Practical Solutions to Machinery and Maintenance Vibration Problems
Chapter 4, Rotor Resonance and Corrections
Section 3, Removing Whip from a Previously Balanced Rotor or Roll
As its basis for discussion, this section will focus on rolls, as rolls are very easy to visualize. However, the same procedures apply for all rotors that are in resonant whirl or "whip." For rotors that are not similar to papermachine rolls, such as multistage turbines, more sophisticated procedures may be necessary, often requiring help from the machine manufacturer. In such situations, the rotor manufacturer may have high speed balancing machines and procedures, especially for whip removal. Most often such manufacturers do not use the expression "whip." Instead, they refer to "flexible rotors."
If shutdown time permits and means are provided for driving the individual roll while in the papermachine, whip can be removed without removing the roll. Otherwise, whip removal is usually performed in a dynamic balancing machine that is capable of running the roll at its whip speeds.
As whip is a phenomenon of rotor resonance, remember that when the roll is mounted in the papermachine, the rpm at which it will whip is most probably different than the rpm at which it will whip when mounted on the balancing machine. Therefore, whip must be measured and removed at the rpm at which it is occurring. The frequency of which whip occurs while the roll is in the papermachine is not the same frequency at which it will occur on the balancing machine. Therefore, the frequency for maximum whip in the papermachine does not apply for the frequency at which whip should be removed while on the balancing machine. Remove whip when the frequency actually occurs during the whip removal process. If, for example, when on the balancing machine whip is properly removed at the roll's first critical speed, it will not whip at its first critical speed in the papermachine. For example, assume that a roll's first critical speed in the papermachine is 1100 rpm and its first critical whip speed on the balancing machine is at 900 rpm. On the balancing machine remove whip at 900 rpm and then re-check in the papermachine at 1100 rpm.
Dynamic unbalance has within it, the components of static (in-phase) unbalance, combined vectorially with couple (180° out-of-phase) unbalance. Visualizing a roll in resonant whirl at its first critical speed (one bow), you can see that the "in-phase unbalance" forces are being resonated but the "180° out-of-phase unbalance" forces are not being resonated. This will be true even if both types of unbalance are combined in dynamic unbalance. The resonant whirl shape actually separates the static unbalance forces from the couple unbalance forces. To review: for resonant whirl at a rotor's first critical speed, the in-phase vibration is drastically magnified, whereas the 180° out-of-phase vibration is not magnified.
In similar manner, if the resonant whirl occurs as the operating speed vibration coincides with the roll's second critical speed, then its characteristic double bend will not magnify the in-phase forces, but will instead resonate the 180° out-of-phase.
This textbook contains only part of the information in our Practical Vibration Analysis seminar.