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mechanical harmonics

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PMD

Well-Known Member
Joined
Apr 11, 2015
Messages
1,745
Location
Martensville SK
Some of you may have noticed we have a rather experienced NVH guy on this site (i.e. Billski) and you might notice I get particularly curious when the issue of re-drives is brought up - mainly due to concerns over how thorough a design may have calculated, tested and PROVEN behaviour in that regard. Often people dismiss this as nit-picking, but I can assure you, it is one of the very basic parts of machine design.

While I am basically a grease monkey, I admit that I ***** myself out to the electrical industry to deal with mechanical components and systems on large and very large power transformers. On a little job to fix a issue at a cogen plant, the attached oil refinery asked if I might look at a small network transformer that had a ridiculous pressure gauge reading. Now, normally, the happens when there has been a disastrous spike in pressure (usually result of a boom-boom inside that exceeds a pressure relief device setting and vents the excess pressure, setting of an alarm and tripping the breaker that connects the transformer. None of this happened, but the gauge read a goofy number. I could find anything at all to explain, so I simply replaced the offending device with a shiny new (EXPENSIVE) on from same manufacturer (that makes nothing but power transformer devices). As soon as I got it mounted, the needle started bouncing all over the place. Large electrical equipment is of course surrounded in a force field (EMF) at 60 CPS (in North America). This will induce physical loads into metallic structures (in this case, the oil tank into which the windings are mounted) with amplitude that will vary with location, but frequency that is very predictable and constant. Now the EEs know all about harmonics, as they calculate and design around them all of the way up into the tens of orders, but it seems they never give much thought to the very same field that makes large electrical devices humm and buzz at 60 CPS.

My report on this issue was that the natural resonant frequency of the Bourdon tube in the compound gauge happened to be either 60 CPS or a harmonic of same (but in this case, I think first order). The resulting vibration over a long period of time might have worn the little rack and pinion and accompanying bearings to the point of where the rack jumped a tooth every once in a while on the pinion.

Why I post that here is to appreciate that EVERYTHING on an airplane engine has the potential to resonate with some immediately or long term destructive consequences. It is why I HATE wiring, tubing, hoses, etc. hung all over the place on Adel clamps. BUT: think of things such as vacuum pumps, prop governors, alternators, magnetos, etc. that are not only subject to large amplitude (due to propellor) induced vibrations, but from the torsional variations of their drive system due to crank angular velocity variations (and on 4 cylinder, 4 cycle or 2 cylinder 2 cycle or less cylinder counts) REVERSALS as well as the resonances of shafts and slop of gear meshes and you can see it is NOT a trivial pursuit.
 
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