Meaning of Amplitude Change and Frequency Shift after Random Vib 1

[fang54]

We ran a random vib. on our component. After the vib test the lab discover that their slip table had some lubrication issues and fixed it, and we ran the post-sweep, and compared it to the pre-test sweep. Everything matched line to line, except one peak (@ ~60Hz)now is lower. we ran another post-sweep, and the 2 post-sweep looked exactly the same. Now we are left to explain the discrepancy to our customers. I want to eliminate and be ready for a few possibilities before blaming it on the unlubed table.

So, I understand that a frequency change of the peaks is indicative of growing of cracks/damage. What about the amplitude change? What does that usually indicate? slippage? or are the amplitude and frequency change usually linked?

Thanks for any insight.

 

[GregLocock]

A change in amplitude without a change in resonant frequency normally implies a change in damping. Assuming the change in amplitude is large then that seems unlikely. I think it is reasonable to give your cutomer the history and say that the rig was probably responsible for the change, as you have double checked the result.

Cheers

Greg Locock


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

http://eng-tips.com/market.cfm?

 

[msquared48]

Sounds like a modal shift to me.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[dktoao]

From my experience you can expect the frequency shifts to align as long as the part is designed to withstand the loading, amplitude shifts on the other hand are much harder to control and to explain when they occur. Most customers will give us a 10% frequency shift spec with no amplitude shift spec. With this type of spec we usually don't have any trouble meeting the requirements. However, on a recent test we had a somewhat tight amplitude shift spec of 20% and a fairly complicated fixture with multiple control accelerometers. I had a heck of a time with the first two parts we tested because we kept missing the amplitude spec and had to constantly call the customer and let them know. The frequency was never off by more than a percent or two and there was not the slightest visual evidence of any damage. We eventually came to the conclusion that the mounting interfaces were "settling". Meaning that a small amount of slipping was occurring and either causing some bolts to come in contact with the sides of the thru holes or just that small inconsistencies in the pressure distribution at the flange interface due to tightening of bolts were evening out. Anyway, after a big to-do with our cust, we mounted the next unit and I told the technician to:

1. Tighten all the screws holding the unit in place hand tight.
2. Once all the screws were in, back each off 1/8 turn.
3. Torque screws while holding the unit to what I judged was the most settled position (i.e. with gravity) to make sure that the turning of the screws did not reposition the unit.

After all of this we got better results than before but still had one or two signatures that went out of spec. This tells me that we were somewhere in the ballpark with our initial conclusion.

Anyway, I am not exactly sure what purpose the amplitude shift spec serves. Maybe someone else can enlighten me on that.

 

[MikeyP]

Is it a peak or a spike?
What frequency is your local electricity supply?

M

--
Dr Michael F Platten