Will a high speed rotor made of Ni-Cr alloy bar machine and hold up as well as
one made from a forging?
The forging rotor may have a more radial structure and be able to spin at a
slightly higher speed, but that's not at issue here.
Bret Cahill
a
There are a variety of issues here.
The right quality bar can replace almost any forging, how high a quality bar
you need depends on the application.
I will address one issue that is sometimes overlooked. The properties of a
heat treated bar are usually measured near the surface, so even if the alloy
has sufficient ruling section to obtain the desired mechanical properties at
the starting size for machining, the properties in the central part of the
bar may be much lower. The ends of the rotor, with bearing journals drive
couplings etc, are probably made from this material. This can be overcome by
heat treating the bar at the proof machined stage but this often increases
costs significantly.
Could the bar stock end up more expensive
than the equivalent number of forgings?
I got a quote for forgings 7 times higher than
the equivalent amount of bar.
Forgings have the advantage of reducing the
overall amount of metal that is removed /
wasted but that might not be an issue for
small rotor disks.
Sounds ok because heat and rotational stress
increase in the radial direction.
I'm guessing by central you mean radial (r=0),
not axial (l=L/2)
I'm guessing this means the harder outer case
of the bar has been removed by, say, turning in
a lathe exposing the softer inner metal.
Might not be necessary. Aren't bearings
mounted on Al and titanium shafts?
Bret Cahill
at
bar
Depends on what you need in bar properties. Probably not if the application
is suitable.
a
alloy
at
the
Yes, bar is generally heat treated in long lengths and then cut to length
for you.
Yes
increases
The last failed high speed (20,000rpm) rotor I looked at, had a bent central
spindle. This central spindle was part of the same forging.
Would it have been appropriate to make the replacement part from a bar where
the centre is half the expected strength, I doubt it.
If you are making parts from drawings, you have to allow for engineering
design assessment (management of change) costs in any changes you make. High
speed usually makes a mess when things fail.
There are other potential issues e.g. radial strength, fatigue etc these
depend on design and answers are specific to the design. I would guess that,
for small production runs (replacement parts), 60-90 % of forgings can
economically be replaced by machined bar / profiled plate parts.
a
Be very sure the bar stock has the strength and all the other properties
that the forgings have if the part is in a place that can cause a serious
problem when it fails. The bean counter that fails to understand the
engineering of the part they are replacing can be held liable for the damage
caused when the parts fail. Sometimes the chance seems safe in every way yet
is sure to fail. If possible get the engineer that designed the part in the
first place to look at the problem if not get a quailed person to evaluate
it.
You don't want to be immortalized in history for causing a failure that
costs someone their life for a few dollars savings. There was a reason it
uses a forging find out why.
I'ld like to drive gas turbines down to the $20 -
$50/kW range, maybe even install them in
hybrids as a small, light weight, temporary
power engine. If it is only used 5% of the time
-- hauling the boat down to the dock -- who
cares if is only 15% efficient?
For general aviation the engine might be more
reliable and cost a little more, maybe $80/kW.
Flying through hubs wastes time and money
and fuel, not to mention those body cavity
searches. It's better to just know all 8 people
on your plane.
But right now I'm just trying to get an idea of
the costs involved.
Some guy at DOE said it would cost 10 billion
to start a gas turbine company. This is nuts.
The wars of the 20th Century caused gas turbines to be developed with unlimited
funding.
As a result the GT is vastly overpriced.
Bret Cahill
You should read Sir Frank Whittle's biography before you talk about
"unlimited funding" !!
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Steve
Frank Whittle was the genius of British technology who ran the group
that developed the Gas Turbine engine for aircraft prior to and during WWII.
As part of the trading of Wartime Secrets, the British donated their
Radar and Gas Turbine work to the USA in exchange for some form of
cooperation in the Atomic Weapons development.
There is some real fascinating technical history involved, if you
respect technological history, that is.
Jim
Some guy at NASA said 60% of the billions in
R & D costs of gas turbines was in materials
science.
It may be possible to prove beyond any doubt
that film cooling will always be necessary for a
high efficiency engine.
Bret Cahill
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