Adam
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Joined: May 2016
Posts: 101
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Post by Adam on May 26, 2016 16:08:27 GMT -5
Hi, I'm building a small gas turbine/ jet engine. Great to find a website with people actively building turbines and experts to help! I bought the Kamps book in 2003, 2nd edition. I first scavenged a turbo from a scrap yard and tried to do the calculations based on the compressor dimensions and came out with some pretty far out nonsense numbers (it seems i'm not the only one), the blade exit height was only about 2-2.5 mm so I put the turbo back together and traded it in at a turbo specialist for the 2037 compressor wheel specified in the book. That was in 2006. I've made a shaft, but the bearing tolerance is no good, so that has to be remade, have four more bolts... I've made a shaft tunnel with the wave washer front bearing preload, I found out about that from the pdf by Bob Englar. But what confuses me is that if the front bearing is preloaded, won't the shaft expand forward pushing the compressor wheel into its cover? A few years ago I started visiting opp shops armed with a ruler and a magnet, and started collecting stainless house ware things to make things easier. I did make a spot welder from a microwave transformer, didn't work very well, I bought a tig welder a few years ago. After experimenting with a charcoal fired furnace made with home made refractory I built a new one with bought refractory, made a propane burner, got some crucibles and greensand, the burner works well but haven't fired up the furnace yet. I was making the diffuser of the glued in vane type from a piece of aluminum I'd found but yesterday when i was cutting the slots with a junior hacksaw i got lazy, reached for the jigsaw and promptly broke it, so its looking like I will go back to my original plan of casting it with wedge type vanes.....
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Adam
Veteran Member
Joined: May 2016
Posts: 101
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Post by Adam on May 26, 2016 16:10:20 GMT -5
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Post by racket on May 26, 2016 18:54:22 GMT -5
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Adam
Veteran Member
Joined: May 2016
Posts: 101
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Post by Adam on May 26, 2016 21:36:34 GMT -5
Hi John, Thanks, yes, unfortunately i'm always put off by the initial subscription fee about $75 NZD, so iv'e never signed up
Cheers Adam
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Post by racket on May 26, 2016 22:34:26 GMT -5
Hi Adam
LOL.........yep , I can appreciate that :-)
There shouldn't be much "endfloat" in the shafting, and both shaft and tunnel will be expanding , so things shouldn't change much .
What clearance between comp and cover are you planning on using ?
Cheers John
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Post by jetjeff on May 26, 2016 22:51:04 GMT -5
Hi Adam,
The learning curve on homebuilt turbines is a steep one for sure. My first Kamps build didn't self sustain, but I did learn a lot with that first build. My advise is to try to finish it, it looks like you're quite along on the build. I'm not familiar with the KJ66 engine, but I think if the front bearing is 'sandwiched' between the shaft tunnel and compressor diffuser, the shaft should not be able to ride forward into the front cover. That problem did happen to me on my Kamps build. but that happened because the epoxy bond on the compressor diffuser blades failed. You could always make the front cover from nylon or Delrin if you think that might happen. I'm not saying nylon wouldn't damage the turbocharger wheel, but your odds are better vs made from aluminum.
I can tell you a 3.5" turbine wheel (347 stainless steel) expands up to .015" when heated with a propane torch, but I'm sure the shaft wouldn't expand nearly as much.
Jeff
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Adam
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Joined: May 2016
Posts: 101
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Post by Adam on May 27, 2016 0:47:08 GMT -5
Thanks for the info guys, The comp clearance specified in the book is 0.3 mm and in this one www.jmaireland.com/build%20your%20own%20turbine.pdf it says 0.2 mm, so the shaft expansion must be negligible, thanks for putting my mind at rest. I'm working on a pattern to cast the diffuser, so once the furnace is up and running I should be able to cast some aluminum to turn the compressor cover out of. The minimum length of aluminum bar stock I can buy is 200 mm and with a diameter of 110 mm it was like over $100 last time I checked, so this is the reason for casting these bits or at least some big enough lumps to carve them out of. I don't know where to buy delrin or nylon yet but I have carbon fiber and epoxy so thats an option. Adam
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Post by racket on May 27, 2016 1:17:48 GMT -5
Hi Adam
I've done a bit of "research" and front bearing preload is the way to go , more for the bearing integrity due to the natural forward forces of the running motor, it does compromise wheel to shroud clearances having to run a bit more than desirable.
Cheers John
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Adam
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Posts: 101
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Post by Adam on May 27, 2016 1:34:56 GMT -5
Thanks John, great to know i'm on the right track
Cheers Adam
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Adam
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Joined: May 2016
Posts: 101
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Post by Adam on May 29, 2016 17:19:35 GMT -5
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Adam
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Joined: May 2016
Posts: 101
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Post by Adam on May 29, 2016 18:13:39 GMT -5
Hi guys, I wasn't sure how to work out the number of vanes I would need. But I saw a post by John saying "go with what looks right", so I decided to use the force. 12 vanes because its an even number and easier to make, because its quite fiddly at this small size. 21 deg to the tangent. 15 deg wedge. 2-2.5 mm tip clearance. The case I'm planning on using is a bit larger than the specified one, 114 mm ID instead of 109.4 mm, but my intuition tells me this isn't going to matter. I have got a wine bottle cooler with an inside skin of 110 mm I could cut up if necessary. So after making up this pattern (vanes about 1-2 mm oversize at the moment) I did a bit of a search on this forum last night and found some information on diffusers. So i started imagining I was an air molecule with a bunch of other air molecules around me and what is happening as we get squashed together and then go through this diffuser, while being mindful of what the the diffuser is supposed to do, like slow us down a bit and get us around the corner. I've made a drawing this morning and worked out the area of where we go IN and OUT. I came up with this number of 1.85. So im thinking thats the diffusion ratio, like the exit is 1.85 times the size of the entry. I remember reading a post by a guru last night that said the diffuser ratio never really goes above 2, so I'm quite happy with this 1.85 at the moment. The entry diameter of the compressor is 42 mm and hub dia is 14.5 mm. exit is 66 mm with a blade height of 5 mm, 12 blades. I hope that's not going to cause some sort of surging effect with my 12 diffuser vanes. Maybe if it does I could increase the tip clearance. Anyway, now that iv'e worked out a way of making the wedges uniformly by cutting a strip on the bench saw at the right angle then slicing that up and gluing them into the disk it's not to much trouble to start again if necessary. Adam
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Adam
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Joined: May 2016
Posts: 101
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Post by Adam on May 29, 2016 19:58:18 GMT -5
Ohh! the 21 deg should be the centre line of the vane! Looking like I might have to start again.....
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Post by racket on May 29, 2016 20:18:35 GMT -5
Having the same number of vanes as comp blades isn't a good idea , it can create all sorts of problems :-(
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Adam
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Joined: May 2016
Posts: 101
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Post by Adam on May 30, 2016 2:00:05 GMT -5
OK, thanks John. I'll have to sort out something else. I just noticed the one on the cover of my Kamps book has 11 quite thick straight edge wedges which would be good for bolts and fuel lines etc, but it looks like its for a different compressor wheel. The one in the drawing for the solid machined option in the book has 15 really thin curved vanes that can only just take an M2 bolt. I was trying to avoid making the glued in type, I don't really trust the epoxy to hold. But i used some aluminum brazing rod a few weeks ago to repair a small casting that worked out quite well so I might try that out on the one I broke.
Cheers Adam
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Post by jetjeff on May 30, 2016 2:56:38 GMT -5
Hi Adam,
I've had poor results using aluminum brazing rods and oxy/acetylene torch. A welding shop using a TIG welder should be able to weld the broken part back on. If you can't find nylon or Delrin locally, you should be able to find it online.
Jeff
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