How vibration analysis can detect alignment problems

Condition monitoring can help to identify misalignment on operating machinery.

By Mike Bacidore, editor in chief

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Detecting alignment problems in operating equipment isn’t difficult, and it can save a plant from expensive repairs and even more costly downtime. The first step to identifying an alignment issue is condition monitoring.

“Vibration is probably the best indicator of an alignment issue,” says Robert X. Perez, author of “Is My Machine OK?” High radial vibration levels on the coupling end of a machine or high axial vibration levels at the thrust end of machine are both excellent indications of alignment issues, he explains. “Vibration issues related to alignment normally show up at either a 1X or 2X vibrational frequency,” says Perez.

vibration1 
Figure 1. After detection, correcting the problem can be accomplished with a laser alignment tool. (Source: Ludeca)

“A good vibration data collector and analyzer can tell you with a high degree of certainty through cross channel phase that you have a misalignment problem, explains Alan Luedeking, vice president at Ludeca and an ISO Level I certified vibration analyst (Figure 1). “Other technologies, like infrared thermography, can be useful, too. Seeing a coupling that’s too hot will alert you that you have a problem, but it won’t necessarily reveal whether the cause is misalignment, insufficient lubrication, or looseness, whereas vibration analysis can reveal that exactly. Your reliability program should include both on-line and handheld vibration monitoring and trending systems to catch these problems long before your spares inventory is unnecessarily affected.”

Spares inventory is no place to look for indications of problems, agrees Heinz P. Bloch, P.E., owner of Process Machinery Consulting, and author of “Improving Machinery Reliability” and “Pump Wisdom,” as well as numerous other books. “I would rather invest my money in training, so the staffers recognize failure patterns,” he advises. “I would look at failed parts early on, not spares history a few years down the pike.”

Is it alignment?

How does a plant differentiate between an alignment issue and something else? “Consider that this perspective differentiates between an ongoing plant monitoring program and a specific troubleshooting job,” advises Bill Watts, senior vibration engineer, Azima DLI. “Generally with the former, vibration data are collected by plant personnel or a remote representative and conveyed to an Internet-based database for analysis and reporting. This approach is quite cost-effective, in that many machines can be assessed on a frequent basis without the vibration analyst being on site. In this scenario, vibration spectral data are directly compared with an established baseline, or occasionally compared indirectly with data from a similar machine. We believe in employing tri-axial accelerometer arrays when feasible, or at least three individual axes of data for at least one test location per major machine component. The diagnosis for shaft misalignment logically is indicated by abnormally high vibration levels at one and/or two times shaft rotational rate frequency — 1X and 2X.”

Theoretically, the misalignment can be categorized as “angular” or “parallel,” explains Watts. “In reality, it is often a combination of the two. For angular misalignment, we look for abnormal 1X axial vibration on both sides of the coupling as the primary evidence. Often the resulting structural response is high 1X amplitudes in one of the other two axes. Usually we want to see the axial amplitude higher than the amplitude in at least one of the other two axes. Alternatively, higher amplitudes in both axes in the plane of rotation than axially would indicate rotor imbalance. Dominant amplitude in one axis only, other than axial, likely indicates a mounting or foundation problem. For parallel misalignment, we look for abnormally high 2X vibration in the plane of rotation on both sides of the coupling. Again, there may be a combination of axial 1X and/or 2X vibration.”

This type of evidence generally indicates a problem, the severity of which depends on absolute amplitudes, margins above baseline values and trending. “While the identified problem is shaft misalignment, it is important to note that a warped shaft and component mounting distortion — soft foot — also can mimic the effects of misalignment and produce similar evidence,” explains Watts. “With respect to vibration testing and analysis, these faults can be more specifically identified through the use of more detailed testing on-site using phase measurements. The bottom line is that, if abnormally high vibration levels are seen as described, there is a problem. The vibration measured at the bearing housing illustrates excessive forces being applied to the bearings, resulting in premature wear. Whether the underlying cause is shaft misalignment, a warped shaft, or improper mounting, the problem needs to be corrected.”

Conversely, if the specific vibration levels are normal, then there is no perceived problem, says Watts. “That is not to say that the shaft alignment is definitely good,” he cautions. “However, a somewhat improper alignment, associated with a coupling which is doing its job of absorbing the misalignment, is not producing additional forces on the bearings. Therefore, no fault is identified if the applicable vibration levels are normal. With this logic, machine type and configuration, and method of mounting, also need to be considered. There are a great many belt-driven machines out there. Using similar logic applied to coupled shafts, one can identify belt sheave runout or eccentricity for either sheave depending on which shaft 1x and 2x vibration levels — drive shaft or driven shaft — are abnormal. Another factor with direct-drive, coupled machines is whether the driven component, usually a pump or fan, is simply supported — bearings on both sides of the rotor — or overhung — cantilevered. In such a case, an imbalance of the overhung driven produces a strong axial force at the driven component which can transmit back to the driver and look somewhat like shaft misalignment.”

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