Subscribe to our

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.