New to the group, could use help with an equation

[racket]

Hi James

You might find that it would be prudent to go with a floating bush , clearances both inside and outside to produce more "flexibility" , our "brass bushes" have considerable radial clearances.

The TV94 rotor has tolerances of 0.003" - 0.007" radial clearance between the bush OD and the shaft tunnel housing , theres also a few thou clearance between shaft and ID of bush , axial thrust bearing clearances of between 0.003" - 0.010", the rotor just flops around in there before the lube is supplied to provide shaft damping for those various "vibrations" which afflict all shafts at high rpm

Heh heh , yeh , brain starain stuff :-)

Cheers
John

[propellanttech]

Aug 20, 2011 21:48:47 GMT -5 racket said:

Hi James

You might find that it would be prudent to go with a floating bush , clearances both inside and outside to produce more "flexibility" , our "brass bushes" have considerable radial clearances.

The TV94 rotor has tolerances of 0.003" - 0.007" radial clearance between the bush OD and the shaft tunnel housing , theres also a few thou clearance between shaft and ID of bush , axial thrust bearing clearances of between 0.003" - 0.010", the rotor just flops around in there before the lube is supplied to provide shaft damping for those various "vibrations" which afflict all shafts at high rpm

Heh heh , yeh , brain starain stuff :-)

Cheers
John

Do you mean a floating air bearing?? I don't know if that can be done at the pressures we are talking about. The area ratio would need to be perfectly matched to insure a true float (inside to outside ratio).

If it's an oil bearing, I would have to see how the bearing is restrained. It is something I haven't seen before, and I get around.

Is an interesting idea.

James

Still working to process the last document.

[racket]

Hi James

The bush does not need to rotate ...................it was just a thought , I don't know enough about air bearings to be able to comment with any certainty .

The ball bearing cartridge in the Garrett turbos have an OD oil damper between the cartridge and the housing , the cartridge is simply "pinned " at the central lube supply .

LOL, theres lotsa interesting turbo documents out there on the Net . ............all food for thought :-)

Cheers
John

[wolfdragon]

I'm going to be very interested in seeing if the air bearings James is talking about can dampen the whirl instabilities that occur with the shaft as it reaches and passes through those critical RPMs. I know that is one reason why the brass bushings with the oil seal are used, the viscosity at those pressures makes for an excellent damper.

[propellanttech]

Aug 21, 2011 9:19:47 GMT -5 wolfdragon said:

I'm going to be very interested in seeing if the air bearings James is talking about can dampen the whirl instabilities that occur with the shaft as it reaches and passes through those critical RPMs. I know that is one reason why the brass bushings with the oil seal are used, the viscosity at those pressures makes for an excellent damper.

I too am interested to see if the bearings will withstand the gyration.

I know the bearings are being used, but I know little more than that.

If the shaft wears the bearing material any, there will be a less efficient cushion, so a few tear downs will be necessary even if it appears to work.

James

[propellanttech]

Ok, so I have been conversing with a well known model turbine designer through email.

He advises I take a different method to my prototyping.

We have come to the conclusion (or at least I think we have), that bleed air bearings are of an unknown possibility. They could work, but they could fail massively.

The method which I have decided to use would be:

Use the guts of a T04 turbo to build a test bed for testing.
The first design will use bearings like a model engine, but have provisions for change.

The bearings will be changed to an external air supply after the unit is functional.
This will allow the bearings to be run at different pressures (and volumes) to find if bleed air would be sufficient.

I do understand there is a huge possibility of the unit being totally destroyed. This could happen more than once.

The main problem I see is the current bearing arrangement will probably not work with the air bearing arrangement. This is not due to the radial component of support, but the axial load on the shaft when running. This will be the hardest hurdle to overcome. I know what must be done, but designing for it, and the ability to change is not so easy. The shaft, inlet, and exhaust cone may all need to be different from roller type bearings to air bearings.

The unit will probably need 4 air bearings, when 2 regular bearings would support the shaft normally.

This will be a uphill battle.

Right now I'm working to put the critical rotational calculations into a spread sheet. This will be the first part to insure the shaft design will withstand the rotational speed.

Wow......it's going to be interesting....

James

[Richard OConnell]

This should be interesting to say the least. I've always followed the belief that you learn more by failures than anything else.
Best of Luck
-Richard

[racket]

Hi Richard

I can second that ................even if its not the most pleasant experience at the time :-(

Cheers
John

[propellanttech]

I hate to ask, but does anyone have a design based on turbocharger turbines?

I thought I hand one, but I don't. I have looked around for the Phoenix design, but can't find anything.

Accurate drawings will help with 3D modeling.

James

[Johansson]

As John has experienced first hand with his Fat Mumma engine there is issues with converting a radial turbocharger turbine to ball bearings, the axial force becomes so high that you need to have a couple of tricks up your sleeve to make the bearings survive.

The reason is that with an axial turbine the combustion pressure acting on the turbine wheel and the thrust sort of equals each other out while with a radial turbine the two forces acts in the same direction and put a very high load on the bearing taking up the axial force.

The Phoenix did work but it was a relatively small engine whereas your larger engine will have a much higher axial load, I am not saying it isn´t doable but it won´t be a walk in the park for sure.

[propellanttech]

I'm going to start off with a much smaller design. It is probably much more like the KJ66 size. I figure this would provide information about if air bearings are possible.

I'm working to draw up the engine in CAD (3D). It is however not an easy task without some reference to the NGV part. I have no idea of the vane angles with respect to the turbine.

James

[racket]

Hi James

Vane angle is that angle that produces the required throat area , that produces the required gas velocity from the available mass flow at the designed gas temperatures and pressures .

Some turbine wheels have "wide" tips , overly large tip heights , this then requires a "shallower" angle or more blades to tighten up the areas to that required , it generally requires some "give and take" when using ready made parts as we do , very rarely will a turbocharger provide ideal gas turbine configuration .

LOL, my "CAD" drawings are rough, scratchy, pencil on paper , which I can never seem to find once the engine is built :-(

Cheers
John

[propellanttech]

Aug 27, 2011 0:00:31 GMT -5 racket said:

Hi James

Vane angle is that angle that produces the required throat area , that produces the required gas velocity from the available mass flow at the designed gas temperatures and pressures .

Some turbine wheels have "wide" tips , overly large tip heights , this then requires a "shallower" angle or more blades to tighten up the areas to that required , it generally requires some "give and take" when using ready made parts as we do , very rarely will a turbocharger provide ideal gas turbine configuration .

LOL, my "CAD" drawings are rough, scratchy, pencil on paper , which I can never seem to find once the engine is built :-(

Cheers
John

Well, if I do ever get it drawn, I will freely share the files.

I'm starting to think a axial turbine may be the way to go. This is mainly because I can not figure a way to use the turbine with a custom shaft.

Are you removing the shaft from the turbines?

James

[propellanttech]

Here it is....the first drawing of mine. This is based on the J68 turbine.

When I get these finished, I will be willing to share the files.



James

[Johansson]

If you are going to build a much smaller engine first you might as well build a well tested design like the KJ66 or MW54, since you cannot simply scale the engine up later it feels like a waste of effort to build one from scratch if all you want to do is to test air bearings in a working gas turbine.

If I were you I would build the full scale engine directly instead of spending years building a scaled down version of it.