Uptime Magazine - October/November 2008 - (Page 55)

supposed to represent a permissible range of misalignment. Now then, suppose a machine is found to be high by 0.002” at the front feet and low at the back feet by 0.002”. Therefore, it is deemed to be in tolerance by this method. If the distance between the feet is 8 inches, this results in an existing angular misalignment of the shaft centerlines of 0.5 mils per inch (which we derive by observing the 4 mils difference in offset over the 8” run between the feet.) If the distance from the front foot to the coupling center is 10 inches, simple rise over run tells us that the resulting offset between the machine shafts at the coupling would be +7.0 mils! This offset is considerably in excess of the ±3 mils of offset at the coupling that is considered the maximum acceptable for an 1800 RPM machine (from Figure 6), yet, with the improperly specified foot correction tolerances this alignment would be erroneously considered to be in tolerance! This is a classic example of where small correction values at the feet do not necessarily mean that you have a good alignment at the coupling. See Figure 9. Moreover, if you extend this centerline over to the other machine, the foot positions of that machine are far in excess of the permissible values of ±2 mils! See Figure 10. So then, how is it possible that a given alignment of the shafts would be considered in tolerance when looked at from the perspective of one machine, yet be completely out of tolerance if looked at from the perspective of the other machine? This example clearly illustrates the self-defeating fallacy of this approach. An equally bad consequence of this ‘tolerances at the feet’ approach is that the opposite scenario Figure 9 - Tolerances expressed in terms of correction values is just as likely to occur: at the feet can lead to gross misalignment even though the Imagine a large machine aligner thinks the alignment is acceptable. (such as a diesel engine), running at 1,200 RPM, whose distance between the feet is 80 inches. alignment at the coupling is only +1.25 mils of The distance from front feet to the coupling is offset and only –0.225 mils per inch of angularity 30 inches. This machine is found to have mis- (–2.25 mils gap difference at a 10” diameter). alignment requiring foot corrections of –8 mils See Figure 11. at the front feet and –26 mils at the back feet. This would be considered way out of tolerance These alignment conditions at the coupling are if we apply a permissible limit of ±2 mils at the already much better than required by the stanfeet. However, in this case, the resulting mis- Our Students Call It CSI for Engineers. We call it “Practical Plant Failure Analysis” It’s a reasonably-priced practical seminar with hundreds of hands-on examples designed for plant people and engineers. You’ll work with gears, bearings, shafts, belts, fasteners, seals, and a selection of corrosion examples. With the hands-on analyses, you’ll learn how and why they fail – and how to diagnose the multiple causes. The public session is three days long and is held in Syracuse,NY in November. Private sessions range from two to four days and can be held at your site. From the Reliability Professionals who “wrote the book on logical and practical plant failure analysis”. For more details contact Dale Gamba at 315-487-4390 or email us at reliable@twcny.rr.com Taking place at the Hilton Clearwater Resort in Clearwater Beach, Florida from January 18th to January 21st, 2009 You’ll learn how to: This is one technology conference you can’t afford to miss! Visit UESystems.com for presenter information or call 800.223.1325 today. THE ULTRASOUND APPROACH www.uptimemagazine.com 55 http://www.UESystems.com http://www.UESystems.com http://www.uptimemagazine.com

Table of Contents Feed for the Digital Edition of Uptime Magazine - October/November 2008

Uptime Magazine - October/November 2008 Contents Upfront Upclose: An Asset Management Oasis in the Desert Information Technology: Streamlining Through Simplicity Lubrication: Phases for Framing a Solid Foundation Infrared: Do Your Homework Before You Buy Maintenance Management: Get It Together by Getting Together Motor Testing: Key Elements in Motor Decision Making Precision Maintenance: Shedding Some Light on Tolerances Reliability: RCM & the Mortgage Meltdown Ultrasound: A Multidimensional Tool Vibration: The Whirling Pump Mystery, Solved with ODS Upgrade: Are You Looking in the Right Place for Your Wear Debris?

Uptime Magazine - October/November 2008

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