Dear Vib expert

I wonder what is the diff between natural freq and critical freq. Any diff or the same?

And why i always find an engine (horizontal machine) that have high vibration on the horizontal only.

Hope you guys can assit me to elaborate on this

thanks

Vibuser,

Natural frequencies are inherent to the machine and depend on the mass quantities and distribution, stiffness and damping. Natural frequencies are what you determine when you perform a bump test.

When any forcing frequency (i.e: could be from an external source) coincides with one natural frequency, you have resonance.

Critical speed is form of resonance where the forcing frequency is the rotation of the rotor. The critical speed is not necesarily the operating speed. It may be a speed you pass through during start up.

As far as finding engine vibrations higher in the horizontal direction, I suspect it may be related to stiffness. It is likely that the stiffness in the horizontal direction is lower and, therefore, the response (or vibration amplitude) is higher.

I hope this helps,

Dear ELias

THanks for your input.

It reminds me back on the principle.
I always forgot this as foundation.

THanks again mate.