Pumps & Systems, February 2008
Your company has recently purchased a laser based alignment system. Your newly trained technicians are asked to align a critical process pump. You witness the alignment. They save the final alignment data. You review the data, but what does it mean? How does it compare to dial indicators? Why did they rotate both shafts when measuring the misalignment? Why are the numbers at the feet higher than you would like? How do you KNOW the alignment is good? The debate begins.
To make sure the alignment is good, you insist on rechecking it with dial indicators. This practical example will illustrate how to compare rim and face dial indicator data to the data produced by laser systems and help you troubleshoot the problem.
Lasers Versus Rim and Face
With most laser systems, you are encouraged to rotate both shafts during the measurement process. This is because we want to use the rotational axis of one shaft as the datum (reference line) and then compare the rotational axis of the second shaft to the datum. If only one shaft is rotated, the datum is dependent on the accuracy of the coupling surface. You can improve the results with indicator methods by rotating both shafts while taking alignment measurements.
The face indicator is used to measure angular misalignment. The face indicator measures the actual gap difference at the diameter that the face indicator travels. With larger diameters you will see larger face indicator readings. Angularity should be expressed as a slope, not a displacement. Therefore, you must divide the face reading by the diameter to calculate the true angular relationship of the two shafts.
The rim indicator is used to measure offset misalignment. When the rim indicator is set to zero and subsequently rotated 180-deg, the total indicator reading is divided by two to determine the actual offset misalignment.
Laser systems report angular misalignment as a slope and the actual offset misalignment at a user-specified axial shaft location. For close coupled machines, the user will typically specify the coupling center as the axial location for offset misalignment because the destructive misalignment forces are generated there.
The Misalignment Example
In this example, we have intentionally misaligned a motor. From a perfect alignment, the rear feet of the motor are shimmed .030-in (30-mils). There is no horizontal misalignment. Misalignment will be expressed in mils: 0.001-in = 1.0-mils. The machine dimensions have been altered slightly to simplify the example.
Graphical Result
Angular Misalignment
The slope relationship of the motor shaft to that of the pump shaft: 30-mils in 15-in.
30-mils/15-in = 2-mils/1-in
Offset Misalignment
The motor shaft is low 20-mils at axial plane of the coupling center.
The motor shaft is low 16-mils at the axial plane where the rim indicator is measuring.
The Laser Result
Angular Misalignment
The vertical slope relationship of the motor shaft to the pump shaft: 2-mils/1-in. The coupling icon indicates that the coupling gap is larger at the bottom. The red color guidance indicates this is condition is not satisfactory. Horizontally, we are reading zero.
Offset Misalignment
Vertically, the motor shaft is low 20-mils at axial plane of the coupling center. The data indicates the actual shaft misalignment not the total indicator value. Again, red means an unsatisfactory value. Horizontally, the offset misalignment is zero.
The Rim and Face Indicator Result
Angular Misalignment
We know the slope relationship = 2-mils/1-in sloping upward away from the coupling and that the face indicator is at a 6-in diameter. Therefore, if the face indicator is set to zero at 12:00 and rotated to 6:00, the dial will read +12
Offset Misalignment
Vertically, the motor shaft is low 16-mils at axial plane of the rim indicator. If the dial is set to zero at 12:00 and read at 6:00, the value will be +32.
Ending the Debate
In summary, the laser reports the angular misalignment as the slope relationship of the two shafts in mils/1-in. The face indicator measures coupling gap difference.
Face Reading / Diameter = Angular Misalignment
The laser reports the absolute shaft offset at the coupling center while the rim dial indicator reads twice the offset.
Rim Reading / 2 = Offset Misalignment
Other Differences to Consider
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The rim indicator values will be slightly different because the laser is calculating the offset at the center of the coupling, while the indicator is measuring at the coupling hub. If the angular misalignment is not zero, measuring in different axial planes will cause small differences on final results.
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The laser has no deflection due to gravitational forces. Consequently, you may see some differences in vertical values if you do not determine the amount of dial indicator bar sag.
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You might note that the values at the feet are larger than those at the coupling. This is simply because even small angular misalignment will result in larger and larger values as you get further from the coupling.
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In the final example the motor shaft is sloping downward away from the coupling. The shaft offset at the center of the coupling is 0.0. The distance from the center of the coupling to the front feet is 10.0-in. The front feet result is: (-.3*10) + 0 = -3.0. The distance from the center of the coupling to the rear feet is 25-in. The rear feet result is: (-.3*25) + 0 = -7.5.