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Curran919

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Reply with quote  #31 
Quote:
Originally Posted by electricpete
(starting about 1:20 in) - I had to do a double-take where the guy is facing us writing on the glass and it still comes out the right direction from our viewpoint.  Either they digitally flipped the video or that guy has got some crazy coordination to write backwards while he's talking. 


Look at the buttons on his shirt. The video is definitely flipped. Quite clever.
MarkL

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Reply with quote  #32 
No, as much as I would like to I get frustrated as I hate to see machines wrecked due to short foresight and the general lack of giving a sthi by most production masters.
John from PA

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Reply with quote  #33 
Quote:
Originally Posted by electricpete

 

Speaking of rigid body motion…

 

(starting about 1:20 in) - I had to do a double-take where the guy is facing us writing on the glass and it still comes out the right direction from our viewpoint.  Either they digitally flipped the video or that guy has got some crazy coordination to write backwards while he's talking. 



Maybe his name is Leonardo? [biggrin]
RustyCas

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Reply with quote  #34 
To clarify just a little on “beats”... if you have two fans that are near each other and both have a significant imbalance, and the running speeds are 2 rpm different, then you will get a beat 2 times per minute. Twice each minute the amplitudes will “add” and you get maximum vibration, and twice per minute the amplitudes subtract and you get minimum amplitude. Typically - but not always - the machine with the highest imbalance will have the highest amplitude. The best way to confirm a beat is simply to watch the spectrum with the analyzer in “live” or “monitor” mode - the peak will rise and fall periodically, in tune with the beat you can feel and hear.

If however you are seeing varying phase readings at 1X, it could be that fan speed is varying. You can’t always “hear” belt slip - they don’t always squeal, squall, or squeak when slipping. Slip is usually not constant and fan speed can vary significantly; the speed will often increase and decrease almost rhythmically. As the fan comes up to full speed, the belts start to slip as the power requirement exceeds the capacity of the belt ‘system’ (loose belts decrease the power capacity), and the fan starts slowing down as the ‘slip’ increases. At some lower speed the power requirements decrease (fan moving less air) and the belts start to grab, and the fan starts speeding up. And the cycle repeats. Once had a fan up on a tower where the resonant frequency was between the top speed and bottom speed of such a belt slip cycle. So it was passing back and forth through resonance. Add in a significant fan imbalance, a flexible tower with all sorts of attached handrails, light masts, air lines, etc. and it was quite a show.

So, finally, a question: is there any discernible beat-like vibration or noise present? Have you verified that fan speed is constant using either a tunable strobe, or by watching a tach signal? I record motor speed and fan speed for every v-belt driven fan I monitor, every time data is collected. Plotting the speeds will show a fixed ratio if the “drive line” (sheaves, belts, belt alignment, and the design) is good. Fans that are “sped up” once in the field (either by VFD or sheave changes) can easily exceed the power capacity of the belt system - seen it many times. Fans are often “designed” to run just below resonance, because this is the cheapest way to build them. Moving the resonance farther away (above) running speed requires more stiffness, that is, more steel, welding, and money. If you speed the fan up, or the system loses stiffness (loose fasteners, broken welds, corroded steel) you will likely run close to resonance.

Vibration isolators simply give the ‘unit’ a low natural frequency and isolate the base from the unit. But you can still have resonances in the unit which you will measure, but which you won’t feel if you’re just standing nearby.

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esresec

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Reply with quote  #35 
Quote:
Originally Posted by RustyCas
To clarify just a little on “beats”... if you have two fans that are near each other and both have a significant imbalance, and the running speeds are 2 rpm different, then you will get a beat 2 times...


Wow Rusty, first of all thanks so much for such as detail explanation. Very instructive for me all the belt capacity and beats description, taking note of that for sure. If tomorrow I can put my ass on the chair, I'll send some "long" (about 1 second maybe) waveforms we have. From historic measurements I can tell yo that 42Hz frequency is always fixed, but I have to check properly.

Also still waiting for CoastDown measurements, I'm afraid that they'll send me when I take my vacancies...
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