> I've got a machine skid with excitation loads from the vendor
> running full-out at 30,000 RPM, and he recommends a linear
> relationship for speeds less than that (I.e. Half the load
> amplitude at 15,000 RPM). Oh, and the skid's sitting on a steel
> platform, which is why this is critical.
Your vendor isn't considering the effects of the supporting
structure. And I'd also quibble with the assumption that the load
varies with the machine speed.
If the loading you're talking about originates with some small
imbalance, the force it generates goes as the rotational speed
squared (the centrifugal force of a rotating mass is Mrw^2 where m is
the mass, r is the distance from the mass to the rotational axis and
w is the rotational speed in rad/sec. But the linear variation with
speed is conservative.
When the machine is mounted to an elastic foundation the loads
imposed on the foundation are greatly magnified at points of
resonance. the magnification can be as high as 50-100 times the
equivalent static load. You can quantify all this by looking through
one of your dynamics textbooks in the chapter of simple harmonic motion.
Don't panic right away though. Most structural frequencies lie
between 1 and 50 Hz. Your machine loading is pretty low in that
frequency range so even though the loading is amplified, you may not
break anything.
30000 rpm is pretty fast. I worked for a guy once who got involved
with spin testing of turbine rotors, which is usually carried out in
a pit. If one of them gets away gyroscopic forces will keep it
upright for quite a while until it slows. Bryant saw one break loose
and he said it just seemed to walk around the test pit and whenever
it touched something like some instrumentation the thing just
vanished. Before slowing down it was walking through concrete block
walls like they weren't even there and flinging stuff all over, like
it was very very angry.
Christopher Wright P.E. |"They couldn't hit an elephant at
chrisw@skypoint.com | this distance" (last words of Gen.
.......................................| John Sedgwick, Spotsylvania
1864)
http://www.skypoint.com/members/chrisw/
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