Uptime Magazine - October/November 2008 - (Page 54)

Shaft Alignment Tolerances (short couplings) Excellent RPM Offset (mils) 5.0 3.0 2.5 2.0 1.0 0.5 Angularity (mils/in) 1.0 0.7 0.5 0.3 0.2 0.1 Acceptable Offset (mils) 9.0 6.0 4.0 3.0 1.5 1.0 Angularity (mils/in) 1.5 1.0 0.8 0.5 0.3 0.2 related to the maximum permissible offset and angularity through the following formula: Shaft Alignment Tolerances (spacer couplings) Angularity (Angles α and β), or Projected Offset (Offset A, Offset B), (mils per inch) RPM 600 900 1200 1800 3600 Excellent 1.8 1.2 0.9 0.6 0.3 0.15 Acceptable 3.0 2.0 1.5 1.0 0.5 0.25 600 900 1200 1800 3600 7200 2*d*r*a* π Where: d = coupling diameter, r = revolutions/time, and a = angle in radians. Figure 6 – Short-Flex Coupling Tolerance Table: Standard Industry Norms each of the machine shafts. Therefore, all that needs to be specified is the maximum angle that may exist between the spacer shaft and each of the machine shafts it connects to. This angle may be specified directly in mils per inch (or milliradians), or in terms of the offset that each individual machine shaft projects to the opposite end of the spacer with respect to the other machine shaft. The first way is called the “angle-angle” method (also sometimes called the “alpha-beta” method), and the second way is called the “offset-offset” (or “offset A-offset B”) method. As can be seen in Figure 7, Angle ß on the right projects Offset B and Angle α on the left projects Offset A on the right. Figure 8 presents the table of values most widely accepted as the standard industry norm for spacer couplings. When offset and angularity exist, the flexing or moving elements in a coupling must deflect, slide or travel by double the amount of the offset and angularity every half a rotation. Since the speed of rotation is defined, then, given a certain amount of misalignment, the maximum velocity that is achieved by the moving element in accommodating this misalignment is, in turn, defined as well. In essence, when you limit the sliding velocity that is permissible, you have, by definition, also limited the offset and angularity (in any combination) that can exist between the coupled shafts. By definition, since this approach only looks at the maximum absolute value of the velocity attained, it is more conservative and is, therefore, more akin to the vector tolerance approach. For 1800 RPM, typically this velocity limit is about 1.13 inches per second for an excellent alignment, and 1.89 inches per second for an acceptable alignment. Again, the best laser alignment systems will let you to apply this approach as well. 7200 Figure 8 – Spacer Shaft Alignment Tolerance Table: Standard Industry Norms and the sign of the values! Therefore, this approach is so cumbersome and error-prone as to be impractical in the field. Since the dimensions between the coupling and the feet and between the feet themselves (front to back) are different for each machine, a tolerance that only describes a maximum permissible correction value at the feet without any reference to the dimensions involved makes no sense. The same correction values, when applied to different dimensions between the support points and to the coupling, can yield vastly different alignment conditions between the rotating centerlines of the shafts. Such a tolerance simply ignores the effects of rise over run, which is essentially what shaft alignment is all about. Furthermore, a tolerance defined generically in terms of corrections at the feet does not take into account the speed of the machines. Such alignment “tolerances” can have two equally bad consequences: the values may be met at the feet yet still allow a poor alignment to exist between the shafts, or, these values may be greatly exceeded yet the alignment between the shafts is still acceptable! This means that in the first scenario, the aligner may stop correcting his alignment before the machines are properly aligned, and in the second case, may be misled into continuing to move machines after they have already arrived in tolerance, thereby wasting valuable time and effort. In addition, meeting “foot tolerances” at one machine almost guarantees violating them at the other machine! Let’s take a look at a couple of examples that illustrate all of the fallacies associated with this approach: Let us assume that the specified alignment tolerance for an 1800 RPM machine is defined as a maximum correction value for the machine feet of ±2 mils. Note that a specific sign (+ or –) is not specified, because a tolerance is, by definition, october/november 2008 Since most flexible couplings have two flex Tolerances Expressed in Terms of planes (or points of articulation), these spacer Corrections Values at the Feet: coupling tolerances may safely be used with all The Wrong Way such couplings, even the ones usually considered “short flex” couplings. The best criterion to This approach is wrong. It is impossible to make the distinction between a short flex and define the quality of the alignment between a spacer coupling is the relation between the shaft centerlines in terms of correction values diameter of the flex planes and the distance at the feet alone, unless one also specifies the between them. Anytime the distance between exact dimensions associated with these specific correction values each time for each machine, flex planes (axial span) is greater than the working diameter of those flex planes, call it a spacer coupling. This will make achieving tolerances easier when performing alignment corrections in the field and in no way diminishes the conservative Offset B nature of these values. Sliding Velocity Tolerances Another approach for specifying alignment tolerances is to describe the maximum permissible limit of the velocity that the moving elements in a flexible coupling may attain during operation. This can be easily SPACER SHAFT Angle α MACHINE SHAFT Angle β Offset A MACHINE SHAFT Figure 7 - Spacer Coupling Tolerances can be measured using the “angle-angle” method or the “offset-offset” method. 54

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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