Driving vs Natural Frequency

[SoDakEng]

Hello, I am structural engineer designing structures for machinery that operates with a rotating imbalance. I understand that I do not what my structure (or any part of it) to have a natural frequency that is the same as the driving frequency, but I have also been told that the natural frequency should not be a multiple of the driving frequency. I was unaware if this means that the driving frequency should not be a fraction of the natural frequency or just a whole multiple, or both. Also, does anyone know of any papers that cover this material?

 

[GregLocock]

http://books.google.com.au/books?id...a=X&ei=m8RpUbvpNIKGiQfI0IDoDA&ved=0CFQQ6AEwAw

Probably worth buying if you like it

Basically yes you must make sure that your first order frequency does not align with a significant mode, but depending on the type of machine both harmonics (multiples of rotational speed) and fractional excitation may be present - for example cylinder to cylinder pressure variations creatr a half order excitation in 4 stroke engines.

If you have a reasonably complex machine operating over a speed range then it becomes impossible to make sure that no modes align with any excitations, that's where the engineering starts.

Cheers

Greg Locock


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[SoDakEng]

The machine will only operate at about 900 RPM (the imbalance is created by a weight spinning about a horizontal shaft). My concern is for the steel structure supporting the machine. The machine itself does not have a natural frequency at the operating speed. Also, the support structure is steel "I" shaped columns and cross beams with steel angle diagonal bracing.

 

[GregLocock]

That sounds ok, but you need to bear in mind you will get 3,5,7... harmonics because your OOB will cause the bearing clearances to take up, so you need to check for them.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

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