April 3, 2014
Can Precision Shaft Alignment Be Too Good?
By Stan Riddle
A shaft alignment tolerance is simply that – an allowable, minimal amount of misalignment. There are many different tolerance tables for shaft alignment. At VibrAlign, we have a good alignment tolerance, based on many years of experience and engineering. Other alignment tools have their own tolerance tables. Many technical and trade organizations have their alignment tolerance values. Coupling manufacturers and machinery manufacturers have tolerances for alignment as well.
Some facilities specify an alignment tolerance of “as close to zero as possible”. With today’s extremely accurate laser alignment tools, you can get VERY close to zero. But to do so, you will probably spend an inordinate amount of time achieving an unnecessary goal.
Many years ago, I aligned a baghouse fan for a power plant. And quite honestly, I wanted to show off – and get “all zeroes” on the alignment. I spent an entire day on this fan, shimming and moving very carefully, until I got repeatable “zeroes” for angularity and offset at the coupling.
We started this fan, and almost immediately heard a loud “growl” from the motor, which also started overheating. When the inboard motor bearing got to 180 degrees, it tripped out. I uncoupled the machines, unwired and removed the motor, and disassembled it. The bore in the inboard end bell was 0.0005″ oversized, which allowed the inboard bearing to slip in the end bell.
So the brand new motor was re-bored, reassembled, re-wired, and re-aligned. But this time, the alignment was left about 2 mils misaligned on the horizontal offset. And it ran smoothly for years.
By trying (and achieving) “perfect” alignment, I can only suppose there was little to no pre-load on the bearing. If I had left it misaligned by only a couple of mils, the bearing probably would not have slipped, and I would not have spent an additional day in repairs and additional alignment.
Modern laser shaft alignment tools are not about how close to zero you can get the alignment, but how quickly you can complete the job, while doing it accurately, repeatably, and easily.