Vibration Measurements

[WARose]

A manufacturer I work with took some measurements of a piece of equipment at the bearings/housing and the supporting framing levels.....and sent them to me to because he wants the framing addressed. (Apparently some support framing was altered during the shut down (on the 4th) and nobody checked it out for vibration.)

Aside from fixing that, one question he has for me is: can I give him an idea as to how unbalanced the machinery currently is? About the only way I know how to do that (aside from bringing in someone who balances machinery for a living) is to model the situation, take a guess at the unbalanced force, and adjust that force to match the measured amplitudes (i.e. data they provided).

But is there a way to answer his question based on the data alone? (Which I could back check once the modeling is done.) I would think that the measurements taken at the housing/bearing level would be magnified by the fact the support is now shaking so badly.

 

[GregLocock]

If you think the machine is operating above the first resonance on the mounting frame, then you could attempt to work it out from the machine mass, ie assume it is free-free rigid body.

If the machine is operating in the stiffness controlled area of the frame then then you could use an estimated frame stiffness

If, as is most likely, you think a resonance is involved then you need to know the damping, is not available which from a single speed measurement'

Bash it with a hammer (even without a load cell) and record the vibration.

Hopefully Walt will chime in.



Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

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[Tmoose]

The equipment ran fine for years before the support framing was modified?
That is interesting, and certainly suspicious, but may not be the whole story. The modifications may have altered alignment as well as stiffness, and maybe some other things I can't think of right now.
The fact that the machine speed was also increased 20% as part of the modifications may be yet to be revealed.

Are there any vibration measurements pre-modification?
Does the vibration data include amplitude, frequency, and phase?

 

[WARose]

The equipment ran fine for years before the support framing was modified?

Yep.

The fact that the machine speed was also increased 20% as part of the modifications may be yet to be revealed.

I asked that question about a hundred times myself......they swear they didn't change a thing with the machine. (And honestly, looking at the steel shop drawings (for the new framing), I think I believe them.)

Are there any vibration measurements pre-modification?
Does the vibration data include amplitude, frequency, and phase?

The answer to all your questions is: yes. But their point on that was that the data is very old.....ergo, they want an update.

 

[Tmoose]

"looking at the steel shop drawings (for the new framing)...."

Are those drawings something you could share with your old buddies here on Eng-Tips ?
Along with some indication what was removed ?

===============

1X phase from Volume 1(The early days) vs 1X phase from today might offer a talking point.

http://lampx.tugraz.at/~hadley/ss1/appendix/resonance/resonance.gif

 

[WARose]

Are those drawings something you could share with your old buddies here on Eng-Tips ?

I don't think I should risk it.....I have a NDA with these guys. (And they are adamant about even their name being mentioned.)

Along with some indication what was removed ?

They basically took out some support beams (under a couple of the skids)....and put in something more shallow. (For the purpose of running some new pipe in a tight space.) I've seen the calcs on the new framing.....it checks out, but they ran numbers for the static loads only.

I appreciate everyone's comments. I think (for now) I'll just go with my original idea of modeling the situation and adjusting the force to match the measurements. (Then working back to figure the unbalance.)

 

[scheah]

I appreciate everyone's comments. I think (for now) I'll just go with my original idea of modeling the situation and adjusting the force to match the measurements. (Then working back to figure the unbalance.)

As Greg pointed out, your modeling estimate would be dependent on your damping guesstimate. I second the idea of a modal test to get the transfer function with driving points at the bearing housing. The unbalance forces are then back-calculated from the operating response. This could be checked for all speeds during shut down.


Best regards,
Jason

 

[WARose]

As Greg pointed out, your modeling estimate would be dependent on your damping guesstimate.

True.....but this being something supported on a (elevated) steel frame.....I don't think the level of damping is any big mystery. (I.e. we are talking about 2%.)

I second the idea of a modal test to get the transfer function with driving points at the bearing housing. The unbalance forces are then back-calculated from the operating response. This could be checked for all speeds during shut down.

I think I may suggest that.....but since it would mean extra dollars, I doubt they will approve.

 

[scheah]

but since it would mean extra dollars, I doubt they will approve.

Yeah, I understand how that works.

This paper may be a useful reference: [URL unfurl="true"]https://sem.org/wp-content/uploads/2016/01/sem.org-IMAC-XVII-17th-173805-The-Identification-Rotor-Unbalance-from-Measured-Foundation-Response.pdf[/url]


Best regards,
Jason

 

[WARose]

Thanks sk.