Propeler fan balancing/structure movement

[spike]

I am not in the same league as most of the forum members, but I will ask
the question anyway.
A balance report on a propeler fan on a cooling tower plotted the initial
magnitude and phase of imbalance. This was followed by magnitude
and phase of trial weights, (5 trial weights). A line was connected to
the initial balance magnitude and phase, the first trial weight magnitude
and phase. and so on to the 5th magnitude and phase. The unbalance
was lowered significantly. However, there was a comment on this plot that
said that these lines indicated structure movement. I thought
that the magnitude and phase would always change with trial weights.
How does the plot indicate "structure movement"?
Thanks

 

[GregLocock]

The response of the structure will be proportional to the imbalance of the fan, and will be at a constant phase lag with respect to the imbalance vector, at each speed, if the system is linear.

That is, if the imbalance is halved, the vibration of the structure at that frequency should halve, if there are no other sources of vibration.

In practice things are rarely that simple, of course.



Cheers

Greg Locock

 

[spike]

Thanks Greg, but I am not sure I follow you. If you connect the magnitude and phase
locations you will have a plot similar to this:


0
1
2 5

3 4
There are four quadrants corresponding to the four possible phase
readings from 0 degrees to 360 degrees. If you connect the initial unbalance magnitude and the phase angle at 0 to point 1, point 2, and so on. How does this indicate structure movement?

 

[GregLocock]

I don't know! rereading your original post I can see what you meant, but it doesn't really help much. Ask the author.



Cheers

Greg Locock