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Thread: Prop Balance

  1. #21

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    Default Re: Prop Balance

    When I pulled my gearbox off- I checked to see if the prop shaft was bent, which it was not. I did not check the crank, my logic was if the prop shaft was straight the crank should be straight too. When Aircore rebuilt the gearbox I am pretty sure he replaced the spring washers behind the dog gear(there was a update on those I think) and when I talked to him a couple of days ago he remembered some galling(or marks) on the dog gear. My rough running issue is vibration, started by gearbox chatter below 2500 rpm. I will try to get some video of it so it can be better understood. Thanks again for all your input.

    Scott

  2. #22
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    Default Re: Prop Balance

    We may be talking semantics here, maybe not but, the prop shaft being straight is different than the prop hub being true and within spec for runout. If the prop hub is not true and/or runout is not within spec it could be caused by a bent prop shaft or the hub itself or within the gearbox or the proper alignment of any of them.

    The prop shaft being straight, does not mean the crankshaft is true or within runout spec as you have the gearbox in between.

    I still think both are worth checking. If it were a gear design issue (even though they may have improved it), I'd think we would have heard about a lot more issues than just the few here, out of the thousands running.

    Greg

  3. #23
    Super Moderator Av8r3400's Avatar
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    Default Re: Prop Balance

    Danzer, you need to understand that my (and Scott's) engines are early 912 UL motors. The new gear set that I have coming is a complete redesign, the material, pressure angles and oil control are completely different.

    This vintage gearbox, per Rotax, is supposed to be rebuilt at 300 hours. (Meaning these parts get replaced then.) Most do not get this done because it isn't needed.

    I have checked runout of the prop flange, prop shaft and crank gear. These are all fine. I'm sure Scott's is too. The combination of many variables has caused this issue. I'm sure the high mass index of the Warp Drive prop that I have is part of it. I'm also sure that the sitting my motor did from the time it was new in 1993 to the first flight of the plane in 2007 has something to do with it.

    From all of the Rotax people I have spoken with, who are very much in the know, this is a gearbox issue.
    Av8r3400
    Kitfox Model IV
    The Mangy Fox
    912UL 105hp Zipper
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  4. #24
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    Default Re: Prop Balance

    It could be - I'm just pointing out possibilities. And to play devils advocate - If they are completely different, new and improved and Scott's was just rebuilt, and Rotax knows of his issue (vibration) and other issues with the older gears on other engines - why didn't Aircore replace the gear set when they rebuilt it? Or, at least recommend that the gear set be replaced? Would have made sense, if that was thought to be an issue or the cause of the problem. Is he supposed to wait until 300 hours to fix the problem (if that is the root of it)?

    Just possibilities and food for thought.
    Greg

  5. #25
    Super Moderator Av8r3400's Avatar
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    Default Re: Prop Balance

    I appreciate the out of the box thinking. Really. That's why this forum is great!


    There are three Rotax authorized service stations in the US. Lockwood, LEAF and California Power Systems. If one of them had done the gearbox rebuild they would have, no doubt, replaced the gears and dog ring if they showed any spauling.

    I don't know Aircore. I'm sure they are very competent mechanics. But they are not Rotax. Speaking with the shop manager (a friend of mine) at LEAF he told me that if there is ANY wear or damage showing on the drive dogs the parts would be rejected immediately. This is a known problem.

    Like a recall done by an auto manufacturer, there needs to be enough failures to make the action (Service Bulletin) economically feasible. These parts are taking care of themselves by attrition. My and Scott's problem are outliers.
    Last edited by Av8r3400; 10-16-2013 at 08:57 PM.
    Av8r3400
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  6. #26
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    Default Re: Prop Balance

    Aircore is a Rotax factory authorized repair center, this type of work does not in any way require the use of a "service center".

    Maybe Scott should ask Aircore why they did not replace (or even recommend replacing) the gear set when it was apart. If, what you say LEAF says "the parts would be rejected immediately" and the guy at Aircore "remembered some spalling" then the gear set should have been replaced under that scenario. Not suggesting either LEAF or Aircore are wrong, but the way I see it, under this scenario, they can't both be right.

    Scott, it would be very interesting to hear what Aircore thinks about it (and LEAFs opinion).

    Greg

  7. #27

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    Default Re: Prop Balance

    Jim Scott (Aircore) will be here Friday. Like Larry said, both of our UL's are very early examples, and both have sat a lot. Jim did the known AD on the gearbox which was replace the spacers and reshim I believe. And like Larry has shown us, the wear on the dog gear does not look that bad. If Jim had called me and given me the choice of $200 and taking a chance or $1500 I would have picked the first option every time Both Larry and I have talked to multiple Rotax experts and we are getting the same story. Could we both have different problems? Absolutely. That is why I am having Jim come work on my plane on Friday. However I will not be surprised if he tells me it is the gearbox. Here is the cool thing- I will update everyone what I find out, and so will Larry, that way we all can learn and know what to look for.

    Scott

  8. #28
    Senior Member SkyPirate's Avatar
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    Default Re: Prop Balance

    Just curious what are the series #'s or serial #'s on your 912? Mine starts 440.... And its also a ul
    Chase
    Model 5 OutBack
    912 UL

  9. #29
    Senior Member SkyPirate's Avatar
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    Default Re: Prop Balance

    From what Larry has said in another post im pretty sure i
    Have the wider gear set
    Chase
    Model 5 OutBack
    912 UL

  10. #30
    Senior Member SkyPirate's Avatar
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    Default Re: Prop Balance

    If my rpms drop short of about 2300 she gets a little rough, but other then that, 2300 to 5400 she is smooth
    Chase
    Model 5 OutBack
    912 UL

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