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Practical Solutions to Machinery and Maintenance Vibration Problems

Chapter 7, Misalignment

Section 24, Coupling/Shaft Running Alignment

Alignment of rotating machines while running is an art rather than a science. Most readers won't even attempt it for fear of the suspected dangers. Update is not recommending that running machine alignment be performed. It is discussed here only for those who are forced to do so because their machine is vibrating very badly due to shaft/coupling misalignment, and no "normal" alignment attempts work for the running conditions. Actually, the method so far has proven to be relatively easy and safe if certain precautions are taken.

The idea originated from the method developed by two men from Du Pont, entitled "Vibration Reduction Techniques." It is embellished with the writer's findings that have enabled running machine alignment on many machines, some over 5000 hp. Practice is suggested on a simple motor driven setup, perhaps on equipment up to 20 hp. Then, when the precautions are understood, fear removed, and experience gained, decide whether to boldly attempt more.

One pulp and papermill in Canada routinely gives a fast alignment using standard methods. Then, after the machine is running, loosens the hold-down bolts of the proper machine and gives a relatively fast trim running alignment. Here are the precautions and details:

• Place dial indicators at all of the moveable machine's foot pads; oriented in the vertical direction to monitor vertical adjustments. Two horizontal indicators are usually enough. All are set to "zero" before loosening hold-down bolts. This enables a return to the original alignment or the dial readings are used to indicate the adjustments to be made for final bolted-down alignment.

• Unless there is no way out, do not loosen the machine section that is connected to pipes, especially hot and rigid pipes. Almost always there is pipe strain. Instead, move the "free" member. For a motor-driven pump, move the motor. For a turbine-driven generator or a gearbox-driven generator, move the generator. Also loosen the bolts to the pipe conduit that encloses the motor's input electric lines.

• After the machine is running, adjust all horizontal jack screws until they either barely touch the machine's feet; or with a feeler gauge, leave a gap of no more than one mil.

• Although the primary vibration amplitude due to misalignment is at a frequency of 1 x rpm, vibration at this frequency can also be combined with rotor unbalance. Therefore, even if the 1 x rpm vibration is largest in amplitude, it is usually the least desirable frequency to monitor for running alignment. Instead, for most machines, the amplitude at 2 x rpm is most often preferred as it is usually free from other vibration sources. The exception is 2 x rpm vibration due to reciprocating part unbalance in reciprocating machinery. For reciprocating machinery, the 3 x rpm frequency may be a good frequency to monitor.

• Another good alignment monitoring frequency is the rpm times the number of protruding segments in a coupling half. For example, if each coupling half has three protruding segments, then 3 x rpm is a good monitoring frequency, even if its amplitude is much less than at 1 or 2 x rpm.

• If possible, monitor bearing temperatures at the same time. If the vibration decreases but bearing temperatures increase, then the running alignment should be distrusted. Temperatures should either remain constant or, in some cases, decrease.

• Since engines, reciprocating compressors and most vibrating screens have a good possibility for higher 2 x rpm vibration, before attempting running alignment, it is best to disconnect the coupling and run the unit alone to determine just how much of this vibration is present without coupling misalignment. If low when the machine is running solo and increased a lot when coupled, then the 2 x rpm may be a good frequency to monitor for running alignment. If the coupling can't be disconnected or if the reciprocating unit is a driven unit, such as a compressor, then it is best to trust the 1 x rpm vibration as the frequency to monitor rather than the 2 x rpm vibration.

• While watching the vibration readings, loosen one hold-down bolt at a time. Sometimes the vibration will increase. Don't worry about that as once all the hold-down bolts are loose, you can make your first real moves by trial and error. Adjust each bolt's tightness to slightly more than "finger" tight.

• If at this point the motor moves on its own, it cannot go upward more than a few mils. It also can't go down, but it can move horizontally. Good -- as the horizontal move seems to always be in the correct direction. It seems the coupling forces tend to align the shafts rather than misalign them. Lubricated shims help this process.

• Small screw jacks, or those made with a larger diameter bolt and nut, are convenient for raising and lowering the member to be moved. Small hydraulic jacks work very well. Some mechanics and millwrights perform very fast running alignment with a technician at each end of the moveable machine, such as the motor, making vertical adjustments with crowbars.

• Rather than removing and adding one shim at a time, it is usually best to do all of the vertical adjusting until the alignment is accomplished, using jacks or crowbars. Remove all of the shims, and measure the gaps with feeler gauges to determine what is required under each foot.

• For very large machine foot pads, feeler gauges should be used at several locations on the same foot, enabling different parts of the gap to be filled by different thicknesses of shims.

• Sometimes a Foot-Related Resonance (FRR) is encountered whereby the foot pad cannot be bolted to the base without the vibration increasing drastically (see section "Considerations For The Probability Of Resonance").

• Prefer to do running alignment only on machinery that does not have large sources for vibration other than that of misalignment. This can be determined through effective vibration analysis and by understanding the machine's history. For example, if there is a possibility that the rotor is appreciably out of balance as well as misaligned, it is best to first balance the rotor.

Instruments that provide a spectrum of amplitude vs frequency, at first, seem to be the best to use for monitoring vibration while running machine alignment moves are being accomplished. However, the instruments should be set up for the fastest response, such as when using the least number of lines without averaging. A simple Data Collector/FFT and a swept filter vibration instrument can be readily used together to obtain the best results.

Procedure:

1. Use the FFT to obtain a spectrum (especially in the range of "lower harmonics"). From the spectrum, choose a harmonic that has a large enough amplitude to be practical for monitoring, such as 2 x rpm, 3 x rpm or number of segments x rpm.

2. Tune swept filter-type vibration instrument (usually with analogue amplitude meter) to the chosen frequency.

3. Make alignment adjustments while first monitoring the results on the swept filter instrument's analogue meter. As the amplitude progressively decreases, occasionally check the spectrum on the slower responding FFT to determine the effects at all the lower harmonic frequencies.

 

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