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At a probably reputable site I read this:
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A loose gearbox has a distinctive “rattle” at low RPM, if allowed to continue, the rattle will get worse and the internal parts will wear progressively faster. Fortunately the Rotax gearbox is easily removed from the engine if any repairs are needed.
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We normally idle at 2000. No rattle there at all.
On the rare occassions where it has briefly dropped down to about 1600 RPM (like after a cold weather start and I released the the choke too early without bringing the throttle up a notch) I DO hear what I would call a rattle. Of course I dont let it stay down there as I (a) it just doesnt cound right and (b) I understood that is hard on the clutch, etc.

But my question is is _that_ a symptom of loose gearbox? Or would it only be considered loose-gearbox symptom if I was hearing it up at 2000 RPM or higher?

My other questions are is if it is a symptom of a loose gearbox --
1) does it seem like something that urgently needs to be addressed (is it causing risk or risk of damage to the engine if I never let it run down that low?)
2) Anyone have any idea how much typically a rotax savvy A&P might charge (parts and labor) to rebuild the gearbox? Or if one removed it and sent it in for rebuild what the typical cost range is?
3) Our 912UL has about 500 hours on it. One rather Rotax experienced A&P guy (the instructor at the 16 hour ELSA inspectors course I took last year) mentioned, if I am remembering right, that it is not unusual for 912s to need the gearbox rebuilt at about 600 to 800 hours.
Can anyone comment on that?


Al
  • Re: Rattle at low RPM symptom of ?

    by » 13 years ago


    Hi Al,

    If you are dropping the 912ULS rpm down to 1600 rpm then what you most likely are hearing and especially feeling is the 10.5:1 compression ratio shaking and chugging. That is one of the reasons not to set an idle for the ULS 100 hp too low. It shakes everything to death and causes the gearbox more wear.
    If you have an older 912 then the gearbox inspection time is 600 hours and if it qualifies for the 2000 HR. TBO then 1000 hrs. (Line Maint. manual section 05-20-00 page 16) That said if you use 100LL all the time then then you will need gearbox maint much sooner because that lead gets into the slipper clutch and when it builds up will bind it up and make it so it functions very poorly and if left too long with too much lead may render is useless. You need to know what the slipper clutch friction torque is and that will help in your decision and if you are getting constant kickbacks during start ups may also be an indicator.
    I'm a believer in when the inspection comes and you have to remove the gearbox get the whole thing done, new bellville washers, shims and bearing. Then your done with it and don't need to worry about doing it again in a couple hundred hours.
    91 octane will help a lot for the entire engine in keeping lead maintenance to a minimum.

    Roger Lee
    LSRM-A & Rotax Instructor & Rotax IRC
    Tucson, AZ Ryan Airfield (KRYN)
    520-349-7056 Cell


  • Re: Rattle at low RPM symptom of ?

    by » 13 years ago


    Thanks Roger,

    Your suggestion that if we take off the gearbox for inspection we might as well do the bearings, etc, at that time makes a lot of sense. Will remember that when the time comes.

    Since ours is an experimental and not REQUIRED to do the recommended gearbox inspection at 600 or even 1000 hours, and we almost never do idle below 2000, never run leaded fuel, change oil regularly, never get kickbacks, and have no symptoms really of anything not perfect in the gearbox --

    --what do you think about _this_ approach:
    If it is passing the slipper clutch test, giving no other symptoms of being below spec, we dont do anything until there is actually reason/symptoms, other than hours, to think it needs inspecting or work.

    Guess its the conflict between the `ounce of prevention is worth a pound of cure` approach and the `if it ain`t broken don`t fix it` approach?

    You wrote, in part --
    >91 octane will help a lot for the entire engine in keeping lead maintenance to a minimum. <

    Not sure I understand that. Some Rotax 912UL owners use 89 or even 87 octane automotive gas (per the manual) and neither of those have lead in them. Lead has long since been banned from automotive fuel. As far as I know the only place you can get gas with lead in it is LL avgas.

    I am guessing when you wrote about 91 octane as _the_ way of avoiding lead you were of the mind that the only two fuels ever used in a Rotax 912 were LL avgas or the non-alcohol automotive gas (which is only available in 91 Octane as far as I know)? Is that right?

    Al

  • Re: Rattle at low RPM symptom of ?

    by » 13 years ago


    Hi Roger,

    In a DIFFERENT recent post of mine you mentioned that the 912UL (which is the engine I have) has no slipper clutch.

    Yet in this thread you wrote

    "...That said if you use 100LL all the time then then you will need gearbox maint much sooner because that lead gets into the slipper clutch and when it builds up will bind it up and make it so it functions very poorly and if left too long with too much lead may render is useless. You need to know what the slipper clutch friction torque is and that will help in your decision and if you are getting constant kickbacks during start ups may also be an indicator. ..."

    So, pardon the really dumb question, but I take it on our 912UL we don't even DO a check of "slipper clutch friction torque",right?

    And doe me letting you know we have the 912UL change any of your recommendations thoughts on this topic?

    Al

  • Re: Rattle at low RPM symptom of ?

    by » 13 years ago


    Hi Al,

    No slipper clutch friction torque check on the 912UL with no slipper clutch. I didn't realize you had the 912UL in the beginning so I kind of got off on a tangent.
    When I wrote about the 100LL and the slipper clutch I wasn't thinking of the non slipper clutch engines like some of the 912UL.(Some engines are certified and some not) I was commenting on the 912ULS with the slipper clutch and the lead build up that affects that slipper clutch and it does need cleaning or it will just clog it up over time. The UL or non slipper clutch engine doesn't have that problem. You still need a gearbox inspection when your time comes. It involves inspecting more than just a slipper clutch. The inspection isn't bad at all.

    The original post was about the vibration. Bottom line is don't idle too low any time and especially on a cold start. It will cause more vibration and depending on the engine mount condition it could shake severely. The Rotax maint manuals list the time periods for the gearbox inspection. See attached page.
    NewRotaxLineMaitmanual7-2010.pdf (You do not have access to download this file.)

    Roger Lee
    LSRM-A & Rotax Instructor & Rotax IRC
    Tucson, AZ Ryan Airfield (KRYN)
    520-349-7056 Cell


  • Re: Rattle at low RPM symptom of ?

    by » 13 years ago


    Thanks again Roger,

    Do I correctly read in that attachment you provided as saying the gearbox needs checking at only 600 hours?

    And do you know of any proceedure sheets of videos on doing that?

    And hey, when you write --
    "non slipper clutch engines like some of the 912UL....."
    does that mean that that my 912UL MIGHT have a slipper clutch?
    If so, I'm right back to my original question, which was --
    Isn't there any way for me to tell from my serial number if MINE has the clutch?


    Alex

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