Regarding the double dial indicator alignment of centrifugal pumps and motors

4. Correction of the concentricity of the pump and motor coupling: Due to installation errors and other factors, the two shafts connected by the coupling often cannot be aligned properly, resulting in a certain degree of deviation. The deviations of pump couplings are divided into three types: radial deviation (△y), axial deviation (gap Δx), and combined deviation (△y and Δx), as shown in Figure 2–67. (1) Select tools and equipment. (2) Select the benchmark for correction. Tighten the bolts securing the pump base, using the pump as a reference for concentricity adjustment and the motor as the object to be adjusted. Remove the coupling bolts symmetrically, leaving two bolts in place to rotate the pump. (3) Select the measurement points. Wipe the outer circle and end face of the coupling clean with a cleaning cloth, and use a scribe to mark 4 symmetric measurement points on the pump’s coupling: front (A), rear (B), bottom (C), and top (D). (4) Set up the dial indicator. Fix the magnetic base of the dial indicator stand to the pump’s coupling, and bring one of the dial indicator probes into vertical contact with the outer diameter of the motor coupling in order to measure the radial deviation ； Another dial indicator probe makes perpendicular contact with the rear surface of the motor coupling, and is used to measure the axial deviation (gap), as shown in Figure 2–68. Adjust the downward pressure of the dial indicator probe to 2 mm, then tighten all the fixing screws of the dial indicator holder. Rotate the bezel of the dial indicator to zero the pointer. Gently pull the measuring rod of the dial indicator and let it rebound to see if the dial indicator still returns to zero ； If it is not at zero, the table stand and the gauge should be readjusted to bring the dial indicator back to zero. Rotate the pump by one full turn and check whether the dial indicator returns to zero ； If it is not zeroed, readjust the watch stand and the watch so that the dial indicator returns to zero. (5) Measure the radial deviation and axial deviation. Use a special wrench to rotate the coupling 90° in the direction of the pump’s rotation. Record the readings from the radial and axial deviation gauges for the front section (B). Rotate it another 90° and record the readings for the lower section (C). Rotate it yet another 90° and record the readings for the rear section (D). Finally, rotate it 90° back to the starting position (A) to check whether the readings on both gauges have returned to zero. If the readings from the two tables are as shown in Table 2-12, then the radial and axial deviations are as follows: Table 2-12: Radial Deviation Table and Axial Deviation Table. Items: Upper (A), Front (B), Lower (C), Rear (D). Radial measurement table readings: 0, 0.24, 0.32, -0.16. Axial measurement table readings: 0, 0.26, -0.44, -0.20. Note: With the zero mark as the reference point, when the pointer rotates clockwise, it indicates that the measuring rod is being pressed in, and the reading is positive ； Conversely, when the hand rotates counterclockwise, it indicates that the measuring rod has been extended, and the reading becomes negative. Radial deviation in the up-down direction: C – A = 0.32 – 0 = 0.32 mm (the motor coupling moves downward). Radial deviation in the front-back direction: D – B = (–0.16) – 0.24 = –0.40 mm (the motor coupling moves forward). Axial deviation in the up-down direction: C – A = (–0.44) – 0 = –0.44 mm (the upper part of the pump-motor coupling opens up). Axial deviation in the front-back direction: D – B = (–0.22) – 0.26 = –0.46 mm (the front part of the pump-motor coupling opens up). (6) Adjust the front-back deviation of the coupling. Loosen the motor’s foot bolts, use a crowbar to move the motor back and forth by the amount of its radial deviation, and then pry the motor back and forth by half of that axial deviation amount. (7) Adjust the vertical deviation of the coupling. First, shims with a thickness of △y = C – A = 0.32 mm (the radial deviation in the up and down direction) are placed simultaneously on the front and rear supports of the motor, thereby moving the motor upward and eliminating the radial up and down deviation of the coupling, as shown in Figure 2–69(II). Secondly, in Figure 2-69(Ⅱ), since the triangles △fgh, △dbc, and △eac are similar triangles with their corresponding sides in proportion, the thicknesses bd and ae of the shims to be added to the front and rear supports of the motor when adjusting the up-and-down axial deviation (difference in opening) of the motor are calculated using the following formula. Thickness of the shims to be used at the front support of the motor: db = dc△x/fh. Thickness of the shims to be used at the rear support of the motor: ae = ec△x/fh. Here, dc is the distance between the end face of the motor’s front support and the coupling of the pump (which can be measured in millimeters) ； ec — the distance between the rear support of the motor and the mating surface of the pump coupling (can be measured in mm) ； fh — maximum outer diameter of the motor coupling (can be measured with a ruler), mm ； △x = C – A —— axial deviation in the vertical direction (C–A has been measured using a dial indicator), in mm. After placing shims of thickness bd and ae on the front and rear supports of the motor respectively, tighten the motor’s foot bolts. (8) Redetect the radial and axial deviations at points A, B, C, and D to verify whether they meet the technical requirements. If the technical requirements are not met, re-measure and adjust until the axial deviation of the pump coupling is no more than 0.06 mm, the radial deviation is no more than 0.08 mm, and the end-face clearance between the two couplings reaches 6–8 mm. (9) Tighten the base bolts and coupling connection bolts. (1 O) Clean up the site and collect tools and equipment.

It is precisely because of this misalignment that there are changes in the readings; that’s why it’s necessary to determine this value of misalignment

Hello! Maybe my question isn’t clear enough. What I’m trying to say is that when the drum rotates, and both the motor shaft and the pump shaft rotate by 90° or 180°, even if there’s misalignment, the position of the dial and the hands doesn’t change; therefore, the reading should remain exactly the same, right? It’s like the distance from my head to the ground doesn’t change just because I rotate 90° relative to the ground, right? Could it be that I had too many ideals while studying, which affected my thinking?

When performing alignment in general, we should first align the axial direction and then the radial direction; read more materials on this topic as well.

For example, take horizontal deviation: you stand on one side of the coupling, and the gauge is zeroed out at that horizontal position. Suppose there is a radial deviation between the pump and the motor in the horizontal direction, with the motor being on your side. When you start turning the shaft, you manually zero the gauge; after turning it 180 degrees, the gauge ends up on the opposite side from you, and its reading represents twice the deviation between the two axes.

Simple yet not simple: (, we’re using lasers now. Applicable!

Your analogy is of a single object, so its rotation doesn’t cause any changes. However, alignment involves two objects: the dial holder is on the motor, and the gauge is on the pump. Therefore, when they rotate simultaneously, if they are not in line with each other, it will alter the reading of the gauge.

Thank you all for your replies. I drew a diagram and now I understand.