|Quoting nicoeddf (Reply 160):|
could you kindly elaborate on the gearbox reliability and the specific case why it is the ultimate on condition maintenance item ?
|Quoting JoeCanuck (Reply 163):|
It's not so much that it would slowly make metal, but the particles themselves are expected to be very small....tiny chips and shavings, which would be detected long before enough metal was lost or broken off that gear failure might become a serious threat.
Correct. These gears are pretty big gears with big teeth. And they get plenty of oil feed, mainly for cooling purposes. So the circulation rate of the oil is fairly high. The gearbox oil sump, and /or return oil line form the gearbox is/are equipped with magnetic Chip Detectors. As these gears (and bearings) wear out, they shed tiny metal particles, which are caught by these Chip Detectors, which now generate a warning. At this point the wear is still perfectly acceptable, and the engine can remain in service.
Meanwhile, maintenance will now remove the Chip Detector, transfer the metallic debris and have a laboratory analysis performed. Typically this lab results are available within 24 hours (can be a little later if the debris sample has to be shipped by currier).
The lab results usually will be able fairly accurately pinpoint the component that has generated the debris, by material specifics. Critical components can even get “marker” elements, so that the exact component can be identified, such as Bearing No.xx or Gear xx.
While initial intervals will be set very conservative, Pratt has a very good understanding on the deterioration rate of each main bearing and gear. Once considerable service time has been accumulated, I would not be surprised that Pratt would allow maintenance actions to be deferred for 50 or 100 flight hours (or even more) before the gearbox has to be replaced, and/or the worn out component be replaced.
Since these gears are pretty big, and for the most part relatively slow in rotation, it can take anywhere between 50 and 500 operating hrs from the first metal generation, to a catastrophic failure. Like I said, initial intervals will probably be quite conservative, but I have no doubt that these will evolve considerably.
So then, why is this the ultimate on-condition machine? Gearboxes, once well designed with good lubrication and thermal transfer, have very good life and don’t need much attention. You can basically run them until the critical component (either a bearing or a gear) starts to wear. Since this can be very well monitored, there is no need for much preventive maintenance (like scheduled overhauls); the gearbox will tell you well in advance of a critical problem that it needs TLC
(Tender., Love, Care). At which point it will probably be overhauled, or refurbished at minimum.
The most critical component in such a reduction gearbox system is the input gear shaft (which is connected to the LPT) and its supporting bearings, since this is the fastest turning gear. All other gears and bearings turn slower, and are therefore bigger.
The bigger the gear, the slower its rate of deterioration.
And what also helps, is that this gear system is in the coolest part of the engine, all the way in the front.
I’m not privy to the design of the GTF
, but I would not be surprised if its gear system is (at least partially) changeable on-wing. That would really make it the ultimate on-condition machine:
* run gear system until it wears out - which could turn out to be 10000 – 20000 hrs;
* When it starts to wear out, it produces warning well in advance;
* Which allows for planning of maintenance, rather than ad-hoc corrective maintenance;
* Change engine overnight, or
* Change gear system on-wing overnight;
* Refurbish engine/gear system;
* Unscheduled down time: minimum, or even non-existing.
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