Does anyone have industrial experience regarding prox probe measured vibration with > 20 inch diameter shafts and .021 inch nominal diametral clearance??

Someday We will be commisioning our new 120 rpm motor with >20 inch dia journal bearings. The MIL specs generally allow for something less than 2 MILS pk (not pk-pk) for >>either<< shaft or housing vibration measurements. My previous hobo's experience with much higher speed fans, turbines and generators with smaller shafts suggest 1 or 2 MILS pk-pk would be the expected running condition. Anyway, we need to submit a test plan that includes some alerts and alarms. Right now I have proposed an arbitrary alert at 4 MILS pk and and alarm at 6 MILS pk, based on 2X and 3X the MIL allowable.

Thanks,

Dan Timberlake

From my understanding a typical alarm limit would be 50% of diametrical clearance. In your case 21/2 = 10.5 mils.

But I don't work with prox probe measurements much, so hopefully someone else will chime in to confirm or provide additional info.

Are the MIL specs looking at shaft relative vibration or integrated casing vibration? 2 mils (Why not pp?) seems like a very low target for the vibration.

It may be difficult getting the shaft runout to less than 0.5 mils pp on a motor of this size; try to specify allowable runout (<=0.75 mils pp, I like <=0.5 mils, but the vendor may take exception.). With this amount of runout 2 mils pp (The standard is pp.) is a low number.

At the low speed, what is the expected oil fim wedge? This will help determine the reasonable vibration. On a 50 or 60 Hz electrical machine of this size, you might expect infinite life on the bearings at 4 mils pp or higher.

Integrated casing readings on a slow speed machine would be entirely different.

MIL Spec = military specification. How about U?

Would you be saying Mill Spec as in your specific location?

As Bill pointed out, the standard is P-P displacement. If your are in the process of developing a specification, I'd go to something more realistic.

At 42 mils clearance you may want to see if the OEM or Bentley has something. My $0.02 worth.

Yep, as far as I'm concerned P-P is (was?) the "standard."

But, Since President Bush signs the checks, we gotta play by his rules.

MILitary standards.
You can join the fun here
http://assist.daps.dla.mil/quicksearch/

Document ID MIL-STD-167-1
Department of Defense
"Mechanical vibrations of shipboard equipment"

Figure 2 has curves for maximum allowable vibration. One for bearing (housing) and another for turbine shaft vibration. Displacement single amplitude - MILS

There is a "Standardization document improvement proposal" form on the last page. I tried to "see instructions - reverse side", but there is no reverse side, or even and address. Otherwise I would have proposed a change in units, among other things.

"From my understanding a typical alarm limit would be 50% of diametrical clearance."
That is a number similar to my recollection, but I'd like to be able to refer to some standards someplace.
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"At the low speed, what is the expected oil fim wedge? This will help determine the reasonable vibration. "
Operating speed is, and must be essentially 0 - max rpm (<<1000 rpm).
The very low speed requirement reportedly has brought on journal roundness and cylindiricity requirements (from a big name bearing designer) that are "challenging" to say the least.
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As always, any help IS appreciated.

Thanks,

Dan Timberlake

I don't know the exact situation and should not be held liable for my ignorance, but running a rotor at 1/2 the diametral clearance would likely damage the bearing in most situations. So, setting the first level alarm there would be a little large. (If radial clearance, 1/2 the clearance would probably be conservative.)

One has to take into account any resaonant amplification (if applicable) for a coastdown from the alarm setpoint(s) and the consequences.

Running with vibration of 1/2 the bearing clearance (diametric) may greatly exceed the normal bearing oil film at low speed. This could be a problem.

Bearing metal thermocouples are a good practice.