Paul,

I read that portion of the book too. Seems like they just rough mill
them then final grind and lap for accuracy. Then again this book was
written in 1969 when cnc equipment sucked so I bet it is much different
today. Also by there manufacturing methods back then lead to much
variance in the rotors as written about in the compression ration of the
rotors in that turbo 2 design paper you sent out. So two point
approximations and such are worthless these days.

I think aluminum is the wrong direction to go. Talk of any of the exotic
alloys that it would take to make it work well will never happen and we
all know it in terms of cost,saftey,etc. With aluminums greater
expansion rate you would have to increase the clearances which means in
start up and part throttle the thing is going to blowby a bunch.
Aluminum does have a fatigue life and is just to damn soft to take any
kind of oops factor either from assembly or coughing seals. Titanium is
way to prohibitive after re-reading the nasa paper on it useage. Special
coatings on the seal grooves and needing chemical machining to take off
the out layers after heat treating is just unrealistic. Like I said
before Steve B. gave me an idea to do a furnace brazed sheetmetal rotor
after he mentioned they did it on the side housing with good results.
Welding is good but brazing is better since you would be able to get all
the fins/vanes/whathaveyou to join instead of just the outer edges where
the weld can go. A multi-piece steel rotor brazed I think is the
direction we should concentrate on. The only thing to beat steel in
strength to weight, modulus to weight,etc is carbon fibers and titaniums
which are not usable. If you went the extra mile and made the rotors out
of 4340 Hardened to RC 48-50 the yield is 180kpsi and ultimate is
230kpsi. Not much is going to touch that. 1020 or 4130 is more realistic
but with steel you have a ton of choices.

Rob Woods

The problem is wall stiffness. To get it down to the weight of aluminum the walls would have to be one third as thick as cast iron. About the thinnest one can cast in iron is about 3/16 of an inch. That means most of the walls and webs would be 1/16 thick. I don't think that is going to be stiff enough for the rotor faces.

What is the NASA paper you are referring to?. Can you upload a copy?

If Ted wants to do Titanium let him try.

BTW Rob please set your left margin to zero so it does not screw up the forwarding.

Paul Lamar

As far as the ringing is concerned I am not to familiar with the ins and
outs but the connection of the ringing would be pathed through the roll
pins of the gear, journal bearings,etc so I would assume that would take
care of it. I am unaware of any system in an engine that doesnt benefit
from stiffness.  I am against titanium for the reasons i listed in the
reply i just sent.

Rob Woods

It is local panel vibration due to the shock of combustion.
The roll pin loads would definitely be different due to the reduced
mass.

Paul Lamar

 Rob,
Can you upload that nasa paper you're reffering to?

Thank You
Ted

Paul,

Dont know what the zero margin thing is. I am just responding in the space yahoo gives me and I dont see any options for this. Please explain to me what I have to do.

I dont follow you on your dimension conversion. If you were to cast the rotor out of aluminum to be equal in stiffness (since cast iron is only a little above aluminum) the aluminum would have to be about 2 times the thickness since the UTS and YS are about 1/2 to 1/3 the strength of nodular cast iron. If you were to go from cast iron to steel the E value is about 2.5 times greater. Not to mention you can get really high UTS and YS number out of it. But lets jst say we do it out  or 1020 then the UTS and YS are about the same but still a stiffer material. So you could go do half the thickness of the cast iron webbing which i think is more than enough. The centrifual loads wont effect anything but the base ring area so all you have to do it provide a 1000 psi load on the combustion chamber in FEA to see what the deformation is which I dont think would be that great. Point being with the increased stiffness and strength of steels over aluminums I dont think that it would be any kind of a problem to go thinner wall. If anything you could always go to a 15 pin for additional strutting if it is an issue. Until it gets modelled and worked out it is all conjecture. I just know which very strong alloys it wont be much of an issue.

1000psi x 15in^2 = 15,000lbs load (but this is only peak) supported over the face thickness and the 4 webs. For comparison a AN-NAS 1/4 bolt can take about 5000 lbs of load in double sheer. That area time three pales in comparison to the amount of metal on a rotor and sheer forces are the worst kind in terms of material strength(parallel vs perpendicular to grain axis).

I sent that paper to you about 4 months ago or maybe it was the one that was too big for you to take. I will copy and paste the good and bad about the ti rotor they did. The negatives outweigh the positive since the amount of post work is prohibitively expensive for small scall operations.

Do you have any scarpped rx-8 rotors handy anyone? I would like to get 2 to cut apart and disect for comparitive engineering purposes.

Rob Woods.

That explains it. Yahoo uses their own email program I think
and I don't think you can set the margin.

Aluminum density is .1 pounds per cubic inch and cast iron is around .3 pounds
per cubic inch. Strength at room temp per se is probably not the problem as 7075 T6 is 80 kips. Strength at 500 F is an issue and that is why you want to use Be/al allow not to mention the density is .076 pound per cubic inch.

If the aluminum was 2 times the thickness of a cast iron  panel ignoring modulus of elasticity for the moment stiffness would be about 100 times stiffer as it is a fourth power function of the panel thickness. We are not talking tensile stiffness here. We are taking about panel stiffness. Roughly thickness to the fourth power. You cannot go to half the thickness of cast iron with steel and expect it to be as stiff.


http://www.engineersedge.com/beam_bending/beam_bending1.htm
This is a simplistic way of looking at it because I can't find
simple steel panels uniform pressure loaded on the internet so far.

Plus the panel support structure enters into the equation.
Nothing less than a good FEA is going to help us out here.
A simple FEA would be a 3 by 3 by 3 box with one side open.
A constant pressure applied to the opposite panel would give
good insight. What we are really interested in is the panel resonant
frequency.


Paul Lamar