Bearing vibration signs - fundamental and harmonics frequencies
Bearing vibration signs - fundamental and harmonics frequencies
(OP)
I'm working with monitoring of bearing vibrations and I need to specify a A/D conversor for aquisition data. I need to know the total amplitute of the frequency harmonics for a fundamental frequency about 1 kHz (like ball bearings).
Thanks
Marcello
Thanks
Marcello





RE: Bearing vibration signs - fundamental and harmonics frequencies
if you want to experiment, you have to mearsure the response of the system to some kind of input, say, impuse input.
then doing fft, you can find the magnitude of frequency response.
RE: Bearing vibration signs - fundamental and harmonics frequencies
Thanks,
Marcello
RE: Bearing vibration signs - fundamental and harmonics frequencies
M
RE: Bearing vibration signs - fundamental and harmonics frequencies
cage: 31.52 Hz
ball spin: 172.96 Hz
outer race: 377.92 Hz
inner race: 582.087 Hz
You should be looking to at least the fourth harmonic of the highest fundamental so I would set the Fmax to an absolute minimum of 2kHz in FFT acceleration or velocity. However there will be a lot more information further up the frequency scale but it will depend on how you mount the accelerometer and what type of accelerometer you are using.
A typical low cost accel will have a natural frequency of between 10 and 20 kHz - and that is only when perfectly mounted on a good flat surface and tightened to the correct torque. If you are using hand-held you should not look at any data above 1kHz. A good 2 bar magnet will give a natural frequency of about 2 to 3 kHz (varies a lot). A "real-world" permanently mounted accel will have a natural frequency of about 5-10 kHz (again depending on how good the installation is).
I would suggest that you permanently mount the accelerometer and take a lot of care when carrying out the installation. Make sure you have a good flat surface and that the threaded hole to accept the accel stud is absolutely perpendicular.
Now you can set your Fmax at 5kHz for FFT (remembering that the sampling rate is about twice the FFT Fmax). I would then record my FFT with 800 lines to get reasonable resolution. I would also take a time domain reading at 10kHz with enough samples to record about 3 revs of the shaft. 1024 samples would give a sampling rate of 10.6kHz. This will let you see the individual impacts from the deteriorating bearing.
If you are looking at a centrifugal compressor you should also be thinking about the gear meshing frequencies - but that is another story.
Ron Frend
tel: 011 44 1253 400541
ron.frend@predicon.net
http://www.predicon.net