Critical Speed: "When the machine rotational speed becomes equal to the system or component's natural frequency"

My 1st question is that why some machines have more than one critical speed?

2nd question is , what does it mean by system natural frequency, would it be the shaft's Fn or bearings, or the whole system/structure?

needs reply?

quote:

why some machines have more than one critical speed?

All machines have multiple critical speeds if you looked at high enough speeds (which may be beyond safe or practical operating limits). Consider a simply supported beam, it has an infinite number of resonant frequencies.. each with different mode shape. But what is usually of interest is the resonant speeds within the operating range. Many slow speed machines are designed to operate below all critical speeds. Some faster speed machines and particularly turbomachines are designed to operate above one or more critical speeds.

quote:

2nd question is , what does it mean by system natural frequency, would it be the shaft's Fn or bearings, or the whole system/structure?

In general, the whole system needs to be considered. There may be some special cases where a very good estimate can be made by considering only a portion of the system, with an idealized representation of the remainder of the system, but it should not be expected in general.

Thanks, Electricpete !