×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Bearing vibration signs - fundamental and harmonics frequencies

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

RE: Bearing vibration signs - fundamental and harmonics frequencies

so, what are your questions?
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

(OP)
Thank's for the answer. Sorry for my english. I was talking about the total frequency band for the measure, becouse I'm specify the A/D conversor, the stage after the acelerometer signal conditioning. In my case, the bearing used is SKF 2309 (number of balls: 12, ball diameters: 15,87 mm, diametral pitch 71,89 mm), rotating at 4800 rpm. I'm looking for any near experience becouse I can't do a preview test to measure. In another experience I found a 10 kHz as a total frequency band to mesasure. All of that harmonics were in this frequency band. Do you have some case like this or know some literature related?
Thanks,
Marcello

RE: Bearing vibration signs - fundamental and harmonics frequencies

For decent time resolution, I would recommend a minimum of eight time points at the maximum frequency of interest. Whether you consider the 3rd 5th or even 7th harmonic frequency to be the maximum frequency of interest is up to you.

M

RE: Bearing vibration signs - fundamental and harmonics frequencies

The fundamental frequencies you will get from this bearing at this speed are as follows:

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

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources