meauring vibration on a rotating disc

[akalmand]

Hi all,

I am measuring acceleration levels on a rotating disc with the help of 4 accels in axial and 4 accels in tangential direction (90 degrees apart). The intention is to capture wobble motion, and in plane radial motion on the the face of the disc.

While I do frequency analysis, I see that the frequencies shift as I move from rotating co-ordinates to stationary co-ordinates using trigonometric functions. The frequencies in some case increase by rotational frequency of the shaft and in some cases decrease by rotational frequency, and this is what is bothering me. I am not able to figure out the physical phenomenon to explain this frequency shift and the reason why does it goes either way (increase in some cases and decrease in other cases).

Can someone help me understand it, and also refer me to a text, book or some literature that I can review on this (measuring in rotating shaft it self and interpreting results in stationary co-ordinates).

I would really appreciate that.

Thanks !!!

Akki