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Driving vs Natural Frequency

Driving vs Natural Frequency

Driving vs Natural Frequency

(OP)
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?

RE: Driving vs Natural Frequency

http://books.google.com.au/books?id=iKC8ZJTLkgwC&a...

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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RE: Driving vs Natural Frequency

(OP)
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.

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