What type of bearing is best suited for extreme rpms 2

[wmike]

Hello, I'm about to build a small gas turbine engine, about the size used in RC models, so now I would like to know what type of bearing I should use. I'm aiming at absolute maximum rpm, the bearing will most likely be the limiting factor, foil bearings and electromagnetic ones are out of the question.

So I'm basically trying to figure out what I'm looking for in a bearing before I go on with this project, here are some aspects to consider.

First things first, roller or regular ball bearing (or other type)?

Seals: there are many sealing materials but I have a feeling no seal at all is best suited here?

Lubriction: we have grease, oil, dry lube and no lubrication at all (and maybe the fuel the turbine will run on).

Play/tolerances between races and balls/rollers/etc, whats the best for max rpm?

Cage: I've seen some plastic materials, metal, and no cage at all, I'm guessing no cage at all is what I want?

Surface quality of balls etc and races.

Silicon nitride/carbide is much lighter than steel, and should have longer service life, I know they make ball bearings in SiC but I dont know if they make other types.



Is there any way to calculate the maximum rpm for a bearing before it fails?

I'm thinkng the ball/roller etc size, weight and position will be important.

Outer diameter of the bearing? Axle/shaft diameter?

Does anybody know any formula to calculate this and what parameters you plug in to this?


I dont know if I posted this in the right section, maybe some of the AERO-forums would have been better. Anyway thanks/ Mike

 

[desertfox]

Hi wmike

I would talk to people like SKF and see what the reccomend
here is the site:-

http://www.skf.com/portal/skf/home/products?newlink=first&lang=en

regards

desertfox

 

[metengr]

What about a babbit sleeve bearing?

 

[wmike]

Thanks for your answers! I have never heard of a babbit sleeve bearing, I have to look it up.

I was going to order the SKF catalog since people told me there were tons of good info there, I know they know their stuff, my brother used to work there (we're in Sweden).

Anyway if anyone has heard of some formula to calculate the max rpm I'm all ears, even if its just a rule of thumb or similar, just need something to start with, need to deceide if the turbine will actually drive/power something or just work as some kind of demonstrator/no load thingy.

 

[wmike]

Ah I see, the babbits dont have moving parts, and a thin oil-film in between the parts??

I guess you can make a similar one out of some cobalt based superalloy, I have worked with that before, its very slick, low friction material, only downside of cobalt based superalloys is the cost. I'll research it some more

 

[hydtools]

How fast do you need to go? How long do you need the bearing life to be? Service temperature?
I have run ball bearings to 32000 rpm in small hydraulic motors. Fully flooded DU bushings to the same speed.

Ted

 

[wmike]

OK

The rotor will be about 28-25mm dia if its made to power something, the prototype will be made in 6al4v Ti. If its just for spinning very fast (no load) I will make a 14mm dia rotor, the mechanical limit for the small one will be somewhere around 2-3million rpm before it goes into orbit and the 25-28 one somewhere 1.2mil (guesstimated), These will only be used to measure efficiency of the design, and to tune in the design.

The Ti turbines will be running on compressed air. I have 2 compressors, one 420hp 28bar 25m3/h and one 30hp 7bar 4.3m3/h.

Then after those initial test (I will test these to destruction) I will build all parts of superalloys, and power the engines with basically any type of fuel that can be burned, the more it expands when burned the better. The inlet temp will be somewhere around 800°C (over that temp the alloys will become weak and rip apart due to the extreme force) I can imagine the bearings will have to endure 2-400C or so depending on how smart I can build and impelent them.

My goal is to beat the MIT guys 4mm microturbine wich revs 2.5 million. :-D could take some time tho... since they had $10mil to play around with.

Anyway this is no joke! I have planned this for about a year now and this is going to be built.

And now back to topic well for a 3-6mm shaft/axle I want a bearing that can handle at least 250k rpm (with cooling and lubrication etc) for well, a long time, preferably 300k

On a sidenote does anyone know of a some kind of metering system (preferably laser) that I can measure rpms over 1 million rpm?

 

[unclesyd]

You should look at air/gas bearings especially the foil air bearing. The foil air bearing has unlimited shaft speed. Here are several sites that have information and leads. The NASA site has some go basic info.

http://findarticles.com/p/articles/mi_qa5322/is_200404/ai_n21346532/pg_1

http://www.grc.nasa.gov/

http://WWW/Oilfree/bearings.htm

http://www.miti.cc/foil-bearings.html

I have several CO2 powered engravers that run 300,000-400,00 on oil-less ball bearings, not sure the type except that they are 440 SS.

http://www.scmsysteminc.com/engraving.php

 

[wmike]

unclesyd how big is the shaft diameter going thru the bearing in your engravers? I know dentist drills rev about 850000 but they only have a 1mm shaft. (too weak for this turbine)

I found a formula or rather rule of thumb on the web, it says 850000/shaft diameter in mm for an "optimal" ball bearing, and thats what I'm trying to find out, what is "optimal" (seals, lube, races, ceramics, cages etc).

 

[unclesyd]

My older model's shaft was 1/8" dia. I haven't looked at the newer model's internals but would suspect it is the same 1/8". Both take the 1/16" dental burr.

I thought that most dental handpiece bearings had a 3.175mm (1/8") bore.

 

[electricpete]

I hope you have thought through the safety aspects for yourself and others nearby.

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[wmike]

If its 3.1 mm thru and its reving 400k you probably have seriuosly good bearings in those engravers, do you know the brand of the bearings? OR if they are off the shelf products at all? sounds like good stuff.

Anyway I have found a way to break MITs 2.5rpm record, I just cant tell you about it yet since I havent patented it yet. I wonder if MIT will hire me? I might make some money out of this I'm not greedy so I'll probably donate 90% to the people that actually needs it.

Any way I have a GOOD FEELING about this project, I'm really exited.

thanks again for all answers. If you got more relevant info , just shoot. I'm all ears.

 

[wmike]

"electricpete"

I hope you have thought through the safety aspects for yourself and others nearby.

----------------------------------------------
No man I'll just rev it up in the school yard and se what happens..

no seriously I have thick titanium sheets that will sorround this engine while running, 420hp 28 bar is no joke, if a hose with that pressure would come loose it would kill you in an instant, I also have 10 mm plexiglass to go with it. Security is actually my number one priority, I'm a machinist by trade and I know the power of even a single kw, no need to worry. I have respect fo the power.

 

[unclesyd]

All these engravers were made in Japan.
The one bearing I've replaced was a Japanese brand that I gave it to a local bearing supplier. He was able to get a replacement that I think was American Made. I've looking around to see if I can find my old sales receipt to get some numbers. It wasn't that expensive, around $11.00. This bearing was oil mist lubricated.

I believe these bearings are the dental handpiece type.

 

[sacem1]

I have not seen any menion to the mecanical integrity of the parts invoñved, the centrifugal force at 2.5 Million rpm's of any turbine should be the critical element of design.

At those speeds the friction of the balls/rollers/needles against their running ways would destroy any type of rolling bearings. The only way you can go for a bearing support will be using oil film floating bearing surfaces and for this surfaces you do not need extra hard materials as the surfaces never come in real contact as they float in the oil film that dinamically develops between shaft and support.

Regards
SACEM1

 

[cbrn]

Hi,
I must admit I'm getting a bit confused about your rotation speeds.
- if it's for "something" running up to 250000 / 300000 rpm, then I'd suggest synthetic "precious stones" like synthetic rubin, running on ultra-polished (Ra = 0.2 micron or so) steel shaft. No need to make a ceramic shaft in this case, because the anular bearing made of rubin is a single crystal whose face in contact with the shaft is a crystal face, thus ensuring extremely low contact friction coefficient (lower than 0.02, easily less than 0.01). Other advantage is very high resistance to high temperature, no property change with temperature, and very narrow constructive tolerances (but bear in mind that, for the fact the crystal can not be machined after its growth, the tighter the tolerance the higher the cost! And other drawback is of course size. With this material, long bearings can not be made: you must split them. The bigger the size (shaft 6 [mm] is somewhat like a max limit, I think) the higher the cost because of uniform "no-defects" growth of the crystal.
- for running speeds up to 2.5 Mrpm (??? are you sure a micro-gas-turbine can run so fast??? I've heard of speeds about 500000 rpm, but not more...), then I really don't know. I suppose you'd have to go to no-contact bearing, oil- or gas-suspended (air-bearings ?).

Regards

 

[wmike]

cbrn

- for running speeds up to 2.5 Mrpm (??? are you sure a micro-gas-turbine can run so fast??? I've heard of speeds about 500000 rpm, but not more...), then I really don't know. I suppose you'd have to go to no-contact bearing, oil- or gas-suspended (air-bearings ?).

----------------------------------------

Honestly I dont know if it can run that fast, things that can happen is that the speed of the inlet gas/air/fuel exceeds the speed of sound for that media (which I dont know yet) and causes some kind of chockwave inside the turbine, then it will certainly break.

Many RC jets/turbines spin at 2-300000rpm so I'm fairly certain we can get it spinning that fast at least with regular ball bearings.

One of my friends calculated the force the rotor would be subjected to and according to him the material would not be the limiting factor, but he could have miscalculated, we'll see how it works out.

The 2.5 million rpm is our goal, but if we get there thats a different story..

here are some links to the MIT turbine, they use a different design, mine would be more of a conventional type.

http://www.memagazine.org/backissues/october97/features/turbdime/turbdime.html

http://web.mit.edu/newsoffice/2006/microengines.html

http://www.tfot.info/articles/49/engine-on-a-chip.html

 

[unclesyd]

Here is another paper and there is also one in Japan that runs at over 1,000,000 rpm.

http://microfluids.engin.brown.edu/Breuer_Papers/Conferences/Hilton Head 00 Bearing.pdf

Some other high speed turbines out there running a high speeds.

http://www.gsfc.nasa.gov/gsfc/spacesci/hst3b/Cryocooler.pdf_1.pdf

http://www.barber-nichols.com/products/turbines/

 

[MrMyers]

Probably an idea to run some better calcs on your rotating components, considering the stress increases on a x^2 relationship with rpm. I bet those critical crack sizes are smaller than inspectable by typical means.



Should you rupture a disc or rotor, plexiglass and titanium sheets probably won't stand in their way.

 

[wmike]

Can someone helt me with calculation at what rpm a 14/28mm diameter rotor (grade5 Ti 6a4v) would rip apart from centrifugal forces? (I'm not an engineer I'm just simple machinist/programmer)

All help would be greatly apprecieated.