Uptime Magazine - October/November 2008 - (Page 52)

precision maintenance upload Shaft vs. Foot Alignment Tolerances A Critique of Various Approaches by Alan Luedeking T he only correct way to express shaft alignment tolerances is in terms of alignment conditions at the coupling. It is entirely incorrect to describe alignment tolerances in terms of the correction values at the machine feet. We will explore this in detail further on, but first let’s examine what our objective is. When two machines are directly coupled via a flexible coupling, any misalignment between their centerlines of rotation can result in vibration which can produce premature wear or even catastrophic failure of the machines’ bearings, seals, the coupling itself, or other rotating components. The worse this misalignment between shaft centerlines, the greater the rate of wear and likelihood of premature failure of the machines. Also factor in a loss of efficiency along with an increase in power consumption. the specific amount of offset and angularity that exists in the horizontal and vertical planes separately, at the location of the coupling. We describe these conditions at the location of the coupling because it is there where the vibration that is so harmful to your machines is created, when misalignment exists. Another way to describe misalignment would be in terms of the sliding velocities resulting from it. However, one way to never describe it is in terms of foot corrections, since those values depend entirely upon the size and geometry of each machine. Since good quality flexible couplings are almost always built to withstand more misalignment than what is good for the machines involved (in terms of the vibration created), it is almost a truism that one should never align to the tolerances allowed for their couplings by the coupling manufacturer, but rather align to a tighter standard. The principal reason why most good flexible couplings permit greater misalignment than what is recommended for the machines is to permit these machines to be deliberately misaligned (sometimes significantly so) in the “cold” and stopped condition, in order to allow for the anticipated changes that will occur to the alignment condition when the machines are placed in service and achieve the “hot” running condition. An excellent alignment of the shafts’ centerlines of rotation does not in itself guarantee absence of vibration because you can still have imbalance of rotating components, resonance, turbulence and cavitation, mechanical looseness or even vibration from other nearby running machines that enters your machines through the foundation or piping. But misalignment of the centerlines of rotation is one of the leading causes of damage to machinery. Absolute perfection in the alignment of the shafts is not realistically attainable, nor is it necessary. Like everything else in life, no matter how hard you try and how long you work at it, you will never achieve perfection, or an absolutely perfect zero-zero alignment. This, of course, begs the question: “If, no matter how hard I try, I will never achieve a perfect alignment and so must always still have some misalignment, how much is too much? How much misalignment can I live with, can I tolerate?” This, by definition, would be your tolerance. To define these limits, it is first necessary to define how we will describe misalignment: The centerlines of rotation of the shafts are simply Anglular two straight lines, sitting someplace in space. The trick is to get the two of Figure 1 - Offset & Angular them to coincide Misalignment so that they form one straight line. If they don’t, then you have either offset misalignment (see Figure 1) or angular misalignment, or a combination of both. Offset A Note on Thermal Growth If the couplings weren’t built to allow for this excessive misalignment (at least for a little while), some machines could never be directly coupled together unless a spacer shaft were installed between them. But this gives rise to the temptation to align to the looser standard the coupling allows rather than to the tighter standards that the machines themselves demand. If you simply ignore the changes to alignment that result from thermal growth, you will have problems. For example, a refinery has a small steam turbine, foot-mounted, and enveloped in insulating blankets. The steel casing temperature while in operation is 455º F, and the distance from centerline to the bottom of the feet is 18 inches. It drives an ANSI pump with a casing temperature of 85º F whose distance centerline to bottom of feet is also 18 inches. Both october/november 2008 Since the shafts exist in three-dimensional space, misalignment can exist in any direction. Therefore, we describe 52

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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