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Old 08-10-2020, 06:09 PM   #11
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Re: Turbine bearings


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Just to show how little it takes to balance an assembly from being unbalanced on assembly ; my Jets Munt Vt80 had a mark on the rear wheel near the retaining nut that almost looked like as if a wrench had slipped and made a slight what appeared to be a scrape mark. This was new out of the box. Being my first turbine I was quite concerned \, my first thought, that some ham fisted guy had assembled the turbine. I contacted the factory with a picture and the explanation was that this was how little it took to grind off metal to balance the rotating assembly. My second turbine ( larger) was the same but with an even smaller "mark" . So it doesn't take much.
The other comment with scribing a mark on any of the components can create a stress point were a larger crack can be created with the changes to extreme heating and cooling of the turbine. The unit is not a toy by any means.

Last edited by stegl; 08-14-2020 at 03:49 PM.
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Old 08-10-2020, 06:21 PM   #12
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Re: Turbine bearings

All too true. Also, even bearings will exhibit very slight variations in true so the assembly may rotate a bit off center. We're likely talking microns or less, but at the speeds mentioned would not take much. This was why with high speed spindles and the like stacked bearings would be matched with marks on both inner and outer races to ensure assembled together so one bearing would not present loads against another in the stack. Even then they would have to be run in and temp monitored.

Worst I saw for this was the spindle of a grinder made in Germany. Due to corrosion in the cooling system it toasted the bearings soon after we received it, (used). A replacement spindle was the equivalent of a fully loaded Mercedes sedan. Added filters and a warning system after that.

As for my mentioning ceramics, we had high speed dremel like tools to dress out our aerospace gears and shafts. These were rated to 100k and specified no air lube. Once worn decided to try repairing ourselves as we went through a lot. Have to say, the smaller and tighter tolerance and the price goes up dramatically.

The problems is at room temp ceramics were extremely fragile to end loads and simply pushing the bearing onto the shaft with fingertips, (very little loading at all) caused them to fail in short order once at speed. Surprising since what they went through during a normal work cycle.
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Old 08-10-2020, 06:33 PM   #13
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Re: Turbine bearings

At the other end of the spectrum... myself and countless others In the GTBA have made home-built turbines, and used a very rudimentary ways to balance. My two engines (MW54 and a KJ66) ran for over ten hours (into the teens IIRC) without failure. I did send my MW54 to Wren after a few seasons, for a professional balance and they said I did pretty good. It ran to 160,000 without issue.

If I had an older turbine, which was in need of bearings, I would try changing them myself as well, assuming you have the knowledge to disassemble and reassemble things correctly.

Ive worked on a few older RAM 750ís and they are still running.
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Old 08-11-2020, 12:47 AM   #14
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Re: Turbine bearings

I donít know about turbine bearings but I know where you can get muffler bearings and blinker fluid.
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Old 08-11-2020, 05:30 PM   #15
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Re: Turbine bearings

Back in the day, the spec was 3 mg-cm... That's a maximum of 3 milligrams at 1 cm from the rotating axis, or 1 milligram at 3cm from the axis. In addition, its a dynamic balance, so you don't wind up with a wobble from even a slight imbalance in the front at 180 phase from a slight imbalance in the rear. So, its balanced on 2 planes (each wheel). Its a ridiculously tight tolerance to hit, and absolutely agrees with seeing what looks like a ding in the wheel.

Some of the balancers used by the GDBA guys could apparently pick up a felt pen mark on the wheel, due to its mass off center.

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Old 08-13-2020, 08:29 PM   #16
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Re: Turbine bearings

Wow, thanks for all the interesting info folks! I have the entire turbine disassembled currently and I'm just waiting for the bearings to come in. Given that this is an older turbine and I didn't pay very much for it, I think I will indeed try to balance it myself. I'll need to do my research first and will get back with everyone when I get into it. Cougar, it's interesting that you mentioned it but my old bearings fell apart as I was pushing them on to try to test something. Will the new bearings be as sensitive or was that just simply a sign that the old bearings were definitely shot? Thanks again guys!
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Old 08-13-2020, 08:36 PM   #17
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Re: Turbine bearings

Quote:
Originally Posted by Major Bladebreaker View Post
Wow, thanks for all the interesting info folks! I have the entire turbine disassembled currently and I'm just waiting for the bearings to come in. Given that this is an older turbine and I didn't pay very much for it, I think I will indeed try to balance it myself. I'll need to do my research first and will get back with everyone when I get into it. Cougar, it's interesting that you mentioned it but my old bearings fell apart as I was pushing them on to try to test something. Will the new bearings be as sensitive or was that just simply a sign that the old bearings were definitely shot? Thanks again guys!
Most of the bearings have a conical shaped race, and are designed to be pre-loaded in one direction to act as a partial thrust bearing. If you put them in backwards, they will press apart and the balls will fall out. There is usually an arrow printed on the outer race to show which direction they are supposed to go. You can tell if you look at either side, which side of the inner race has a thicker looking hub. The bearings go towards one another so the preload spring tries to press the outer races outwards, in the direction the bearings take the thrust load. If you are backwards, then the bearings can just pop apart.
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Old 08-13-2020, 09:28 PM   #18
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Re: Turbine bearings

It should be the thicker side of the inner race points away from the spindle, correct?
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Old 08-14-2020, 11:06 AM   #19
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Re: Turbine bearings

Shame it was taken apart without marking the “clocking” of the parts, it would be less work to balance. At least it should still be somewhat close (unlike a new build where the parts would need serious balancing after casting/machining)

GTBA has a lot of information on tools and procedures if I understand correctly, but costs £25 to join. I was going to join when I considered building my own turbine (have a decent hobby machine shop) but I ended up finding a used turbine instead. https://www.gtba.co.uk/

As I mentioned earlier the Wren54 kit manual also may be a resource, as it gives a static balancing procedure that has been used successfully by many. The only catch is that you need a straight and smooth tube to hold the bearings. Trivial if you have/know someone with a metal lathe and a short scrap of appropriate aluminum tubing.
See section 18. https://www.rchelicopterfun.com/supp...structions.pdf
Also a very good read if you want to understand part terminology and how the parts work.

Also, having dissembled a turbine before, make sure any lube passages are correctly lined up (it may be possible to assemble such that essential lube doesn’t flow), and make sure to torque the turbine wheel and compressor nuts to the correct torque with a torque wrench. “Gudntight” may not actually correctly seat the parts, especially if the turbine wheel and/or compressor are a good thermal interference fit.
Also, keep the shaft tunnel and bearings religiously clean...

Good luck, hopefully that turbine will have many more hours of life yet to come!

D.
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