(19)
(11)EP 2 984 298 B1

(12)EUROPEAN PATENT SPECIFICATION

(45)Mention of the grant of the patent:
02.09.2020 Bulletin 2020/36

(21)Application number: 14810362.5

(22)Date of filing:  06.02.2014
(51)International Patent Classification (IPC): 
F01D 11/22(2006.01)
F04D 29/08(2006.01)
F02C 7/28(2006.01)
(86)International application number:
PCT/US2014/015063
(87)International publication number:
WO 2014/200575 (18.12.2014 Gazette  2014/51)

(54)

GAS TURBINE ENGINE RAPID RESPONSE CLEARANCE CONTROL SYSTEM WITH AIR SEAL SEGMENT INTERFACE

SPIELRAUMSTEUERUNGSSYSTEM MIT SCHNELLER REAKTION FÜR EINEN GASTURBINENMOTOR MIT LUFTABDICHTUNGSSEGMENTSCHNITTSTELLE

SYSTÈME DE COMMANDE DE DÉGAGEMENT À RÉACTION RAPIDE DE MOTEUR À TURBINE À GAZ AVEC INTERFACE DE SEGMENT DE JOINT D'ÉTANCHÉITÉ À L'AIR


(84)Designated Contracting States:
AL AT BE BG CH CY CZ DE DK EE ES FI FR GB GR HR HU IE IS IT LI LT LU LV MC MK MT NL NO PL PT RO RS SE SI SK SM TR

(30)Priority: 12.04.2013 US 201361811546 P

(43)Date of publication of application:
17.02.2016 Bulletin 2016/07

(73)Proprietor: United Technologies Corporation
Farmington, CT 06032 (US)

(72)Inventors:
  • BLANEY, Ken F.
    Middleton, New Hampshire 03887 (US)
  • LUTJEN, Paul M.
    Kennebunkport, Maine 04046 (US)
  • PELLETIER, Brian R.
    Berwick, Maine 03901 (US)

(74)Representative: Dehns 
St. Bride's House 10 Salisbury Square
London EC4Y 8JD
London EC4Y 8JD (GB)


(56)References cited: : 
DE-B- 1 291 560
US-A- 4 127 357
US-A- 5 049 033
US-A- 5 096 375
US-A- 5 601 402
US-A1- 2010 215 477
JP-A- S62 142 808
US-A- 4 131 388
US-A- 5 054 997
US-A- 5 096 375
US-A1- 2003 185 674
US-A1- 2010 313 404
  
      
    Note: Within nine months from the publication of the mention of the grant of the European patent, any person may give notice to the European Patent Office of opposition to the European patent granted. Notice of opposition shall be filed in a written reasoned statement. It shall not be deemed to have been filed until the opposition fee has been paid. (Art. 99(1) European Patent Convention).


    Description

    BACKGROUND



    [0001] The present disclosure relates to a gas turbine engine and, more particularly, to a blade tip rapid response active clearance control (RRACC) system therefor.

    [0002] Gas turbine engines, such as those that power modern commercial and military aircraft, generally include a compressor to pressurize an airflow, a combustor to burn a hydrocarbon fuel in the presence of the pressurized air, and a turbine to extract energy from the resultant combustion gases. The compressor and turbine sections include rotatable blade and stationary vane arrays. Within an engine case structure, the radial outermost tips of each blade array are positioned in close proximity to a shroud assembly. Blade Outer Air Seals (BOAS) supported by the shroud assembly are located adjacent to the blade tips such that a radial tip clearance is defined therebetween.

    [0003] When in operation, the engine thermal environment varies such that the radial tip clearance varies. The radial tip clearance is typically designed so that the blade tips do not rub against the BOAS under high power operations when the blade disk and blades expand as a result of thermal expansion and centrifugal loads. When engine power is reduced, the radial tip clearance increases. To facilitate engine performance, it is operationally advantageous to maintain a close radial tip clearance through the various engine operational conditions.

    [0004] US 5049033 discloses a blade tip clearance control apparatus using cam-actuated shroud segment positioning mechanism, and US 5601402 A discloses a turbo machine shroud-to-rotor blade dynamic clearance control.

    SUMMARY



    [0005] An active clearance control system for a gas turbine engine according to one disclosed non-limiting embodiment of the present disclosure is claimed in claim 1.

    [0006] A further embodiment of any of the foregoing embodiments of the present disclosure includes, wherein said lugged aperture includes a transverse split through said lugged aperture.

    [0007] A further embodiment of any of the foregoing embodiments of the present disclosure includes, wherein said multiple of lugs each define a lug engagement surface engageable with an aperture engagement surface of said lugged aperture.

    [0008] A further embodiment of any of the foregoing embodiments of the present disclosure includes, wherein said lug engagement surface defines a semi-spherical profile.

    [0009] A further embodiment of any of the foregoing embodiments of the present disclosure includes, wherein said aperture engagement surface defines a frustro-conical profile.

    [0010] A further embodiment of any of the foregoing embodiments of the present disclosure includes a chamfer on an insertion surface of said multiple of lugs and a chamfer on said lugged aperture opposite said aperture engagement surface.

    [0011] A further embodiment of any of the foregoing embodiments of the present disclosure includes, wherein said lugged aperture includes three (3) lug apertures.

    [0012] A method of active blade tip clearance control for a gas turbine engine, according to one disclosed non-limiting embodiment of the present disclosure is claimed in claim 7.

    [0013] A further embodiment of any of the foregoing embodiments of the present disclosure includes, wherein the lugged contact interface includes a spherical interface.

    [0014] A further embodiment of any of the foregoing embodiments of the present disclosure includes, wherein the lugged contact interface includes a frustro-conical interface.

    [0015] A further embodiment of any of the foregoing embodiments of the present disclosure includes, wherein the lugged contact interface is a spherical to conical interface.

    [0016] A further embodiment of any of the foregoing embodiments of the present disclosure includes rotationally fixing the puller at the lugged contact interface.

    [0017] The foregoing features and elements may be combined in various combinations without exclusivity, unless expressly indicated otherwise. These features and elements as well as the operation thereof will become more apparent in light of the following description and the accompanying drawings. It should be understood, however, the following description and drawings are intended to be exemplary in nature and non-limiting.

    BRIEF DESCRIPTION OF THE DRAWINGS



    [0018] Various features will become apparent to those skilled in the art from the following detailed description of the disclosed non-limiting embodiment. The drawings that accompany the detailed description can be briefly described as follows:

    Figure 1 is a schematic cross-section of one example aero gas turbine engine;

    Figure 2 is an is an enlarged partial sectional schematic view of a portion of a rapid response active clearance control system according to one disclosed non-limiting embodiment;

    Figure 3 is an enlarged perspective view of a circumferential portion of the rapid response active clearance control system with two air seal segments;

    Figure 4 is an enlarged partial sectional schematic view of one of a multiple of air seal segments with the rapid response active clearance control system in an extended radially contracted BOAS position;

    Figure 5 is an enlarged partial sectional schematic view of one of a multiple of air seal segments with the rapid response active clearance control system in a retracted radially expanded BOAS position;

    Figure 6 is a partial perspective view of a puller for one air seal segment of the rapid response active clearance control system;

    Figure 7 is a partial perspective view of a puller rod;

    Figure 8 is an expanded perspective view of a multiple of lugs on the rod of Figure 7;

    Figure 9 is a generally cold side directed view of an air seal segment with a bridge hook; and

    Figure 10 is a generally hot side directed partial sectional view of the bridge hook to illustrate an aperture engagement surface for the lugs of the puller rod.


    DETAILED DESCRIPTION



    [0019] Figure 1 schematically illustrates a gas turbine engine 20. The gas turbine engine 20 is disclosed herein as a two-spool low-bypass augmented turbofan that generally incorporates a fan section 22, a compressor section 24, a combustor section 26, a turbine section 28, an augmenter section 30, an exhaust duct section 32, and a nozzle system 34 along a central longitudinal engine axis A. Although depicted as an augmented low bypass turbofan in the disclosed non-limiting embodiment, it should be understood that the concepts described herein are applicable to other gas turbine engines including non-augmented engines, geared architecture engines, direct drive turbofans, turbojet, turboshaft, multi-stream variable cycle adaptive engines and other engine architectures. Variable cycle gas turbine engines power aircraft over a range of operating conditions and essentially alters a bypass ratio during flight to achieve countervailing objectives such as high specific thrust for high-energy maneuvers yet optimizes fuel efficiency for cruise and loiter operational modes.

    [0020] An engine case structure 36 defines a generally annular secondary airflow path 40 around a core airflow path 42. Various case structures and modules may define the engine case structure 36 which essentially defines an exoskeleton to support the rotational hardware.

    [0021] Air that enters the fan section 22 is divided between a core airflow through the core airflow path 42 and a secondary airflow through a secondary airflow path 40. The core airflow passes through the combustor section 26, the turbine section 28, then the augmentor section 30 where fuel may be selectively injected and burned to generate additional thrust through the nozzle system 34. It should be appreciated that additional airflow streams such as third stream airflow typical of variable cycle engine architectures may additionally be sourced from the fan section 22.

    [0022] The secondary airflow may be utilized for a multiple of purposes to include, for example, cooling and pressurization. The secondary airflow as defined herein may be any airflow different from the core airflow. The secondary airflow may ultimately be at least partially injected into the core airflow path 42 adjacent to the exhaust duct section 32 and the nozzle system 34.

    [0023] The exhaust duct section 32 may be circular in cross-section as typical of an axisymmetric augmented low bypass turbofan or may be non-axisymmetric in cross-section to include, but not be limited to, a serpentine shape to block direct view to the turbine section 28. In addition to the various cross-sections and the various longitudinal shapes, the exhaust duct section 32 may terminate in a Convergent/Divergent (C/D) nozzle system, a non-axisymmetric two-dimensional (2D) C/D vectorable nozzle system, a flattened slot nozzle of high aspect ratio or other nozzle arrangement.

    [0024] With reference to Figure 2, a blade tip rapid response active clearance control (RRACC) system 58 includes a radially adjustable BOAS system 60 that operates to control blade tip clearances inside for example, the turbine section 28, however, other sections such as the compressor section 24 may also benefit herefrom. The radially adjustable BOAS system 60 may be arranged around each or particular stages within the gas turbine engine 20. That is, each rotor stage may have an associated radially adjustable BOAS system 60 of the blade tip RRACC system 58.

    [0025] The radially adjustable BOAS system 60 is subdivided into a multiple of circumferential segments 62, each with a respective air seal segment 64 (also shown in Figure 3) and a puller 68. In one disclosed non-limiting embodiment, each circumferential segment 62 may extend circumferentially for about nine (9) degrees and includes an associated puller 68. It should be appreciated that any number of circumferential segments 62 may be utilized and various other components may alternatively or additionally be provided.

    [0026] Each of the multiple of air seal segments 64 is at least partially supported by a generally fixed full-hoop mount ring 70. That is, the full-hoop mount ring 70 is mounted to, or forms a portion of, the engine case structure 36. It should be appreciated that various static structures may additionally or alternatively be provided to at least partially support the multiple of air seal segments 64 yet permit relative radial movement therebetween.

    [0027] Each air seal segment 64 may be manufactured of an abradable material to accommodate potential interaction with the rotating blade tips 28T within the turbine section 28 and includes numerous cooling air passages 64P to permit secondary airflow therethrough.

    [0028] A forward hook 72 and a hook 74 of each air seal segment 64 respectively cooperates with a forward hook 76 and an aft hook 78 of the full-hoop mount ring 70. The forward hook 76 and the aft hook 78 of the full-hoop mount ring 70 may be segmented (Figure 3) or otherwise configured for assembly of the respective air seal segment 64 thereto. The forward hook 72 may extend axially aft and the aft hook 74 may extend axially forward (shown); vice-versa or both may extend axially forward or aft within the engine to engage the reciprocally directed the forward hook 76 and the aft hook 78 of the full-hoop mount ring 70.

    [0029] With continued reference to Figure 2, each air seal segment 64 is radially positioned between a contracted BOAS position (Figure 4) and an expanded BOAS position (Figure 5) with respect to the full-hoop mount ring 70 by the puller 68. The puller 68 need only "pull" each associated air seal segment 64 as a differential pressure from the core airflow biases the air seal segment 64 toward the extended radially contracted BOAS position (Figure 4). For example, the differential pressure may exert an about 1000 pound-force (4544 newtons) inward force on each air seal segment 64.

    [0030] With reference to Figure 6, the puller 68 generally includes a rod 80 with a multiple of lugs 82 (Figure 7) that interfaces with a bridge hook 84 of each respective air seal segment 64. The rod 80 may extend to, or be a portion of, an actuator 86 (illustrated schematically) that operates in response to a control 88 (illustrated schematically). It should be appreciated that various other control components such as sensors, actuators and other subsystems may be utilized herewith.

    [0031] The actuator 86 may include a mechanical, electrical and/or pneumatic drive that operates to move the respective air seal segment 64 along an axis W so as to contract and expand the radially adjustable blade outer air seal system 60. That is, the actuator 86 provides the motive force to pull the puller 68.

    [0032] The control 88 generally includes a control module that executes radial tip clearance control logic to thereby control the radial tip clearance relative the rotating blade tips. The control module typically includes a processor, a memory, and an interface. The processor may be any type of known microprocessor having desired performance characteristics. The memory may be any computer readable medium which stores data and control algorithms such as logic as described herein. The interface facilitates communication with other components such as a thermocouple, and the actuator 86. In one non-limiting embodiment, the control module may be a portion of a flight control computer, a portion of a Full Authority Digital Engine Control (FADEC), a stand-alone unit or other system.

    [0033] With continued reference to Figure 6, the multiple of lugs 82, in one disclosed non-limiting embodiment, includes three (3) equally spaced lugs about a distal end 90 of the rod 80 (Figure 7). The multiple of lugs 82 define an outer diameter less than an outer diameter of an upper section 92 of the rod 80. The upper section of the rod 80 connects to, or forms a part of, the actuator 86 to facilitate a seal between the upper section 92 of the rod 80 and, for example, the engine case structure 36 and/or the full-hoop mount ring 70 (Figure 2).

    [0034] A lug engagement surface 94 on each of the multiple of lugs 82 may be of a semi-spherical profile (Figure 8). That is, the multiple of lug engagement surfaces 94 defines a portion of a sphere. It should be appreciate that other lug engagement surfaces such as a frustro-conical surface may also be defined by the multiple of lugs 82.

    [0035] The bridge hook 84 of each respective air seal segment 64 is located between the forward hook 72 and the aft hook 74. The bridge hook 84 bridges a forward rail 96 and an aft rail 98 from which the respective forward hook 72 and the aft hook 74 extend. The bridge hook 84 includes a lugged aperture 100 (Figure 9) that corresponds with the multiple of lugs 82. That is, in the disclosed non-limiting embodiment, the lugged aperture 100 includes three (3) lug apertures 102 arranged to respectively receive the multiple of lugs 82.

    [0036] The bridge hook 84 may be integrally formed with the air seal segment 64 or may be separately manufactured and welded thereto. The bridge hook 84 may also include a transverse split 104 through the lugged aperture 100 for stress relief.

    [0037] The lugged aperture 100 includes an aperture engagement surface 106 (Figure 10) that contacts the lug engagement surfaces 94 when the multiple of lugs 82 are inserted through the lug apertures 102 then rotated so that the aperture engagement surface 106 is in contact with the lug engagement surfaces 94. The rod 80 is then rotationally fixed by a clip 110 that engages a slot 112 in the upper section 92 of the rod 80 (Figure 7). Flats 114 on the outer periphery 116 of the clip 110 rotationally fixes the clip 110 to the engine case structure 36 and thereby the rod 80. In one disclosed non-limiting embodiment, the rod 80 may be, for example, a piston rod of a pneumatic actuator system and the clip 110 may in alternative embodiments not be required as the pneumatic actuator system, for example, provides anti-rotation.

    [0038] In one disclosed non-limiting embodiment, the aperture engagement surface 106 is frustro-conical. It should be appreciated that other aperture engagement surfaces such as a semi-spherical surfaces profiles may alternatively be provided. Since the aperture engagement surface 106 and the lug engagement surfaces 94 form a lugged contact interface that, in the disclosed non-limiting embodiment, is spherical to conical, a high degree of freedom is provided for the air seal segment 64. That is, the aperture engagement surface 106 (Figure 10) and the lug engagement surfaces 94 (Figure 8) essentially define a ball joint contact interface that provides significant freedom which will not overly constrain the RRACC system 58. Furthermore, as the bridge hook 84 and the multiple of lugs 82 are displaced from an inner surface 118 of the air seal segment 64, the interface has minimal - if any - effect upon the cooling scheme and cooling air passages 64P.

    [0039] With continued reference to Figure 6, to assembly each puller 68 to each respective air seal segment 64, the full-hoop mount ring 70 is assembled into the engine case structure 36 followed by the multiple of air seal segments 64. The rod 80 with the multiple of lugs 82 are inserted through the lug apertures 102 then rotated so that the aperture engagement surface 106 is in contact with the lug engagement surfaces 94. That is, once the multiple of lugs 82 are inserted through the lug apertures 102, the rod is indexed 60 degrees - for a three lug arrangement - so the lug engagement surfaces 94 (Figure 8) contact the aperture engagement surface 106 (Figure 10). A chamfer 120 on an insertion surface 122 of the multiple of lugs 82 (Figure 8) and a chamfer 124 on the lugged aperture 100 (Figure 9) facilitates blind assembly of the rod 80. The clip 110 then rotationally fixes the rod 80 with respect to the engine case structure 36.

    [0040] The puller 68 of the RRACC system 58 provides thermal and aerodynamic isolation from the respective air seal segment 64 and permits significant freedom of movement with the ball-joint interface.


    Claims

    1. An active clearance control system (58) of a gas turbine engine (20) comprising:

    an air seal segment (64) with a bridge hook (84) having a lugged aperture (100);

    a puller (68) with a multiple of lugs (82) engageable with said lugged aperture (100); and

    a forward hook (72) and an aft hook (74) that each extend from said air seal segment (64), said bridge hook (84) between said forward hook (72) and said aft hook (74).


     
    2. The system (58) as recited in claim 1, wherein said lugged aperture (100) includes a transverse split (104) through said lugged aperture (100).
     
    3. The system (58) as recited in claim 1, wherein said multiple of lugs (82) each define a lug engagement surface (94) engageable with an aperture engagement surface (106) of said lugged aperture (100).
     
    4. The system (58) as recited in claim 3, wherein said lug engagement surface (94) defines a semi-spherical profile, and/or wherein said aperture engagement surface (106) defines a frustro-conical profile.
     
    5. The system (58) as recited in claim 3 or 4, further comprising a chamfer (120) on an insertion surface (122) of said multiple of lugs (82) and a chamfer (124) on said lugged aperture (100) opposite said aperture engagement surface (106).
     
    6. The system (58) as recited in claim 1, wherein said lugged aperture (100) includes three (3) lug apertures (102).
     
    7. A method of active blade tip (28T) clearance control for a gas turbine engine (20), comprising:

    engaging a puller (68) with an air seal segment (64) through a lugged contact interface;

    inserting a multiple of lugs (82) that extend from the puller (68) into a lugged aperture (100); and

    rotating the puller (68) to obtain the lugged contact interface.


     
    8. The method as recited in claim 7, wherein the lugged contact interface includes a spherical or frustro-conical interface.
     
    9. The method as recited in claim 7, wherein the lugged contact interface is a spherical to conical interface.
     
    10. The method as recited in claim 7, 8 or 9, further comprising:
    rotationally fixing the puller (68) at the lugged contact interface.
     


    Ansprüche

    1. Aktives Spielraumsteuerungssystem (58) eines Gasturbinentriebwerks (20), umfassend:

    ein Luftabdichtungssegment (64) mit einem Brückenhaken (84), der eine mit Nocken versehene Öffnung (100) aufweist;

    eine Ausziehvorrichtung (68) mit einer Vielzahl von Nocken (82), die in die mit Nocken versehene Öffnung (100) eingreifen können; und

    einen vorderen Haken (72) und einen hinteren Haken (74), die sich jeweils von dem Luftabdichtungssegment (64) aus erstrecken, wobei der Brückenhaken (84) zwischen dem vorderen Haken (72) und dem hinteren Haken (74) liegt.


     
    2. System (58) nach Anspruch 1, wobei die mit Nocken versehene Öffnung (100) einen Querspalt (104) durch die mit Nocken versehene Öffnung (100) beinhaltet.
     
    3. System (58) nach Anspruch 1, wobei die Vielzahl von Nocken (82) jeweils eine Nockeneingriffsfläche (94) definieren, die in eine Öffnungseingriffsfläche (106) der mit Nocken versehenen Öffnung (100) eingreifen kann.
     
    4. System (58) nach Anspruch 3, wobei die Nockeneingriffsfläche (94) ein halbkugelförmiges Profil definiert und/oder wobei die Öffnungseingriffsfläche (106) ein kegelstumpfförmiges Profil definiert.
     
    5. System (58) nach Anspruch 3 oder 4, ferner eine Fase (120) an einer Einsetzfläche (122) der Vielzahl von Nocken (82) und eine Fase (124) an der mit Nocken versehenen Öffnung (100) gegenüber der Öffnungseingriffsfläche (106) umfassend.
     
    6. System (58) nach Anspruch 1, wobei die mit Nocken versehene Öffnung (100) drei (3) Nockenöffnungen (102) beinhaltet.
     
    7. Verfahren der aktiven Spielraumsteuerung einer Laufschaufelspitze (28T) für ein Gasturbinentriebwerk (20), umfassend:

    Ineingriffbringen einer Ausziehvorrichtung (68) mit einem Luftabdichtungssegment (64) über eine mit Nocken versehene Kontaktschnittstelle;

    Einsetzen einer Vielzahl von Nocken (82), die sich von der Ausziehvorrichtung (68) aus erstrecken, in eine mit Nocken versehene Öffnung (100); und

    Drehen der Ausziehvorrichtung (68), um die mit Nocken versehene Kontaktschnittstelle zu erhalten.


     
    8. Verfahren nach Anspruch 7, wobei die mit Nocken versehene Kontaktschnittstelle eine kugelförmige oder eine kegelstumpfförmige Schnittstelle beinhaltet.
     
    9. Verfahren nach Anspruch 7, wobei die mit Nocken versehene Kontaktschnittstelle eine kugelförmige bis kegelförmige Schnittstelle ist.
     
    10. Verfahren nach Anspruch 7, 8 oder 9, ferner Folgendes umfassend:
    rotatorisches Sichern der Ausziehvorrichtung (68) an der mit Nocken versehenen Kontaktschnittstelle.
     


    Revendications

    1. Système de commande de dégagement actif (58) d'un moteur à turbine à gaz (20) comprenant :

    un segment de joint d'étanchéité à l'air (64) avec un crochet en pont (84) ayant une ouverture à oreilles (100) ;

    un dispositif de traction (68) avec de multiples oreilles (82) engageables avec ladite ouverture à oreilles (100) ; et

    un crochet avant (72) et un crochet arrière (74) qui s'étendent chacun depuis ledit segment de joint d'étanchéité à l'air (64), ledit crochet en pont (84) étant entre ledit crochet avant (72) et ledit crochet arrière (74).


     
    2. Système (58) selon la revendication 1, dans lequel ladite ouverture à oreilles (100) comporte une division transversale (104) à travers ladite ouverture à oreilles (100).
     
    3. Système (58) selon la revendication 1, dans lequel lesdites multiples oreilles (82) définissent chacune une surface d'engagement d'oreille (94) engageable avec une surface d'engagement d'ouverture (106) de ladite ouverture à oreilles (100) .
     
    4. Système (58) selon la revendication 3, dans lequel ladite surface d'engagement d'oreilles (94) définit un profil semi-sphérique, et/ou dans lequel ladite surface d'engagement d'ouverture (106) définit un profil tronconique.
     
    5. Système (58) selon la revendication 3 ou 4, comprenant en outre un chanfrein (120) sur une surface d'insertion (122) desdites multiples oreilles (82) et un chanfrein (124) sur ladite ouverture à oreilles (100) opposé à ladite surface d'engagement d'ouverture (106).
     
    6. Système (58) selon la revendication 1, dans lequel ladite ouverture à oreilles (100) comporte trois (3) ouvertures d'oreille (102).
     
    7. Procédé de commande de dégagement actif de pointe de pale (28T) pour un moteur à turbine à gaz (20), comprenant :

    l'engagement d'un dispositif de traction (68) avec un segment de joint d'étanchéité à l'air (64) par le biais d'une interface de contact à oreilles ;

    l'insertion de multiples oreilles (82) qui s'étendent depuis le dispositif de traction (68) dans une ouverture à oreilles (100) ; et

    la rotation du dispositif de traction (68) pour obtenir l'interface de contact à oreilles.


     
    8. Procédé selon la revendication 7, dans lequel l'interface de contact à oreilles comporte une interface sphérique ou tronconique.
     
    9. Procédé selon la revendication 7, dans lequel l'interface de contact à oreilles est une interface sphérique à conique.
     
    10. Procédé selon la revendication 7, 8 ou 9, comprenant en outre :
    la fixation en rotation du dispositif de traction (68) au niveau de l'interface de contact à oreilles.
     




    Drawing





























    Cited references

    REFERENCES CITED IN THE DESCRIPTION



    This list of references cited by the applicant is for the reader's convenience only. It does not form part of the European patent document. Even though great care has been taken in compiling the references, errors or omissions cannot be excluded and the EPO disclaims all liability in this regard.

    Patent documents cited in the description