This thread was inspired by a statement made in the "Unbalance or ?" thread. I quote:

"It could be that there is a natural frequency near 1140 cpm, but it is the fan shaft and not the support base. Typically when I balance a fan, I impact test the rotor in H, V, A directions. It helps to understand rotor response to unbalance weight and to predict a good trial weight location.

Walt "

In theory, for a single degree of freedom system there is just one resonance frequency. Sometimes, even without background vibration, we see several peaks in an impact test indicating that it is a multi degree of freedom system. Rotor is a big piece of a machine greately affecting the resonance frequency obtained from a bump test.

In this regard I wonder whether the resonance frequency properly obtained from a bump test (assuming dual channel test with phase and coherence capabilities) is affected by the mass of the rotor? (Let's narrow down a rotor type to the one resting on antifriction bearings with no oil film related damping.)

Putting my question in a different way will be as follows. Is there a need to bump a rotor separately? Isn't that true that a machine support and rotor together make ONE mass-spring-damper system? I am not talking of different machines, instead, I mean to perform, for the same machine, a machine support vs. separate rotor bump test with rotor fastened by bearings to the machine as oppose to one hanging free.


From your practical experience, did a rotor impact test produce different results in comparison to that of machine support? If yes then why?

Thanks,
David

This message has been edited. Last edited by: David_G,

quote:

From your practical experience, did a rotor impact test produce different results in comparison to that of machine support? If yes then why?

I just had the opportunity to shadow one of our lead guys on a modal analysis of a cooling tower. We impacted everything first looking for possible 'sweet spots.' Each blade was around 260Hz, the hub was 11.2Hz, the gearbox at one point was 30.4Hz, the motor was around 22Hz at one point, the support member that connected everything was around 23Hz, etc., etc. So why was everything different? I'd imagine it's mainly due to how everything is fastened together--how rigid everything is--and material composition. It all goes back to mass and stiffness.

David,

A fan structure is not a single "one mass-spring-damper" system. It has many natural frequencies or mode shapes, because it consists of many mass, spring and damper elements. I have conducted impact tests on rotors in machine, on blocks and on slings. The mounting conditions do affect the rotor natural frequencies (shaft bending and wheel modes). It is unually convenient to impact test a fan rotor when installing balance weights. The actual rotor frequencies (bending mode or critical speeds) can vary slightly from impact testing under static condtions due to changes in bearing dynamic stiffness and gyroscopic stiffening of the rotor. This is just a short response to your questions.

Walt

One way of looking at it is to consider the following:

a)The rotor is resonant at, or close to running speed i.e critical speed - the approach in this situation would be to address the rotor resonance by changing its stiffness (design change) or altering the runnning speed - beefing up the supports might change the response as measured on the casing but is unlikely to sufficiently alter the criticial speed of the shaft to alleviate the problem.

b) The supports are resonant at or near to running speed. The approach here would be to change the support stiffness (up or down to move the resonance away from the running speed excitation frequency) or change the running speed if possible.

The hard part is identifying which part is resonant in the first place so that a fix can be implemented