Foundation Design for Vibrations due to Machine/Fan 2

[AbhijitM]

How to interpret the Ratio of Resonanace frequency to Natural frequency (What it means if the ratio is greater than 1 or less than 1) What is the acceptable limit?

Thanks in Advance

Regards,

AbhijitM

 

[csd72]

There have been a number of posts on similar topics in this forum, just do a search at the top of the page.

csd

 

[AbhijitM]

Thanks csd72. I am also looking to estimate natural frequency of foundation. I am designing foundation for a fan of 1200RPM & 1800RPM and Natural frequency I got are close to 8Hz and 9Hz. That means my ratio of frequencs are 0.4 & 0.3. The same foundations are also satisfy the weight ratio: foundation mass to machine mass ratio's are between 3 to 5. But I have told that I need natural frequency of foundation should be 1.2 times of the Fan frequency that means I need 24Hz & 36Hz of foundation frequency respectively. Your thoughts on it are really aapreciated.

Thanks,

 

[csd72]

I can see where that is coming from, When the machine starts up and slows down its frequency will be less than its operating frequency.

Look up slide rule eras website

csd

 

[AbhijitM]

Thanks cds!! That means my design, as mentioned in my post above, is OK at start ups & slows down (Since frequencies are less then operating frequencies and are almost 1/3 of opertating frequency) Also my foundation frequency includes soil factor (damping) in calculation of frequency. so I think natural frequency should be always less than operating frequency. I guess thats what we want??? Any thoughts????

Thanks,

 

[csd72]

No, thats not what I meant. The operating frequency will start at zero and increase to the operating frequency then will reduce again to zero (with all frequencies between).

csd

 

[AbhijitM]

Thanks cds. u r right, frequency chages and so the ratio of natural frequency to machine frequency will change. Natural frequency is not changing as soil & foundation paramters remain unchanged. At what frequency, we should design the foundation.

Does that means we should check maxi. amplitude at resonance and not worry about the frequency ratio. Please help.

Your help is greatly apprecited!!

Thanks,

 

[sdz]

Ideally your natural frequency should be higher than the operating frequency so that machine will not pass through the resonant range. If you *could* do a full dynamic analysis you might well find that the damping is enough to prevent problems; especially if out of balance forces are small. Try not to go down that path!
Rules of thumb will usually provide a satisfactory solution. Make sure base width is at least equal to height from underside to c/l of machine.