Categories: Shaft Alignment

Repeatability vs. Desired Outcome in Precision Shaft Alignment, Part 1

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By on October 14, 2015

When mechanics align rotating machinery, they expect two outcomes. One, they expect their alignment values to be repeatable. Two, they expect shimming and moving machinery to produce a desired outcome-namely, to be in tolerance.

In this paper, we’ll discuss repeatability, which is, performing the same measurement twice, and getting the same results.

If alignment results are not repeatable, after measuring two or more times, most people blame the tool! In fact, a laser alignment tool is simply a calculator with laser beams for measuring, and it simply does not have the capacity to lie! On my calculator, if I put in 2+2, it will ALWAYS equal 4! If it doesn’t, then I didn’t do something right. If your alignment results aren’t repeating, it could be something you’re missing.

So, what should you do if you measure twice, one right after the other, and your values are not repeatable?

1. THINK! What could be changing (moving)? What could you have done differently?

2. Check for looseness. In the brackets, rods, and transmitter/detectors. In the coupling hubs (if you are mounted on them). In the movable and stationary machine foot bolts.

3. Observe if there is any vibration from external sources, such as from a nearby machine. Perform a repeatability test if your laser system has this function. Shut the offending machine down momentarily, if you can.

4. Make sure you are rotating the shafts in the same direction during the measurement process. If you really want to troubleshoot for accuracy, try to measure from the same angular positions each time. Use inclinometer values.

5. Make sure the transmitter/detector lenses are clean. Dirt and dust can refract the laser beams.

6. Confirm that the bearings in the machines you are aligning are OK by performing a lift check.

7. Make sure you are not using the brackets as “handles” to rotate the shafts. And make sure they aren’t bumping against bolts, conduit, oilers, guards, or anything else that could move them.

Lastly, you need to be realistic about how “repeatable” is “repeatable”. If your coupling values are changing a few ten thousandths of an inch, should you care? If you measure three times, and all three measurements are minimally different, but each one is in tolerance, shouldn’t you just save it, and pack it up?

Next time, we’ll talk about desired outcomes, or what happens when 2 minus 2 doesn’t equal zero!

About the Author

Stan Riddle joined VibrAlign in 2008. He has over 35 years experience in aligning industrial machinery. Stan received his AAS Degree in Machinist Technology from Surry Community College in Dobson, NC, and also holds a diploma in Industrial Systems Technology from Forsyth Technical Community College in Winston-Salem, NC, where he was also an instructor in the program.

Stan began his maintenance career working as a machinist and millwright for companies such as Weyerhaeuser, R.J. Reynolds, and Tyco Electronics. He also has over 25 years experience in Predictive Technologies, such as vibration analysis, thermography, oil analysis, and ultrasonic inspection. He is a certified Level III Vibration Analyst with the Vibration Institute, and is a Past Chairman and Board Member of the Piedmont Chapter.

Stan and his wife live in Yadkinville, NC.

One response to “Repeatability vs. Desired Outcome in Precision Shaft Alignment, Part 1”

  1. […] outcome-namely, to be in tolerance. In this paper, we’ll discuss repeatability, […] Read More Source: The Alignment […]