May anybody point out how to select hammers for natural frequency determination with a single channel analyser? I don't have only day tom day use hammers for maintenance activities. Also, at many occassions , Ihave found peaks to be very small (of the orders of 0.2mm/sec or so). May we decide these peaks to be a symptom of resonance? Or, we should always go for multichannel analyser for bump test?

If you only use 1 channel and otherwise also it´s important that your hammer introduce signal when you hit that excite the freq. range of interest. So for normal machinery I use 2"x4" wood hitting with the end. With a hammer you have to fiddle with the various thingys you can fit. In lo freq. cases I stood on the foundation jumping up and down like a fool. Works great. Largest "hammer" I used was a near empty reel of a papermachine in the overhead crane and 2 ropes. So adapt to your need. Olov

I whack some trial whacks while watching a time wave form.

If I see something like the upper trace I figure my hammer and I are good to start taking data.

http://upload.wikimedia.org/wikipedia/en/thumb/
8/87/Damped_Free_Vibration.png/350px-Damped_Free_Vibration.png

I agree with responses above.
I've used 2"X4", rubber mallets, etc. to get a decent response on a single-channel.
But be careful of what you assume. For small peaks, no coherence measurement equals no idea of where it comes from. But then, if I'm whacking a machine with a 2"X4", I'm looking for large peaks that make resonance determination obvious. Repeat several times with peak hold and then compare with background.

Also be sure to move your whacker around (no pun intended) - to make sure you are not at a node point.
Regards
Jim P

When you "ring" teh suspected component, you can also temporarily add mass or stiffness, to look for a frequency change. As an example, if you bump a channeled pump base, and get what you suspect to be the resonant frequency, temporarily add a wedge halfway between the support points, and bump again, to see if you get a frequency shift.