Maintenance Fundamentals Episode 1 Part 5 potx

and a vibration analysis of the machinery in operation shows the vibration effects caused by misalignment to be within the manufacturers’ specifications or accepted industry standards. Note that manufacturers’ alignment specifica- tions may include intentional misalignment during ‘‘cold’’ alignment to compen- sate for thermal growth, gear lash, etc. during operation. COUPLING ALIGNMENT VERSUS SHAFT ALIGNMENT If all couplings were perfectly bored through their exact center and perfectly machined about their rim and face, it might be possible to align a piece of machinery simply by aligning the two coupling halves. However, coupling ec- centricity often results in coupling misalignment. This does not mean, however, that dial indicators should not be placed on the coupling halves to obtain alignment measurements. It does mean that the two shafts should be rotated simultaneously when obtaining readings, which makes the couplings an exten- sion of the shaft centerlines whose irregularities will not affect the readings. Although alignment operations are performed on coupling surfaces because they are convenient to use, it is extremely important that these surfaces and the shaft ‘‘run true.’’ If there is any runout (i.e., axial or radial looseness) of the shaft and/or the coupling, a proportionate error in alignment will result. Therefore, prior to making alignment measurements, the shaft and coupling should be checked and corrected for runout. ALIGNMENT CONDITIONS There are four alignment conditions: perfect alignment, offset or parallel mis- alignment, angular or face misalignment, and skewed or combination misalign- ment (i.e., both offset and angular). PERFECT ALIGNMENT Two perfectly aligned shafts are colinear and operate as a solid shaft when coupled. This condition is illustrated in Figure 7.1. However, it is extremely rare for two shafts to be perfectly aligned without an alignment procedure being Figure 7.1 Perfect alignment. Keith Mobley /Maintenance Fundamentals Final Proof 15.6.2004 4:57pm page 74 74 Maintenance Fundamentals performed on them. In addition, the state of alignment should be monitored on a regular basis to maintain the condition of perfect alignment. OFFSET OR PARALLEL MISALIGNMENT Offset misalignment, also referred to as parallel misalignment, refers to the distance between two shaft centerlines and is generally measured in thousandths of an inch. Offset can be present in either the vertical or horizontal plane. Figure 7.2 illustrates offset, showing two mating shafts that are parallel to each other but not colinear. Theoretically, offset is measured at the coupling centerline. ANGULAR OR FACE MISALIGNMENT A sound knowledge of angular alignment, also called face misalignment,is needed for understanding alignment conditions and performing the tasks associated with machine-train alignment, such as drawing alignment graphics, calculating foot corrections, specifying thermal growth, obtaining target specifications, and determining spacer-shaft alignment. Angular misalignment refers to the condition when the shafts are not parallel but are in the same plane with no offset. This is illustrated in Figure 7.3. Note that with angular misalignment, it is possible for the mating shafts to be in the same plane at the coupling-face intersection but to have an angular relationship such that they are not colinear. Figure 7.2 Offset misalignment. Figure 7.3 Angular misalignment (no offset). Keith Mobley /Maintenance Fundamentals Final Proof 15.6.2004 4:57pm page 75 Shaft Alignment 75 Angularity is the angle between the two shaft centerlines, which generally is expressed as a ‘‘slope,’’ or ‘‘rise over run,’’ of so many thousandths of an inch per inch (i.e., unitless) rather than as an angle in degrees. It must be determined in both the vertical and horizontal planes. Figure 7.4 illustrates the angles involved in angular misalignment. From a practical standpoint, it is often difficult or undesirable to position the stems of the dial indicators at 90-degree angles to the rim and/or face surfaces of the coupling halves. For this reason, brackets are used to mount the devices on the shaft or a non-movable part of the coupling to facilitate taking readings and to ensure greater accuracy. This is a valid method because any object that is securely attached and rotated with the shaft or coupling hub becomes a radial extension of the shaft centerline and can be considered an integral part of the shaft. However, this somewhat complicates the process and requires right- triangle concepts to be understood and other adjustments (e.g., indicator sag) to be made to the readings. Compare the two diagrams in Figure 7.5. Figure 7.5a is a common right triangle and Figure 7.5b is a simplified view of an alignment-measuring apparatus, or fixture, that incorporates a right triangle. The length of side ‘‘b’’ is measured with a tape measure and the length of side ‘‘a’’ is measured with a device such as a dial indicator. Note that this diagram assumes the coupling is centered on the shaft and that its centerline is the same as the shaft’s. Angle ‘‘A’’ in degrees is calculated by A ¼ tan À1 a b This formula yields the angle ‘‘A’’ expressed in degrees, which requires the use of a trigonometric table or a calculator that is capable of determining the inverse Figure 7.4 Angles are equal at the coupling or shaft centerline. Keith Mobley /Maintenance Fundamentals Final Proof 15.6.2004 4:57pm page 76 76 Maintenance Fundamentals Figure 7.5 Common right triangle and simplified alignment-measuring apparatus. tangent. Although technically correct, alignment calculations do not require the use of an angle value in degrees. Note that it is common industry practice to refer to the following value as ‘‘Angle-A,’’ even though it is not truly an angle and is actually the tangent of Angle ‘‘A’’: ‘‘Angle-A’’ ¼ a b ¼ rise run Figures 7.6 and 7.7 illustrate the concept of rise and run. If one assumes that line O-A in Figure 7.6 represents a true, or target, shaft centerline, then side ‘‘a’’ of the triangle represents the amount of offset present in the actual shaft, which is referred to as the rise. (Note that this ‘‘offset’’ value is not the true theoretical offset as defined in Chapter 2. It is actually the theoretical offset plus one-half of the shaft diameter (see Figure 7.5), because the indicator dial is mounted on the outside edge of the shaft as opposed to the centerline. However, for the purposes of alignment calculations, it is not necessary to use the theoretical offset or the theoretical run that corresponds to it. Figure 7.7 illustrates why this is not necessary.) Figure 7.7 illustrates several rise/run measurements for a constant ‘‘Angle-A.’’ Unless ‘‘Angle-A’’ changes, an increase in rise results in a proportionate increase in run. This relationship allows the alignment calculations to be made without using the theoretical offset value and its corresponding run. Keith Mobley /Maintenance Fundamentals Final Proof 15.6.2004 4:57pm page 77 Shaft Alignment 77 Figure 7.6 Concept of rise and run. run 2 run 3 run 4 run 1 O c Angle - A rise 1 rise 2 rise 3 rise 4 B A Figure 7.7 Rise// run measurements for constant angle. Therefore, the calculation of ‘‘Angle-A’’ can be made with any of the rise/run measurements: ‘‘Angle-A’’ ¼ rise 1 run 1 ¼ rise 2 run 2 ¼ rise 3 run 3 ¼ rise 4 run 4 For example, if the rise at a machine foot is equal to 0.5 inches with a run of 12 inches, ‘‘Angle-A’’ is ‘‘Angle-A’’ ¼ 0:5 00 12:0 00 ¼ 0:042 If the other machine foot is 12 inches away (i.e., run ¼ 24 inches), the following relationship applies: 0:042 ¼ X 24:0 00 where X or rise ¼ 1 inch Keith Mobley /Maintenance Fundamentals Final Proof 15.6.2004 4:57pm page 78 78 Maintenance Fundamentals COMBINATION OR SKEWED MISALIGNMENT Combination or skewed misalignment occurs when the shafts are not parallel (i.e., angular) nor do they intersect at the coupling (i.e., offset). Figure 7.8 shows two shafts that are skewed, which is the most common type of misalignment problem encountered. This type of misalignment can occur in either the horizontal or vertical plane, or in both the horizontal and vertical planes. For comparison, see Figure 7.3, which shows two shafts that have angular misalignment but are not offset. Figure 7.9 shows how an offset measurement for non-parallel shafts can vary depending on where the distance between two shaft centerlines is measured. Again, note that theoretical offset is defined at the coupling face. Figure 7.8 Offset and angular misalignment. Figure 7.9 Offset measurement for angularly misaligned shafts. Keith Mobley /Maintenance Fundamentals Final Proof 15.6.2004 4:57pm page 79 Shaft Alignment 79 ALIGNMENT PLANES There are two misalignment planes to correct: vertical and horizontal. Therefore, in the case in which at least two machines make up a machine-train, four types of misalignment can occur: vertical offset, vertical angularity, horizontal offset, and horizontal angularity. These can occur in any combination, and in many cases, all four are present. Vertical Both angular misalignment and offset can occur in the vertical plane. Vertical misalignment, which is corrected by the use of shims, is usually illustrated in a side-view drawing as shown in Figure 7.10. Horizontal Both offset and angular misalignment can occur in the horizontal plane. Shims are not used to correct for horizontal misalignment, which is typically illustrated in a top-view drawing as shown in Figure 7.11. This type of misalignment is corrected by physically moving the MTBM. STATIONARY MTBM (MACHINE TO BE MOVED) Figure 7.10 Vertical misalignment. STATIONARY MTBM (MACHINE TO BE MOVED) Figure 7.11 Horizontal misalignment. Keith Mobley /Maintenance Fundamentals Final Proof 15.6.2004 4:57pm page 80 80 Maintenance Fundamentals ACTIONS TO BE TAKEN BEFORE ALIGNMENT It is crucial that alignment procedures be performed correctly, regardless of what method from Chapter 3 is used. Actions to be taken before alignment are discussed in the following sections, which cover the preparatory steps as well as two major issues (i.e., soft-foot and indicator sag corrections) that must be resolved before alignment can be accomplished. This section provides procedures for making these corrections as well as the proper way to tighten hold-down nuts, an important procedure needed when correcting soft-foot. Preparatory Steps The following preparatory steps should be taken before attempting to align a machine train: 1. Before placing a machine on its base, make sure that both the base and the bottom of the machine are clean, rust free, and do not have any burrs. Use a wire brush or file on these areas if necessary. 2. Common practice is to position, level, and secure the driven unit at the required elevation prior to adjusting the driver to align with it. Set the driven unit’s shaft centerline slightly higher than the driver. 3. Make all connections, such as pipe connections to a pump or output shaft connections on a reducer, to the driven unit. 4. Use only clean shims that have not been ‘‘kinked’’ or that do not have burrs. 5. Make sure the shaft does not have an indicated runout. 6. Before starting the alignment procedure, check for ‘‘soft-foot’’ and correct the condition. 7. Always use the correct tightening sequence procedure on the hold- down nuts. 8. Determine the amount of indicator sag before starting the alignment procedure 9. Always position the stem of the dial indicator so that it is perpen- dicular to the surface against which it will rest. Erroneous readings will result if the stem is not placed at a 90-degree angle to the surface. 10. Avoid lifting the machine more than is absolutely necessary to add or remove shims. 11. Jacking bolt assemblies should be welded onto the bases of all large machinery. If they are not provided, add them before starting the alignment procedure. Use jacking bolts to adjust for horizontal offset and angular misalignment and to hold the machine in place while shimming. Keith Mobley /Maintenance Fundamentals Final Proof 15.6.2004 4:57pm page 81 Shaft Alignment 81 CORRECTING FOR SOFT-FOOT Soft-foot is the condition when all four of a machine’s feet do not support the weight of the machine. It is important to determine if this condition is present prior to performing shaft alignment on a piece of machinery. Not correcting soft- foot prior to alignment is a major cause of frustration and lost time during the aligning procedure. The basis for understanding and correcting soft-foot and its causes is the know- ledge that three points determine a plane. As an example, consider a chair with one short leg. The chair will never be stable unless the other three legs are shortened or the short leg is shimmed. In this example, the level floor is the ‘‘plane’’ and the bottom tips of the legs are the ‘‘points’’ of the plane. Three of the four chair tips will always rest on the floor. If a person is sitting with his or her weight positioned above the short leg, it will be on the floor and the normal leg diagonally opposite the short leg will be off the floor. As in the chair example, when a machine with soft-foot is placed on its base, it will rest on three of its support feet unless the base and the bottoms of all of the feet are perfectly machined. Further, because the feet of the machine are actually square pads and not true points, it is possible that the machine can rest on two support feet, ones that are diagonally opposite each other. In this case, the machine has two soft-feet. Causes Possible sources of soft-foot are shown in Figure 7.12. Figure 7.12 Diagrams of possible soft-foot causes. 1, Loose foot. 2, Cocked foot. 3, Bad shim. 4, Debris under foot. 5, Irregular base surface. 6, Cocked foot. Keith Mobley /Maintenance Fundamentals Final Proof 15.6.2004 4:57pm page 82 82 Maintenance Fundamentals Consequences Placing a piece of machinery in service with uncorrected soft-foot may result in the following:  Dial-indicator readings taken as part of the alignment procedure can be different each time the hold-down nuts are tightened, loosened, and retightened. This can be extremely frustrating because each attempted correction can cause a soft-foot condition in another location.  The nuts securing the feet to the base may loosen, resulting in either machine looseness and/or misalignment. Either of these conditions can cause vibration, which can be dangerous to personnel as well as to the machine.  If the nuts do not loosen, metal fatigue may occur at the source of soft- foot. Cracks can develop in the support base/frame and, in extreme cases, the soft-foot may actually break off. Initial Soft-foot Correction The following steps should be taken to check for and correct soft-foot:  Before setting the machine in place, remove all dirt, rust, and burrs from the bottom of the machine’s feet, the shims to be used for leveling, and the base at the areas where the machine’s feet will rest.  Set the machine in place, but do not tighten the hold-down nuts.  Attempt to pass a thin feeler gauge underneath each of the four feet. Any foot that is not solidly resting on the base is a soft-foot. (A foot is considered ‘‘soft’’ if the feeler gauge passes beneath most of it and only contacts a small point or one edge.)  If the feeler gauge passes beneath a foot, install the necessary shims beneath that foot to make the ‘‘initial’’ soft-foot correction. Final Soft-foot Correction The following procedure describes the final soft-foot correction:  Tighten all hold-down nuts on both the stationary machine and the MTBS.  Secure a dial-indicator holder to the base of the stationary machine and the MTBS. The stem of the dial indicator should be in a vertical position above the foot to be checked. A magnetic-base indicator holder is most suitable for this purpose.  Set the dial indicator to zero. Completely loosen the hold-down nut on the foot to be checked. Watch the dial indicator closely for foot movement during the loosening process. Keith Mobley /Maintenance Fundamentals Final Proof 15.6.2004 4:57pm page 83 Shaft Alignment 83 [...]... sag factor Figure 7 .14 Dial indicator sag 86 Maintenance Fundamentals Example 1: Assume that the sag factor is À0.006 inch If the indicator reading at 6 o’clock equals þ0. 010 inch, then the true reading is: Indicator reading À sag factor ( þ 0: 010 00 ) À ( À 0:00600 ) ¼ þ0: 016 00 Example 2: If the indicator reading at 6 o’clock equals À0: 010 inch, then the true reading is: ( À 0: 010 00 ) À ( À 0:00600... readings Dial Indicators Figure 7 . 15 shows a common dial indicator, which is also called a runout gauge A dial indicator is an instrument with either jeweled or plain bearings, precisely finished gears, pinions, and other precision parts designed to produce accurate measurements It is possible to take measurements ranging from one-thousandth (0.0 01 inch or one mil) to 50 millionths of an inch The point... that sweeps the dial of the indicator It yields measurements in (þ) or (À) mils 88 Maintenance Fundamentals Figure 7 . 15 Common dial indicator Measurements taken with this device are based on a point of reference at the ‘‘zero position,’’ which is defined as the alignment fixture at the top of the shaft— referred to as the 12 o’clock position To perform the alignment procedure, readings also are required... Rim readings are taken simultaneously at each of the four positions (12 , 3, 6, and 9 o’clock) for the movable machine (MTBS/MTBM) and the stationary machine The measuring device for this type of alignment is a dual-dial indicator, and the most common configuration is that shown in Figure 7 .16 90 Maintenance Fundamentals Figure 7 .16 Typical reverse-dial indicator fixture and mounting Mounting Configuration... during the alignment procedure The numbers (1, 2, 3, and 4) should be permanently marked on, or near, the feet It is generally considered a good idea to tighten the nuts in an ‘‘X’’ pattern as illustrated in Figure 7 .13 Always tighten the nuts in the sequence in which the positions are numbered (1, 2, 3, and 4) Loosen nuts in the opposite sequences (4, 3, 2, and 1) Use a torque wrench to tighten all nuts... 7 .14 illustrates this problem Indicator sag is best determined by mounting the dial indicator on a piece of straight pipe of the same length as in the actual application Zero the dial indicator at the 12 o’clock, or upright, position and then rotate 18 0 degrees to the 6 o’clock position The reading obtained, which will be a negative number, is the measure of the mounting-bracket indicator sag for 18 0... procedure should be used for base plates Always tighten the nuts as though the final adjustment has been made, even if the first set of readings has not been taken 4 2 1 3 Figure 7 .13 Correct bolting sequence for tightening nuts Shaft Alignment 85 CORRECTING FOR INDICATOR SAG Indicator sag is the term used to describe the bending of the mounting hardware as the dial indicator is rotated from the top position... stationary and movable components) With the rim-and-face method, one dial indicator is mounted 92 Maintenance Fundamentals perpendicular to the shaft, which defines the offset of the movable shaft The second indicator is mounted parallel to the shaft, which registers the angularity of the movable shaft Figure 7 .17 illustrates the typical configuration of a rimand-face fixture Mounting As with the reverse-dial... of the indicator reverse method of alignment If there is not enough room on the shafts, it is permissible to attach brackets to the coupling hubs or any part of the coupling that is solidly attached to the shaft Do not attach brackets to a movable part of the coupling, such as the shroud Note that misuse of equipment can result in costly mistakes One example is the improper use of magnetic bases, which...84 Maintenance Fundamentals      If the foot rises from the base when the hold-down nut is loosened, place beneath the foot an amount of shim stock equal to the amount of deflection shown on the dial indicator . couplings Figure 7 . 15 Common dial indicator. Keith Mobley /Maintenance Fundamentals Final Proof 15 .6.2004 4 :57 pm page 88 88 Maintenance Fundamentals when checking alignments during predictive maintenance. not been taken. 4 13 2 Figure 7 .13 Correct bolting sequence for tightening nuts. Keith Mobley /Maintenance Fundamentals Final Proof 15 .6.2004 4 :57 pm page 84 84 Maintenance Fundamentals CORRECTING. MTBM (MACHINE TO BE MOVED) Figure 7 .11 Horizontal misalignment. Keith Mobley /Maintenance Fundamentals Final Proof 15 .6.2004 4 :57 pm page 80 80 Maintenance Fundamentals ACTIONS TO BE TAKEN BEFORE