Uptime Magazine - October/November 2008 - (Page 53)

initially started up at the same ambient temperature. The differential in their growth is (0.0000065 inches per inch per deg. F) x 18 inches x (455 – 85) deg. F = 0.043 inches. If these two machines have their shafts aligned center-to-center when ‘cold’, this amount of offset would be certain to throw this equipment train into the frequent failure category. Using the “80/20” rule, it is safe to assume that 20% of our machinery population eats up 80% of our maintenance money. This pump train would be a “problem child” in the 20% group. Aligning center-to-center without paying attention to thermal growth is one of the big factors that keeps companies among those whose maintenance departments are repair-focused. Using laser shaft alignment tools and paying attention to thermal growth is a mandatory requirement for companies whose maintenance focus is, instead, reliability-based. coupling diameter. The latter method is popular because it relates directly to what the mechanic is used to detecting with his feeler gauges between the coupling faces (see Figure 3.) A modern laser shaft alignment system can be set to describe this angle in whatever format is preferred by the user. “Standard” versus “Vector” Tolerances at the Coupling It must be noted that the Figure 4 - ‘Standard’ tolerances applied offset & angularity approach can have two different interpretations. If we describe the permissible offset between the driver and driven shafts as being of X amount, does this mean (1) X amount in any direction, or (2) X amount seen individually and separately for both the horizontal and vertical planes? These two alternatives are not the same! The first example is called “vector tolerances” and is more conservative. The second approach is called “standard tolerFigure 5 - ‘Vector’ tolerances applied ances” and is the more As can be seen, vector tolerances are more commonly used. If you never want conservative and therefore safer for very critimore than X amount of offset to cal machinery. A good laser shaft alignment exist between the shafts in any system will allow you to apply this distinction direction, then you should apply and specify exactly which type of tolerances you vector tolerances since using “stan- wish to use. dard” tolerances can lead, in some circumstances, to more offset than Figure 6 presents the tolerance table most you really intended to allow. See widely accepted as the standard industry norm for short couplings, and you can choose whether Figures 4 and 5 for an illustration of this. to apply the values individually to each plane Figure 4 illustrates a case where applying (vertical and horizontal) or more conservatively, standard tolerances to an offset of 2.5 mils as vector values. horizontally and 2.7 mils vertically is deemed acceptable, because the permissible limit for Spacer Coupling Tolerances either of these offsets, individually, is 3.0 mils. However, the actual offset between the shafts is Spacer coupling tolerances generally are ex3.7 mils , which is unacceptable if your absolute pressed as limits to the angle that may exist limit is 3.0 mils. This is derived from the tra- between each machine shaft and the spacer ditional formula a2 + b2 = c2; thus ( 2.52 + 2.72 ) shaft between them. Since the spacer shaft (or = 3.7 mils. This result can be seen as a vector spool piece) connects to the end of each of the machine shafts at either end, by definition, there tolerance, in Figure 5. shouldn’t be any offset between the spacer and “Short-Flex” Coupling Tolerances This is one of the most common ways of defining alignment tolerances. The offset tolerance simply describes the maximum separation that can exist between two centerlines of rotation at a specific location along the shaft axes, usually the coupling center. The angularity tolerance describes the rate at which the offset between the shaft centerlines may change as we travel along the axes of the shafts. See Figure 2 for an illustration of how such a tolerance envelope looks. Figure 2 - Tolerance Envelope The angularity may de described either directly, as the rate of change in the offset, in mils per inch (or milliradians), or as a gap difference at a particular A A Figure 3 - Angularity Measurement www.uptimemagazine.com 53 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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