Excellent work, great to see the Kermit is so GREAT! I see a question earlier about the life of the Rotax C gearbox, and your response about HP.
In discussing what a particular type gearbox limits are, although it may seem convenient to conversationally express what has been found in the field on motor "
xyz", putting out a rumored
n HP, it is not used in the actual calculations for bearing life.
Although the bearings may be able to run under the total dynamic loads listed in the mfr spec sheets, and stand up to static loads of a lesser factor, speed plays an enormous factor in how long they will stand up to the load placed on them.
The life of the gearbox is determined by three things:
- Rotating inertial mass, determined by diameter and weight of the prop blades, that the prop shaft can handle cantilevered out distance x from the main bearing
- The gears themselves
- Bearings, top and bottom
There is a worksheet posted here
https://www.rotaryforum.com/filedata...hotoid=1127238
which provides the details required to arrive at bearing life in the gearbox under IDEAL conditions (shown side-by-side with the larger gearboxes we normally use on YG4 conversions). Note that in the spreadsheet horsepower doesn't appear. That is because HP is not a force, but rather the amount of work done over a period of time. The force is torque, and in the American standard engineering numbers that is lbs of force (lbf) divided by distance from the axis about which the force is being applied, in feet or inches. Engine torque is measured in "foot pounds", ft.lb. - not to be confused with lbf which is force applied regardless of distance.
In our case, the radius of the drive gear tooth face in the PSRU is 1", and the YG4 puts out an 94 ft lb torque = 1128 in lb @ 12 inches, running 7000 SRPM and 8330 ERPM. The resultant lbf applied to gear tooth, and therefore bearings, at 1 inches R, is 1080 in lb x 12 in/1 in = 13,536 in. lb. Bearing loads are measured in the universal standard kilo Newtons, kN. and the conversion yields 5.52 kN
Assuming running temps < 150°C and suitable, clean lubricant (anything from red tranny oil up to 85/140W gear oil) under ideal conditions the Rotax C can be expected to last a little over 1000 hrs with a prop up to 74 inches diameter weighing a bit over 10 bs total, with hub.
The model allows for the regular 6% increase of forces seen at the bearing due to contact angle of the gear teeth, (not to be confused with helical gear angle) but only allows an extremely conservative 4% wear factor for vibration/harmonics. In other words, it does not accurately account for prop harmonics and gear vibrations, let alone constructive wave formation of the two which will have serious impact on bearing life.
In many gearbox bearing life models it can be found that, due to the wear factor determined by real-time, accurate measurements of harmonics and constructive wave formation, a reduction in bearing life of > 50% is not uncommon.
With bearings expected to last 1000 hrs under ideal conditions, it is a crap shoot whether or not any particular installation will last even 500 hours as of this writing, since I haven't yet written the software to interpret the wave output on my PC's MIDI data screen for the transducer I have had since the 1980's when I was in a bar band and needed to amplify my upright piano for the stage.
As for KERMIT:
With the very low power settings and ERPM you fly currently in the light single-place gyro "Kermit", it is easy to imagine the gearbox lasting past recommended 1000 hr TBO to change the bearings.
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I don't recall if Kermit uses an RK400 clutch or not. If you bought an RK400, you need to remove and inspect it every 75 hours. You should already know this, one would hope, since it is discussed in the mfr installation instructions. (
We know Teal Jenkins never told you about this mfr requirement because I had to tell him about it myself, just last month, publicly in Avidfoxfliers - even though he has been selling them as part of his RX1 conversion kit since 2014.)
If instead you are using a Rotax rubber donut (Hardy disk), at low power settings you might be OK, but as warning: we just found out last spring that on a YG4 140hp at 75-80% the donut cracked through and through after just 12 hours.
(Teal Jenkins learned of this the same day I did. We waited several days for him to come forth and post the news, but he kept it quiet instead. I finally had to be the one to do it. To this day he has not so much as posted a response nor re-post of the info in any forum on which he is routinely active.)
When using a Rotax C box, the drive gear bearing shim must be put in the correct position in order to balance the axial shaft loading between both bearings. Since you must tear down the gearbox to inspect the RK400 or the rubber donut every 75 hours, this becomes even more important to pay attention to, since incorrect shim reassembly at any one of those times will result in bearing failure very quickly following thereafter.
(Teal Jenkins hid this failure for many months, intentionally neglecting to inform the public and his customers about it. It was not until after I came forward, and first made the announcement, that Jenkins found himself forced to finally admit and discuss it publicly, offering any plausible explanation and providing any useful public information on how to hopefully avoid such an imminent, catastrophic gearbox failure.)
(
Do not look to Teal Jenkins for any answers or any attempt at determining the expected life of the Rotax C on a Yamaha conversion. He either won't or cannot do it. Jenkins has had fully at least a year by now to do his homework and figure things out after learning about the Rotax C failure - to determine bearing life, etc - yet he has done nothing. Instead he is in the middle of designing, ground-up, a gearbox for the YG4i 150 HP which he now claims will be good to 200+ HP. The only way to make such a claim is if one figured out the processes involved and did the math to determine bearing loading and expected life for a minimum 1000 hrs TBO. His claim would seem to indicate that he knows how to arrive at these conclusions, do the bearing life calculations on his own. Why, then, the man has not published any information regarding expected life for his existing kit with the Rotax C is anyone's guess. Either he is making unsubstantiated claims about the design he is now involved in, or he has neglected to figure out what his existing kit is worth in terms of safety and duration.)
(I will take another look at the prop shaft bearings and will post in the above mentioned thread if they are significantly lower life than the drive shaft bearings or not. I challenge Teal Jenkins to figure it out for you instead, but seriously doubt he has a clue - or if he does, that he is willing to be forthcoming with any information, given his checkered past on such delicate matters in every single situation we have learned about regarding his kit's equipment to date.)