Motor Shaft Thrust & How It Relates To Apparent Erroneous Alignment Readings
Recently at a customers plant (a gasoline pumping station in PA) we were aligning a 1750 HP Reliance motor that had been coupled to a high capacity pump. Having aligned the unit with a laser based system the week before, vibration results were unchanged. When the motor was started an extreme vibration appeared. Based on vibration analysis results, the problem was an alignment error.
The alignment had been performed uncoupled, and the numbers were repeatable. The results indicated that there was a large misalignment. Since the results were repeatable, moves were made to correct the error. After the moves had been made the system was coupled together. We then swept the coupling hub of the motor using an indicator attached to the coupling hub; the center element had been removed. The readings varied considerably between the indicator and the laser.)
The pumping station handles gasoline and other petroleum products at the rate of 4000 GPM. We mounted the laser back on the motor and found that the computer told him to essentially reverse all the moves that we had performed the week before. The only difference this time was that the shafts were coupled. We undid the moves that had been done before. This time, keeping the laser and target mounted, we continued to take readings at 3 positions, first clockwise then counter clockwise and so on. We repeated to within a thousandth every time. The final center and slope readings from the top of the screen read as follows:
First set of Readings:
Horizontal Center .001" Slope 000"/" Vertical Center -.006" Slope .001"/"
Second set of Readings:
Horizontal Center .000" Slope .000"/" Vertical Center -.006" Slope .001"/"
Third set of Readings:
Horizontal Center .001" Slope .000"/" Vertical Center -.006" Slope .001"/"
Fourth set of Readings:
Horizontal Center .001" Slope .000"/" Vertical Center -.006" Slope .001"/"
Two more sets gave the same results. We were still stumped! Given the repeatability of the entire setup, we figured that something else was at fault. On a hunch, we set up a Hamar L-723 tri scan geometry laser to check axial thrust. We placed an A-517 target on the motor side of the coupling, zeroed it and rotated the shaft. A maximum of .024" in axial thrust was indicated. We then did the same thing to the pump side of the coupling. Zero, no axial thrust was shown. I asked the maintenance man what the axial thrust was when the motor was uncoupled. He told me .625" – 5/8". Then it dawned on both of us: When readings are taken uncoupled, and the shaft changes position axially, the target gathers all of the information and results will show (all other things being equal) the true slope of the shaft, relative to the fixed unit. Something else is being told about vertical offset. As the target continues to advance toward the laser due to the axial thrust of the motor shaft, the apparent target height continuously falls until the final reading is taken, in the calculation mode. In the alignment screen the configuration is being shown lower by some thousandths than it actually will be in its running condition. What is actually happening by having axial thrust is that the shaft is moving along a sloped line, and the center readings are taken. This will only be the case if there is some angular misalignment. What happened was that the center reading had been taken down along a sloped line.
What must be done to correct the problem is to limit the axial thrust of the motor and to take readings when the axial position of the shaft most closely shadows it’s true running position. We installed bronze bushings on the motor shaft upon rebuild to eliminate the thrust. The question of why the indicator readings were so far off from the laser readings was also answered. The indicator was placed on the pump shaft, which was then rotated. The motor shaft, which was not rotated, was swept and indicated. Since the motor was not rotated, and since there was no noted axial movement shown on the pump, thrust never played into the equation and did not have to be compensated for.
Another very important point was made: If any system, be it mechanical indicator or laser, describes an alignment that is too good (or in this case) to bad to be true. Sit down, scratch your head and think about it. Don't take anything for granted. No alignment system will ever replace common sense.