richardm
Senior Member
Joined: June 2022
Posts: 413
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Post by richardm on Mar 21, 2024 16:06:10 GMT -5
I keep wondering how you got to weld in between the blades, especially on the hub with such a narrow spacing . Special TIG torch ? For sure a special skill...
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Post by Johansson on Mar 21, 2024 16:23:43 GMT -5
I keep wondering how you got to weld in between the blades, especially on the hub with such a narrow spacing . Special TIG torch ? For sure a special skill... The hub welding was actually very easy, I fitted and welded one blade at a time so it was only the last blade that was a bit snug.  I run a gas lens cup and 2.4mm thoriated electrodes, I use that setup for everything from 1mm stainless to 4mm aluminium. |
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Post by racket on Mar 21, 2024 17:20:57 GMT -5
LOL..............most of us can only dream of welding as good as you , you make it look easy, which it ain't :-(
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Post by wannabebuilderuk on Mar 21, 2024 17:23:16 GMT -5
I say stick the lathe in highest gear possible and spin her up 🤣
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richardm
Senior Member
Joined: June 2022
Posts: 413
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Post by richardm on Mar 21, 2024 17:41:57 GMT -5
The first ever flying lathe! LOL
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Post by Johansson on Mar 22, 2024 0:11:15 GMT -5
LOL..............most of us can only dream of welding as good as you , you make it look easy, which it ain't :-( I´ve done my fair share of thin sheet welding, practice makes....decent. I say stick the lathe in highest gear possible and spin her up 🤣 That is the best idea I´ve heard so far today! It pulls a bit of wind even at the 160rpm setting, might blow the tools off the lathe at 10x that. Imagine the havoc when the NGV works itself free from the chuck and starts bouncing around the walls in the workshop... |
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Post by Johansson on Mar 22, 2024 15:40:51 GMT -5
Another hour in the lathe and the clearance is ok, ~1.5mm with a little variation due to blade height.  I will put the turbine wheel in the lathe and trim the blades to a uniform tip radius, a quick job with the air grinder in the lathe tool holder.  I´ve also moved the rotor shaft so that it protrudes in the front like it is supposed to do.  Time to start making drawings for the flame tube build, another CNC laser project since I refuse to drill the gazillion of holes needed in 1mm stainless.  Have a nice weekend guys! /Anders |
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richardm
Senior Member
Joined: June 2022
Posts: 413
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Post by richardm on Mar 22, 2024 17:09:55 GMT -5
Just curious once again..Is the 1.5 mm clearance an arbitrary measure or is it calculated? It seems quite tight to me given the thermal expansion of the disk and blades. Here's a calculator that might prove me right or wrong I dont know the numbers and alloy to do it .. goodcalculators.com/thermal-expansion-calculator/Have a nice week end! Richard. |
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Post by racket on Mar 22, 2024 21:46:14 GMT -5
Hi Richard
I can probably take responsibility for the clearance sizing as I felt <2mm was the amount without too many tip losses , 1.5mm should be OK as the NGV/shroud will expand at a slightly faster rate than the wheel due the gases being a tad hotter going into the NGV , this will hopefully expand the wheel shroud and keep things from rubbing , lotsa unknowns though :-(
A 212mm Allison turb wheel only has ~0.5mm clearance ( 1 mm diametrically) , so with a 500 mm wheel as Anders is using , it'd equate to ~1.2mm
Cheers
John
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Post by Johansson on Mar 23, 2024 2:08:25 GMT -5
A very good pointer Richard, I would rather err on the generous side since a rub would probably bend the blades.
Using the calculator a blade will become 1.1mm longer when heated to 700C, and I have no idea if the NGV cover ring will hold the NGV expansion back or not.
Perhaps setting it closer to 2mm than 1.5mm would be wise, as you say John lots of unknowns and the blade segments might settle a tenth or two in the bolt holes when things start to stretch from rotation.
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richardm
Senior Member
Joined: June 2022
Posts: 413
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Post by richardm on Mar 23, 2024 5:13:39 GMT -5
That was just my first impression when I read 1.5mm for such a large wheel. My concern was that the NGV might expand at a different rate. I see it this way: The NGV outer ring is made of a flat pieces that will expand in all directions but most noticeably in length. Since its formed in a circle that linear expansion will translate in a diameter increase related by pi. or about 1/3 the actual linear expansion.
This brings forward more questions: What happens to the NGV blade expanding while restricted at both ends? .Should you leave some free expansion room for the NGV outer ring, depending on how you attach it to the engine? How about segmenting it ?
I dont want to ruin your week end with my silly questioning. Just a few neurones getting too excited about your project...
Maybe just try it the way it is and keep those questions in mind for later if a problem occurs.
Have a nice week end.
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Post by Johansson on Mar 23, 2024 14:18:18 GMT -5
Not silly at all, I appreciate questions and suggestions from people with know-how in the trade. I am stubborn as hell so I might end up just ignoring them, but I appreciate them none the less. Segmenting the NGV ring is an interesting idea, but it won´t work since it is the support for the engine cover and will have a flange welded to it later. The heat expansion in the NGV will be pretty complex with hot and cold areas but I will try it out and see how it works. The freepower NGV for the bike was made like this and I ran it glowing hot without any signs of damage, during the 306km/h run the Allison C20 turbine wheel rim cracked from excessive heat but the homemade NGV survived. It took two hours to make a set of 12.9 shoulder bolts for the turbine blades.  I bought 40 long ones and threaded and cut them to length, now there is a flat surface that supports the blade hole.  With the turbine blades properly torqued down I can fit it in the lathe and get the clearance right.  I just had to give it another spin before I called it a night, I will probably wear the bearings down before I get a chance to start the engine...  Cheers! /Anders |
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Post by enginewhisperer on Mar 24, 2024 20:31:15 GMT -5
you need an abrasive NGV ring so the turbine can clearance itself |
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Post by Johansson on Mar 25, 2024 0:22:13 GMT -5
you need an abrasive NGV ring so the turbine can clearance itself That would be nice, perhaps add a diamond dust coating to the blade tips so they wear a track for themselves in the metal. |
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richardm
Senior Member
Joined: June 2022
Posts: 413
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Post by richardm on Mar 25, 2024 5:18:25 GMT -5
They dont use an abrasive NGV ring but actually grind it when needed.. you might have to do it after a few thousand hours of operation...LOL
But seriously I dont know why they have to do it . Is the shroud deforming over time or growing tighter from repeated thermal cycles or what else ?
Just nice to know.. Knowledge is a bit like butter or jam ..The less you have the more you spread it...
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