What is the source of the vibrations apparent the FFT

Question

I have a system which is composed of 3 Rotating components • Motor i.e. Shaft (running at around ~7rev/s) • 3 Gears of x16 teeth mounted in series • shaft hold Bearings (number of rolling elements unknown)

I am doing an FFT using Matlab on the accelerometer readings, mounted over the shaft holding bearing.

i stack these FFTs and plot them in time(called sample in the image) and get the image below

Now my questions are 2:

• This is the main question: what is the source of vibration indicated in frequency range 200-250Hz, since i know the gear meshing frequency ~120Hz (clear in the FFTs image), i can eliminate it being the gears. But maybe it is the gear in the middle meshing with the 2 other gears on each side? or is it just noise ? any help on this one be much appreciated

• second question: Why do the frequencies drift to the right, i.e. the meshing frequency seems to be increasing. my take on this is the motor heating up after running and so drawing less current, therefore less torque is applied and then the motor runs at higher speed ?

Answer 1

First, I find the 3D waterfall style plots pretty hard to interpret. My preferred method is a 2D color map (e.g. pcolor() in matlab). It's a lot easier to see your frequencies.

To your first question, it's going to be hard to answer that without knowing a lot more about your system. Vibration at twice gear mesh frequency is certainly possible. It's also possible that you have a structural mode. The first thing to do to investigate this is to do a frequency sweep of your drive speed. Look to see whether this higher frequency tracks with the drive speed. If it is synchronous with the drive speed, then you know it is definitely related to something rotating.

Your second question is essentially impossible to answer without knowing a ton more information about your system. e.g. what kind of motor is it? permanent magnet DC? 3 phase AC induction? How are you controlling it? Is there any load on the gearbox or is it spinning freely? You'd probably need to post all of the electric schematics, pictures of the entire setup, and more data. I doubt it's the kind of question that would be easily answered on this kind of forum.