Gear Box Failure @ 43 Hours

Discussion in 'General Auto Conversion Discussion' started by TXFlyGuy, Oct 31, 2019.

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  1. Nov 11, 2019 #141

    Mad MAC

    Mad MAC

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    One does read this and think there is a gap in the market for a generic gearbox monitoring unit with oil temp, oil level, chip detection and maybe a strain guage or two. Torque indication would make big inroads to this problem.

    Have been a Flight test engineer on a R22 with a strain guage for torque indication once, really cool when we hit the asperation limit i thought it was broken the read out was so smooth, it wasn't.
     
    Last edited: Nov 11, 2019
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  2. Nov 11, 2019 #142

    wsimpso1

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    First we have to deal with your prejudices about plastics. LOL. That's a joke son.. (in my best Foghorn Leghorn voice) My wife hates it when someone says a material is plastic. There are some really wimpy and some really stout polymeric materials, and people lump them all together as "plastics". As to using them in powertrain stuff, when I left the business, we were using lots of polymer cages in clutches, dampers, transmissions, transfer cases, engines, final drive cases, and so on. In many bearings, the cages will last longer and do less damage to the rolling elements when made of a high temperature polymer than say out of steel or brass.

    If we look in the the NTSB report, the analyst cited the plastic cages as having melted and in the sump. That tells us something about the temperatures reached - temps exceeded the melting point of the polymers in the cage. It would tell us a lot more if they told us what the melting point of these cages were. Was it an appropriate material in this bearing? A lot of times, yeah, it is. Even after the polymeric bearing cages failed and fell into the sump, the bearings held together and the shafts were still located by the bearings, so I will suggest that the bearing design and failure mode management appears adequate. The bearings lasted longer than the gears did after whatever the overheat event was...

    As to a good oil test, it might have helped but in this case I gotta say I doubt it. Now maybe in your airplane, you would have picked up on the bad oil, sent a sample off for analysis, and tore it down to figure out what evil has befallen your airplane. So we have a couple paths to follow, each with its sub-paths, and so on.

    The owner who has checkitis recognizes that the PSRU must be watched, looks at the oil on the dipstick of engine and PSRU for every flight, finds that the oil is getting dark and has sparkles in it. Tear down and investigate. No need for the oil temp gauge, oil level sensor, or chip detector with this guy.

    The non-diligent owner is ignoring the PSRU oil, flying off his test cards, and having a ball. Somewhere in there, the PSRU packs up and we all get a crunched airplane to talk about. This owner is not doing oil analysis or looking at the dipstick, partly because these Kiwi boxes have a great reputation. The tranny oil would be ignored, just like in cars, until something bad happens. Now what makes you think that a guy who does not look at the tranny oil is going to run a sample out for spectragraphic analysis every 25 hours? Who says it would have caught this problem before the crunch? Maybe the overheat event occurred at 32 hours, and the box took the next 11 hours to come apart. No oil analysis at 25 hours would have caught this sort of issue for the oblivious user.

    Now let's get to the rest of us, who are somewhere between guy who does not pay attention to the PSRU and the Checkitis guy who KNOWS the airplane has some parts in it that are trying to kill people. We know that the gearboxes from NZ are pretty good, we hit the dipstick once in awhile, but have been looking at what we think are higher priority stuff, like our control linkages and heat exchanger hoses. We would like to know if something goes out of range and be told about it by the Dynon when it happens, not 50 hours later... If somehow we got the airplane together and try to do the first engine start without filling the PSRU, the low oil alarm comes on and saves us a bunch of money and heartache. If the PSRU goes right through 250F on a flight or the oil disappears or the chip detector lights up, we can get on it now, not after a dead stick landing becomes very present.

    If you want to run oil analysis too, knock yourself out. But usually a well designed gearbox with good bearings and good gears all conservatively sized and full of oil will run not much higher than 150F for years with no attention, and you are going to stop paying attention to it at some point. Why? well, the oil analysis never shows anything... And those three little sensors will wait patiently until some variable goes off kilter and then warn you. Just like the big boys do with jets and turbines. I think that they have something there...

    Billski
     
    Last edited: Nov 12, 2019
  3. Nov 12, 2019 #143

    TFF

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    I have to say no to plastic races in these types of applications. No matter how hot it gets, the balls or rollers have to stay in place. Does not matter if they are skidding, the bearing needs to be the last to break in hopes it keeps the shafts in place. When a gearbox breaks it needs to get to the point of blowing the housing apart when it stops moving. Does not matter how trashed it is, it has to stay together. Flying around in a helicopter it’s all you got because you are suspended from it.
     
  4. Nov 12, 2019 #144

    Toobuilder

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    Big difference between a bearing "race" and a "cage".
     
  5. Nov 12, 2019 #145

    AdrianS

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    If the bearings are running hot enough to melt the (plastic) cage, there's a serious / terminal problem with the box.

    I don't think metal cage bearings would have stopped the box from failing, unless the original bearings were waay under specced.
     
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  6. Nov 12, 2019 #146

    mullacharjak

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    Cant see any 'race' in this bearing or is it an alternate terminology. BearingConstructionComponents001.jpg
     
  7. Nov 12, 2019 #147

    AdrianS

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    Race is synonymous with "ring" in your drawing.
     
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  8. Nov 12, 2019 #148

    TXFlyGuy

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    This has certainly sparked some deep insight, speculation, and analysis from the residents of the forum.

    The accident represents the only total failure to date that I am aware of that directly involves the PSRU. The total fleet size of T-51's flying is 135-140. Total hours on this exact same model gearbox will be in the thousands.

    Being as objective as possible looking at this, it appears that the Autoflight New Zealand PSRU is pretty darn good based on the historical data.
     
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  9. Nov 12, 2019 #149

    DreamersE/AB

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    Consider using telltale temp tabs and making examination of them part of your pre-flight. You'll need to consider what a given surface's temperature equates to internally. Several temp ranges can be used during development to determine what an operating "normal" is for each location. Telltale temp tabs are used in transport category airplane applications on powerplant and hydraulic system components. I've also seen them used in racing and farm applications as well as industrial machinery.
    John
     
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  10. Nov 12, 2019 #150

    mullacharjak

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    Any chance we can have a look at pictures inside the autoflight NZ Psru ?
     
  11. Nov 12, 2019 #151

    TFF

    TFF

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    I meant cage.
     
  12. Nov 12, 2019 #152

    BBerson

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    Might get better service if the gear box shared engine oil like a GO-300.
    My younger brother learned a gear case lesson the hard way. He tried to drain the engine on his mini 4x4 truck but accidentally drained the transfer case. He couldn't figure how to refill it so he drove it to Quik Lube for them to do it. Just five miles ruined the transfer case because he didn't have free wheel hubs.
    So gearboxes get neglected more than engines, I think.
     
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  13. Nov 12, 2019 #153

    wsimpso1

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    Go back to the OP, follow the picks to the NTSB report. All the photo-evidence and descriptive text that we have is there.

    Billski
     
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  14. Nov 12, 2019 #154

    Dana

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    Not BS at all. Plastic cages have become the norm for lots of bearings for years now, and not just for cost reasons; it often makes good engineering sense. In this particular case, it looks like the melted cage was caused by, not the cause of, the larger failure. And from what I read, it doesn't look like the bearing itself even failed when its cage went.

    Sure, oil analysis might have predicted the problem before a catastrophic failure, or it might not have. But while OE is starting to become mainstream for engines, it's not as commonly thought of for gearboxes.
     
  15. Nov 13, 2019 #155

    Dana

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    No, not BS. You're saying, with no facts to back it up, that a plastic cage is inherently inferior. It's not. It may or may be inferior in a particular application, but that's not true across the board. In many applications, properly selected plastics can be far superior to metals.

    I'm not saying a plastic caged bearing was the best choice here, but regardless, the report gives no indication that the cage failure was in any way related to the the gearbox failure. By the time the box ran dry, destroying the gears and creating enough heat to melt the cage, a metal cage on the bearing wouldn't have made a bit of difference.
     
  16. Nov 13, 2019 #156

    D Hillberg

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    More B.S. look to the aviation industry even with the Robinson with all its Teflon bearings the main clutch support/fan bearings have metal cages, the entire gear train in helicopters are metal caged -
    the report wasn't a full teardown with the manufacture but they did call out the hardness but only after the failure and with no cross section - experience with drive systems and seeing the damage ?
    1 - cage goes loose from design error or other issues
    2 - bearing elements no longer support shaft
    3 - gear teeth stop meshing as designed / chatter & ride high on the teeth = friction and heat increase well beyond design limits. (even with low oil is shouldn't look this bad)
    4 - increased ware and loss of temper of mating parts result in catastrophic failure.

    - No a plastic caged bearing was not the better choice here.
     
    Last edited: Nov 13, 2019
  17. Nov 13, 2019 #157

    Mad MAC

    Mad MAC

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  18. Nov 13, 2019 #158

    piepermd

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    This might be a dumb question, but why not put a Continental GTSIO-520 in the T-51? 375 hp in a certified, proven aircraft engine with no PSRU or conversion worries. Dry weight is 484 lbs without accessories. Is it too wide for the cowling at 34”?
    A new one would be pricey, but not an overhauled engine...
     
    Last edited: Nov 13, 2019
  19. Nov 13, 2019 #159

    TXFlyGuy

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    This might be a stupid answer...but, a small block V8 fits. An air cooled opposed 6 simply will not fit. A new LS3 is around $6500, direct from GM Performance Parts.

    Plus the Continental is no match for this sound.....





     
    Last edited: Nov 13, 2019
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  20. Nov 13, 2019 #160

    piepermd

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    So the 520 is too wide? What is the available width of the cowling in the T-51?
     

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