What does the peaks in my FFT figure mean?

I plotted FFTs of a vibration signal, with and without load. The readings were taken from a rig with a rotating shaft. The rig has extra vibrations on its own. Can someone please tell me what the two peaks mean?

Answer 1

[Star Strider]

The peaks are the frequencies at which the vibration amplitude is maximal. Here, they appear to be at about 900 Hz, and about 1200-1300 Hz. The peaks are about the same in both experiments, however greater in the 400 N load.

If you want to find the amplitudes, use the Signal Processing Toolbox findpeaks function on both experiments. See the documentation on findpeaks function for all the options the function has.

[Dhyan Hariprasad]

The motor used for this experiment is 55Hz. So is the frequency range wrong?

[Star Strider]

The frequencies are not wrong. The vibration can be different than the shaft rotations angular velocity. Consider a tuning fork — it does not turn at all, and can vibrate at several hundred Hz.