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.
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.”
In troubleshooting this case on-site, vibration phase measurements synchronized with a tachometer signal can clarify the diagnosis, advises Watts. “For example, with shaft misalignment, the axial 1X vibration at the driver likely will be 180° out of phase with that at the driven component,” he says. “If the abnormal 1X axial vibration are in phase with each other at both the driver and driven components, then the fault is likely imbalance of the overhung rotor. With regard to structural mounting, soft foot would be associated with a solid, hard mounting. If the component is on flexible mounts, there may a single damaged mounting element.”
4 steps to diagnosis
Jason Tranter, managing director of Mobius Institute, offers four steps to determining whether this issue is alignment, some of it corroborating Watts’ suggestions. “First you have the issue of angular, sometimes referred to as ‘gap’ misalignment versus “offset,” sometimes referred to as parallel misalignment,” he concurs. “Typically a machine will be misaligned, to some degree, with a combination of angular and offset misalignment. Angular misalignment creates stronger axial vibration, mostly at the running speed, that is, 1X vibration, whereas offset misalignment generates mostly radial — some vertical and horizontal — vibration.”
|Mike Bacidore is chief editor of Plant Services and has been an integral part of the Putman Media editorial team since 2007, when he was managing editor of Control Design magazine. Previously, he was editorial director at Hughes Communications and a portfolio manager of the human resources and labor law areas at Wolters Kluwer. Bacidore holds a BA from the University of Illinois and an MBA from Lake Forest Graduate School of Management. He is an award-winning columnist, earning a Gold Regional Award and a Silver National Award from the American Society of Business Publication Editors. He may be reached at 630-467-1300 ext. 444 or firstname.lastname@example.org or check out his Google+ profile.|
Second is the issue of just how the vibration will change as a result of offset misalignment. “In the classic wall charts, it will be shown as a 2X peak,” explains Tranter. “That may happen, but many other patterns are also possible. The nature of the coupling — tire, grid, gear, 3-jaw, 4-jaw — is going to have a significant bearing on the vibration patterns witnessed. But certainly it is possible to witness an increase at 1X plus new or increased amplitude peaks at 2X, 3X, 4X, and/or 5X of running speed when a machine is misaligned. This is a detailed subject; you may or may not see certain peaks. But generally if any combination of those peaks increase in amplitude you have to consider misalignment.”
Third, remember that there are other reasons why those peaks can increase in amplitude. “The machine itself may have rotating elements — lobes, vanes, blades — that will generate that vibration,” explains Tranter. “And other fault conditions, for example, unbalance, bent shaft, eccentricity, cocked bearing, and various forms of looseness, can also cause the vibration to increase in the range from 1X to 5X. And, to top it off, any natural frequencies lurking in the range 1X to 5X will cause one or more of those peaks to increase in amplitude more than it should if there were no natural frequencies. The time waveform can provide some indication, but there is no doubt that phase is your friend.”
To be confident in your diagnosis, axial and radial phase readings should be collected, says Tranter. “They can be taken on either side of the coupling, for example, on the motor and pump,” he explains. “They can be taken on the face of the component, at up to four points around the shaft, or along the component, for example, drive end versus non-drive-end. And they can be used to compare vertical versus horizontal vibration. These readings will help confirm misalignment and rule out the other possible fault conditions. Therefore analysts must not jump to conclusions if they see high 1X axial vibration or high 2X radial vibration. Careful examination of the vibration around the machine and the use of phase readings can ensure that a correct diagnosis is made.”
Figure 2. Always employ precision alignment practices, and don’t assume your alignment practices are acceptable just because the vibration reading don’t indicate a fault. (Source: Ludeca)
What do you do once you’ve identified a misalignment problem? “The typical report will cite an appropriate degree of shaft misalignment if the abnormal vibration is seen in the data,” explains Azima DLI’s Watts. “The recommendation and included commentary suggest that inspections be carried out in order of practicality and convenience. For example, one can begin by using a dial indicator to check for shaft axial or radial runout and inspecting the mounting and foundation integrity. Then the shaft should be checked for misalignment versus the specification for that machine. The coupling may need to be disassembled and inspected. If the problem remains unsolved, or depending on the machine and the circumstance, it may be worth the additional effort and expense to perform more detailed testing and troubleshooting on-site using phase analysis.
Mobius Institute’s Tranter offers one more important comment to consider. “There is no guarantee that the vibration will change if the machine is misaligned,” he notes (Figure 2). “Experiments have been performed, on test rigs but also on larger real machines, where the machines have been deliberately misaligned, and yet the vibration did not change either at all or in a significant enough way that would alert a vibration analyst. Therefore, the golden rule is to employ precision alignment practices and not to assume that your alignment practices are acceptable just because the vibration readings do not indicate that a fault exists.”