Exciting at nodal point

[deesans]

Hey all,

I have a question about resoance frequency on a simple cantilever beam. I am wondering what would happen if I excite a beam at a nodal point.

I am tryig to understand the mode shapes at different frequencies. I know this can be done by a simple modal analysis, by using accelerometer and impulse hammer. If I excite the beam near a nodal point how will the mode vectors change w.r.t exciting at antinodal point.

Thanks
Deepak

 

[FeX32]

The mode shapes never change.
If you excite at a node point of a particular mode shape you simply are not exciting that mode. (but you are exciting others)
I don't think that will work with a hammer test though. You will always get the sums in this case.


Fe

 

[GregLocock]

Yup. Rather more exciting is if you are mounting a longitudinal powertrain and can move the trans mount to the node of first bending. Suddenly a large lump of noise just goes away.

Cheers

Greg Locock


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[SomptingGuy]

Or as was the case with the first vehicle I worked on, if you have the mount near the 1st bending antinode (i.e. the end) and connect it to a nice mobile piece of the bodywork, you get to sit inside a speaker cavity.

- Steve

 

[GregLocock]

XJS or XJ40 (can't remember which, it might have been both), Ford Falcon, Sherpa van all suffer from that.

The screwdriver test is the ideal tool for locating the rear mount on a longitudinal installation, sadly the Health and Safety implications mean that an accelerometer survey is more appropriate (and I grudgingly admit might give more useful results).



Cheers

Greg Locock


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[drmuh]

Hi,
Nodal points will change for every frequency at which you are exciting the beam (different mode shapes). However, I would like to know how do you know beforehand where the nodal point will be with a hammer hit for different frequencies? I think a shaker will be more useful.
Thanks
Mert

http://www.novosim.com

Product Development Services (cad/cae/test)
Medical Device R&D

 

[GregLocock]

You don't know. A shaker can be more useful, but can have its own problems, for example you might be exciting at a node. There are other issues with shakers.



Cheers

Greg Locock


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

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[drmuh]

I think a CAE analysis will give good insight beforehand to estimate the nodal points before starting modal testing to determine acc. and excitation locations.

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Product Development Services (cad/cae/test)
Medical Device R&D

 

[izax1]

You can find the nodal points for a simple cantilever in almost any structural dynamics textbooks.
For mode2 the node is at 78%
For mode3 the nodes are at 50% and 87%