I'm carrying out an experimental modal analysis on the fixture of rotating beams. The rotary beams are attached to a D/C motor and I measured the vibrations at the fixture of the motor. I got peaks at very low frequencies before and after turning on the motor.
Thanks in advance
I understand that you are measuring acceleration at the rotor bearing housing, if I read your photo correctly. Also, it seems that you are studying a 2-blade rotor. Apparently, you have an unbalanced rotor. This should show up at 1X-rpm, however, I do not know, what is the rotating speed. The subsynchronous side peaks at low speeds may be attributed to bearing, yet I am just guessing as I do not know hat kind of bearing is there.You may wish to provide more information, so your question can be properly answerd. I hope this will help,
Dear Dr Yehia,
Thank you for your comment.
Indeed, I want to study the balance of the rotor by measuring the acceleration of vibrations at the housing.
I add masses in the beam and then measuring the amplitude value at the rotation frequency.
The rotational frequency is 215 rpm/ 3.3 Hz.
Hi
Try and tap your setup with a rubber hammer or similar while slowly rotating the blad with your hand to make sure that nothing is loose. If the low frequency peak is there during this 'bump test', then it is a support resonance (though it looks to be a bit too low in frequency for this).
Next, look at the peaks wrt your machine orders, where Order N=1 is RPM/60 and N can be 1, 2,3, .... It may well be that you get a 1/N behaviour which would be the result of uneven load or driving conditions.
/Claes
Instrumentation fit for purpose?
Did you check the accelerometer you use provides valid data at such low frequency?
Most ICP type accelerometers can not be used below 2 Hz.
Test set-up fit for purpose ?
Are you sure there is no rocking or tilting of the entire set-up? This looks like it could be a rigid body motion of the entire assembly.
215 rpm / 60 sec = 3.58 rad/s
3.58 rad/s /(2*pi)= 0.57 Hz which seems consistent with your first peak. That is the vibration induced by the unbalanced rotor.
I hope this helps.
Cammarano, take care....if you divide RPM by 60 s then you will get the frequency directly. Multiply this frequency by two pi then you will get angular frequency.
According to your example, unit of 215/60 = 3.3 Hz.
Dear Ahmed,
This setup of fixture of rotating beams has a one per rev of 3.58 Hz. Are you getting low frequencies at 1/rev or still below? If its below 1/rev, then try to check for any play or looseness in the setup or sensor mounting itself. Also check the mounting base for its attachment rigidly to the ground. Hope it gives some clue..
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