Handbook of Shaft Alignment Part 8 potx

many more that are logical, easy, and inexpensive to do. If your alignment system limits you to a few choices, there is a possibility that you will eventually run into trouble. Honestly, if you do enough alignment jobs, I can virtually guarantee, you will run into a problem with a limited number of solutions. To my knowledge, there is no more effective way to correct misalignment than the methodology explained in this chapter. Still today, some people who align rotating equipment will do trial-and-error alignment. They install some shim stock under the feet and move the machinery sideways a little bit, take another set of readings, and see if the measurements got any better. This sophisticated technique called guessing will eventually produce frustration, anxiety, and anger if continued for long periods of time. To a certain extent I applaud their effort. At least they made an attempt to improve the misalignment condition; many others do not even try. There happens to be a much better way to determine how to accurately position the machinery instead of guessing. And there happens to be a better way than having a limited computer software program telling you what to do, particu- larly if there could be a simpler way to solve the misalignment condition. Even for people who align rotating machinery on a regular basis, it is very difficult to visualize exactly where the centerlines of rotation are by just looking at dial indicator, laser, or optical encoder measurement data. Your goal is to position each machine so that both shafts FIGURE 8.1 Representing the centerlines of rotation of machinery shafts as straight lines. Piotrowski / Shaft Alignment Handbook, Third Edition DK4322_C008 Final Proof page 320 6.10.2006 12:13am 320 Shaft Alignment Handbook, Third Edition run in the same axis of rotation and you invariably begin to wonder—Is one shaft higher or lower than the other one, is it to the west or is it to the east, and if so, how much? Alignment models can be as simple or as complex as the drive system itself. If you are trying to align two pieces of machinery such as a motor and a pump, the alignment model can be constructed to show both of those shafts. If you are trying to align an eight-element drive system with a right angle in the drive, the alignment model can be constructed to show every one of the shafts including the right angle turn the drive system makes. This chapter is intended to introduce you to modeling a two-element drive system. More complex drive systems are covered in Chapter 16 and Chapter 17. 8.1 GRAPHING AND MODELING ALIGNMENT TECHNIQUES Regardless of the device used to measure the positions of the centerlines of rotation (be it dial indicators, optical encoders, lasers, and the like), virtually every alignment measurement system utilizes one (or a slight variation) of the following measurement approaches: 1. Reverse indicator method (Chapter 10) 2. Face and rim method (Chapter 11) 3. Double radial method (Chapter 12) 4. Shaft to coupling spool method (Chapter 13) 5. Face–face method (Chapter 14) To understand how each of these techniques work, dial indicator readings will be used to illustrate how each method can be graphed or modeled to determine the relative positions of each shaft. All of these techniques can be graphed or modeled by hand. Typically all you need is some graph paper (20 division=in. is a good choice), a straightedge, and a pencil (with an eraser just in case). You do not even really need graph paper; all that is required is a scaled grid or some sort of measurement device like a ruler, but graph paper helps. 8.2 BASIC ALIGNMENT MODELS The graphical shaft alignment modeling techniques use two different scaling factors. One scaling factor proportions the overall dimensions of the machinery drive system to fit within the boundaries of the graph paper and another different scaling factor is used to exaggerate the misalignment between the machinery shafts. If we limit our discussion to horizontally mounted rotating machinery drive trains for now, there will be two graphs that need to be drawn. As depicted in Figure 8.2, one graph will show the exaggerated positions of each shaft in the side view illustrating the up and down or vertical positions of the machinery. Another graph will be constructed in the top view that will illustrate the side-to-side or lateral positions of the machinery. Figure 8.3 shows a three- dimensional view of the drive system misalignment. Keep in mind that the shaft centerline positions shown in the side and the top views are exaggerated to help visualize the misalignment condition. Once the relative positions of the machinery shafts are constructed on the graph, a wide variety of different solutions can be determined to bring the centerlines of rotation in line with each other. The benefit of modeling rotating machinery is to visually represent an exaggerated, but accurately scaled picture of the misalignment condition so you can easily ascertain what positions the machinery could be moved that would make it easy to align the shafts within the boundary conditions imposed by the baseplate or foundation and the allowable lateral restrictions between the machinery casing bolts and the holes drilled in the machine cases (a.k.a. ‘‘bolt bound’’ conditions). Piotrowski / Shaft Alignment Handbook, Third Edition DK4322_C008 Final Proof page 321 6.10.2006 12:13am Alignment Modeling Basics 321 Additionally the modeling technique can include other measurement parameters such as improperly fit piping, air gap clearances between stators and armatures, and fan rotor to shroud clearances, for example. Finally, the graph is a permanent record of the alignment of the machinery and can be kept for future reference. In summary, this chapter will review the following key steps in correcting the misalignment situation (refer to Step 6 in Chapter 1): 1. Determine the current positions of the centerlines of rotation of all the machinery. 2. Observe any movement restrictions on the machines at the control and adjustment points (usually the machinery feet and hold down bolts). 3. Plot the restrictions on the graph or the model. Side view Top view East Scale: 4 in. 10 mils Scale: 4 in. 10 mils Up FIGURE 8.2 Side and top view alignment models showing an exaggerated misalignment condition between the two shafts in the vertical direction (side view) and in the lateral direction (top view). Piotrowski / Shaft Alignment Handbook, Third Edition DK4322_C008 Final Proof page 322 6.10.2006 12:13am 322 Shaft Alignment Handbook, Third Edition 4. Determine the moves for either or both of the machinery casings on the graph or the model that will be feasible to perform. We will first begin by illustrating the basic principles of how to construct the relative positions of the two centerlines of rotation and then show how you can determine the wide variety of movement options available to you when repositioning misaligned machinery. 8.3 SCALING THE DRIVE SYSTEM ONTO THE ALIGNMENT MODEL There are several key positions on your drive system where distances need to be measured for scaling onto the graph paper. The most important ones are Top view Scale: Scale: 4 in. 10 mils 4 in. 10 mils east Side view up FIGURE 8.3 (See color insert following page 322.) Three-dimensional view of the side and top views. Piotrowski / Shaft Alignment Handbook, Third Edition DK4322_C008 Final Proof page 323 6.10.2006 12:13am Alignment Modeling Basics 323 1. Where the foot and the hold down bolts are located on each machine 2. Where the measurements are taken on the machinery shafts Other critical dimensions that may need to be taken are 1. Where measurements have been taken to observe how the machinery moved from off- line to running conditions (refer to Chapter 16) 2. Where the piping connections are made 3. Where lateral adjustments are made (assuming they are not exactly where the foot bolts are located) 4. Internal clearances between rotating and stationary components in each machine To begin modeling your alignment problem, the drive system is scaled onto the graph as shown in Figure 8.4 and Figure 8.5. The distances you measure along the length of the drive train should be accurate within 1=4’’ if possible or at least with an accuracy of +1% of the overall length of the drive system. Motor Pump 15Љ 5.5Љ 7Љ 14.5Љ Up PumpSide viewMotor 10Љ 15Љ Scale: 5Љ 5.5Љ 7Љ 14.5Љ10Љ FIGURE 8.4 Scaling the machinery feet and measurement positions onto the graph. Piotrowski / Shaft Alignment Handbook, Third Edition DK4322_C008 Final Proof page 324 6.10.2006 12:13am 324 Shaft Alignment Handbook, Third Edition 8.4 CARDINAL ALIGNMENT GRAPHING AND MODELING RULES Alignment modeling can be confusing when you first attempt it but there are a few rules that apply when constructing an alignment model using any of the alignment measurement methods: 1. Only plot measurements that have been compensated for bracket sag. 2. Only plot half of a rim dial indicator reading. 3. Positive (þ) dial indicator readings means the shaft is ‘‘low.’’ 4. Negative (À) dial indicator readings means the shaft is ‘‘high.’’ 5. Zero the indicator on the side that is pointing toward the top of the graph paper. 6. Whatever shaft the dial indicator (or any other measuring device) is taking readings on is the shaft that you want to draw on the graph paper. 7. Superimpose your boundary conditions. 8. Select an alignment correction line (a.k.a. overlay line or final desired alignment line) that is possible and easy to do. 8.4.1 PLOT MEASUREMENTS THAT HAVE BEEN COMPENSATED FOR BRACKET SAG As discussed in Chapter 6, gravity will have an effect on a mechanical bracket when measuring shaft positions on horizontally mounted machinery. Bracket sag typically only Side view Label which view you are looking at and the direction pointing to the top of the graph paper. Label where each machine is located. Mark down the scale factor from left to ri g ht. You could use an X to indicate where the center of the bolts are located. 15Љ 5ЉScale: 5.5Љ 7Љ 10.25Љ 14.5Љ Motor Pump Up FIGURE 8.5 Scaling the machinery feet and measurement positions onto the graph. Accurately scale off the distances between the inboard and outboard feet of both machines, the distances from the inboard feet of both units to the point where the dial indicator plungers are touching (i.e., taking readings) on both shafts, and the distances between measurement points along the graph centerline from left to right. Piotrowski / Shaft Alignment Handbook, Third Edition DK4322_C008 Final Proof page 325 6.10.2006 12:13am Alignment Modeling Basics 325 affects the measurements taken from the top to the bottom of a shaft and will come into play when plotting the shafts in the side view alignment model. Usually the amount of bracket sag is the same on both sides of a shaft and therefore the sags cancel each other out. The readings taken from one side of a shaft to the other side are plotted in the top view alignment model. If you are not sure how to compensate for bracket sag in your measurements, review the section in Chapter 6 and specifically Figure 6.56 and Figure 6.57. 8.4.2 RIM READINGS ARE ALWAYS TWICE THE OFFSET AMOUNT Remember, anytime a rim or circumferential reading is taken, the amount measured from one side to the other side of the shaft (1808 of rotation) is twice the amount of the actual distance between the centerlines of rotation at that point. Refer to Figure 6.44 and Figure 6.45 to understand why this happens. 8.4.3 PLUS MEANS ‘‘LOW’’ AND MINUS MEANS ‘‘HIGH’’ Where and how to position the rotating machinery shafts on the alignment model will make far more sense if you reason out what the measurement sensor has told you. If you zero a dial indicator at the top of a shaft, sweep to the bottom of that shaft and your indicator has registered a þ40 reading, it is low by 20 mils at that point. The sign of the number tells you which way the shaft is, the number tells you how far away it is. This is a vector problem. It has an amount and a direction. The sign tells you the direction, the number tells you the amount. For example, Figure 8.6 shows a bracket attached to the shaft on the left holding a dial indicator that is measuring the shaft on the right. When the bracket and indicator are rotated to the bottom, the stem of the dial indicator was pushed in as it traversed from the top to the bottom of the shaft on the right. When indicator stems set is pushed in, the needle sweeps in a clockwise direction, producing a positive number. Therefore the shaft on the right is ‘‘low’’ with respect to the shaft on the left. The body of the dial indicator stays at the same distance from the centerline of rotation of the shaft it is attached to. FIGURE 8.6 Positive reading indicates that the shaft you are measuring is ‘‘low.’’ The stem of the dial indicator was pushed in as it traversed from the top to the bottom of the shaft on the right. When indicator stems set is pushed in, the needle sweeps in a clockwise direction producing a positive number. Therefore the shaft on the right is ‘‘low’’ with respect to the shaft on the left. Piotrowski / Shaft Alignment Handbook, Third Edition DK4322_C008 Final Proof page 326 6.10.2006 12:13am 326 Shaft Alignment Handbook, Third Edition Figure 8.7 shows a bracket attached to the shaft on the left holding a dial indicator that is measuring the shaft on the right. When the bracket and indicator are rotated to the bottom, the stem of the dial indicator traveled outward as it traversed from the top to the bottom of the shaft on the right. When indicator stems travel outward, the needle sweeps in a counter- clockwise direction producing a negative number. Therefore the shaft on the right is ‘‘high’’ with respect to the shaft on the left. The quotation marks around the words ‘‘low’’ and ‘‘high’’ are there for a reason. ‘‘High’’ and ‘‘low’’ are relative terms and only apply if you are viewing horizontally mounted shafts when looking at them in the side view (up and down direction). If for example, you are looking at the shafts in Figure 8.6 from above and the top of the page is north, the shaft on the right in Figure 8.6 would appear to be to the south of the shaft on the right (positive (þ) indicator reading). Likewise if you are looking at the shafts in Figure 8.7 from above and the top of the page is north, the shaft on the right in Figure 8.7 would appear to be to the north of the shaft on the right (negative (À) indicator reading). This sounds very simple but in fact more people have trouble plotting shafts in the top view. Again, it is important to understand what happens to the stem of the indicator as you traverse from one side of a shaft to the other side. Does the stem get pushed in (i.e., go positive) or does it have to travel outward (i.e., go negative)? 8.4.4 ZERO THE INDICATOR ON THE SIDE THAT IS POINTING TOWARD THE TOP OF THE GRAPH PAPER In a horizontally mounted drive system, when you are viewing the alignment model in the side view, you will only need to plot the dial indicator measurements you got on the top of the shaft and on the bottom of the shaft. The readings you got on each side (north and south or east and west or left and right) only come into play in the top view. Classically when people initially set up their alignment measurement system the dial indicator is placed on the top of a shaft in the twelve o’clock position, zero the indicator The body of the dial indicator stays at the same distance from the centerline of rotation of the shaft it is attached to. FIGURE 8.7 Negative reading indicates that the shaft you are measuring is ‘‘high.’’ The stem of the dial indicator traveled outward as it traversed from the top to the bottom of the shaft on the right. When indicator stems travel outward, the needle sweeps in a counterclockwise direction producing a negative number. Therefore the shaft on the right is ‘‘high’’ with respect to the shaft on the left. Piotrowski / Shaft Alignment Handbook, Third Edition DK4322_C008 Final Proof page 327 6.10.2006 12:13am Alignment Modeling Basics 327 there and sweep through 908 arcs for the other three measurements as shown in Figure 6.35 through Figure 6.38. 8.4.5 WHATEVER SHAFT THE DIAL INDICATOR IS TAKING READINGS ON IS THE SHAFT THAT YOU WANT TO DRAW ON THE GRAPH PAPER Again, when viewing the alignment model in the side view you want to plot the measurements you got from the top to the bottom of the shaft. Since you typically zero the indicator on the top and sweep to the bottom, you will plot half of the bottom rim reading onto the alignment model. A line representing the centerline of rotation of the pump shaft is drawn from the position where the bracket was attached to the motor shaft through the point where the dial indicator measured the position of the pump shaft as shown in Figure 8.8. Note the scale factor in the lower left corner of the alignment model. Remember, you only plot half of the bottom reading onto the graph. Also remember that whatever shaft the dial indicator is taking readings on is the shaft that you want to draw on the graph paper. In this case, it is the pump shaft. Motor Pump Side view Scale: 20 mils 5 in. 10 mils Pump Up Motor Sag compensated readings Plot half (10 mils) of this measurement here. T B W −10+30 +20 0 Pump E FIGURE 8.8 (See color insert following page 322.) Plotting the pump shaft in the side view. Piotrowski / Shaft Alignment Handbook, Third Edition DK4322_C008 Final Proof page 328 6.10.2006 12:13am 328 Shaft Alignment Handbook, Third Edition A line representing the centerline of rotation of the motor shaft is drawn from the position where the bracket was attached to the pump shaft through the point where the dial indicator measured the position of the motor shaft as shown in Figure 8.9. Remember, you only plot half of the bottom reading onto the graph. Also remember that whatever shaft the dial indicator is taking readings on is the shaft that you want to draw on the graph paper. In this case, it is the motor shaft. Figure 8.9 now shows an exaggerated picture of the misalignment condition of the motor and pump shafts in the up and down direction. But where are the shafts in the side-to-side direction? When viewing the alignment model in the top view you want to plot the measurements you got from one side of the shaft to the other side of the shaft and here is where a lot of mistakes are classically made. Since you did not zero the indicator on one of the sides, how do you handle the side readings? Real simple, zero the indicator on the side that is pointing toward the top of the graph paper. Motor Pump Side view Scale: 20 mils Motor Plot half (20 mils) of this measurement here. Sag compensated readings 0 T B WE −40 +10−50 5 in. 20 mils Motor Pump Up FIGURE 8.9 (See color insert following page 322.) Plotting the motor shaft in the side view. Piotrowski / Shaft Alignment Handbook, Third Edition DK4322_C008 Final Proof page 329 6.10.2006 12:13am Alignment Modeling Basics 329 [...]... / Shaft Alignment Handbook, Third Edition DK4322_C0 08 Final Proof page 340 6.10.2006 12:13am Piotrowski / Shaft Alignment Handbook, Third Edition DK4322_C009 Final Proof page 341 9 26.9.2006 8: 42pm Defining Misalignment: Alignment and Coupling Tolerances 9.1 WHAT EXACTLY IS SHAFT ALIGNMENT? In very broad terms, shaft misalignment occurs when the centerlines of rotation of two (or more) machinery shafts... FIGURE 9.4 Definition of shaft misalignment Figure 9.4 shows a typical misalignment situation on a motor and a pump By projecting the axis of rotation of the motor shaft toward the pump shaft (and conversely the pump shaft rotational axis toward the motor shaft) there is a measurable deviation between the projected axes of rotation of each shaft and the actual shaft centerlines of each shaft where the power... Piotrowski / Shaft Alignment Handbook, Third Edition DK4322_C009 Final Proof page 345 345 Defining Misalignment: Alignment and Coupling Tolerances Driver shaft Maximum alignment deviation occurs here Driver offset (in mils) 26.9.2006 8: 42pm Driven shaft Driven offset (in mils) Misalignment is the deviation of relative shaft position from a colinear axis of rotation measured at the points of power transmission... further divided into 60 parts called seconds of arc Therefore there are 21,600 min of arc and 1,296,000 s of arc in a circle Piotrowski / Shaft Alignment Handbook, Third Edition DK4322_C009 Final Proof page 343 Defining Misalignment: Alignment and Coupling Tolerances 26.9.2006 8: 42pm 343 Parallel misalignment Angular misalignment “Real world” misalignment usually exhibits a combination of both parallel and... However, as the misalignment is expressed in mils per inch, whether the deviations are measured where the readings were taken, or at the ends of the shafts, or at the flexing points Piotrowski / Shaft Alignment Handbook, Third Edition DK4322_C009 Final Proof page 3 48 3 48 26.9.2006 8: 42pm Shaft Alignment Handbook, Third Edition Motor Side view Up Fan 30 mils Motor shaft centerline 15 mils Fan shaft centerline... the inboard foot of the other machine case could be used as pivot points By Piotrowski / Shaft Alignment Handbook, Third Edition DK4322_C0 08 Final Proof page 334 6.10.2006 12:13am 334 Shaft Alignment Handbook, Third Edition Side view Up Motor Pivot here Raise 48 mils up Fan Overlay line Raise 1 38 mils up Pivot here Motor shaft centerline Scale: Fan shaft centerline 5 in 30 mils FIGURE 8. 14 (See color... 0.500 8 0.600 0.600 Zero elevation line 0.700 0 .80 0 0.700 0 .80 0 0.900 0.900 Best fit catenary curve 1.000 1.000 HP Turbine 1.100 LPA Turbine 1.100 up LPB Turbine 1.200 Generator 1.200 Exciter Side view looking east LPC Turbine Piotrowski / Shaft Alignment Handbook, Third Edition DK4322_C009 Final Proof page 352 26.9.2006 8: 42pm Shaft Alignment Handbook, Third Edition Piotrowski / Shaft Alignment Handbook, ... definition, let us dissect each part of this statement to clearly illustrate what is involved Piotrowski / Shaft Alignment Handbook, Third Edition DK4322_C009 Final Proof page 344 344 26.9.2006 8: 42pm Shaft Alignment Handbook, Third Edition Collinear means in the same line or in the same axis If two shafts are collinear, then they are aligned The deviation of relative shaft position accounts for the... view Piotrowski / Shaft Alignment Handbook, Third Edition DK4322_C009 Final Proof page 349 26.9.2006 8: 42pm 349 Defining Misalignment: Alignment and Coupling Tolerances If you rotate this shaft only, you will align the centerline of rotation with the centerline of the improperly bored coupling hub, not the other shaft centerline of rotation 10 _ 0 + 10 20 20 30 30 40 50 40 Centerline of rotation To align... centerline of the bore of the coupling hub on the shaft on the right but it is not in line with the centerline of rotation of the shaft on the right By its purest definition, shaft alignment occurs when the centerlines of rotation are collinear This is a very important point in aligning rotating machinery that a vast number of people overlook It is possible to align the centerlines of rotation of machinery shafts . centerlines of rotation of machinery shafts as straight lines. Piotrowski / Shaft Alignment Handbook, Third Edition DK4322_C0 08 Final Proof page 320 6.10.2006 12:13am 320 Shaft Alignment Handbook, . / Shaft Alignment Handbook, Third Edition DK4322_C0 08 Final Proof page 324 6.10.2006 12:13am 324 Shaft Alignment Handbook, Third Edition 8. 4 CARDINAL ALIGNMENT GRAPHING AND MODELING RULES Alignment. Third Edition DK4322_C0 08 Final Proof page 3 28 6.10.2006 12:13am 3 28 Shaft Alignment Handbook, Third Edition A line representing the centerline of rotation of the motor shaft is drawn from the