Properly aligned shafts will do more to increase bearing, seal, and rotor life than any other single thing you can do after lubrication. Unfortunately, many maintenance departments in smaller plants still think that alignment is only needed for large, high-speed shafts on somebody else’s equipment. Many have no idea how to align two shafts beyond using a straight edge to get them close. Besides, the guy who sells the couplings says that the coupling can take up to one degree of misalignment and not hurt anything.
Begins an article written on the Alignment & Vibration Services blog.
Couplings ain’t round.
Suppose we have two shafts that are perfectly aligned but one has a coupling improperly mounted on a shaft so that there is some error in colinearity of the axes. As we approach the maching the error is only in the horizontal direction. Let’s set up the dial indicator so that it reads the outside of the coupling then rotate only the shaft on which the indicator is mounted, leaving the coupling still. When we get to position 1 the dial indicator will be extended to, say, -0.010 inches due to the error. Turning to position 2 will bring the dial indicator back to zero. At position 3 the indicator is compressed due the error and reads +0.010. If we believe that the coupling is square on the shaft and the axes are collinear we will believe that there is a TIR or 0.020 inches in the horizontal direction. Acting on that information will cause us to misalign the shafts by 0.010 inches – introducing vibration and potential bearing damage.
There are four other errors mentioned. The article doesn’t go into too much detail, but it gives an excellent overview.