Heavy Spot and High spot after 1 critical Speed

This is a pretty major topic; here is a summary.

The heavy spot is the physical unbalance (the sum of all the unbalance into one vector of angle and mass). The heavy spot does not move.

The high spot is the peak of the waveform detected as a result of the heavy spot.

Well below critical, the high spot is in phase with the heavy spot. For simplicity's sake, there is no lag.

Above critical, theoretically, the high spot is opposite the heavy spot. It is a long discussion about exactly how much lag there will be, but for simplicity, it is opposite.

At resonance, the high spot lags the heavy spot by 90 degrees.

As the machine increases in speed, it will transition from in-phase, through 90 degrees lag to 180 degrees lag.

What happens through second critical is an even longer story but the high spot (of that mode) does not return to in-phase, instead the high spot related to the second mode transitions from in-phase to 180 degree lag. But that is a very long story! Join us on Cat IV for a detailed animated discussion!

You will learn more about this on the Cat III course using a pretty fancy simulator.


Often, high spot and heavy spot are oversimplified.   Imbalance is distributed, especially in a rotor system with multiple natural frequencies.  Similarly, there are multiple measurement points - or points that have a measurement, whether measured or not.

Mode shape is under appreciated when it comes to the high spot heave spot relation.  The location of the measurement compared to the force in a simple case of only one imbalance is important. 

If using this information for balancing -- and it can be very successfully used -- all this must be put together.  The fact that the phase is not exactly 0, 90, or 180 degrees is important too; so location with respect to a particular natural frequency is important.

Thinking of the imbalances (forces) as modal entities can help.  What type of forces drive the modes as observed?  What are the modeshapes?  Be careful when there is a node in a modeshape between the force and the measurement. 

If thinking in modal space the 0 to 180 shift works for each mode.  However, one must think about superposition of the modes to get back to the measurements and resultant force locations (or corrective force locations).

Also, if measuring in velocity the 0 - 180 doesn’t work because velocity leads displacement by 90 degrees.

These concepts can be used to great advantage if the details are minded.  (Mind the Gap as I have heard too often.)

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