Bonjour
Bien, pour répondre à une demande exprimée dans des termes remarquables, voire curieux, je laisse le soin de les qualifier à d'autres, ce n'est pas mon genre de reprendre ce type de débordements !
Et comme, même si je pense être un excellent mécanicien, je ne me pose pas en ingénieur Aero ! Ce qui se dit est bien loin de contredire ce dont j'ai pu parler depuis 2 mois, en particulier je n'ai jamais dit que la désintégration du Disk de l'IPT était la cause primaire , de fait c'est une conséquence de l'assemblage qui à lâché !
Donc je laisse la parole à des posteurs hyper qualifiés chez PPRune !
Voila 6 pages (Et plus) de discussion de haut niveau concernant le T900 !
De fait je reprend la suite de ce qui à été exposé précédemment ... plus qq pages nouvelles, le débat continue chez PPRune
Qq extraits seulement, mais tout à lire pour les courageux !
A mon avis toutes les réponses sont là , il suffit de savoir lire et de faire fonctionner les neurones !
Ce n'est pas un procès de RR en aucune manière, juste un point de vue, bien étayé, par des gens sérieux !
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Vibrations
Des exemples T500 et T900
http://www.eng.ox.ac.uk/samp/pubs/clifton_transfer.pdf
Des Commentaires intéressants chez PPRUNE, des commentaires d'ingés qui savent de quoi ils parlent !
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De Bearfoil
The Supporting Structure of the Bearings and its shortcomings are well known, they are addressed by RR themselves, I believe. The IPT Case containing the IPT Disc and the LP/NGVs with Platform are supported by a center bore that is itself part of the Casing. These are ten Radial and hollow struts that are fastened to the Case with special fasteners (bolts). The AD does address the chronic loosening of these bolts, and requires their re-torquing at specific intervals. "Re-Torquing", "Borescoping", "Oil Inspections"; these are known "Palliatives". Palliative in the sense of treatment, not cure. The underlying consideration for the life of this powerplant is the rehabilitation back to profiles certified in its original certificate.
The squishy part is determining, and by whom, what constitutes Safety in pursuance of keeping this engine on wing. I can simply repeat that the cause of the chronic wear and shortened life of this machine is and was known. It is Vibration, and resonance; a "novelty" dependent on whom one asks. The core bores, bearings, and webs are subject to not additional stress, but normal stress that was unaddressed. It may be pedantic, but I think it is accurate. It is not a mystery, it never was. The engine landed on wing early, with insufficient test and a lurking problem that cannot be controlled given present design, not insofar as adherence to the certificate is involved. Decisions made about the future of the TRENT are forthcoming. Replacing moving mass every three hundred cycles may treat the disease, but it won't cure it.
http://www.pprune.org/tech-log/437978-qantas-a380-uncontained-failure-23.html#post6229915
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De Turbine D
I've looked at the recent posts and informative sites regarding vibrations and what can result. I think Bear gives some good information regarding support structures and what may be going on in these newer engines.
First, if you think about the fan and all related rotating airfoils attached to one end of a cylinder, and the LPT rotor attached to the other end, 7 feet away, you have a dumb bell in a static sense. If you pick up the dumb bell at one point near the center, both ends will sag downward, the LPT end more so as it will generally be heavier. So then, to prevent this from happening in reality, a series of static structures are designed and placed to provide the backbone of the engine. These structures must maintain alignment between rotors and stators and their design is dictated by the need for stiffness, more so than strength. The major sources of load for an engine translates into forces on the structures such as maneuver loads (turning), CAT (clear air turbulence), acceleration/deceleration, gyroscopic forces, moments through the bearings into the frame hubs that support the bearings, unbalance in the rotors, internal variable pressures in the engine flowpath and lastly, thermal differences induced by the airfoils compressing the air and then the hot gasses flowing from the combustor through the turbine components. There is also the dynamic behavior of the engine and interactions with the airframe to which it is affixed. All of these features must be combined with design features and analysis to enable the static structure components to handle different types of loads.
For the designer, there are all kinds of analysis programs to assist in determining the adequacy of the structure design, redundant structural model, finite analysis model and a complete engine structural model and a first engine bending model. In addition,designs are verified by static load testing that can include built-in defects to test design robustness. Extreme engine testing can test for structural capability under extreme vibration. Low cycle and high cycle fatigue capabilities must be examined. Based on all of this, limit conditions can be determined and set so that a well designed structure will not show any sign of deformation or contribute to a loss of performance even if the limit is exceeded in a one time event. So as you can see, the engine structures are key to having successful engine capable of meeting the mission advertised to the customers.
Good structures must be capable of absorbing vibrations and not amplifying them and there are many techniques that can be incorporated to accomplish this. I used the word "robust". Care must be taken to not reduce weight of structures that could compromise long term structural integrity although this is often looked at during weight reduction/saving campaigns to improve performance.
Vibrations and rotor unbalances are always present in an engine and they change as the total engine cycles grow over time. So the engine backbone must be capable of handling all of these variables as they occur.
http://www.pprune.org/tech-log/437978-qantas-a380-uncontained-failure-23.html#post6230386
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De Bearfoil
Additionally, knowing that the Ball Bearings act as Thrust bearings and Axial bearings under load, we can describe the "Limit" Sphere as certainly Three Dimensional. The Balls operate in a "Tube", and react against their race depending on loading. If there were only thrust loads to attenuate, the Wear Print would be at the "Forward and Aft" corners of each metal run. A total of four separate areas. If Radial loads present, these four areas would expand, as the force of the Shaft expresses itself in radial directions as well as in axial stress. If these loads are balanced, (reasonably), there could be an expectation of this design working through and beyond its service life.
This is not the case. Mechanical loads vary widely depending on several designed for aspects: Axial shift depending on translation of Thrust, RPM and lubrication, plus vibration, put variable and tested for loads on this bearing.
As above, if the individual balls are demanded to snub stress in three dimensions, the wear print will eventually affect the entire face of the Race. Under constant load in three dimensions, Oil can be wiped, and metal to metal can accelerate wear. The "C" mod AD requires an initial inspection at 200 cycles, then the next at 100 cycles after it. Why?? As installed, the 'new' bearing is at minimal tolerances. With use, the tolerances widen. As before, I describe a logarithmic failure graph with wear. The more it has worn, the faster (rate) it wears. 200 cycles is about a year, 100 about six months.
Inspections are ordered to find wear. That means, wear is expected to be found. Initially, wear is less concerning, but with sloppier tolerances, inspections are demanded more often. Eventually, the bearing will require rebuilding, or replacing with new. How this affects the original Certificate is between RR and the operators and regulators. And the travelling Public.
So the "A" of the abcde failure cascade is Duff bearing. Not support structure, not oil pipe, not EEC, not Oil Fire. Think..... The Oil Fire caused Aft migration of the IP Shaft? No.
Fire is an instant and chaotic failure, but it cannot wear out bearings and Splines in mere seconds. Especially one meter forward.
http://www.pprune.org/tech-log/437978-qantas-a380-uncontained-failure-22.html#post6223647
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FAG Schaeffer, inexpérimentés dans les turbines, et retenus par RR pour le T900
Digging Deeper
"Solutions for vibration problems involving the excitation of a natural frequency (resonance) are frequently difficult to obtain based solely upon experimental data." Thats what FAG says and moreover:
"Finite Element Analysis (FEA) is a numerical technique that can be used to approximate the structural dynamic characteristics of vibrating mechanical systems, the understanding of which is paramount to any root-cause failure study involving excessive vibrations."
The problem is that FAG have little experience in this field compared to SKF.
This is where METROLOGY began, the original paper from 1955:
http://mdrl.mne.psu.edu/papers/bearinganalyzer.pdf
This is how RR service agents themselves tackle vibration testing in the field, after the engine has left the factory and needs fixing.
http://www.skf.com/files/446130.pdf
And this is a dynamic exploded view of the bearings as they fit into a gas turbine, which is the smoothest running machine known to man: it HAS to be just to stay together.
Engine & gearbox - SKF.com/Industries/Aerospace/Applications
As Annex has told us as reported in his local newspaper:
"since SchaefflerGroup / FAG as the bearing supplier has denied a failure of their bearing(s) as a primary cause"
They did not say this was the first time they had designed and built a series of bearings for an aerospace gas turbine or indeed the entire bearing mounting system from one end of the engine to the other.
So how come they got the contract? Well YES how come?
Thier design had less mass, the mounts were "vibration reducing" and they saw the big cake of the EA business in N America for the B787. They even set up a manufacturing facility in Canada to serve EA. They want to be up there with the new generation of lean engines.
Not a good start!
All replies most welcome
http://www.pprune.org/tech-log/437978-qantas-a380-uncontained-failure-22.html#post6219127
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Je crois que dans les 5-6 pages de ce blog, pages 20, à 25, on peut avoir une très bonne idée de ce qui s'est passé dans le T900, de vrais experts et de vrais ingénieurs, pas du pipeau ! Bonne lecture !
JPRS
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par Invité Sam 18 Avr 2009 - 20:44
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