I am struggling with a strange problem when measuring a roundness profile of large rotor in a horizontal grinding machine. The rotor is a paper mill where both rotor ends are ground. The rotor is attached between centres (headstock/tailstock). Rotor alignment is supposed to be in tolerance. When I measure the roundness (not exactly the roundness, it is the total radial displacement though) with capacitive non-contact sensors I obtain the displacement waves.
The problem arises when measuring with at the same time with two sensors. One is located at 0º and the other at 180º in radial direction, both in the headstock end. The measured displacement profile is not the same for both readings. When analysing the data, there is a clear difference in the 2x harmonic component.
While the 1x (runout related) remains the same, the 2x harmonic component disappears for the 180º reading. The 2x is dominant for the 0º reading. I wonder how is possible to have different readings for 0º or 180º in the 2x component. I have tried with 90º and 2x appears too, but not with the same amplitude as in 0º.
I have tried this in two different machines and two different rotors and the same conclusion applies.
Any thoughts about this?