Danny,

Watch out now. This ole WV boy is headed that way next week. I know where you live.

D

Just checking to see if you are watching this one.

I understand the relationship of WV to VA. I am not sure about the Arkansas to Texas.

WV is that illegal state that the yankees got during the war for independence with the north, and no one complained from VA. We still haven't quite won that one.

Bill,

Around here some people still refer to it as the War of Northern Aggression.

Living so near to Richmond, we are very well versed in one way of looking at it.

I wouldn't want to claim Arkansas as home... that's where Clinton came from.

Whoops... Bush came from Texas. On second thought, I think it's a bad idea to judge a state by its presidents.

"This couldn't have been an instability could it?"

Wouldn't a bearing that completely unloads as the unit heats up cause an "instability"? If not, I'm not sure what you mean, Bill. I think there may have been oil whirl involved, but it was 15 years ago, so I can't remember exactly. Yes, we tried using our meters with the shaft riders and didn't conclude anything. That's why we brought in an "expert" (at least they charged enough that we thought they were experts). I learned a long, long time ago that when I don't have an answer, I just say "I don't know."

I don't have any problem with being born and raised in Arkansas. I am perhaps a little embarrassed at having gotten my college education in Mississippi though. One of the things I like about Arkansas (most of the South actually), is everyone trusts you until you give them a reason not to. Also, while we don't think we are as dumb as everyone thinks we are, we also don't think we are as smart as everyone thinks they are.

Do you know what being smart (intelligent) gets you? Absolutely nothing.... unless you have common sense and the ability to work hard to go along with it.

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Whoops... Bush came from Texas. On second thought, I think it's a bad idea to judge a state by its presidents.

Yea, like Washington and Jefferson.

I have a feeling that Bush and Clinton's tombstones will highlight being president. Jefferson found other accomplishments that were noteworthy rather than list President.

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Do you know what being smart (intelligent) gets you? Absolutely nothing....

Of course you're speeking from experience as usual here.

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Wouldn't a bearing that completely unloads as the unit heats up cause an "instability"?

This depends upon the situation. Instabilities are system issues, and one bearing unloading can cause an instability in some situations but not always. Look at poor vertical electric motors with fluid film bearings - sometimes it might be good that they aren't perfectly aligned (to the magnetic forces too), but with proper bearing design these can be stable.

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Is bushy boy #4? Bushy boy is a yankee from Conn

I believe the term carpet bagger still applies.

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Sounds like rate reduction time!

What, exactly do you mean, Sam?

Rusty, You could relable this post "old wives tales" and get similar responses!

I vote for casing measurements...

Walt

Hi all,
Interesting to learn about American history but now to the point gentlemen, there are some readers from outside USA too.
Beware, in Europe we have much longer history.
The basic consideration should be: what type of measurement method provides the best signal-to-noise ratio, at acceptable costs, for a given machine? My rule of thumb: if machine is running well above it's critical speed(s) then prox probes are preferred (often these type of machines can easily justify the costs associated with proximity probes and signal conditioning stuff).
Regards,
Arie Mol, from the old Continent

By Golly, I'm both amazed and amuzed at the responses that has dragged this thread to a point where one wonders...What was the original questions again? I guess Rusty must be wondering where did I go wrong, in posting the question.
I've always believed in the 'KISS' philosophy, if casing measurements had worked for me for other similar units installed at site. It would be difficult for me to justify both to myself and my superiors, why I need to spend more money on a different system that may or may not add value.
Hence, my vote will be against prox. probe installation.

Cheers...Rajan Muthukrishnan

Arie,

The original question was about large centrifugal fans. THese fans have motor, flexible coupling, and fan rotor on two bearings. Bearings are typically journal type and with water cooling. Common design practice (USA) has been to design rotor 1st critical to be 20% above service speed. This is rarely the case in practice, since I have seen 1st critcal to be above, below, and at service speed.

These fans typically have steel/cast bearing pedestals and concrete foundations. The concrete foundation may rest on bedrock, concrete/wood piles, or be a floating mat on compacted fill. I have seen natural frequencies near/at service speed that are dominated by deflection in pedestals and/or foundation.

The rule-of-thumb (old wives tale) about rotor/casing weight ratio for selecting proximity probes is a poor choice for this application. If someone has their heart set on installing proximity probes on large fans, then that's fine. Bearing housing sensors (accelerometer or seismic velocity) are very adequate for permanent monitoring, fault diagnostics and in-place rotor balancing.

Walt

This is still an interesting discussion, even if we 'drift' a little....

One fan vendor, who builds a lot of fans and probably does more field service work, proposed in their proposal the installation of vibration "switches"... you know the kind, large metal box, stuck onto the side of the pedestal, which almost never work. That tells you what their state-of-the-art is. This is from a company that works on these fans every day. I doubt they've even seen a fan like this with prox probes on it.

Hi all once more.
Just an addition to my yesterday's posting and Walt's response. Over the years I have balanced ++MW fans of the type and size we are talking about here, mostly in power stations all across the globe. Housing accell/seismic sensors everywhere. With one exception: in the US only I met prox probes (example: 4P, WI)! So, isn't local tradition playing a decisive role too?
Regards, Arie Mol, NL