EP3226080B1 - Barrel system for a timepiece - Google Patents

Description

Technical area

The present invention relates to the field of watchmaking. It relates more particularly to a self-lubricated barrel system for a timepiece.

State of the art

In a traditional mechanical watch, the energy allowing its operation is typically stored in one or more barrels acting as a driving source for the regulating member. This energy is transmitted between the barrel and the regulating member by means of a finishing gear, which also serves to drive the display members.

A conventional barrel has a hollow drum coaxial with a barrel shaft, and a cover which serves to close the drum. Inside the drum is a barrel spring, spiral, which links the shaft to the inner wall of the drum. In order to ensure the connection between the outer end of the spring and the drum, the latter is provided with a flange, attached to, or come integrally with, said outer end. This flange can be of the fixed type, which is typically intended for a manual winding watch, or sliding, which is typically intended for a self-winding watch. In order to operate the timepiece, this spring is loaded and applies a torque allowing the maintenance of the movement by unwinding. If the torque supplied by the spring is not sufficiently constant, the isochronism of the watch is not guaranteed.

Inside the barrel, there are several surfaces which come into contact with each other and therefore generate friction. For example, the coils of the spring may rub against each other, the flange as well as the outer coil may rub against the inner wall of the drum, and the upper and lower faces of the spring may rub against the cover and / or against the flat inner face of the drum. drum.

In order to limit this friction, the barrel is traditionally greased. Such lubrication, usually organic, is very sensitive to external factors such as changes in temperature, slight variations in humidity, and chemical changes in the grease which occur over time. The latter, which lead to a degradation of the tribological properties of the lubricant, require that the barrel be subjected to regular periodic maintenance, typically over a period of a few years. Such maintenance requires disassembly of the part, as well as complete disassembly and cleaning of the barrel and the spring.

Some solutions have already been presented aimed at avoiding the use of a wet lubricant, by reducing the coefficient of friction between the elements of the barrel system which rub against each other (such as the flange and the wall of the drum) by depositing self-lubricating materials on at least one of these components. In this regard, the document GB 864 531 reveals a barrel spring coated with a thin layer (<1µm) of a synthetic resin or plastic material which constitutes a dry lubricant. The documents US 2,979,417 and GB 987 702 offer similar solutions. The document FR 1 249 231 proposes, on the other hand, to use a layer of a salt of stearic acid for the same purpose, and the document EP 2 270 612 offers a polycrystalline diamond coating. Finally, the documents WO 02/204836 , WO 2013/011032 and US 6,755,566 provide a diamond-like carbon deposit ("Diamond-Like Carbon", DLC), which is a form of amorphous carbon, on a barrel spring, or on the wall of a barrel drum, as the case may be. In the case of the document WO 2009/043391 , the two surfaces of the barrel spring which come into contact with each other are each provided with a layer of DLC, each layer of which has a different orientation of the crystal planes.

Although these dry lubrication solutions represent tribological advances over a standard greased barrel, they are not entirely satisfactory in terms of the coefficient of friction and the wear of the components that slide over each other and are therefore subject to friction. Consequently, it is sometimes necessary to add grease to maintain satisfactory operating conditions.

The aim of the invention is therefore to provide a barrel system in which the aforementioned defects are at least partially overcome.

Disclosure of the invention

More specifically, the invention relates to a barrel system for a timepiece. This system comprises a barrel comprising a barrel shaft, a barrel drum coaxial with said shaft, as well as a barrel spring disposed inside said barrel. The drum can, if necessary, be closed by a cover. In known manner, said spring is connected by a first end, internal, to said shaft, and connected by a second end, external, to said drum by means of a flange. The latter also comprises a first layer of amorphous carbon, of a first type, on at least part of its surface.

According to the invention, at least one component of said barrel, such as for example the inner cylindrical wall of the drum, the surface of the cover (if present) and / or of the drum facing the spring, a bung arranged on the shaft, comprises a second layer of amorphous carbon, of a second type different from said first type, on at least part of its surface intended to be in contact with said barrel spring. Said first layer and said second layer are arranged to at least partially come into contact with each other, and exhibit differences in terms of proportions of sp ² -sp ³ hybridizations of their carbon atoms. As such, the proportions of sp ² -sp ³ hybridizations of one of said first layer and said second layer are between 5% and 40% of sp ² hybridization carbon atoms and between 60% and 95 % of sp ³ hybridization carbon atoms, while the proportions of sp ² -sp ³ hybridizations of the other layer are between 60% and 90% of sp ² hybridizations carbon atoms and between 10% and 40% sp ³ hybridization carbon atoms. More particularly, the proportions of sp ² -sp ³ hybridizations of one of said first layer and said second layer are between 5% and 20% of sp ² hybridization carbon atoms and between 80% and 95% sp ³ hybridization carbon atoms, while the proportions of sp ² -sp ³ hybridizations on the other layer are between 65% and 75% sp ² hybridization carbon atoms and between 25% and 35% sp ³ hybridization carbon atoms.

Surprisingly, the use of two layers of amorphous carbon exhibit dissimilarities in terms of the proportions of sp ² -sp ³ hybridizations of their carbon atoms substantially reduces the coefficient of friction and, therefore, makes it possible to avoid any application. fat. The performance as well as the life of the system are thus improved, and the assembly of the system is facilitated because no grease has to be applied.

It is noted that the tests showed a preference (not mandatory) for a higher proportion of sp ³ hybridization for the first layer, that is to say that located on the spring, compared to the proportion of hybridization sp ³ in the second layer.

Advantageously, said first layer is placed on at least part of said spring flange and / or at least part of the turns of said spring. Furthermore, said second layer is advantageously arranged on at least part of the internal wall of the drum, and / or at least part of the flat internal face of said drum opposite said spring and / or at least part of a bung. that said shaft comprises, and / or at least part of a surface of said cover facing said spring.

In one embodiment, said first layer is disposed on the spring flange as well as on the surface of the coils of the spring, said second layer being disposed (i) on at least part of the inner wall of the drum and is intended to be in contact with said first layer on the flange and (ii) on a bung that comprises the barrel shaft and to which the inner end of the spring is linked.

In another particular embodiment, said first layer is arranged on said spring flange as well as on the outer surface of the coils of the spring (that is to say the surface facing the wall of the drum), said second layer is disposed on at least part of the internal wall of the drum and is intended to be in contact with said first layer, a third layer of amorphous carbon, of a third type different from said first type, is arranged on the internal surface of the turns of the spring (i.e. the surface facing the barrel shaft). Said third type is preferably identical or substantially similar to said second type, but it may also be different from the two types mentioned above.

Consequently, the advantages of the invention can be obtained not only at the level of the contact between the flange and the wall of the drum, but also between the turns of the spring. Furthermore, a fourth layer of amorphous carbon, of a fourth type different from said third type, can be arranged on a bung that the barrel shaft comprises and to which the inner end of the spring is linked, said fourth type being of preference substantially similar to said first type. The invention can therefore be applied to any contact between the spring, the wall of the drum and the shaft.

In the case of a system comprising a third and a fourth layer as mentioned above, the proportions of sp ² -sp ³ hybridizations of one of said third layer and said fourth layer may be between 5% and 40 % sp ² hybridization carbon atoms and between 60% and 95% sp ³ hybridization carbon atoms, while the proportions of sp ² -sp ³ hybridizations of the other of said third layer and said fourth layer may be between 60% and 90% of sp ² hybridization carbon atoms and between 10% and 40% of sp ³ hybridization carbon atoms. More particularly, the proportions of sp ² -sp ³ hybridizations of one of said third layer and said fourth layer may be between 5% and 20% of sp ² hybridization carbon atoms and between 80% and 95% of sp ³ hybridization carbon atoms, while the sp ² -sp ³ hybridization proportions of the other layer can be between 65% and 75% of sp ² hybridization carbon atoms and between 25% and 35% sp ³ hybridization carbon atoms.

Advantageously, each of said layers has a thickness of between 100 nm and 3 μm, preferably between 250 nm and 950 nm.

Brief description of the drawings

• La fig. 1 est une vue schématique en coupe d'un système de barillet selon un mode de réalisation exemplaire de l'invention ;
• La fig. 2 est une diagramme de phase ternaire du carbone amorphe ;
• La fig. 3 est une vue schématique d'un détail du principe de l'invention sous forme schématique ; et
• Les fig. 4 et 5 sont des vues schématiques de variantes de l'invention.

Other details of the invention will emerge more clearly on reading the following description, given with reference to the appended drawings in which:

• The fig. 1 is a schematic sectional view of a barrel system according to an exemplary embodiment of the invention;
• The fig. 2 is a ternary phase diagram of amorphous carbon;
• The fig. 3 is a schematic view of a detail of the principle of the invention in schematic form; and
• The fig. 4 and 5 are schematic views of variants of the invention.

Embodiments of the invention

The figure 1 schematically illustrates an exemplary barrel system 1 according to the invention. As regards the overall construction of the barrel system 1, it is quite conventional, any known construction can be taken. In the illustrated variant, the system 1 comprises a barrel 3, the latter comprising a drum 5 mounted in rotation and coaxial with reference to a barrel shaft 7 in a known manner. The drum 5 has peripheral teeth 9, but can also be smooth in order to be used in combination with a rocket system. The drum 5 is typically closed by means of a cover 11, which may be secured either to the drum 5 or to the barrel shaft 7. A ratchet wheel 13 is also provided, integral in rotation with the barrel shaft 7. According to a known variant of construction, the ratchet wheel 13 can be positioned on the shaft 7 in order to act as a cover 11.

Inside the barrel 3, in the space defined by the drum 5 and the cover 11 is housed a barrel spring 15. The inner end 15a of the spring 15 is linked to a bung 14 arranged on the shaft, and its outer end 15b has a flange which connects it to the cylindrical inner wall 5a of the drum 5, in a known manner. The flange 15b may come integrally with the spring 15, or may be an additional part attached to the outer end in known manner.

The main sources of friction in such a barrel are as follows. First, each of the turns of the spring 15 can rub against its neighboring turn when the spring unwinds. The flange 15a and the outer coil of the spring 15 the inner end 15a of the spring 15 can rub against the plug 14 of the shaft 7. Moreover, if the spring 15 can bear against the flat inner face 5b or the face of the cover 11 opposite the spring 15, this can also cause friction. At least part of the spring 15 and part of the barrel 3 are coated with a self-lubricating layer, as will be explained in more detail later.

Before detailing the way in which the self-lubricating layers of the system 1 according to the invention differ from those of the prior art, it is first necessary to examine the properties of amorphous carbon (DLC).

To this end, the figure 2 schematically represents a phase diagram of amorphous carbon. Carbon atoms can be organized according to several different geometries, exhibiting different covalent bond orientations. In crystalline diamond, atoms organize themselves according to a face-centered cubic structure, in which the covalent bonds orient themselves according to tetrahedra. These bonds involve sp ³ hybridizing carbon atoms, and this diamond structure is on the upper tip of the diagram. In the case of graphite, the atoms organize themselves in sheets. In each sheet, the covalent bonds orient in regular hexagons lying in a single plane, and each sheet is linked to its neighbor by Van der Waals forces. The hybridization of the carbon atoms in each sheet is of the sp ² type. This structure is on the left tip of the diagram. The third point, right, of the diagram represents hydrogen.

• a-C : carbone amorphe substantiellement dépourvu d'hydrogène et présentant une hybridation dominante du type sp² (typiquement 60-90% sp² et 10-40% sp³) ;
• ta-C : carbone amorphe tétraédrique substantiellement dépourvu d'hydrogène et présentant une hybridation dominante du type sp³ (typiquement 60-95% sp³ et 5-40% sp²) ;
• a-C:H : carbone amorphe comprenant une proportion non négligeable d'hydrogène et présentant une hybridation dominante du type sp² (typiquement 60-90% sp² et 10-40% sp³ de la proportion du total qui est constitué de carbone) ; et
• ta-C:H : carbone amorphe tétraédrique comprenant une proportion non négligeable d'hydrogène et présentant une hybridation dominante du type sp³ (typiquement 60-95% sp³ et 10-40% sp² du total du carbone).

Starting at the tip of the diagram representing hydrogen, which is gaseous at normal temperature and pressure, increasing the proportion of carbon in any type of hybridization, we go through a strip of polymers (indicated by "Poly") , and then we enter the domain of hydrogenated carbon. Areas of particular interest for their tribological properties are indicated on the diagram by ovals schematically indicating the respective proportions of hydrogen, sp ³ and sp ² hybridizations, which represent (from left to right):

• aC: amorphous carbon substantially devoid of hydrogen and exhibiting a dominant hybridization of the sp ² type (typically 60-90% sp ² and 10-40% sp ³ );
• ta-C: tetrahedral amorphous carbon substantially devoid of hydrogen and exhibiting dominant sp ³ hybridization (typically 60-95% sp ³ and 5-40% sp ² );
• aC: H: amorphous carbon comprising a not insignificant proportion of hydrogen and exhibiting a dominant hybridization of the sp ² type (typically 60-90% sp ² and 10-40% sp ³ of the proportion of the total which consists of carbon); and
• ta-C: H: tetrahedral amorphous carbon comprising a not insignificant proportion of hydrogen and exhibiting a dominant hybridization of the sp ³ type (typically 60-95% sp ³ and 10-40% sp ² of the total carbon).

These DLC layers can be deposited by PVD (Physical Vapor Deposition) process, or by CVD (Chemical Vapor Deposition), PACVD / PECVD (plasma assisted CVD), or any other technique allowing such a deposition. By varying the parameters during the deposition, layers having the proportions of sp ² -sp ³ hybridizations as well as the desired proportion of hydrogenation (s), can be deposited, and then determined by current measurements. This is well known in the literature, and therefore should not be described in more detail here.

The inventors have determined that, surprisingly, if a DLC layer is deposited on each of the surfaces of two components of the barrel system 1 which rub against each other, and that these two DLC layers are different l On the other hand, the coefficient of friction is significantly reduced. This eliminates any wet lubrication, and also generates a significant reduction in component wear.

• Aucun problème de répartition de la graisse. En effet, la graisse doit être bien répartie sur toutes les surfaces qui sont en contact, et a tendance à fluer de manière indésirable. Plus particulièrement, il existe un effet de décantage, par lequel la graisse a tendance à fluer et se retrouver en bas du barillet, ainsi qu'un phénomène de migration de graisse entre les spires du ressort qui peut les faire coller les unes contre les autres grâce à la tension superficielle de la graisse.
• Montage simplifié, car aucune étape d'application de graisse n'est plus nécessaire.
• Effet lubrificateur dès le début. Par contre, la graisse nécessite un certain temps d'utilisation afin de se répartir de manière homogène, ce qui engendre une phase de rodage au début du fonctionnement de la pièce, et après que la pièce soit laissée inutilisée pendant un laps de temps prolongé.
• Protection des pièces contre les attaques environnementales, car les surfaces revêtues de DLC sont isolées de l'environnement par une couche qui est exempte d'oxygène. L'étape de nickelage communément effectuée dans ce but est simplement remplacée par celle du dépôt de DLC, et par conséquent le nombre d'étapes de fabrication du ressort reste inchangé, et les composants du barillet peuvent être des composants standards à l'exception des revêtements en carbone amorphe.
• Protection contre l'usure entre les spires, si la totalité des surfaces du ressort qui sont soumises à des frottements est revêtue.

In general, dry lubrication using DLC has the following advantages in the context of a barrel system:

• No fat distribution problem. Indeed, the grease must be well distributed over all the surfaces which are in contact, and has a tendency to creep in an undesirable manner. More particularly, there is a decanting effect, by which the grease tends to flow and end up at the bottom of the barrel, as well as a phenomenon of grease migration between the coils of the spring which can make them stick together. thanks to the surface tension of the fat.
• Simplified assembly, as no grease application step is required.
• Lubricating effect from the start. On the other hand, the grease requires a certain time of use in order to be evenly distributed, which generates a running-in phase at the start of the operation of the part, and after the part is left unused for a prolonged period of time.
• Protection of parts from environmental attack, as DLC coated surfaces are isolated from the environment by a layer that is free of oxygen. The nickel plating step commonly carried out for this purpose is simply replaced by that of DLC deposition, and therefore the number of manufacturing steps of the spring remains unchanged, and the components of the barrel may be standard components except for the amorphous carbon coatings.
• Protection against wear between the turns, if all the surfaces of the spring which are subject to friction are coated.

More particularly, the figure 3 schematically illustrates part of a barrel system 1 according to the invention. In this variant, a first layer of amorphous carbon 17 of a first type is deposited on the surface of the flange 15b which comes into contact with the cylindrical internal surface 5a of the drum of the barrel 5. This cylindrical internal surface 5a is also coated with a second layer of amorphous carbon 19 of a second type, different from the first type. This same principle can also be applied to the plug 14 and to the inner end 15a of the spring 15, and / or to the upper and / or lower surfaces of the spring 15, if these surfaces risk coming into contact with the cover 11 or drum 5 respectively. In particular, said first layer 17 can be deposited at least two places, in particular on the surfaces of the two ends 15a, 15b intended to be in contact with the plug 14 and the wall 5a of the drum 5 respectively, said second layer 19 being deposited on the corresponding surfaces of the plug 14 and of the wall 5a drum 5.

Typically, these two layers 17, 19 differ in their proportions in sp ² -sp ³ hybridizations of the amorphous carbon, while other differences are still possible, particularly in the degree of hydrogenation.

One of the two layers 17, 19 contains for example a proportion of sp ² hybridization of between 5% and 40%, preferably between 5% and 20%, more preferably substantially 5, 6 or 7% (more generically ta -C or ta-C: H) and a proportion of sp ³ hybridization of between 60% and 95%, preferably between 80% and 95%, more preferably substantially 95% (more generically aC or aC: H). The other of the layers 19, 17 has for example a proportion of sp ³ hybridization of between 5% and 40%, preferably between 25% and 35%, more preferably substantially 30% and a proportion of sp ² hybridization of between 60% and 95%, preferably between 65% and 75%, more preferably substantially 70%.

In this case, the layer comprising a major proportion of sp ³ hybridization (e.g., ta-C or ta-C: H) is preferably the first layer 17 of the spring 15, and one comprising a major proportion of sp hybridization ² (eg aC or aC: H) is preferably the second layer 19 on the barrel 3, but the reverse arrangement is also possible.

It is also possible that one of the layers 17, 19 has a hybridization ratio of about 50% sp ² and 50% sp ³ , hydrogenated or not, while the other layer 19, 17 essentially comprises ta- C, ta-C: H, aC or aC: H.

The first layer 17 can advantageously be deposited over the entire surface of the spring 15, including its flange 15b, which simplifies its manufacture and avoids carrying out a masking step before depositing the first layer 17 (see below). This variant is illustrated on the figure 4 , and is the easiest to manufacture. Furthermore, if the spring 15 risks rubbing against the cover 11 or the bottom of the drum 5, said second layer 19 may also extend over these surfaces. A second layer 19 can also be deposited on the bung 14.

Even more advantageously, the variant illustrated in figure 5 provides the first layer not only on the surface of the flange 15a intended to be in contact with the surface 5a of the drum 5, but on the entire contiguous surface, that is to say on the outer surface of the set of turns of the spring 15. The opposite surface, that is to say the inner surface of each turn facing the barrel shaft 7, is coated with a third layer 21 of DLC, of a third type different from said first type. This third type may be substantially similar to said second type, or different from the first and second types. Those skilled in the art know how to deposit these two different layers 17, 21, for example by means of successive conventional masking and deposition steps, carried out for example before slipping of the spring 15, or alternatively carried out with the spring 15 supported. stretched out and lying in a jig.

At the center of the spring, the plug 14 may include a fourth layer of amorphous carbon 23, dissimilar to the third layer 21, which may for example be of a type similar to said first type. Alternatively, in view of the fact that the contact between the spring and the bung 14 is less critical than between the flange 15b and the drum 5, the fourth layer of DLC may be similar to said second type, or may be omitted.

With regard to the upper and lower faces of the spring 15, if they risk rubbing against the cover 11 or the bottom of the drum 5, the same considerations as those discussed in connection with the figure 4 also apply here.

The variant of figure 5 has the advantage that each surface which comes into contact with another surface rubs against a surface having a layer of dissimilar DLC, therefore maximizing the friction reduction effects brought about by the invention.

For all the variants mentioned above, the layers 17, 19, 21, 23 typically have a thickness between 100 nm and 3 μm, ideally between 250 nm and 950 nm, and are deposited by one of the methods mentioned above. above (PVD, CVD, PACVD, PECVD ...) at a temperature typically between 80 ° C and 220 ° C. These temperatures are sufficiently moderate so as not to interfere with the hardening of the spring 15.

The advantages conferred by the invention have been demonstrated experimentally. The Applicant has submitted two types of barrel systems 1, having their spring 15 and the internal wall 5a of their drum 5 coated according to the variant of the figure 4 of the present invention, to accelerated aging tests corresponding to twenty-eight years of wear. Two types of barrels 3 were tested: the first barrel 3 had a fixed flange 15b (also called "manual barrel"), while the second barrel 3 had a sliding flange 15b ("automatic barrel"). The measurements and aging were specific for each of them, so as to best represent the actual conditions of use.

These accelerated aging tests have shown that the resistance to wear of the barrel systems according to the invention is much greater than that of standard barrels. After the aging phase corresponding to 28 years of use under standard conditions, the barrel systems 1 according to the invention show no decrease in their functional properties. In addition, after the tests, the internal wall 5a of the drum 5 had only undergone very moderate wear, despite significant stress during the aging cycles. No material removal or formation of wear particles was observed.

Traditional barrel systems 1 have, for their part, suffered a loss of functionality from five to seven years and therefore require service every two to five years, of course involving often preventive dismantling of the watch movement. It should also be noted that the accelerated aging tests are to the advantage of standard barrel systems 1, because the grease does not have time to age, and the high rotation speeds used promote good distribution. The interval between services of the movement is then advantageously increased by virtue of the present invention.

Although the invention has been described above in connection with specific embodiments, additional variants can also be envisaged without departing from the scope of the invention as defined by the claims. Furthermore, the principle of the invention can also be used for other components of a watch movement which are not the subject of the claims, such as, for example, a finger on a column wheel, toothings of mobiles, cogs. and gears, forks and pegs entries or even driving fingers.

Claims (8)

1. Barrel system (1) for a timepiece, said system (1) comprising:

   - a barrel (3) comprising a barrel shaft (7), a barrel drum (5) coaxial to said shaft (7);

   - a barrel spring (15) situated inside said barrel (3), said spring (15) being linked by a first end (15a) to said shaft, and linked by a second end (15b) to said drum via a bridle that said second end (15b) of said barrel spring (15) comprises, said spring (15) comprising a first layer of amorphous carbon (17) of a first type on at least a part of its surface,

   characterized in that at least one component (5, 7, 11) of said barrel (3) comprises a second layer of amorphous carbon (19) of a second type, different to said first type, on at least a part of its surface intended to be in contact with said barrel spring (15), said first layer (17) and said second layer (19) being arranged to enter into contact on at least a part of their respective surfaces, said second layer (19) differing from said first layer (17) with respect to proportions of hybridizations of the carbon atoms in each of said layers (17, 19),
   and in that the proportions of sp²-sp³ hybridizations of one of said first layer (17) and said second layer (19) lie between 5% and 40% of sp² carbon atoms and between 60% and 95% of sp³ hybridization carbon atoms, whereas the proportions of sp²-sp³ hybridizations of the other of said first layer (17) and said second layer (19) lie between 60% and 90% of sp² hybridization carbon atoms and between 10% and 40% of sp³ carbon atoms.
2. System (1) according to the preceding claim, wherein the proportions of sp²-sp³ hybridizations of one of said first layer (17) and said second layer (19) lie between 5% and 20% of sp² hybridization carbon atoms and between 80% and 95% of sp³ hybridization carbon atoms, whereas the proportions of sp²-sp³ hybridizations of the other of said first layer (17) and said second layer (19) lie between 65% and 75% of sp² hybridization carbon atoms and between 25% and 35% of sp³ covalent hybridization carbon atoms.
3. System (1) according to one of the preceding claims, wherein said first layer (17) comprises a proportion of sp³ hybridization carbon atoms higher than that of said second layer (19; 17).
4. System (1) according to one of the preceding claims, wherein said first layer (17) is situated on at least one of:

   - at least a part of said bridle (15a) of said spring (15);

   - at least a part of the turns of said spring (15) .
5. System (1) according to one of the preceding claims, wherein said second layer (17) is situated on at least one of:

   - at least a part of the inner wall (5a) of the drum (5);

   - at least a part of the flat internal face of said drum (5);

   - at least a part of a boss (14) that said shaft (7) comprises;

   - at least a part of a surface of a cover (1) that the barrel (3) comprises, said surface facing said spring (15).
6. System (1) according to Claims 4 and 5, wherein:

   - said first layer (17) is situated on said bridle (15b) of the spring (15) and on the outer surface of the turns of the spring (15);

   - said second layer (19) is situated on at least a part of the inner wall (5a) of the drum (5) and intended to be in contact with said first layer (17);

   - a third layer (21) of amorphous carbon, of a third type different to said first type, is situated on the inner surface of the turns of the spring (15), said third type being preferably substantially similar to said second type;

   - a fourth layer of amorphous carbon (23), of a fourth type different to said third type, is situated on a boss (14) that the shaft (7) comprises and to which the internal end (15a) of said spring is linked, said fourth type being preferably substantially similar to said first type,

   wherein the proportions of sp²-sp³ hybridizations of one of said third layer (21) and said fourth layer (23) lie between 5% and 40% of sp² carbon atoms and between 60% and 95% of sp³ hybridization carbon atoms, whereas the proportions of sp²-sp³ hybridization of the other of said third layer (21) and said fourth layer (23) lie between 60% and 90% of sp² hybridization carbon atoms and between 10% and 40% of sp³ hybridization carbon atoms.
7. System (1) according to the preceding claim, wherein the proportions of sp²-sp³ hybridizations of one of said third layer (21) and said fourth layer (23) lie between 5% and 40% of sp² hybridization carbon atoms and between 60% and 95% of sp³ hybridization carbon atoms, whereas the proportions of sp²-sp³ hybridizations of the other of said third layer (21) and said fourth layer (23) lie between 65% and 75% of sp² hybridization carbon atoms and between 25% and 35% of sp³ hybridization carbon atoms.
8. System (1) according to one of the preceding claims, wherein each of said layers (17, 19, 21, 23) has a thickness of between 100 nm and 3 µm, preferably between 250 nm and 950 nm.

Images