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Practical Solutions to Machinery and Maintenance Vibration Problems

Chapter 4, Rotor Resonance and Corrections

Section 2, Understanding Resonant Whirl Correction for All Types of Susceptible Rotors through "Whip" Correction of Papermachine Rolls

With so many papermachines already running at much higher speeds than when originally commissioned, any further increase in speeds results in a relatively high probability of resonant whirl in the more slender rolls. The most common non-technical word used to describe this condition in the United States or Canada, is "whip." Common expressions are "the roll has whip in it." Or, "the roll is whipping at the new operating speed."

Although whip is common, most papermill engineers and supervisors don't relate whip to a rotor's resonant condition and, therefore, look for other sources for the roll's seemingly "bent" shape. For example, whip used to be explained by the fact that even average rolls are very far out of balance before balancing and that the balancing counterweights are usually placed in correction planes too distant from the planes of actual unbalance. The high centrifugal forces at operating speed were supposed to have caused the rolls to bend at operating speed, whereas the smaller forces at lower speeds were supposed to have been small enough to allow the rolls to remain straight.

However, the writer's experience has forced him to reject the above explanation for whip, except in very rare instances of very long, small diameter, slender rolls with exceptionally large initial unbalance. Instead, whip is a common phenomenon that is the result of one of the roll's critical speeds resonating to a vibration frequency equal to its own rpm. For example, the vibration source could originate from its own residual unbalance. But if the roll's first critical speed is, for example, 1200 rpm, the roll will resonate to its own residual unbalance in the 1200 rpm resonant range (about 20 percent of 1200 rpm, plus or minus). This is a typical resonant whirl situation. Recognizing that a roll's whip is almost always a resonance problem, one way of solving the problem is to run at a different operating speed. As this was too often interpreted as only lowering the speed, the usual objection was due to the resulting lower production rate. However, if the process will allow it, the rotor can operate in a whip-free condition at higher speeds as well. But what if speed change is not too desirable or practical, and the optimum running speed is right in the rotor's frequency range?

There are several solutions, one being to balance the roll ahead of time so that it will not whip even though resonant. (See chapter on "Update Method For Easier Balancing of Papermill Rolls with the Benefit of Reducing or Eliminating the Possibility of Whip.") Unfortunately for most people, the whip condition isn't discovered until the new, higher operating speed is first reached. The rotor has been previously balanced with the regular balancing procedures. Therefore, instead of using a balancing procedure to prevent whip, a procedure must be used to remove whip from the already balanced rotor.

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