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What are the downsides of having a company "optimize" a turbine wheel through lightening. Say they take a standard Q-trim wheel and attempt to lighten and balance it. Does anyone have information on any con's to this? I could only think of the fact that it might spin down quicker but I'm sure we'd all take quicker response and give that up. Kinda of like a lightened flywheel. Anyhow, any info appreciated.
 

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If I had a turbo and somebody told me they could balance it better than who built it, I'd think the original builder screwed up. If you have a top quality turbo, shouldn't it already be super well balanced? Now trimming the wheel may change performance for better or worse and that is not an uncommon mod though some comments make me think it's controversial.

In work we've made valves and parts from Inconel and Titanium and from touchy feely I had the impression Inconel was a bit denser/heavier than most other stainless steels. I checked a handbook and for example:
Inconel = 8.25gm/cm2
321SS = 7.86gm/cm2
316LSS = 7.95gm/cm2

Now titanium is way lighter and also good for very high temp. So I think Inconel is used for better high temp and corrosion performance. Here's an interesting article, including links to turbo apps and tubing parts etc.

http://www.burnsstainless.com/TechArticles/Inconel_article/inconel_article.html
 

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Discussion Starter #4
What I was referring to was mostly lightening an already reasonably light shaft. I hear they hollow it out but still keep it balanced. It reduces the wheels already light weight even more to make it spool even faster.
 

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silvino said:
What I was referring to was mostly lightening an already reasonably light shaft. I hear they hollow it out but still keep it balanced. It reduces the wheels already light weight even more to make it spool even faster.
The difference between a normal turbine weight and a "lightened turbine" would be very minute if even noticable. There is not enough material on the shaft to be hollowing it out I would stick with leaving the turbine wheels and shafts as they are instead of sacrificing their durability for such a small gain. If you want to improve spool you should mess with sizing on the turbine housing or different bearing assemblies.

Inconel turbine wheels are used on the exhaust side because of their incredible high heat properties and titanium is a very durable/lightweight metal but it doesnt hold up to high heat very well. Most of the titaniums that are used in high heat applications are an alloy mixed with certain ceramics to allow it to put up to heat much better then a pure form of titanium would. For the most part your only going to see titanium on the compressor side.
 

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ThomsonCharm said:
Inconel turbine wheels are used on the exhaust side because of their incredible high heat properties and titanium is a very durable/lightweight metal but it doesnt hold up to high heat very well. Most of the titaniums that are used in high heat applications are an alloy mixed with certain ceramics to allow it to put up to heat much better then a pure form of titanium would. For the most part your only going to see titanium on the compressor side.
What you said intruiged me because of the titanium midpipes that some people are developing now. I just googled around a little and it seems Titanium has a much higher melting point than most other metals, pure titanium has a mp of about 1670 degrees celcius, while Inconel has a mp of about 1370 celcius. Titanium also has lower thermal expansion coefficient than inconel. Is there something else about titanium that makes it unsuitable for turbine applications? I'm assuming there must something or I would figure many more turbines would use the material, being half the density of inconel.
 

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Drilling out the shaft does not make any sense to me. Since the turbine is a rotating mass, what affects its angular acceleration is moment of inertia, not weight. The contribution to moment of inertia of each infinitesimal element is related to the square of its distance from the axis of rotation. Therefore, the shaft adds the least to the moment of inertia. Now, if you could reduce weight by shaving off the outer parts of the turbine, then it would be a different story.

Hope this makes sense.

Cheers
Rich B. 94 BPU 6-spd
 

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Discussion Starter #9
Thanks. I'm always skeptical of optimizations that sound too good to be true.

Rich B said:
Drilling out the shaft does not make any sense to me. Since the turbine is a rotating mass, the issue is moment of inertia, not weight. THE contribution to moment of inertia of each infinitesimal element is related to the square of its distance from the axis of rotation. Therefore, the shaft contributes the least to the moment of inertia. Now, if you could shave off the Outside. then it would be a different story.

Cheers
Rich B. 94 BPU 6-spd
 

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Rich B said:
Drilling out the shaft does not make any sense to me. Since the turbine is a rotating mass, what affects its angular acceleration is moment of inertia, not weight. The contribution to moment of inertia of each infinitesimal element is related to the square of its distance from the axis of rotation. Therefore, the shaft adds the least to the moment of inertia. Now, if you could reduce weight by shaving off the outer parts of the turbine, then it would be a different story.

Hope this makes sense.

Cheers
Rich B. 94 BPU 6-spd
Excellent point, dont know why I didnt think of that when I first read this, hallowing the shaft would most likely only weaken it and offer no advantages at all
 

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Ray745 said:
What you said intruiged me because of the titanium midpipes that some people are developing now. I just googled around a little and it seems Titanium has a much higher melting point than most other metals, pure titanium has a mp of about 1670 degrees celcius, while Inconel has a mp of about 1370 celcius. Titanium also has lower thermal expansion coefficient than inconel. Is there something else about titanium that makes it unsuitable for turbine applications? I'm assuming there must something or I would figure many more turbines would use the material, being half the density of inconel.
I dont remember where exactly I got that information but looking back I believe it was from a Burns Stainless catalog I was sent a while back comparing the different metals and their usefullness in different applicaitons. I cant find the table on the website but here in my catalog I will write out for you the differences in the alloys they use for their tubing. For the most part I think the reason titanium is not used is because of its higher Coefficent of Thermal Conductivity. But here are the numbers so you have something to compare, I will compare 4 significant metals and the properties in a number in the following order. Aluminum 6061-T6, then 321 Stainless Annealed (Steel Alloy), Titanium CP-2 Grade 2 ASTM B338, and finally Inconel 625.

Tensile Strength in lb/in^2 @ 70 degrees F: 45000, 90000, 50000, 140000
Yield Strength in lb/in^2 @ 70 degrees F: 40000, 35000, 40000, 77000
Elongation: 12%, 55%, 20%, 47%
Density in lb/in^3: .098, .290, .163, .305
Modulus of Elasticity x10^6 lb/in^2: 10, 28, 15, 30
Coefficent of Thermal Expansion in/in-Fx10^-6(@70 degrees F): 13.1, 9.6, 4.8, 5.5
Coefficent of Thermal Conductivity BTU/ft-hr-F(@70 degrees F): 96.50, 9.30, 12.00, 5.65

I think the last one here is the significant number. I believe this meaning titanium will not get rid of the heat quick enough in an enclosed engine bay to be an efficent header or turbine wheel.

Hope this helps,
 

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Titanium is used to make the turbines in jet engines, which live at around 2500 degrees. FYI, the melting point of most metals melt at around 2200 degrees.
 

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Are you sure that is pure titanium though and not a titanium alloy? I know Top Fuel Dragster also use ALL titanium parts too but I believe they are alloys that allow the normal metal titanium to change its properties to put up with heat better.
 

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Thanks alot Thomson, very useful information, but the higher the Coefficient of Thermal Conductivity is, the more heat it will get rid of. So I don't think that would be the reason it isnt used, maybe it is the relatively low tensile strength compared to steel and inconel.

Alright I just did some research in the middle of this post and apparently pure Titanium loses most of its strength after 470 degrees celsius, so any turbine application must be an alloy to allow it to keep its strength under that amount of heat, some titanium alloys are able to increase its useful temperature range very high, up to about 1300 degrees celsius. The titanium alloys however are still very light, usually under 5 g/cm^2, a little more than 3 g/cm^2 less than Inconel. It seems to me that certain alloys would be fine for turbine applications, however I have no idea what the cost would be of such alloys, I'd assume more expensive than Inconel, but if it wasn't all that much it would be a wonderful thing to have for the wonderful spool up from the ~35% or so weight reduction in rotating mass.
 
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