torsional and lateral criticals

[upm]

Gents,
I do not know the defference between the torsional and lateral critical speeds ? when do we need to consider either? for new draft forced fan, do we need to do such an analysis as recommended by the API? can anybody explain the ifference in a simple way??

 

[electricpete]

For lateral critical speed the shaft bends in the shape of a rainbow. If shaft speed approaches that speed there is a resonance causing large defections and vibrations.

In considering torsional vibration, the shaft (or coupling etc) twists (like the stripes of a barber pole).

I don't think there is such a thing as torsional critical speed. There will be a torsional resonant frequency. But just operating the machine at a certain speed does not excite torsional vibration to my knowledge. (maybe someone can explain if I am wrong?).

api 684 = Tutorial on API Standard Paragraphs... An Introi to Train Torsional Analsysis and Rotor Balancing

 

[upm]

electricpete
Thnaks for the help, as i do not have API-684, do you know any links i can learn about these frequencies???
this is very important to me as the results for lateral/torsional analysis in one of our gear box indicates that we are running closer to lateral speed??? therfore, i need to learn more about it
thanks for the help

 

[benucci]

I would recommend to plot a Campbell diagram, to make sure the operating speed and harmonics are not to close to the natural frequencies. The possible excitation frequencies have to be determined depending on the type of driver, and also based on the design of the fan(blade pass frequecy ...)Following the API recommendation is a good approach.