SHAFT VERSUS FOOT ALIGNMENT TOLERANCES A Critique of the Various Approaches by Alan Luedeking, Ludeca, Inc.

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1 SHAFT VERSUS FOOT ALIGNMENT TOLERANCES A Critique of the Various Approaches by Alan Luedeking, Ludeca, Inc. The 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. 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 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 two straight lines, sitting someplace in space. The trick is to get the two of them to coincide 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. Figure 1 Figure 2 Since the shafts exist in three-dimensional space, misalignment can exist in any direction. Therefore, we describe 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 machines! 2008 Ludeca, Inc. Page 1 of 10

2 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. 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 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 455 degrees 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 85 degrees F whose distance centerline to bottom of feet is also 18 inches. Both initially started up at the same ambient temperature. The differential in their growth is ( inches per inch per deg. F) 18 inches (455 85) deg. F = 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 repairfocused. Using laser shaft alignment tools and paying attention to thermal growth is a mandatory requirement for companies whose maintenance focus is instead reliabilitybased. 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 3 for an illustration of how such a tolerance envelope looks Ludeca, Inc. Page 2 of 10

3 Figure 3 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 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 4.) A modern laser shaft alignment system can be set to describe this angle in whatever format is preferred by the user. Figure 4 Standard versus Vector Tolerances at the Coupling It must be noted that the offset & angularity approach can have two different interpretations. If we describe the permissible offset between the driver and driven shafts as being of χ amount, does this mean (1) χ amount in any direction, or (2) χ 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 tolerances and is the more commonly used. If you never want more than χ amount of offset to exist between the shafts in any direction, then you should apply vector tolerances since using standard tolerances can lead, in some circumstances, to more offset than you really intended to allow. See Figures 5 and 6 for an illustration of this Ludeca, Inc. Page 3 of 10

4 Figure 5: Standard tolerances applied Figure 5 illustrates a case where applying standard tolerances to an offset of 2.5 mils horizontally and 2.7 mils vertically is deemed acceptable, because the permissible limit for either of these offsets individually is 3.0 mils. However, the actual offset between the shafts is 3.7 mils, which is unacceptable if your absolute limit is 3.0 mils. This is derived from the traditional formula a 2 + b 2 = c ; thus ( ) = 3.7 mils. This result can be seen as a vector tolerance, in Figure 6: Figure 6: Vector tolerances applied 2008 Ludeca, Inc. Page 4 of 10

5 As can be seen, vector tolerances are more conservative and therefore safer for very critical machinery. A good laser shaft alignment system such as the Rotalign Ultra or Rotalign Pro will allow you to apply this distinction and specify exactly which type of tolerances you wish to use. Figure 7 presents the tolerance table most widely accepted as the standard industry norm for short couplings, and you can choose whether to apply the values individually to each plane (vertical and horizontal) or more conservatively, as vector values. SHAFT ALIGNMENT TOLERANCES (SHORT COUPLINGS) RPM Offset (mils) EXCELLENT Angularity (mils/inch) Offset (mils) ACCEPTABLE Angularity (mils/inch) Figure 7: Short-Flex Coupling Alignment Tolerance Table: Standard Industry Norms Spacer Coupling Tolerances Spacer coupling tolerances generally are expressed as limits to the angle that may exist between each machine shaft and the spacer shaft between them. Since the spacer shaft (or spool piece) connects to the end of each of the machine shafts at either end, by definition, there should not exist any offset between the spacer and 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. See Figure Ludeca, Inc. Page 5 of 10

6 Figure 8 As can be seen here, Angle ß on the right projects Offset B and Angle α on the left projects Offset A on the right. Figure 9 presents the table of values most widely accepted as the standard industry norm for spacer couplings. SHAFT ALIGNMENT TOLERANCES (SPACER COUPLINGS) Angularity (Angles α and β), or Projected Offset (Offset A, Offset B) (mils per inch) RPM EXCELLENT ACCEPTABLE Figure 9: Spacer Shaft Alignment Tolerance Table: Standard industry norms Since most flexible couplings have two flex planes (or points of articulation), these spacer coupling tolerances may safely be used with all such couplings, even the ones usually considered short flex couplings. The best criterion to make the distinction between a short flex and a spacer coupling is the relation between the diameter of the flex planes and the distance between them. Anytime the distance between 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 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 2008 Ludeca, Inc. Page 6 of 10

7 attain during operation. This can be easily related to the maximum permissible offset and angularity through the following formula: 2 d r a π Where: d = coupling diameter, r = revolutions/time, and a = angle in radians. 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 be 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. Tolerances Expressed in Terms of Corrections Values at the Feet: The Wrong Way This approach is wrong. It is impossible to define the quality of the alignment between shaft centerlines in terms of correction values at the feet alone, unless one also specifies the exact dimensions associated with these specific correction values each time for each machine, and the sign of the values! This approach is therefore so cumbersome and errorprone 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 still the alignment between the shafts is 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 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 2008 Ludeca, Inc. Page 7 of 10

8 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, 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 10. Figure 10 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 11: 2008 Ludeca, Inc. Page 8 of 10

9 Figure 11 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 is just as likely to occur: Imagine a large machine (such as a diesel engine), running at 1,200 RPM, whose distance between the feet is 80 inches. The distance from front feet to the coupling is 30 inches. This machine is found to have misalignment requiring foot corrections of 8 mils at the front feet and 26 mils at the back feet. This would be considered way out of tolerance if we apply a permissible limit of ±2 mils at the feet. However, in this case, the resulting misalignment at the coupling is only mils of offset and only mils per inch of angularity ( 2.25 mils gap difference at a 10" diameter). See Figure 12. Figure Ludeca, Inc. Page 9 of 10

10 These alignment conditions at the coupling are already much better than required by the standard industry norms for 1,200 RPM, yet, using the improperly specified tolerance values at the feet of ±2 mils, the aligner would be misled into doing unnecessary work to bring the machines to these values. Again the fallacy of this approach is indisputable. Figures 5, 6, 10, 11 and 12 created with the assistance of Alignment Center and Alignment Explorer software by Prüftechnik Ludeca, Inc. Page 10 of 10