Rigid vs floating vibration measurement

[pugap]

We are looking for the correct means of measuring vibration in terms of should the object be rigidly mounted or can it sit on a foam pad.
The objects are either small psc motors (42, 48 frame) or motor/blower wheel assemblies. Based on different person's experiences, we debate on whether the object should be rigidly secured (ideally like in the end application, which isn't always possible) or if sitting it on a foam pad is acceptable.
Doing relative measurements (motor 1 vs motor 2) I suppose it doesn't matter, but to trouble shoot issues or establish a repeatable baseline measurement I would think it does.
I'd like to get some other people's experience plus reference to standards that are clear. I say clear because anything that seems to reference NEMA motor frame sizes and test methodology get a little wishy washy in these frame sizes.

 

[GregLocock]

I'd prefer to see it mounted as it is used, but that implies that you have some control over the details of installation.



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[reidh]

I agree with GregLocock. Even if your only looking at doing a relative measurement, it provides another source of error in that the foam pad is yet another variable. This is especially true if you have to repeat testing in the future.

 

[Tmoose]

Found a decent source here -

http://www.hq.nasa.gov/office/codej/codejx/Vibration_Spec.doc

based on the GM spec.
It has some rules as to what resiliently mounted motor really means, and how to do it, and what reasonable tolerances might be.
About the only danger is if the final as-used mounting has resonances, in which case good motors will act badly, but no test system can prevent that.

 

[electricpete]

I'm not sure what you mean about NEMA being wishy-washy, it is very clear. NEMA MG-1 says you can test resliently mounted or rigidly mounted. They define resilient and rigidly mounted and they give different limits for each case (0.12 ips for resiliently mounted and 0.15 ips for rigidly mounted).

I agree with the above comments that as a user, you want to see it mounted as close as possible to the way it's going to be used.

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