May 4, 2012
Foot Tolerances vs. Coupling Center Tolerances
By Patrick Lawrence
Pardon the hiatus last week, folks. We did a little maintenance on the blog–be sure to check out the new tabs up top designed to help you navigate better. Let’s jump back into it–foot tolerances vs coupling center tolerances. It’s a hotly debated topic in some circles. I’m no great debater but let’s take a look at this from the two sides we hear most often:
Argument 1: You’re Crazy
We all know tolerances are expressed at the coupling center. The calculated values at the feet have nothing to do with the quality of the alignment. Alignment tolerances are expressed as an angle and offset in the vertical and horizontal planes at the point where the two shaft centerlines intersect. Why in the world would you continue to ‘correct’ an alignment when it is already within industry accepted tolerances? We know how difficult these pieces of equipment can be to align. Tempting fate and continuing to adjust the equipment just for a thousandth of an inch here or there is ludicrous. It does nothing to substantially increase the reliability of the equipment. Furthermore, shooting for values at the feet that are, say, 0 ±0.002”, just shows a lack of understanding of the geometry involved in shaft alignment. Didn’t you read ‘Don’t Look At Your Feet‘?
Argument 2: You’re Lazy
The idea behind foot tolerances is to drive you to a precision state. If there’s a correction that can be made, why not attempt it? Who cares if you’re in tolerance? Why not leave it in as great condition as we can before starting it up—this may be the only chance we get for a while. Putting in a little extra effort goes a long way. We are using accurate measuring tools, following an effective procedure and have taken great care in our prealignment steps to remove sources of measurement errors. Part of the reason we take great care in those prealignment steps is so we can tweak the equipment and get an appropriate response. So what if it takes an extra set of moves to get there. If we’re honest with ourselves, most of the equipment we align is relatively easy to move around. Now get off your duff and help me pull these shims out!
Well, maybe I wouldn’t make a great politician but these are pretty much the conversations when this topic comes up.
What’s your position on this and—more importantly—can you defend it?
The problem seems to occur when the foot tolerances are taken too literally. For example, I have seen alignments done to very low coupling tolerances in a short time yet the aligner being told to go back to correct another mil (or even tenths) at a foot and it takes them an additional 3 hours to get the foot value where they think it needs to be but the actual coupling tolerances to be a little worse. To me both alignments would be great but 3 hours were wasted for no gain. This leads to a lot of frustration and sometimes attempts to cheat the measurement.
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