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

 

[BrianPetersen]

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.

 

[jistre]

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.

 

[jistre]

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

 

[wmike]

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.

 

[MrMyers]

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.

 

[unclesyd]

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

 

[cbrn]

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

 

[jistre]

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.

 

[wmike]

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?

 

[jistre]

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?

 

[vpl]

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 for tips on how to make the best use of the Eng-Tips Forums.

 

[jistre]

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!

 

[btrueblood]

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.

 

[wmike]

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.

 

[btrueblood]

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.

http://www.testdevices.com

 

[Tmoose]

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.