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Re: DYNAMICS: Machine Run-Up

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On Dec 10, 2008, at 12:55 PM, Bill Polhemus wrote:

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(--nospam--at)   | this distance" (last words of Gen.
.......................................| John Sedgwick, Spotsylvania 1864)

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