Why Can’t I Just Use My Straight Edge?!

Recently, I was at NC State University observing one of our training classes. This question came up in the beginning of the class by one of the students, and I’m sure this question comes across a lot of people’s minds when they consider making an investment in laser or dial indicator precision shaft alignment tools. This same question ran across my mind when I did my first shaft alignment as well. So, I am going to demonstrate what happens when you only use a straightedge.

Observe the coupling below. Look closely… is it aligned (vertically) or not? The problem is that it is impossible with the human eye to tell the difference between a coupling with angular misalignment and one that is perfectly aligned. After all, we are aligning to tolerances that are smaller than a human hair.

So let’s put the laser alignment system on and see what it has to say…

Everything is within tolerance except the vertical angularity, as is indicated by the red coupling icon.
The problem with the straightedge is this… it simply isn’t long enough to accurately show angular misalignment; but let me explain why, before you run to the store to buy a longer one. If you were to remove the coupling, cut open both the motor and the pump and expose both shafts completely then you could “possibly” align to tolerance with a straightedge.

In that made-up scenario, you would then be able to visibly see the drop or rise of the tail-ends of the shafts. This rise/drop is what creates the angular misalignment at the coupling. This misalignment simply isn’t measurable by the human eye at the coupling with a straightedge. Remember, acceptable tolerances for these alignments are thinner than a human hair.

The straightedge still has its place however, use your straightedge to get you close in your rough alignment. This will save you time when it comes time to put the lasers on for the “precision shaft alignment”.

Submitted by Jonathan Dissinger at VibrAlign.