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Post by jetjeff on Aug 19, 2016 22:45:43 GMT -5
Hi Alain,
Indeed JetCat makes quality turbines and they know way more about turbines than me. My explanation of why JetCat uses a light force fit is GRW full compliment ceramic bearings won't tolerate a lot of axial force when you try to remove the shaft (as in when the overhaul is performed), the inner and outer races will separate and you'll find the balls rolling all over your workbench.
I actually got a O.D. reading of 1.258 after heating, I'll check it again but 1.258" to 1.255" was in the ballpark hot.
I do appreciate any and all information forum members contribute to my project. That's whats great about this forum, as many people here have 'been there, done that'.
Jeff
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dieselguy86
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Joined: September 2014
Posts: 186
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Post by dieselguy86 on Aug 20, 2016 0:28:22 GMT -5
Speaking of bearings and clearences, im re-engineering small frame diesel turbos (garrett gt15-gt22) to ball bearing. Garrett is already doing it with their newest generation of diesel vnt turbos, but that doesnt help us who constantly are whiping out our turbos on 2.7-2.8L diesels. Anyways i contacted GMN bearings when i couldnt find exactly what i was looking for. I gave them some basic info such as turbo idle rpm, operating temp, shaft and bearing housing material etc. they require an interference fit of .008mm/.0003" on the shaft and a .005mm/.0002" clearence of the housing. That is very very tight tolerences, dang near a tight hand sliding fit on the shaft. With those in mind i dont see how anything more than .001" interference fit would be needed on the shaft. If anyone is interested heres a pic of the custom bearings GMN made for me, 10mmx22mmx6mm ABEC7 angular contact, full complement. If you look really close youll see a really tiny hole, thats a direct oiling port that supplies oil to the non-thrust side of the balls. Bleed air will be piped in between the rear bearing and turbine piston ring to keep the bearing cool and to make sure bearing lube oil doesnt pool up behind the bearing, causing excessive drag. With custom molded high-temp o-rings to dampen the bearings as well as seal the direct-lube ports, these bearings can operate at a sustained 600°F as long as the oil can flow enough to keep from being cooked in the channels. Sorry jetjeff if i hijacked your thread, just thought id share some info straight from a bearing manufacturer. And this is about the only place i know that could possibly ooh and aah over some bearings. By the way i see your in Mi, im in NW Ohio, Tiffin to be exact, i came across the JFS 100 also.
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dieselguy86
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Posts: 186
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Post by dieselguy86 on Aug 20, 2016 0:29:23 GMT -5
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Post by racket on Aug 20, 2016 2:13:40 GMT -5
LOL, thats why I went to "brass" with bearing clearances in multiples of thous of an inch , a good rattly fit ;-)
Those RC engine alloy bearing housings/shaft tunnels expand at twice the rate of cast iron and bearing material , which makes it kinda hard to reach a compromise when we don't know exactly how hot the different parts get during running .
On my FM-1 and 9/94 engines which used "balls", the bearing was a hand push on fit over the shaft , whilst a loose fit into the outer bronze cups that controlled preload and endfloat , the bearings themselves were really sloppy ball races that rattled badly even though they'd just come out of an Allison 250 aero engine where they supported the gas producer turbine wheels.
Yep, its a very complicated business getting high speed ball races to work just right and have reasonable life spans .
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Adam
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Joined: May 2016
Posts: 101
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Post by Adam on Aug 20, 2016 3:35:08 GMT -5
HI guys,
Interesting reading about the bearing tolerances etc. In the updated drawings I have for the KJ66 Kamps type engines the front bearing outer is riding in an O ring that sits in a small grove machined into the shaft tunnel, it's meant to take up the slack as the tunnel expands. In one description I have read it says both bearings ride in O rings, but all the drawings I have only show an O ring for the front bearing. Does anyone have any experience with this? I haven't made the groves in my tunnel yet, I was planning to take it down to the seal specialist to get the exact size and material O rings.
Cheers Adam
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ripcrow
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Joined: December 2015
Posts: 114
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Post by ripcrow on Aug 20, 2016 4:34:46 GMT -5
My immediate thought would be the o rings would melt by the time the bearing heated up enough to expand. I'm not very smart though I like the idea as it might dampen any vibration
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Post by jetjeff on Aug 20, 2016 4:37:47 GMT -5
Hi Adam,
I wouldn't think a rubber O ring would survive at the rear bearing.
Jeff
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Adam
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Joined: May 2016
Posts: 101
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Post by Adam on Aug 20, 2016 4:51:40 GMT -5
Hi Jeff, Yes, thats what I thought too, It must just be meant for the front bearing. Apparently it does dampen the vibration. I had thought that those wound spring type rings might be better as it wouldn't have any problem surviving the heat. Adam
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Post by racket on Aug 20, 2016 5:08:38 GMT -5
Use silicone O rings if worried about the heat
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Adam
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Posts: 101
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Post by Adam on Aug 20, 2016 5:13:13 GMT -5
Ok, cheers, thanks John. Adam
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Post by jetjeff on Aug 20, 2016 18:13:10 GMT -5
Hi All,
I installed the front bearing in the shaft tube and balanced the turbine using my Jet Joe balancer, today. Final assembly to begin.
There is wind in the forecast, so I don't expect to get much done on the turbine tomorrow, windsurfing time,,,lol.
Jeff Pittel
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Post by smithy1 on Aug 24, 2016 16:31:33 GMT -5
Hi Jeff and Adam, I have plenty of these little engines and have repaired/overhauled heaps of them.... the bearings are indeed a snug sliding fit on the shaft and in the shaft tunnel, The very early Wren MW54 engines did have an "O" ring in the rear shaft housing but they deleted it in later versions of the same engines. The front bearing rides in an "O" ring.
There are a couple of engines such as the early AMT Olympus which used a press fit of the rear bearing on the shaft but I'm not sure if they still do this.
My late model Kingtech K180G engine has "sliding fit" angular contact bearings. The pre-load wave spring has a load of ~3-4kg/mm and is located behind the front bearing to help keep the correct bearing loads, this helps prevent the balls from "skidding" in the races, which is obviously a bad thing. The very early engines of 10-15 years ago had a coil spring located forward of the rear bearing but this proved problematic as thrust numbers increased and would slightly "unload" the front bearing leading to the balls skidding in the races and hence premature bearing failure. On the low thrust engines this wasn't an issue but as the thrust figures increased during development of the engines, it became obvious and they had to devise a better system.
As I understand it, most of the RC type jet engines use a wave type pre-load washer behind the front bearing these days.
Cheers, Smithy.
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Adam
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Posts: 101
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Post by Adam on Aug 24, 2016 17:52:30 GMT -5
Hi Smithy, Thanks for the info. That clears it up for me Cheers Adam
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Post by jetjeff on Aug 24, 2016 18:02:01 GMT -5
Hi Smithy,
Thanks for the info. The turbine shaft on my Kingtech K-80 spins freely, but it's not like if you give the shaft a spin, it will spin for several minutes. You definately detect a hint of resistance. Must be due to the preload.
Jeff
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Post by smithy1 on Aug 25, 2016 16:38:17 GMT -5
Hi Smithy, Thanks for the info. The turbine shaft on my Kingtech K-80 spins freely, but it's not like if you give the shaft a spin, it will spin for several minutes. You definately detect a hint of resistance. Must be due to the preload. Jeff Hi Jeff, Yep, my K80E is exactly the same, the K80's have ~2.5kg/mm springs in them and can be "shimmed" to give the correct loads as measured on a force meter etc... "Some" manufacturers of larger engines have coil springs behind the front bearing, but the effect is the same. Pre-load is a good thing for these angular contact ceramic bearings....lower pre-load can give less/better free spin ability but at the sacrifice of bearing integrity at higher rpm....the better we can hold the balls in the races correctly and prevent skidding or "rattling", the longer they will last. Imbalance vibration is big killer in these engines too.... and balancing is critical....I balance all my own turbines on a Schenck CAB-690H balancing machine down to "silly" tolerances...I use 1mg-in as my maximum tolerance on the smaller engines, K80 and smaller.... and under 5mg-in for the larger ones such as my K180G. There's no such thing as "perfect balance" but time taken to get a nice balance will be rewarded with longer bearing life and arguably better performance. Most RC type engines come from the factory at ~50-70mg-in....which is still reasonably good but not great. If you can feel any "buzz" at idle, it'll not be good at higher rpm's. I find the balance tends to be "harmonic" as the rpm's change and the turbine can often cycle through good and bad vibrations. Many "real" turbine engine manufacturers specify their engines not to be operated at certain rpm's settings due to this phenomena and ask that the operator "transition" quickly through these harmonic zones to help prevent transient vibration issues. Cheers, Smithy.
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