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What type of bearing is best suited for extreme rpms
2

What type of bearing is best suited for extreme rpms

What type of bearing is best suited for extreme rpms

(OP)
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

RE: What type of bearing is best suited for extreme rpms

What about a babbit sleeve bearing?

RE: What type of bearing is best suited for extreme rpms

(OP)
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.

RE: What type of bearing is best suited for extreme rpms

(OP)
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

RE: What type of bearing is best suited for extreme rpms

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

RE: What type of bearing is best suited for extreme rpms

(OP)
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. big smile 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?
 

RE: What type of bearing is best suited for extreme rpms

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/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

 






 

RE: What type of bearing is best suited for extreme rpms

(OP)
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).

RE: What type of bearing is best suited for extreme rpms

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.    

RE: What type of bearing is best suited for extreme rpms

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.

RE: What type of bearing is best suited for extreme rpms

(OP)
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.

RE: What type of bearing is best suited for extreme rpms

(OP)
"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.

RE: What type of bearing is best suited for extreme rpms

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.  

RE: What type of bearing is best suited for extreme rpms

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  

RE: What type of bearing is best suited for extreme rpms

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

RE: What type of bearing is best suited for extreme rpms

(OP)
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

 

RE: What type of bearing is best suited for extreme rpms

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.

RE: What type of bearing is best suited for extreme rpms

(OP)
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.

RE: What type of bearing is best suited for extreme rpms

There is no single answer to that question because it depends on the shape of the part in every extreme detail. There is more to it than just a rotating disk, because the blades of the turbine would each be trying to rip themselves apart from the disk that they are mounted to.

The smallest automotive turbochargers are around twice the diameter you are talking about, and run somewhere near 150,000 rpm with oil-pressure-fed plain bushings and non-contact seals.

RE: What type of bearing is best suited for extreme rpms

Don't get yourself killed.  No kidding around here.

At those rotational velocities any failure of the rotating mass is going to be catastrophic, as the kinetic energy stored in that rotating assembly is likely to be pretty large relative to its volume.  As pointed out before, the energy formula for a rotating mass is analogous to that of linear motion, so the energy stored rises with the square of the velocity if the moment of inertia stays constant.  Go on the internet and look for what can happen when a hard disc or a CD-ROM platter shatters.  Those things are rotating at a max of 15000 rpm.  Imagine we take the same platters and get them up to 2.5M rpm.  We've just stored about 28000 times more energy in them.  Now, granted, a CD or HD platter has a relatively high moment of inertia because it's a flat disc, but the illustration holds.  Depending on the mass and geometry of your rotor, you could be getting into some REALLY dangerous stuff.  

Also, at those velocities, any tiny flaw in the metal can be an initiation point for a rupture.  On the surface you MIGHT be able to see them.  If the flaw is internal, though, you're going to need things like radiographic testing to even know the flaw is there, much less decide if it will make the rotor unsafe.  Inclusions in bar stock happen all the time, and they'll get you hurt or killed by shrapnel just as easily as a surface crack will at 2.5M rpm.

If you insist on doing this, get someone who can really engineer the rotor to keep your experiment safe.  Make sure that someone runs some calculations on your containment vessel, too, to make sure it will hold up to the impact of an exploding rotor instead of just guesstimating on how much you'll need.  There's a lot to consider and worrying about your bearings holding up is a moot point if your rotor explodes all on its on and destroys everything around it because it can't handle the rotational velocity and the containment was insufficient.  

I just want you to be REALLY careful here and don't underestimate what can happen when a rotating mass fractures.

RE: What type of bearing is best suited for extreme rpms

That being said, how are you planning to spin the rotor up?  I'm curious.

RE: What type of bearing is best suited for extreme rpms

(OP)
New Postjistre (Mechanical)      
20 May 08 16:08
That being said, how are you planning to spin the rotor up?  I'm curious.
----

It a tesla/boudary layer disc turbine (I dont have access to 5 axis machines so this is waht I can do), the rotor spins by pushing air in beween discs, the action is viscosity/adhesion,  
if I tinker little with nozzles/ disc spacing etc I can probably get 30%or so efficiency. The rotor circumference can never spin faster than the inlet gas pushing it, it usually spins at 90% of the speed of the inlet gas.

RE: What type of bearing is best suited for extreme rpms

Regarding the calculations, a typical method is to conduct a detailed FEA stress analysis (both mechanical and thermal transients) followed by a fracture mechanics approach to understand failure and to illustrate suitable life.

This is typically carried out for both the rotor/disc and the blade root fixings.

In saying that, there are some simple approximations you may be able to live with to understand what ball park you are playing in, but these would require big assumptions and may therefore not be acceptable.

We do this commonly and unfortunately it is not a cheap or simple exercise.  I'd suspect outside of turbine OEM's there are only a handful of companies globally that can carry out said analysis appropriately.

RE: What type of bearing is best suited for extreme rpms

I think the OP is trying to use the Tesla solid rotor concept for his turbine.  

There is a theoretical analysis of a disk turbine in two parts in issue 15 and 16.

http://www.teslaengine.org/main.html#order

RE: What type of bearing is best suited for extreme rpms

Hi,
this convinces me that for such an application, either you go to fluid-film plain bearings (air bearings as mentioned) or, if you want something much simpler to assemble, synthetic rubin / zaphire bushings running on a tungsten carbide synthered shaft. Why this:
- the bushings can be done with very good precision by specialized manufacturers which are not difficult to find
- the tungsten carbide shaft can be bought from tools manufacturers because they build many of their mills with this material (ITT, Freisa, Hitachi, and many many many others). If you want to step over the tools manufacturers, you can purchase a sample carbide bar directly from the carbide manufacturer (be sure to ask an "ultra-fine grain" grade) such as Konrad-Friedrichs, then have it ground on a centerless machine (with proper care, you can achieve total runout tolerances down to 1 micron over a 100 mm length)
This would not be the "material pair" giving you the lowest friction coefficient, because tungsten carbide is not smooth by nature, but such a shaft would be extremely hard even without coating. As an alternative, you can build the shaft in steel.
Just my opinion, though. I have no direct experience with extreme rpms.

Regards

RE: What type of bearing is best suited for extreme rpms

Hmm.  By my calculations, a 1 inch diameter rotor spinning at 2.5M rpm is going to have a linear surface speed of about 7400-7500 mph.

That's fast.

RE: What type of bearing is best suited for extreme rpms

(OP)
Ok this is what I calculated.

3.14 x 28(mm) x 2500000
-------------------- = surface speed in m/s (3663.33.....m/s)
   1000 x 60


Just dont know how should plug in the values, should I calculate how much a mm2 of the material weighs and take that x the surface speed ^2 (similar to calculating bullet energies) to get the force it will be subjected too.

Also what figures are the important ones at matweb? Yeild tensile strenght?

RE: What type of bearing is best suited for extreme rpms

Okay, I just read farther up where you were looking at 14mm for the test item diameter.  That brings the linear surface speed down to about 4100 mph.  

linear surface speed = angular velocity * radius
                     = 2.5e6 rev/min * 2*pi rad/rev * 7mm
                     = 4.1e3 mph

What's staggering to look at is the radial acceleration a point on the surface of a 14mm diameter rotor will see when rotating at 2.5M rpm

Radial acc. = (angular velocity)^2 * radius
            = [(2.5e6 rev/min)*(2*pi rad/rev)]^2 * 7 mm
            = 1.6e9 ft/s^2
            = 49e6 g

Can someone doublecheck these for me to make sure I'm not just being rock-brained and am missing something in the dynamics that is making my values come out much higher than they actually are?

RE: What type of bearing is best suited for extreme rpms

jistre

I think he's using 28 mm diameter rotor (based on his calculation) which is a bit higher than your original 1" assumption.  He came up with ~8195 mph, which seems in the ballpark.

all

Forgive me if I'm a bit dubious on this project.  I guess it's possible for an individual, who does not have a scientific background, to come up with an idea that will "blow away" what engineers and scientists at MIT have worked on four 10 years.  

But really?

His original question was answered.  As he doesn't want to share the details as it's not yet patented, I think we should let him finish his design.
 

Patricia Lougheed

Please see FAQ731-376: Eng-Tips.com Forum Policies: Eng-Tips.com Forum Policies for tips on how to make the best use of the Eng-Tips Forums.

RE: What type of bearing is best suited for extreme rpms

Oh, I definitely wish wmike the best and am not really trying to get deep into the design.  I'm just trying to drive home the safety part.

Good luck with the project and keep us posted!

RE: What type of bearing is best suited for extreme rpms

Just one more thought:  a piece of steel or even titanium moving at more than 6000 mph is somewhere around 3 times that of a high-power rifle bullet.  Whatever your protection scheme is to surround this turbine during a spin test, it should be capable of stopping that fragment.  I don't think a few tens of mm of poly-anything is going to do it.  Think multiple layers of steel and concrete or sand bags.  Take a gander at some of the videos (burst tests) on the Barber-Colman site that UncleSyd posted.

RE: What type of bearing is best suited for extreme rpms

(OP)
Hello Patricia,

The engine will be a type of tesla rotor (where the discpak rotates) plain and simple its just that (I just have some thoughts about how to implement various types of bearings that I just can go into, or someone will steal the design) thats why I wanted to know the which was the fastest bearings I could get hold of to get a head start.

EVERYTHING else I can discuss, no problems there, we're 2 on this, my partner, Dr Greger (different field tho) has been in the biotech sphere for 10 years so he'll be doing marketing, and the knows all the dirty tricks we will encounter and how to evade them, he also comes with other tips etc, we will be machining, thinking out tests etc for it toghether, without him pushing me to get stuff done I would still be at step one.

RE: What type of bearing is best suited for extreme rpms

Ok, Barbor-Nichols was the website I was thinking of, and they don't have the videos I was thinking of.  But this site does, as well as a lot of other useful information regarding high-rpm spin testing.  

www.testdevices.com

RE: What type of bearing is best suited for extreme rpms

T*st d*v*c*s used to test flywheels etc in a steel pit lined with lead bricks (to deform and also slide if necessary to act like a brake and dissipate the rotational energy).  The lid on the pit needed to be bolted securely since the lead would extrude upwards HARD during a powerful event.

The definition of "securely" got upgraded a bit after at least one spectacular event.

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