WO2020043982A1

WO2020043982A1 - Resin injection modulator, resin injection circuit and associated methods

Info

Publication number: WO2020043982A1
Application number: PCT/FR2019/051960
Authority: WO
Inventors: Christophe Ravey; Olivier FOUSSARD; Loïc SORGNARD; Adrien TOUZE; Romain Venat; Nicolas DROZ
Original assignee: Safran
Priority date: 2018-08-31
Filing date: 2019-08-26
Publication date: 2020-03-05
Also published as: FR3085298A1; FR3085298B1

Abstract

The invention concerns a resin injection modulator (54, 54') for a circuit injecting resin into a mould for manufacturing a composite part, in particular a turbomachine composite part, said modulator being connected to said resin injection circuit and having a resin storage capacity (58) with a determined volume configured to allow feeding of the manufacturing mould with said resin, and a means (60) for pressurising the resin provided by said capacity (58), characterised in that the means (60) for pressurising the resin comprise control means (66) configured to vary the pressure and the duration of application of said pressure.

Description

RESIN INJECTION MODULATOR, RESIN INJECTION CIRCUIT AND RELATED METHODS

The invention relates to a resin injection modulator for a resin injection circuit used for molding a composite part of a turbomachine, means for controlling such a modulator and a method for controlling these means. control.

STATE OF THE PRIOR ART

The RTM molding process, the initials of which refer to the acronym for Resin Transfer Molding, is a well-known process for producing parts in composite materials based on resin-impregnated fibers, these materials having resistance characteristics high as well as reduced mass.

In known manner, such a method, when applied to a part of a turbomachine such as a fan blade, comprises several successive operations. We start by weaving fibers to obtain a preform blank in three dimensions, then we cut said blank to obtain a preform having substantially the shape of the blade to be obtained. This preform is then placed in an injection mold, which is closed. Then, resin is injected in the liquid state while maintaining a pressure on the injected resin while the part is polymerized by heating.

The resins used are very fluid resins which are able to penetrate the fibers of the preform well, even when they are injected under reduced pressure. During the polymerization, under the effect of heat, the injected resin passes successively from the liquid state to the gelled state and finally to the solid state.

The continuous supply of the injection mold with resin under pressure is a primary requirement which aims to guarantee quality parts, without defects and without porosity. Indeed, the resin tending to degas during polymerization, it is necessary to maintain the pressure of the resin until complete polymerization of the part in order to prevent gas releases from compromising the integrity of the resin.

In conventional RTM injection systems, it is known to maintain the pressure in the injection circuit by means of an injector cylinder which is connected to the circuit, which contains the resin, and which ensures its distribution and delivery. under pressure. Once the resin is injected, it is then necessary to keep the injector cylinder connected to the injection circuit in order to maintain the pressure in the mold until complete polymerization of the resin.

Such systems have the disadvantage of not making it possible to achieve high production rates, since the time of immobilization of the injector cylinder in the injection circuit, for which it cannot be used on another circuit of injection, naturally depends on the time of polymerization of the resin.

In addition, in a system of this type, there is a non-negligible risk of polymerization of the resin inside the injection circuit, which makes the cleaning operations of said circuit complex. In particular, if the circuit is not heated, the resin contained in the injection circuit between the mold and the injector cylinder may have to polymerize before the resin contained in the mold, because the thickness of resin in the mold is more important. This freezing of the resin in the injection circuit, besides making it difficult to clean, does not allow transmission of the pressure exerted by the injector cylinder into the mold either, which results in the presence faults in the room.

To overcome these drawbacks, there has been proposed in document WO-2013/068666, a pressure maintenance device for a resin injection circuit in a manufacturing mold, comprising on the one hand isolation valves allowing '' isolate an injector cylinder upstream of the mold then to dismantle it, a resin trap arranged downstream of the mold, and on the other hand pressurizing means which are capable of injecting a gas under pressure into pipes of the injection circuit located between these valves and the mold downstream and out of the mold. To this end, T-pipes, interposed in the injection circuit between the isolation valves and the mold, are connected to a source of pressurized gas and the circuit has vertical pipes which are supposed to prevent the gas from reaching in the mold. The injector cylinder can then be disconnected from the circuit while the pressure is maintained on the resin by means of pressurization.

Such an injection circuit comprising such a pressure maintenance device is intended to allow the isolation of the injector cylinder. When the injector cylinder is insulated, it is no longer able to play the role of resin capacity. This design has the disadvantage that the injection circuit which is isolated is not capable of proposing on its own a capacity of resin capable of countering the shrinking of the resin in the mold during its polymerization. It can therefore result in the appearance of resin deficiencies on the finished part.

In addition, given the fluidity of the resin and the low capacity of resin contained in the circuit and between the source of pressurized gas and the mold when the injector cylinder is disconnected, pressurized gas from the gas source under pressure can also travel in the circuit and be injected into the mold in the form of bubbles, and consequently cause the appearance of defects on the surface of the part.

Finally, the cooling of the resin in the conduits between the source of gas under pressure and the mold tends to rapidly increase its viscosity, so that the source of gas under pressure, whose pressure is continuous, is quickly no longer able effectively solicit the resin to maintain it under pressure. STATEMENT OF THE INVENTION

In this context, the invention aims to propose a means enabling the pressure in the injection circuit to be maintained while avoiding any risk of parasitic injection of gas into the injection circuit, by means of an additional capacity of resin able to counter the phenomenon of resin shrinking in the mold during its polymerization, and able to pressurize the resin capable of opposing the rapid cooling of the resin in the pipes, preventing it from stagnate.

To this end, the invention proposes a resin injection modulator for a resin injection circuit in a mold for manufacturing a composite part, in particular a composite part of a turbomachine, said modulator being connected to said injection circuit. of resin and comprising a resin accumulation capacity of a determined volume which is configured to allow the manufacturing mold to be force-fed with said resin, and a means for pressurizing the resin supplied by said resin accumulation capacity , characterized in that the means for pressurizing the resin comprises control means configured to vary the pressure and the duration of application of said pressure.

According to other characteristics of the modulator:

the resin accumulation capacity communicates with the resin injection circuit and the resin pressurization means comprises a source of pressurized gas connected to said resin accumulation capacity and capable of subjecting the resin contained in said resin accumulation capacity at the pressure of said gas,

the source of gas under pressure comprises a booster, supplied by a source of air at constant pressure, of which an outlet air pressure is controlled by said control means,

- The modulator comprises at least one enclosure which is connected to said circuit and inside which is movably sealed a movable wall means, said movable wall means delimiting in said enclosure a first cavity, forming said resin accumulation capacity, said movable wall means forming the means for pressurizing the resin, and the control means controlling the movements of said movable wall means in the enclosure,

the control means actuate the movable wall means directly and mechanically,

the mobile wall delimits in the enclosure a second cavity and the control means comprise a source of pressurized gas connected to said second cavity and capable of subjecting the mobile wall means to the pressure of said gas,

- the control means are manually controlled,

the control means are automated and they are configured so that the means for pressurizing the resin follow a sequence of pressure stages each having a determined duration.

The invention also relates to a method for controlling means for controlling an injection modulator of the type described above during resin injection into a manufacturing mold, characterized in that it comprises a sequence of pressure levels comprising:

a first sub-sequence comprising at least a first pressure level at a first maximum level pressure,

- a second sub-sequence comprising at least a second pressure level at a second non-zero minimum level pressure,

- A third sub-sequence comprising at least a third pressure level at a third level pressure, intermediate between the first maximum level pressure and the second minimum level pressure.

According to another characteristic of the process, the third subsequence comprises an alternation of bearings at the third intermediate pressure and of at at the first maximum pressure, each said steps being of shorter duration than the first subsequence.

The invention will be better understood and other details, characteristics and advantages of the present invention will appear more clearly on reading the description which follows, given by way of nonlimiting example and with reference to the appended drawings, in which:

FIG. 1 is a schematic view of an injection circuit and of a mold according to a first prior state of the art,

FIG. 2 is a schematic view of a first embodiment of an injection circuit according to the invention,

FIG. 3 is a schematic view of a first embodiment of an injection circuit according to the invention shown in a step of injecting the resin,

FIG. 4 is a schematic view of a first embodiment of an injection circuit according to the invention shown in a step of feeding the mold with the modulator,

- Figure 5 is a schematic view of a second embodiment of an injection circuit according to the invention;

FIG. 6 is a schematic view of a first embodiment of a modulator according to the invention,

FIG. 7 is a schematic view of a second embodiment of a modulator according to the invention,

FIG. 8 is a schematic view of a third embodiment of a modulator according to the invention,

FIG. 9 is a schematic view of a fourth embodiment of a modulator according to the invention,

FIG. 10 is a block diagram representing the steps of a process for manufacturing a part of a turbomachine according to the state of the art,

FIG. 11 is a block diagram representing the steps of a method for manufacturing a part of a turbomachine according to the invention, FIG. 12 is a diagram representing the sequences of the pressure levels delivered by an injection modulator during the process according to the invention.

In the following description, identical reference numerals designate identical parts or having similar functions.

FIG. 1 shows an RTM 10 injection installation according to a first prior state of the art.

In known manner, the RTM injection installation 10 comprises an injection circuit 12 and a mold 14 intended for the manufacture of a part such as a blade of a turbomachine fan from a woven preform 18. The mold 14 comprises in particular a lower part 18 comprising an imprint 20 intended to receive the preform 16, preferably produced by weaving three-dimensional composite fibers, such as carbon fibers, and to trap this preform 16 in said mold 14 in closing on the lower part 18 an upper part 22 of said mold 14, in order to inject into said mold 14 a polymeric resin permitting to impregnate the fibers of the preform 16.

The elements of the injection circuit 12 will be described from upstream to downstream with respect to the direction of flow of the resin in the circuit, that is to say from the right to the left of FIG. 1.

In known manner, the injection circuit 12 comprises at least one resin injector cylinder 24, which is intended to supply the circuit 12 with resin and to subject this resin to a determined pressure allowing the injection of said resin through the circuit 12 to the mold 14.

The injection circuit 12 comprises, downstream of the resin injector cylinder 24 and upstream of the mold 14, conventionally, a resin preheating device 32 which is interposed in the circuit 12 in order to allow the preheating of the resin before its injection into the mold 14. The mold 14 is also placed in an oven 34, as shown in Figure 1, or alternatively (not shown), under a heating press.

To connect these elements, the circuit 12 includes a first circuit branch 38 connecting an outlet of the resin injector cylinder 24 to an inlet of the resin preheating device 32. The circuit 12 also includes a second circuit branch 38 connecting an outlet of the resin preheating device 32 to an inlet of the injection mold 14 via a primary resin opening / closing valve 40, a third circuit branch 42 connecting an outlet of the resin preheating device 32 to an outlet of the injection mold 14 by means of a secondary valve 44 for opening / closing of resin, and a fourth branch 46 of circuit connecting a exit from the injection mold 14 to the resin injector cylinder 24, by means of a tertiary valve 48 for opening / closing of resin.

To simplify the connections of the different branches of the circuit 12, the second and third branches 38, 42 are connected to each other at the outlet of the heating device 32 by a T-connector, and the third and fourth branches 42, 46 are connected to each other at the outlet of the mold 14 by a T-connector 52.

In this configuration, the injection of resin into the mold 14 is carried out successively in several stages, as illustrated in FIG. 10. During a first step E1, the primary and tertiary valves for opening / closing the resin are opened. 40, 48 and the secondary resin opening / closing valve 44 is closed, and the resin injector cylinder 24 is actuated. The resin passes through the first circuit branch 36, passes through the preheating device 32, passes to through the second circuit branch 38, fills the mold 14, leaves the mold 14 and returns to the injector cylinder 24 of resin via the fourth circuit branch 46.

Then, during a step E2, the tertiary resin opening / closing valve 48 is closed and the secondary valve is opened opening / closing resin 44, eî maintains the pressure in the circuit 12 using the resin injector cylinder 24 until the resin polymerizes in the mold 14.

As a result, the resin contained in the mold 14 is kept under pressure, which in principle makes it possible to avoid the appearance of porosity defects in the final part, the maintenance of the pressure making it possible to avoid degassing of the resin.

However, it has been found that this design has a number of drawbacks.

First, it was found that it was necessary to circulate a large amount of resin through the mold 14 and through the first, second and third branches 36, 38 and 46 during the first step E1 before reaching eliminating any gas bubble in the circuit 12. Furthermore, this objective is not necessarily achieved, since bubbles can nevertheless remain in the third branch 42, bubbles which may be entrained in the mold 14 during the second step E2.

The presence of these bubbles in the circuit 12 can lead to deposits of gas bubbles on the surface of the preform 16, with the consequence of the appearance of defects on the finished part.

Secondly, it has been observed that during the polymerization of the resin in the mold 14, the latter was subject to a phenomenon of shrinkage in the mold 14 and consequently in the first and third branches of the circuit 38, 42. By Consequently, the resin contained in these branches may not be sufficient to absorb the resin deficit in the mold 14, thus creating defects in the finished part.

Thirdly, the second step E2 is a long-term step, which has the drawback of immobilizing the injector cylinder 24 to maintain the pressure in the resin circuit 12, even if the latter could be used in another RTM injection installation. It is therefore important to propose a new design of an injection circuit 12 allowing, on the one hand, keeping the second and third branches 38, 42 of the circuit under pressure without requiring immobilization of the injector cylinder 24 and allowing, d on the other hand, an improved supply of resin aimed at overcoming the consequences of the shrinking of the resin in the mold 14

To this end, the invention proposes a new design of an injection circuit 12 for an RTM injection installation 10 comprising at least one injection modulator 54 making it possible to maintain the resin pressure in the circuit 12 in place and place of the injector cylinder 24, in variable intensity and duration.

In a first embodiment of an injection circuit 12 as shown in FIGS. 2 to 4, the injection modulator 54 is connected to the first branch 36 between the resin injector cylinder 24 and the heating device 32 by via a connection element 56 such as a T or a 3-way valve 56

In a second embodiment of an injection circuit 12 'as shown in FIG. 5, this comprises at least one resin injector cylinder 24', which is intended to supply the circuit 12 'with resin and to subjecting this resin to a determined pressure allowing the injection of said resin through the circuit 12 'into the mold 14'.

The injection circuit 12 'comprises, downstream of the resin injector cylinder 24' and upstream of the mold 14 ', a resin preheating device 32' which is interposed in the circuit 12 'in order to allow the resin to preheat before injection into the 14 'mold

To connect these elements, the circuit 12 'comprises a first circuit branch 36' connecting an outlet of the resin injector cylinder 24 'to an inlet of the resin preheating device 32'. The circuit 12 'also includes a second circuit branch 38' connecting an outlet of the resin preheating device 32 'to an inlet of the injection mold 14' via a first primary opening / closing valve 4G of resin and a second valve 43 'for opening / closing 4G of resin. A third branch 45 comprising a third valve 47 and an injection modulator 54 'is connected to the second branch 38' between the first and second valves 4G, 43 '. Downstream of the mold 14 ', a fourth branch 49' is connected by the intermediary of a fourth valve 5G to a vacuum source 53 'and to a pressure regulator 55', capable of maintaining a static resin pressure in the 14 'mold.

The injection modulator 54 ′ is therefore in this case, arranged downstream of the heating device 32 ′.

Like a pressure regulator known per se from the prior art, the injection modulator 54, 54 ’is connected to the resin injection circuit 12 and 12’.

Different examples of injection regulators 54, 54 ′ have been shown in FIGS. 6 to 3.

Whatever the embodiment of the injection regulator 54, 54 ′, this includes a capacity 53 for accumulating resin of a determined volume which is configured to allow the manufacturing mold 14, 14 'to be force-fed with the resin, and a means 60 for pressurizing the resin provided by said resin accumulation capacity. In accordance with the invention, the means 60 for pressurizing the resin comprises control means 66 configured to vary the pressure and the duration of application of said pressure.

According to first and second embodiments of the injection modulator 54, 54 'which have been shown in FIGS. 6 and 7, the resin capacity 58 communicates with the resin injection circuit 12 and the pressurizing means 60 of the resin comprises a source of gas 62 under pressure connected to said resin accumulation capacity 58 and capable of subjecting the resin contained in said resin accumulation capacity 58 to the pressure of said gas.

More particularly, in the first embodiment of the injection modulator 54, 54 'of FIG. 6, the resin capacity 58 is formed directly in the conduits of circuit 12, that is to say in the first, second and third branches 36, 38, 42 of circuit 12 previously shown in Figures 2 to 4. This configuration makes it possible to dispense with a tank of resin accumulation but requires taking into account the capacity of resin which can be contained in the first, second and third branches 36, 38, 42 of the circuit 12 The source of pressurized gas 62 comprises a booster 64, supplied by a constant pressure air source P1, an outlet air pressure P2 of which is controlled by said control means 66 and is therefore adjustable in intensity and duration. The air pressure P2 is therefore likely to be applied according to a variable intensity and frequency, with pressure stages ranging from one second to several minutes.

In the second embodiment of the injection modulator 54, 54 ′ in FIG. 7, the resin capacity 58 is formed in a reservoir 68 which communicates with the resin injection circuit 12 and, in a similar manner to the previous case, the means 60 for pressurizing the resin comprises a source of gas 62 under pressure which is connected to the reservoir 68, also formed by a booster 64, which is capable of subjecting the resin contained in said accumulation capacity 58 of resin in the reservoir 68 at the pressure of said gas. This configuration guarantees a constant supply of resin, without taking into account the capacity of circuit 12.

According to third and fourth embodiments of the injection modulator 54, 54 'which have been shown in FIGS. 8 and 9, this comprises at least one enclosure 70 which is connected to the circuit 12 and inside which is movably sealingly movable wall means 72. This movable wall means 72 has been shown in the form of a rigid piston 72, but it will be understood that it could also act as a deformable membrane. The movable wall means 72 delimits in the enclosure 70 a first cavity, forming the capacity 58 for accumulating resin and it is this which forms the means for pressurizing the resin, and the control means 680 control the movements of said movable wall means 72 in the enclosure.

In the third embodiment of the injection modulator 54, 54 ′ which has been represented in FIG. 8, the control means 66 actuate the movable wall means 72 directly and mechanically. For example, the control means comprise an actuator electromagnetic or hydraulic (not shown) which determines the stroke and the frequency of going and returning of a connecting rod 74 connected to the piston forming the movable wall 72.

In the fourth embodiment of the injection modulator 54, 54 ′ which has been shown in FIG. 9, the movable wall 72 delimits in the enclosure a second cavity 76 and the control means comprise the source of gas under pressure 62 connected to said second cavity 46 and therefore capable of subjecting the movable wall means 72 to the pressure of said gas. The movable wall means 72 is then able to subject the resin capacity to mechanical pressure.

The advantage of these last two embodiments is that they make it possible not to subject the resin capacity 58 to a source of gas under pressure, either because the pressure to which the resin capacity 58 is subjected by means of movable wall 72 is purely mechanical as is the case of the third embodiment, either because the movable wall means 72 isolates the source of gas under pressure 62. These two configurations make it possible to guarantee the absence of gas bubbles in the circuit 12 and especially in the mold 14.

The control means 66 can be manually controlled. In this case, an operator is in charge of their order.

As a variant, the control means 86 can be automated and be configured so that the means 60 for pressurizing the resin follow a sequence of pressure stages each having a determined duration.

During the manufacture of a turbomachine blade, an injection circuit 12 comprising an injection regulator 56 can be controlled, as illustrated in FIG. 11 according to a method different from the method known from the prior art.

During a first step EΊ, the primary and tertiary resin opening / closing valves 40, 48 are opened and the secondary resin opening / closing valve 44 is closed, and the resin injector cylinder 24 is actuated, as shown in FIG. 3. The resin passes through the first circuit branch 36, passes through the preheating device 32, passes through the second circuit branch 38, fills the mold 14, leaves the mold 14 and returns to the resin injector cylinder 24 via the fourth circuit branch 46.

Then, similar to the method known from the prior art, during a step E 2 (not shown), the tertiary valve for opening / closing resin 48 is closed and the secondary valve for opening / resin closure 44.

The resin is then pressurized throughout the circuit by the resin injector cylinder 24.

The method then comprises a new step E'3, shown in FIG. 4 during which the resin injection modulator 54, 54 'is actuated by putting it under pressure and during which the resin injector cylinder is disconnected 24. The pressure remains variably maintained by the pressure modulator 54, 54 '.

We see here all the advantage of the design according to the invention, which allows to maintain the pressure in the injection circuit 12 by varying it in duration and intensity while allowing the disconnection of the resin injector cylinder 24 in order to '' allow cleaning before the resin polymerizes. The modulation of the pressure makes it possible to delay the solidification of the resin and to allow the mold to be force-fed more effectively than would allow it to be put under static pressure.

Furthermore, this design advantageously makes it possible to reuse the resin injector cylinder 24 in another injection circuit before even that the resin has not polymerized in the mold 14 which was the subject of the previous injection.

Similarly, the circuit which has been represented in FIG. 5 would allow not only the disassembly of the resin injector cylinder 24, but also of the preheating device 32.

During the third step E'3, as illustrated in FIG. 12, the modulator 54 can be controlled according to a sequence S of pressure stages comprising a first sub-sequence SS1 comprising at least a first pressure stage PP1 at a first maximum level pressure, a second sub-sequence SS2 comprising at least a second pressure level PP2 at a second minimum non-zero level pressure, and a third sub-sequence SS3 comprising at least a third pressure level PP3 at a third bearing pressure, intermediate between the first maximum bearing pressure PP1 and the second minimum bearing pressure PP2.

Preferably, the third subsequence SS3 is not carried out at continuous pressure. It includes alternating stages SS3 at the third intermediate pressure PP3 and stages SS4 at the first maximum pressure PP1, each of said stages being of shorter duration than the first sub-sequence SS1.

Of course, other sub-sequences implementing other stages can be envisaged.

The invention therefore advantageously makes it possible to improve the quality of manufacture of the composite parts of a turbomachine while increasing the production rates.

Claims

1. Resin injection modulator (54, 54 ') for a resin injection circuit (12) in a manufacturing mold (14) of a composite part, in particular a composite part of a turbomachine, said modulator being connected to said resin injection circuit (12) and comprising a resin storage capacity (58) of a determined volume which is configured to allow the manufacturing mold (14) to be force-fed with said resin, and means (60 ) pressurizing the resin supplied by said capacity (58),

characterized in that the means (60) for pressurizing the resin comprises control means (66) configured to vary the pressure and the duration of application of said pressure.

2. Injection modulator (54, 54 ') according to the preceding claim, characterized in that the resin capacity (58) communicates with the resin injection circuit (12) and in that the setting means (60) under resin pressure comprises a source (62) of pressurized gas connected to said resin accumulation capacity (58) and capable of subjecting the resin contained in said resin accumulation capacity (58) to pressure (P ₂ ) of said gas.

3. Injection modulator (54, 54 ') according to the preceding claim, characterized in that the source (62) of pressurized gas comprises a booster (64), supplied by a source of air at constant pressure (Pi) , an outlet air pressure (P ₂ ) of which is controlled by said control means (66).

4. Injection modulator (54, 54 ') according to claim 1, characterized in that in that it comprises at least one enclosure (70) which is connected to said circuit (12) and inside which is sealingly movable movable wall means (72), said movable wall means (72) delimiting in said enclosure a first cavity forming the resin accumulation capacity (58), said movable wall means (72) forming the means (60) for pressurizing the resin, and the control means (66) controlling the movements of said movable wall means (72) in the enclosure.

5. Injection modulator (54, 54 ’) according to the preceding claim, characterized in that the control means (66) actuate the movable wall means (72) directly and mechanically.

6. injection modulator (54, 54 ') according to claim 4, characterized in that the movable wall (72) defines in the enclosure a second cavity (76) and in that the control means (66) comprise a pressurized gas source (62) connected to said second cavity (76) and adapted to subject the movable wall means (72) to the pressure of said gas.

7. Injection modulator (54, 54 ’) according to one of the preceding claims, characterized in that the control means (66) are manually controlled.

8. Injection modulator (54, 54 ') according to one of the preceding claims, characterized in that the control means (66) are automated and in that they are configured so that the means for pressurizing the resin follow a sequence (S) of pressure stages each having a determined duration.

9. Method for controlling control means of an injection modulator (54, 54 ') according to one of claims 1 to 8 during injection of resin into a manufacturing mold (14), characterized in that that it comprises a sequence (S) of pressure steps comprising:

a first sub-sequence (SS1) comprising at least a first pressure level at a first pressure (PP1) of maximum level,

- a second sub-sequence (SS2) comprising at least a second pressure level at a second pressure (PP2) of non-zero minimum level,

a third sub-sequence (SS3) comprising at least a third pressure level at a third level pressure (PP3), 8 intermediate between the first maximum level pressure (PP1) and the second minimum level pressure (PP2)

10. Control method according to the preceding claim, characterized in that the third sub-sequence (SS3) comprises alternating bearings at the third intermediate pressure (PP3) and bearings at the first maximum pressure (PP1), each of said bearings being of shorter duration than the first subsequence (SS1).