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Hello,

I have started to do impact test and ODS on site with Me'scope and different devices. I don't have real issues to manipulate for now software and devices. The issue comes with setup and taking real data on real structure.
For example for the attached baseplate I would like to do an impact test to check the natural frequencies of the structure. I have selected a frequency span of 200 Hz with 1600 lines ot have good resoltion and because 200 Hz span should be enough with macihne runnning at 1500 rpm.
My problem is to define the type of hammer I could use and the points of measurements. Where should I impact the structure to get good results ? in all X,Y,Z directions ? is it, in that case better to rove the hammer or to rove the accelerometer or it doesn't change ? Am i sure that if I impact on one point of the structure I will get good result on a point "far away" ? will it be possible that I need to check my structure for that.
What I find not easy, is that you can understand how to use the software and hardware but those "practical" things, once on site, aren't easy to put in practice.
I would appreciate from you, some help, or advice ?
Thank you for your time
Nicolas

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You bring up a good point here.
I've also wondered about the advantage/disadvantage of roving the accel vs. roving the impact.
Reciprocity would suggest it doesnt matter but I wonder if that is true from a practical sense in some structures and modes.
If I suspect very little energy would be seen at a point/direction when the impact is in another direction, is it a valid test?
For your particular case, my "guess" is that the base would probably only be of interest in the vertical direction. Perhaps an ODS would be the first order of business....just to see what motions may be causing you headaches.

Regards
Jim P
RM
Reciprocity is a true property. After all we measure magnitudes ratio and relative phase. I prefer to use roving accelerometer for the following reason: one wants to make sure that he is NOT impacting a node prior to the test. Once you have verified that it is not the case and you ARE in fact inparting energy, you can proceed with the test. Fixed impact location also is convenient to ensure that direction of impact has components in all 3 axes.
RM

regarding the impact test, spend some time bumping the machine in different locations while moving the accel. Watch the response in all three axis and eventually you will be able to determine a good driving point for the entire test. Once you find the driving point that excites all your measurement points in each planer you can use that for the fixed location.

 

I usually use 400 lines for a 200hz fmax. that gives you frequency resolution of 30 lines which is more than adequate resolution and reduces collection time considerably. use more resolution if your impact response data suggest closely spaced natural frequencies.

 

 

RM

Hi Nicolas,

I like Jim's post here with regard to your question - in particular 400 lines of resolution being applied - definitely more than enough resolution and huge time saving.

 

For baseplates and larger surfaces I have use a small fractional hp variable rpm d/c motor. I have a fly-wheel type rotor fitted to the shaft with a known amount of unbalance weight to create vibration.

 

I place the motor in position - secure it and begin tuning (via attached rheostat controller) through different rpm ranges while measuring in the different locations and in each axis - don't be surprised to find at times only one direction is an issue. Tracking the rpm of the motor (forcing frequency)can be done via a photo tach or strobe light. The flywheel attachment I made has drilled and tapped holes equally spaced around the diameter for the addition of extra weight if needed. Of course this is dependent on the amount of mass involved as to exciting any natural frequency and in some cases the old "dead blow" sledge hammer is required. 

 

I find this way I get consistent force vs. using a hammer or other method to excite natural frequencies. I have used this for identifying natural frequency of objects and for nodal testing on structures and piping. Word of caution is to be sure the unit is properly secured to whatever you are testing - in particular piping. If you question the safety of your mounting - go back to your "dead-blow" hammer process. If you like I can post a pic of the unit I put together quite some time ago.  

RM
Last edited by Registered Member

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