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RBT

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Reply with quote  #16 
My 4x4 hammer works great!!!

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Vibe-Rater

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Reply with quote  #17 
Trick is to excite the wn of interest. For me that is usually a paper machine felt roll whipping. So a fist will do. You could use a 4x2 but mind the double bounce. I set up an 8 average normal data collection point and away I go. Wait for the ring down to be complete and strike again until finished.  Works nicely. rgds

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RustyCas

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Reply with quote  #18 
How do you know when the ringdown is finished?  


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Curran919

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Reply with quote  #19 
Quote:
Originally Posted by Vibe-Rater
Trick is to excite the wn of interest. For me that is usually a paper machine felt roll whipping. So a fist will do. You could use a 4x2 but mind the double bounce. I set up an 8 average normal data collection point and away I go. Wait for the ring down to be complete and strike again until finished.  Works nicely. rgds


If you have a machine with one low natural frequency and low damping such that it rings for 10s, how it is struck indeed matters less. The impact fidelity indeed has a stronger effect on the first few periods of the measurement. With damping >5%, that's all you get, so you gotta have a nice impact. If its ringing out longer, just cut off the first second and you can get some nice results and won't even have to average over many impacts (also important then to have nice consistent bumps).

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Originally Posted by Rusty

How do you know when the ringdown is finished?  


If the unit is so lowly damped that the vibe is above the noise floor after 10s, then I find it entirely acceptable to use the hand damping method, especially if you are going to be using exponential windows on the data. Just hold on to it and the vibe should disappear (if the object is small enough). This won't affect the previous acquisition, and gives you a 'fresh slate' for the next acquisition.
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Reply with quote  #20 
Quote:
Originally Posted by RustyCas
How do you know when the ringdown is finished?  



By looking at the waveform live as per above plot.  Also by feel.  But I am used to a particular component - Felt rolls. they tend to have a wn ranging from about 25 - 35 Hz. So inother words quite low. rgds
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