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Practical Solutions to Machinery and Maintenance Vibration Problems

Chapter 7, Misalignment

Section 1, Reasons for Vibration Due to Coupling/Shaft Misalignment

Here is a major vibration source that could be prevented with only a small amount of craft training, but until alignment methods improve, there will still be a major need for analysis. Coupling and shaft misalignment vibration analysis is too often confused with vibration due to unbalance, as misinformation has been repeated in articles on vibration. Further confusion comes from not using phase readings as a means to determine how a part is shaking (modes) as well as how much a part is shaking (amplitudes). There are also misconceptions concerning the predominant frequencies produced by shaft misalignment. Unbalance is too often blamed for the 1 x rpm vibration that also originates from misalignment. Once phase and modes were used to more definitely separate vibration from misalignment and vibration due to unbalance, for example, it was found for most vibrating machinery that vibration due to misalignment was to blame at least as often (and sometimes more) than vibration due to unbalance. Most specialists agree that misalignment is the source for 50 percent or more of all vibration troubles.

Most companies do not have definite standards for what is considered "good alignment." When asked, supervisors and craftsmen usually give somewhat the same answer. "We just line it up as good as possible." Or, "We line it up to within a few mils." Yet alignment tolerances should depend on such things as classification of the equipment, size, type of coupling, diameter of the coupling, operating speed, length of operating time between shutdowns, cost of downtime and how crucial the machine is to the process.

There is a general misconception that machines with flexible couplings do not need to be aligned as carefully as those with non-flexible couplings. While it is true that misaligned, flexible couplings will transmit torque and perhaps cause a little less vibration, the vibration can still be high enough to cause excessive wearing of the bearings and seals. Almost all machinery manufacturers recommend that machines with "flexible" couplings be aligned as carefully and accurately as those with solid couplings.

Another cause for so much trouble due to shaft misalignment is the method used for alignment. Update has conducted vibration training seminars throughout the industrial world and questioned those craftsmen who actually perform the alignment. Even within the same company, there are often two or three methods of alignment, using different tolerances, and in some cases, not actually measuring alignment at all but instead, coupling eccentricity! At this point in time, the most accurate and fastest methods are reverse indicator and laser based alignment. However, too often even the right tools and methods are not used properly, and the result is mediocre or even poor alignment. This usually comes from a poor understanding of the methods used by the technicians and is not discovered by managers or supervisors due to their own vague understanding of the subject. Too often precision alignment is assumed but not actually obtained.

The alignment process can become drastically more difficult if adequate prealignment checks and sound alignment procedures are not used. For example, the most precise measurement between the shafts will not allow for the exact corrections at the machine's feet if soft feet, poor shims, and base deflections exist. The readings themselves may be distorted by coupling backlash, bracket sag or dirty prism detector surfaces, to name a few. The calculations for alignment corrections are too often taught in a complicated manner which, though technically correct, can intimidate the technicians who do not use a great deal of math in their everyday work. Many systems are now sold with computers that handle the calculations, without the technicians understanding how this is done. Too often, this leads to entering data incorrectly, not having confidence in the results, or simply not trusting the method at all and reverting back to more intuitive, though less accurate methods. Computers and formulas are also inflexible and cannot always be readily applied to unusual machine configurations or problems.

As a result of these difficulties and many others, alignment has historically been a difficult and frustrating task. Mediocre alignment has been "as good as you're going to get," and the results have not been perceived as a "problem." With this perception deeply ingrained in most people's minds, the largest single source for machinery vibration will not be sufficiently reduced. Effective training in this area, however, will make precision alignment obtainable in almost every situation, usually requiring far less time than in the past.

 

 

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