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RustyCas

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I had a customer that recently reconfigured a large bridge crane in a mill.  On the aux hoist drive, they removed a large magnetorq and added a long jack shaft in its place between the drive motor and the gearbox.  Said they were having "high vibration" at full speed (900 rpm @ 60 Hz) but not at 75% of full speed.

Data (below) confirms high vibration at 1X rpm and a bump test (bottom plot) confirms resonance.  Do you think it would be possible to design a system with resonance this close to running speed?  I worked out the necessary reinforcement while we were up there and they were working on it when I left.  We'll see tomorrow how it turned out.  Hope I was right, because it required considerable welding working from a manlift about 50 ft up.

Aux_Hoist_Drive.png  Aux_Hoist_Drive_Bump.png 


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Danny Harvey

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That's a pretty good shot.

It looks like there is another about 100 cpm lower. Hopefully, the modifications will take care of both.

Maybe that's why manufacturers always say to consult with their engineering department before making modifications.

What is the component that has the 891 natural freq?
RustyCas

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The data point in both plots is motor inboard horizontal. About a 200 hp motor, sitting atop a metal pedestal (more of a “stand” really) about 24” tall, which is welded to a 5/16” deck plate with minimal support underneath (there were some stiffeners of 1/4” plate). Really only had to look at it to know what was going to happen when they turned it on.

Manufacturers are given way too much credit for their understanding of stiffness and resonance.

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MarkL

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I'd agree with that last sentiment Rusty, I had just today, a vertical 300Kw seal oil motor in a CCGT power plant showing resonance again, mainly high 1x run speed in one direction(red) highest amplitude at the top bearing.

Time and time again these large vertical motors are used in power generation industry on either seal oil or boiler feed water and they seem to be ripe for resonance issues, but still, they persevere using them. Evernote Snapshot 20191015 123620.png

RustyCas

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That bump test was done using a 4x4 about 2 ft long as my “impact hammer.”

I throw shade quite often on MA because of the simple problems that most of the videos demonstrate, especially on LinkedIn. I tend to think of MA as a shortcut, used to reduce the “real work” of vibration analysis. But then, faced with a suspected resonance, rather than going to the truck and pulling out a force hammer, cables, accels, and adapters to do a proper “impact test”, I usually just grab a rubber mallet or chunk of wood and do a quick “bump test”. So really, I’m no better I guess, except maybe for not paying $30K for my shortcut.

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OLi

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[thumb]
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fburgos

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Well done the old 4x4 never gets Rusty
MarkL

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Quote:
Originally Posted by RustyCas
That bump test was done using a 4x4 about 2 ft long as my “impact hammer.”

I throw shade quite often on MA because of the simple problems that most of the videos demonstrate, especially on LinkedIn. I tend to think of MA as a shortcut, used to reduce the “real work” of vibration analysis. But then, faced with a suspected resonance, rather than going to the truck and pulling out a force hammer, cables, access, and adapters to do a proper “impact test”, I usually just grab a rubber mallet or chunk of wood and do a quick “bump test”. So really, I’m no better I guess, except maybe for not paying $30K for my shortcut.

Not Sure who mentioned Motion amplification? but I don't have that luxury, have seen it and it's impressive alright but for now the old fashioned methods will have to suffice.
the good old lump of timber always works well. 
ivibr8

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Rusty

IMHO  whatever provides you with the simple truth is usually good enough.

So if you can identify a resonance problem with 4x4, then hats off to you; you are a practitioner of the KISS principle.  At this point in the solution, that may be all that's needed.  So stiffen the structure (or loosen if it makes sense) and move on to the next problem.  Congrats!  [biggrin] 

Jim P
RustyCas

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I wish I’d gotten deeper into impact testing and FRFs. It’s interesting to see just how little force it takes to excite a resonance. I always wanted to do a FRF on a “flexible” fan system, and then add a weight to generate a known (calculated) force to see if the response is as suggested by the FRF. Hopefully I have - or just ‘take’ - more opportunities to do that before I take down my shingle.
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OLi

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My customers are rarely prepared to pay for the fancy stuff, they just want to get it "balanced" and not vibrating even if that is not the problem. If they do want to know it is a many hundred T hydropowerplant that doesn't even flinch by a 10"x10" 5m long "hammer" so you rarely get that happy. I once made a ODS using a VFD balancing training kit borrowed from the ste and bolted to the frame on a 13m hi Calender stack that was fun and actually worked ok. I hit it with a Tambour also for good measure it also gave a nice result but not so detailed.
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Danny Harvey

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Reply with quote  #12 
I would NEVER use a 4 x 4! It's too big to get a good grip.[wink]

"Manufacturers are given way too much credit for their understanding of stiffness and resonance."

I agree in general but in this case it sounds like there wasn't a problem until your client made some modifications without the manufacturers input. That makes it kind of hard to say that THEY don't understand stiffness and resonance.

So was the 891 the reed frequency of the pedestal?
electricpete

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Quote:
Do you think it would be possible to design a system with resonance this close to running speed

Haha, I know a guy that can do that easily. His name is Murphy.  (he has a law). 

Sounds like a good diagnosis.  The thing I wondered about is whether the 900rpm resonance you’re seeing might be a critical depending heavily on jackshaft flexibility without much influence from support stiffness, such that support stiffening wouldn't help (i.e. support is already much stiffer than the shaft before stiffening).  

Out of curiosity, do you have the length and diameter of the jackshaft? 

electricpete

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Quote:
electricpete wrote:The thing I wondered about is whether the 900rpm resonance you’re seeing might be a critical depending heavily on jackshaft flexibility without much influence from support stiffness, such that support stiffening wouldn't help (i.e. support is already much stiffer than the shaft before stiffening). 

In case it wasn't clear what I meant, look at Figure 7 here https://pdfs.semanticscholar.org/b36a/49a722acc3d42b2dec169cb819dfab9a8aa3.pdf
The horizontal axis is bearing stiffness and vertical axis is resonant frequency. Let's say the critical is the first critical way over on the right side of the curve.  The slope of the curve is almost horizontal there... changing bearing stiffness (same as support stiffness in this simple model) isn't going to change resonant frequency much.

Knowing the jackshaft diameter and length you could estimate the simply supported critical speed of that shaft.  That is the resonant frequency with infinite support stiffness under this model.    If that simply supported resonant frequency is somewhere just above 900rpm, then stiffening probably won't help much. 
ivibr8

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Reply with quote  #15 
Pete
I have used M Leader's paper as a go-to guide for the very reasons you mention. 

So I had to re-think myself and this may be a case where bearing stiffness has little to no effect on the changes Rusty was seeing. 
If the impact was on the structure though, I wonder how it may have transmitted the energy to the rotor shaft if it was near a node?? 

Rusty, maybe I didn't read this thru properly -->was the impact on the rotor itself or the "structure"? and where did you measure response?

In any case, I do like the idea of using Malcolm's paper to "estimate" a rough rotor resonance using various values for the modal mass to see it this is a possibility


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