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Post by madrocketscientist on Jan 27, 2018 19:16:04 GMT -5
Hi John, I pm'd you.
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Post by madrocketscientist on Jan 27, 2018 19:37:02 GMT -5
And in other news, I have made further progress on a basic test setup of the Xicoy FADEC and associated birds nest of wires. The Glow igniter works so now I need to make up the nozzle and fittings for the fuel to be injected across the hot part! Shannon.
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Post by madrocketscientist on Feb 1, 2018 4:31:14 GMT -5
I have been thinking about how to make the shaft tunnel and after reading about numerous issues with bearings spinning in the housing and o-rings sticking... I have decided to make bearing sleeves at both ends of the shaft tunnel. The front end will be a sliding fit with only a small amount of rearward movement available against the preload spring. The rear bearing sleeve might also be a sliding fit but it can be a tight one as it doesn't need to move. I am considering ducting some cooling air between the outside of the rear sleeve and the shaft tunnel to keep the sleeve from expanding faster than the bearing and letting it spin. The case may be however, that the sleeve will be more likely to stay at bearing temperature if the NGV heats up that end of the shaft tunnel itself. The bearings themselves can be a very light press fit in the sleeves, a heavy one risks distorting the outer raceway. I haven't made any effort to reduce the weight of these parts as its only a concept test and a 3D model after all. Shannon
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Post by smithy1 on Feb 1, 2018 19:06:23 GMT -5
Hi Shannon, Many of the RC turbines these days use a wave washer for bearing preload..."All" 5 of my RC turbines use them. None of them use bearing sleeves either, just a nice sliding fit into the aluminium shaft tunnel.
Cheers, Smithy.
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Post by madrocketscientist on Feb 1, 2018 20:05:46 GMT -5
Hi Smithy, I am thinking of using the bearing sleeves after seeing all the spun bearings in the homebuild threads on various forums. From what I have read, the sleeves do improve bearing alignment and hopefully bearing life. I can also machine a new bearing fit without turning a whole new shaft tunnel A couple of things I like about springs are that I can make them myself to suit, and also that the compressed length is not as critical as the compressed length of a wave or Bellville washers, the force is closer to the required preload over a wider range, that is, unless a really short spring is used. AMT uses a pair of back to back Bellville washers, these could give a more even push on the bearing than a wave washer. Sometimes a big stack of them are used in ATC toolchange spindles. en.wikipedia.org/wiki/Belleville_washerMore than one way to skin this cat Shannon
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Post by madrocketscientist on Feb 4, 2018 3:47:16 GMT -5
Build progress for today, I spun a couple of pieces of 0.5mm stainless for the fronts of the combustion chambers. I used some 25mm acrylic (pmma) as the buck not knowing how well it would hold up. I am pretty pleased with the result! I kept the speed of the lathe quite slow to limit the amount of heat buildup in the stainless steel as I was forming it. The combustion chamber sides are made out of 0.25mm stainless sheet for now (it cuts really easily with scissors). I made a temporary shaft tunnel out of some PVC high pressure water pipe so I can get a sense of how everything is fitting together so far. Still to do is working out the hole areas and arrangement for the first trial combustion chamber. Edit: Also still thinking about how to flange the rear of the outer CC down to fit closely to the NGV A few pictures.... Shannon.
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Post by jetjeff on Feb 4, 2018 10:18:11 GMT -5
Hi Shannon,
Nice work on the spun burner can parts. I couldn't get .016" stainless to form at all on my lathe, gave up and just pressed them out.
Regards
Jeff
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Post by madrocketscientist on Feb 4, 2018 21:05:38 GMT -5
Still working on the combustor hole areas, the rule of thumb seems to be as follows.
Primary zone hole area 30% of compressor inducer area (this includes the evaporators hole area) Secondary zone hole area 20% of inducer area Tertiary zone hole area the remaining 50% of inducer area.
I worked out the hole areas for a few engines I have the drawings for, I have the KJ66 plans but left it out as this engine is known to be not as optimized as it could be, (hence all improved copies).
MH54 (WREN54) Primary 27% Secondary 30% Tertiary 44%
GR180 Primary 35% Secondary 26% Tertiary 39%
GR130 Primary 34% Secondary 29% Tertiary 37%
I think I will aim for my hole sizes, primary 35%, secondary 25%, tertiary 40% as a starting point and modify things from there after testing.....
Shannon.
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Post by madrocketscientist on Feb 7, 2018 1:49:26 GMT -5
My Holset HX50 compressor wheels arrived today, I intend to machine these down from the 99mm exducer to 93mm which with a 67mm inducer gives a trim of 52. Should work much better since these wheels have a PR of the required 3.8:1 at the design point. 93mm might turn out to be slightly too big but it is always easier to take a bit off than stick more back on.... unless you own a laser sintering 3D printer I already have some magnet rings from Xicoy for the rpm sensing, its just as well I purchased the larger 8mm ID rings as the new compressor wheels have a 8mm bore rather than the 1/4" of the originals. Its a win there I have also been doing some playing around with some trim maps for the same compressor wheels, imported to CAD to see what the effects of changing trim and diameter have. From what I can work out the Holset HX50/55 map I have is for a wheel with 65mm inducer with a 43 trim. The information on this map is quite lacking and on one forum it was called a ghetto map because its so bad. Image below is the same map as a few posts back... I was originally looking for a compressor wheel that I didn't need to machine to size, thinking that it wouldn't need re-balancing. It is clear to me now that the balance for a turbine needs to be a lot better balanced than what is normally applied to a compressor wheel intended for use in a turbo houseing. The wheels will need a really fine balance regardless of if they are cut down or not! John, let me know if I have this right, I assume that reducing the trim on the GT4294 compressor wheel by machining the exducer to 93mm and the inducer down to 67mm for a 52 trim, would also mean this would reduce the mass flow from 70lb/min to the 60lb/min that I need? Ta! Shannon.
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Post by racket on Feb 7, 2018 3:11:17 GMT -5
Hi Shannon
Yep , thats all you need to do .
Be careful with the "curve" though as you probably need to maintain similar inlet to exit area ratios .
It could/might be possible to "adjust" the map though , its scroll A/R is designed/calibrated for turbo use where a broad map is desirable , by using a vaned diffuser with a smaller throat area than the scroll throat it will shift the best efficiency island closer to the surge line and up to a higher pressure ratio .
Cheers John
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Post by madrocketscientist on Feb 7, 2018 3:40:54 GMT -5
Thanks John,
I will stick with reducing the exducer size on the Holset HX50 wheels that I have here for now. Just getting my head around how things change with various dimensions.
Is there a 'rule of thumb' for the tip height? I have been wondering how the tip height things. I have read a few papers on it but still a bit in the dark about if it needs to be bigger or smaller?
Shannon.
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Post by madrocketscientist on Feb 9, 2018 0:55:36 GMT -5
Spent some time on the lathe today and machined the new compressor wheels to 93mm OD. I cut the nose slightly shorter and machined the magnet recess at the same time. I cut the nose to a taper to match the angle of the rest of the wheel and sandblasted the machined section to match. In hindsight I should have used glass beeds rather than garnet for the sandblasting these, the garnet grit was a little bit too aggressive whereas the glass beed is a lot gentler on the aluminium. Anyway it's done now! I used an offcut of black HDPE turned for a push fit into the magnet recess to keep the front surface from getting blasted. Dimension changes to the wheels are: Exducer from 89.90 to 93.0mm (shifts the trim from 46 to 52) Length from 45.0 to 41.0mm Tip height from 6.6 to 7.2mm (this includes the backwall which is 1.2mm thick) Weight from 195g down to 187g A couple of interesting observations with these compressor wheels, the balancing for the front is a small holed drilled and filled with lead. The back is the usual trimming of the backwall. and the vanes don't quite go all the way to the outside of the compressor backwall, They stop short approx 0.5mm. It doesn't matter for me as I have cut these completely off Now to re-design the diffuser to suit... Shannon.
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gtbph
Veteran Member
Joined: August 2013
Posts: 101
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Post by gtbph on Feb 9, 2018 11:29:27 GMT -5
Hi Shannon,
The stainless pieces look very nice! It's interesting that blasting the wheel produced a finish that looks the same as the original finish. I'll have to remember that, but I don't have a blasting cabinet at the moment.
When looking at the pictures of the bearing sleeves you posted on Feb, 1, they could easily be modified to act as fluid film dampers. A bit of clearance, some oil channels and O-rings and they would dampen the vibrations. Don't know if it would work well or have negative consequences, it's just an idea, but I'm going to try that in my build.
Regards, Alain
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Post by madrocketscientist on Feb 9, 2018 18:25:35 GMT -5
Hi Alan, I currently use an old stainless sink with some clear plastic taped to the top for my blasting cabinet. It doesn't really keep the grit entirely inside but I just sweep the floor before I start so that I can sweep up and reuse the grit that escapes. I have plans to make a decent cabinet out of a 44 gallon drum, but that is another one for the 'get around to it' list! My sand blaster is just a cheap handheld like this.. For the oil film sleeve damping, I guess you could just use a feed branch from the bearing oil line. The o-rings would keep most of the oil between the sleeve and housing, and any that does escape just ends up in the shaft tunnel. I would use Viton o-rings as they are fuel resistant. It seems most of the model jet engine builders have gone away from using o-rings because they stuck or dried out. It does make me wonder if they were using plain rubber instead of Viton as I haven't seen the type of rubber mentioned. There are also silicon o-rings available that could be used in the rear section. I have used these in my air rifles that I built and they handle very high temperatures just fine (the squeaking they made drove me nuts tho ) I think I will just do a sliding fit to start with, it should end up with a thin oil film there anyhow, as the stuff goes everywhere!! Let us know how you get on, it it always good to have more ideas and tricks up the sleeve. Shannon.
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gtbph
Veteran Member
Joined: August 2013
Posts: 101
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Post by gtbph on Feb 10, 2018 16:12:12 GMT -5
Hi Shannon,
Thanks for the info! Maybe I'll make the cabinet myself too, they are quite expensive, even when already used, here. Yes, I'll probably use Viton O-rings on the compressor side and silicone on the hot end, I heard Viton can release nasty stuff when overheated. The danger could be exaggerated though, seems to happen a lot with safety-related issues. On the other hand, why not silicone, so I'll try that first.
Looking forward to see more updates!
Alain
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