Practical Solutions to Machinery and Maintenance Vibration Problems
Chapter 4, Rotor Resonance and Corrections
Section 4, Whip Correction Procedure for Resonant Whirl at Rotor's First Critical Speed
As most rotor resonance occurs at its 1st critical speed, it will be treated first. A point to remember is that when the previously balanced roll bends to its resonant whirl mode, a new unbalance is caused as the roll's mass is displaced radially. Even though it usually appears to be large, this unbalance must be ignored. The balancing machine operator must focus all attention on whip only. As whip is removed, the rotor straightens. As it straightens, the unbalance also decreases proportionately. When the roll's whip is finally removed, the roll would again be straight. The vibration at the roll journals should be ignored until the whip is finally removed. After the whip is removed, release the restraints at the bearings and recheck the vibration in the planes of the bearings. If not acceptable, finish by regular balancing with weights added in the end planes.
1. As much as is practical, the balancing machine's end supports are to be physically restrained from vibrating. Means should be added to restrain the rotor itself when running and whipping at its critical speed. Try to duplicate the final running conditions as much as possible, such as by using similar bearings, etc.
2. Run the roll at the peak of its 1st critical speed. Measure the phase angle and amount of whip. The whip mode should be as shown.
3. As shown above, place trial whip correction weights in the two planes, 1/4 the rotor's length from the bearings. Add two equal counterweights at each end, both diametrically opposite the whip correction weights. For most rotors being corrected the first time, whip removal is a cut and try method. The counterweights should be equal to the whip correction weights.
4. The inboard whip correction weights should be put inside the roll. Care must be taken so that the oz•in or gram•mm value of the trial whip correction weights can be replaced by the same value as the final permanent whip correction weights. As the permanent weights in the roll are placed at a smaller radius, the final weights will weigh more than the temporary trial weights.
5. After the whip correction weights and counterweights are placed inside the roll, the roll is again checked for balance at normal low balancing speed. It should require only a trim balance, via either small balancing weights on the ends of the roll, or weight removal by drilling. It usually is not necessary to make further whip corrections as the larger forces that caused the whip should now have been removed. The small forces due to small inaccuracies of weighing and making permanent corrections should not be large enough to cause whip. However, a check run at operating and also at maximum whip speed should be made.
This textbook contains only part of the information in our Practical Vibration Analysis seminar.