April 15, 2015
The Benefit of a “Hot” Alignment Check
By Brad Case
There have been numerous posts on TheAlignmentBlog.com dealing with Thermal Growth and Dynamic Movement of rotating machinery. http://thealignmentblog.com/blog/category/alignment-troubleshooting/dynamic-movement/. Thermal Growth continues to be a subject that is discussed and taught in our training classes, though on a daily basis is not always correctly compensated for in industry.
Many maintenance personnel performing precision shaft alignments use thermal targets or offsets which have been calculated, however the question we VibrAlign Trainers get asked frequently is “just how accurate are the calculations?” The answer; they may or may not be that accurate, there are just so many variables that can affect a rotating machine’s position from a “cold” to “hot” position that it is best to take alignment measurements at each to verify.
For clients with very critical applications our Fixturlaser NXA Pro or Ultimate Systems has the optional OL2R App and precision brackets available to determine very accurate dynamic movement targets but what if your laser shaft alignment system doesn’t have this capability?
A “Hot” Alignment Check is a fairly accurate method to determine if the thermal targets given for the “Cold” Alignment were accurate, however a hot alignment check will not show other operational influences as does the OL2R app and brackets.
The key for accurate Hot Alignment Checks is the speed to which the machines can be shut down, locked out, tagged out, guards removed, laser equipment mounted, and alignment reading taken. The quicker the more accurate the hot alignment readings. If a machine can not be locked out/tagged out quickly (and safety) then a hot check will not be accurate (the machine is already cooling off) and the OL2R option will be the better choice.
During a recent shaft alignment training class the mechanics took alignment readings to verify the “hot” position of several refrigeration compressors. These mechanics followed their company’s lockout/tag out safety protocols, however they had “their ducks in a row” with their tools ready to go before shutting down the machines resulting in them acquiring hot alignment results very quickly.
This first example shows the actual “cold” target of -5.5 mils low for the vertical offset that the machine was aligned to.
The “hot” alignment was checked and found to be in tolerance verifying the -5.5 vertical offset cold target was correct and that there was very little horizontal movement.
Another hot alignment check on a different machine showed the vertical alignment to be good, however the machine did experience movement horizontally leaving it slightly out of 3600 RPM tolerance.
After a quick horizontal correction the machine was placed in operation to “heat it back up”, afterwards a second hot alignment check indicated an excellent precision shaft alignment.
As is shown the best way to know if you have a good “HOT” alignment is to measure it, if at all possible.
So what if the actual hot alignment readings show a large disparity from the anticipated hot position?
We have a Hot Check App for that! My next blog will discuss using the free “Hot Check” App to compare a “Cold” position vs the “Hot” position. “Hot Check” is also a program in some of the Fixturlaser Shaft Alignment Systems.