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637CHAPTER 12 Machinery Alignment12 M isalignment problems have plaguedmachines ever since the need evolved to transmit torque from one mechanicaldevice to another. Early industrial machines did not have extensive alignmentproblems due to low speeds, low horsepower, and compliant connections. How-ever, as machinery sophistication developed over time, so have the requirementsfor improved alignment. Modern machinery trains usually consist of primarydrivers directly coupled to the driven equipment. The induction motor drivingthe process pump, the gas turbine driving the feed gas compressors, and thesteam turbines driving the generator all share a common characteristic. All ofthese machinery trains require alignment of components, internals, and coupledshafts for safe and reliable operation.To meet the needs for improved machinery alignment, a number of techni-cal approaches have been devised, and many good references are available to themachinery diagnostician. One of the earliest techniques to employ electronicinstrumentation for this task was the 1968 paper by Charles Jackson 1 . Morerecently, John Piotrowski 2 authored the latest revision of his book that includesthe details of laser tools for improved alignment measurements and accuracy.These techniques have been further refined with the use of portable computersthat interface directly with the measurement system. These devices allow consis-tent recording of the moves, plus automated calculations and graphing.It should be self-evident that proper alignment is critical to the life of amachine, and the consequences of misalignment can be seen through the train.Coupling wear or failure, bearing failures, bent rotors or crankshafts, plus bear-ing housing damage are all common results of poor alignment. The extent of thedamage is directly related to the magnitude of the misalignment. For example, ageneral purpose motor driven pump with a slight shaft misalignment mightexperience premature seal failure, bearing damage, or coupling wear. In thiscase, the marginal shaft alignment decreases the time between failures, andincreases the annual repair costs. However, a severe misalignment between thispump and motor could be potentially destructive to the plant surroundings, and 1 Jackson, Charles. “Shaft Alignment Using Proximity Probes,” ASME Paper 68-PET-25, Dal-las, Texas (September 1968). 2 Piotrowski, John, Shaft Alignment Handbook - 2nd ed., (New York: Marcel Dekker, Inc., 1995). 638 Chapter-12 the attendant personnel. In fact, there have been many documented cases of cou-plings that have traveled for several thousand feet after a failure. In this sce-nario, the primary concerns include personnel safety, plus extensive repair costs.In the overview, alignment consists of three distinct categories that areidentified as shaft, bore, and position alignment. Shaft alignment is the mostcommon form of alignment performed on machinery. There are many procedures,practices, and tools available to obtain a precision shaft alignment. Bore align-ment addresses the position of the internal machine components with relation tofixed items such as the main bearings. Bore alignment is used for tasks such aslocating diaphragms with respect to bearing centerlines. Position alignment isprimarily reserved for machine location or elevation. It is also commonly used tomeasure and correct for thermal growth of the machinery. Within this chapter,the fundamentals of machinery position, bore, and shaft alignment will be dis-cussed, and descriptive case histories will be presented. P RE -A LIGNMENT C ONSIDERATIONS Prior to embarking on any alignment project, the diagnostician must evalu-ate the machinery installation, and select the method, tools, and procedures tobe applied. Since each machinery installation differs is size, speed, power, loca-tion, and function, it is necessary to integrate all of the alignment variables in acohesive plan before commencing the actual work. The fundamental items to beaddressed are summarized as follows: r Machine arrangement, type, bearing configuration, and viewing position. r Coupling type, condition, runout, speed, and transmitted torque. r Potential thermal growth or shrinkage. r Potential pipe strain. r Condition of foundation, baseplate, sole plate, and anchor bolts. r Location and condition of leveling bolts and jack bolts. r Shim selection, and soft foot checks. r Obstructions to alignment work. r Machinery alignment offsets and tolerances. Machine arrangement often dictates the alignment process and method.On any train, it is necessary to identify the fixed plus the moveable machines.The fixed equipment is the unit that will not be moved during the alignmentwork. Conversely, the moveable machines will be moved to obtain the correctalignment. For instance, in a typical pump-motor application, the pump remainsfixed, and the motor is the moveable machine. In a turbine driven compressortrain, the turbine remains fixed, and the compressor is the moveable unit. If agear box is included between the turbine and compressor, the gear box becomesthe fixed machine, and the turbine and the compressor both become moveable.The criteria for determination of which machine is fixed or moveable is basicallya decision defining the moveability of the various machines. A motor typically Pre-Alignment Considerations639 has no external forces or pipe strain, and it may be easily moved. Turbines aresensitive to external forces, and they usually become the fixed units. A gear boxin a machine train almost always becomes the fixed equipment. If the gear box ismoved, the alignment moves required farther down the machine train may notbe possible. Certainly there are exceptions, such as reciprocating compressorsthat are driven through a gear. In this arrangement, the compressor becomes thefixed equipment, and the gear box plus driver become the moveable units. Machine types and specific bearing configurations can greatly influ-ence the alignment process due to special requirements or considerations. Sincemost alignment techniques are performed at zero speed, it is necessary to antici-pate and accommodate the machinery behavior between zero speed and full oper-ating conditions. For example, large machines with sleeve bearings willexperience a shift in shaft centerline position as the journal progresses from arest position to full running speed. The shaft will rise on the oil film in the direc-tion of rotation, and this centerline change should be considered in the coldalignment offset. As another example, tilting pad bearings can cause problemsduring shaft alignment. Machines with load between pads (LBP) bearings retainthe shaft during alignment. However, machines equipped with load on pad (LOP)bearings can cause problems if the shafts are turned in different directions dur-ing alignment sweeps. This may cause the bottom pad to pivot, and this couldcause the shaft to shift horizontally, which would corrupt the alignment data.Special conditions exist for heavy rotors such as industrial gas turbines, orlong generator rotors. These types of assemblies will sag due to gravity when theshaft is not turning. Once the machine is running, centrifugal and gyroscopicforces will straighten the rotor. However, the static gravitational sag, or bow, willcause the shaft ends to deflect upward outboard of the journal bearings. It is nec-essary to know this shaft deflection to set the machine at the proper at rest loca-tion. If the catenary curve describing the static rotor is not known, then it maybe computed with the analytical techniques described in chapter 5 of this text.Overhung machines have a similar problem, due to the fact that the over-hung wheel often pulls the rotor down at zero speed. In essence, the shaft pivotsacross the wheel end bearing, and this action forces the coupling end journal tothe top of the available bearing clearance (or vice-versa depending on specificrotor geometry). To compensate for this motion, it is common practice to push theshaft back down into the bearing with a pair of rollers positioned on top of thecoupling end journal. Gear boxes may also provide difficult situations during thealignment process. As shown in Figs. 10-1 and 10-2, the direction of the gear con-tact forces are opposite for each gear. One element tries to sit down in the bear-ing, and the other gear wants to climb to the top of its respective bearing. Again,this actual running position should be taken into account during alignment.Although radial offsets generally command most of the attention duringmachinery alignment, the diagnostician must also consider the axial position ofthe respective rotors. For instance, gear boxes with double helical elements willtypically have only one thrust bearing on the bull gear. The pinion will centeritself in the helix, and this axial running position must identified in order to setthe proper axial coupling spacing for the high speed pinion. Similarly, the mag- 640 Chapter-12 netic center for motors must be identified so that the motor rotor may be prop-erly located in the running condition. This knowledge will allow the correctspacing to be established between the motor and the driven coupling hub.Although axial spacing is important on any machine, it is essential to maintainthe proper axial dimensions on units equipped with diaphragm couplings.The viewing position of the machinery to be aligned is critical to anyalignment work. The fundamental concepts of up-down , left-right , and fore-aft become totally meaningless unless a definitive observation or reference point isused. This observation point or direction must be established at the start of thealignment work, and that point is maintained throughout the entire project. Fur-thermore, the final alignment documentation must reflect this viewing direction.It is also desirable to reference the compass points for additional clarification.This viewing position often varies between centrifugal and reciprocatingmachines. Hence, complete documentation of the machinery layout is the onlyway to maintain historical continuity. Coupling types will have a significant influence on alignment, and it isimportant to understand how a particular coupling works. Many technicalpapers have been published on the various coupling types. In addition, excellentoverviews on the entire topic are presented in the 1986 book by Jon Mancuso 3 plus the 1994 text by Mike Calistrat 4 . As mentioned in the previous paragraph,part of shaft alignment is the proper setting of coupling spacing, and/or shaft endgap. This type of information, plus the associated tolerances, are generally speci-fied on the certified coupling drawing. Coupling types will also govern the config-uration of the dial indicator alignment bracket(s). Also, it is important todetermine how the shafts will be rotated to take the alignment readings. Onflange type gear couplings, the flange bolts work well to turn the shafts, but dueto their size they often require the use of larger brackets.Furthermore, in planning the alignment job, consideration should be givento the handling and intermediate storage of the coupling parts. For instance, agear coupling may have two dozen coupling bolts to be removed, and saved forthe reassembly. However, some diaphragm couplings with intricate spool piecedesigns fall into the category of 1,000 bolt couplings. These units require a lot oftime to disassemble, and even more time to put back together. Coupling bolts areusually body fit bolts that are matched and balanced. If someone happens to losea coupling bolt or nut, the entire project becomes unavoidably delayed.The physical condition of the coupling can greatly affect the alignment. Aworn or damaged coupling may produce erratic readings, or have high runouts.It is also easy to misdiagnose vibration data as misalignment when the realproblem is a damaged coupling. In all cases, the coupling should be thoroughlyinspected prior to beginning any alignment job. Check the gear teeth, shimpacks, grid members, bolts, or whatever components exist in the coupling assem- 3 Jon R. Mancuso, Couplings and Joints - Design, Selection, and Application , (New York: Mar-cel Dekker, Inc., 1986). 4 Michael M. Calistrat, Flexible Couplings, Their Design Selection and Use , (Houston: CarolinePublishing, 1994).
