June 1, 2017
Roughing In Before Precision Shaft Alignment
By Stan Riddle
Some people believe that precision laser shaft alignment has evolved to the point where pre-alignment steps, such as roughing in the alignment with a straightedge, are no longer needed. Well, quite frankly, those people are wrong!
The reason for roughing in the alignment has little to do with how accurately the lasers/sensors measure. It has nothing to do with how big the laser detector is, or how sensitive the detectors are, or how fast they are.
Roughing in isn’t simply to “get it close”. It is to minimize errors due to the following:
- Radial stresses (bending) in the shafts and couplings.
- Reduced clearances in bearings due to radial stresses (side loading).
- Assembly errors in the coupling.
- Imparting strain in mechanical systems, such as mechanical seals, bearings, gears, etc.
Think about it. Just because a laser shaft alignment tool has a 30mm or larger detector DOES NOT MEAN you don’t have to rough in. I would suggest that if two shafts are over an inch out of alignment, you couldn’t assemble the coupling in the first place. To assemble the coupling, you would have to get it fairly close.
But getting it close enough to assemble the coupling may not be close enough either. There are many variables that must be considered, such as:
- The type of coupling, and its flexibility, or lack thereof. Some couplings flex quite easily. Some have very little flex.
- The length of the shaft from the bearing to the end of the shaft. Long shafts extending from the machine may be somewhat tolerant of bending without inducing strain. Short shaft extensions would bend minimally, if at all. The diameter of the shaft, and the shaft metallurgy would have an effect as well.
- Start-up torque, soft start vs. hard start, specific gravities, static loads-all play a part.
Granted, if you are aligning two shafts, and the coupling has not been assembled, none of this would play a part. But not all coupling types can be assembled after the alignment is completed, or if they are, the coupling hubs must be slid back on the shafts. Very few aligners would try to align an interference fit coupling, and then heat it up to position it. And even “slip fit” couplings don’t always “slip” easily.
So, regardless of the measuring tool, roughing in before performing a precision shaft alignment is always a good idea. It doesn’t have to be perfect, however it will be beneficial to get it close, within 15 to 20 mils. And that can be done quickly with a straightedge.