June 24, 2013
IT’S HAS TO BE ALIGNABLE!
By Brad Case
One of the issues precision shaft alignment technicians face is whether a machine is alignable. There will be times that a machine cannot be aligned in its current condition.
A recent Fixturlaser GO Basic training class ran into this issue when performing alignment checks on 4 small centrifugal pumps. All pumps had 10 HP electric motors, 2 operate at 3600 RPM and 2 at 1800 RPM.
The two alignment teams found all pumps to be slightly out of tolerance. 3 of the 4 pumps were aligned to tolerance within 1 hour total time, the 4th pump not so.
The 4th pump’s coupling values in the vertical plane where not repeating, the angular value was changing by almost 1 mil/1”, and the offset by approximately 8 mils (1.0 mil = .001”) Something had to be moving.
When it comes to precision shaft alignment 2 of the biggest culprits that affect repeatability are looseness and backlash (coupling looseness). If it moves, the alignment system will see it. (Sometimes in the heat of battle folks forgot to tighten the sensor fixtures tightly! It happens more than you think).
In this case the looseness was not with the sensor fixtures or coupling backlash but within the electric motor. The class found the motor to have excessive play in the bearings. So how did they find it? By a simple lift check of the shafts using the displayed detector values of the GO Basic.
While in the measuring screen the sensors were rotated to the vertical plane, while looking at the S Sensor value they lifted or pulled up slightly on the pump shaft. The S detector value changed by less than 1.0 mil, just a few 10ths actually.
When performing the same lift check on the motor shaft, while now viewing the M sensor the detector value, the displayed value changed by 5.0 mils.
This excessive play caused the motor’s rotational centerline to vary each time a set of alignment measurements are taken. This movement is easily illustrated in the graph below.
The red line represents the reference rotational centerline of the pump extended across the page. The green line is the electric motor rotational centerline from the first set of measurements. The blue dashed line shows how much the 5.0 mils of looseness in the motor shaft bearing affects the position of the rotational centerline. The vertical angular misalignment drops to almost 0 mil/1” while the vertical offset increases to 8.5 mils at the coupling center.
This machine will not be alignable to the tolerance specified for 1800 RPM, (max. allowable angle of +/- 0.7mil /1” and max. allowable offset of +/- 4.0 mil) until the motor is replaced. In its current condition the angular misalignment is within tolerance however the offset misalignment is almost 2 times tolerance.