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Practical Solutions to Machinery and Maintenance Vibration Problems

Chapter 5, Unbalance

Section 15, Update Method for Easier Balancing of Papermill Rolls with the Benefit of Reducing or Eliminating the Possibility of Whip

Basic Facts to Understand Before Proceeding

Dynamic unbalance is the vectorial summation of its static and couple unbalance components.

For most rolls with dynamic unbalance, the static unbalance component is considerably larger than the couple unbalance component.

Most papermill rolls run at a speed that is 25 percent below their first critical speeds or less and, therefore, will not go into resonant whirl ("whip" at first critical speed of the roll).

For the higher speed, smaller diameter rolls where operating speeds fall into their critical speed ranges (resonant whirl), it is almost always the first critical that is reached. Very rarely is the second critical speed reached, except in extraordinarily long and slender rolls. For such rolls, it is usually reached only at relatively high speeds.

Resonant whirl (rotor resonant to a vibration frequency equal to its own rpm) at the first critical speed causes the roll to bow or whip with its largest deflection in the center.

Roll resonant whirl at its first critical speed resonates the 1 x rpm vibration component due to its static unbalance. It does not resonate its component due to couple unbalance. (Couple unbalance vibration would be resonated by a roll running at its second critical.)

For a roll subject to resonance, separation of static unbalance from couple unbalance can provide a much simpler basis for preventing or correcting whip than the usual dynamic balancing.

The machining of roll shells (in preparation for assembling roll heads) usually causes very large static unbalance. In the lathe, both ends are usually displaced eccentrically, in phase with each other. Or, the pipe or tube from which the shell is machined will have more probability of a bow in the center, which when machined straight, removes much more material on one side of the roll than on its opposite side.

A roll subject to resonant whirl or whip, requires the placing of its balancing weights inside the roll, at correction planes inward from each of its two bearing locations, approximately one-fourth of the total roll length. (Roll length is measured from bearing center to bearing center.)

Balancing Procedure

After the roll's shell is machined to receive its heads, do not assemble the heads before balancing. Instead, place the machined (or rough machined) shell on an ordinary static balancing stand, rolling on the outside diameter of the shell. If one is not available, use the dynamic balancing machine as a static balancing stand by disconnecting the drive.

Determine the correction planes by obtaining the expected distance between the roll's bearing centers. The correction planes will be at a distance of one-fourth of this length, measured inward from each bearing center.

Determine the shell's static unbalance. A relatively rough tolerance may be used, since after the heads are put in place, the roll will again become statically unbalanced (but not as much as the shell's original static unbalance). As the roll's largest static unbalance originated in the machined shell, two static counterbalance weights can now be readily placed inside in the correct planes the open-ended shell.

After the shell is statically balanced, complete the assembly of the roll by adding the heads. Perform whatever further machining is required.

Dynamically balance the assembled roll as you would any other type rotor, by making corrections on the roll ends or heads. Usually the corrections at this stage are considerably smaller than the original static balancing correction. It may not be necessary to put additional balancing weights inside the roll.

Suggestions for Driving Rolls While Still Mounted in the Papermachine

A. Air motor, pulley and wrap around belt.

B. A boat trailer wheel and tire, driven by any variable speed source.

C. A dc motor, powered by an arc welder dc engine/generator.

If drive source does not permit constant speed, set vibration instrument filter for frequency to desired balancing speed. Run variable speed drive rpm to about 10 percent, or more, above the desired balancing speed. Decelerate the rotor through the desired balancing speed, noting phase and amplitude at that speed only. The amplitude meter will "peak" as the rotor reaches the desired balancing speed. When the peak is reached, mentally record phase and amplitude. (It will be changing with change in speed.)

If phase change occurs too quickly to be read practically, then switch filter to out/off position as the peak is approached. The filter out/off readings of phase and amplitude will be valid even though the strobelight image or digital phase reading might be slightly erratic, but still readable.


 

 

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