November 27, 2012
Defining Angularity in Shaft Alignment
By Stan Riddle
There are only two types of measurements for shaft misalignment – offset, which may be called parallel or rim, and angularity, which is sometimes called slope, or face. Offset (parallel or rim) is easy to understand. One shaft is simply higher, lower, or to the left or right of the other one.
But angularity can sometimes be confusing.
It means the shaft centerlines intersect at an angle. With dial indicators, we measure it as a gap difference between the top & bottom and/or the side to side of the coupling. If for instance, a 6″ coupling has a gap of 12 mils across it (top to bottom) that is its slope.
It’s important to note that, in this example, a slope of 0.012″ across a 6 inch coupling face results in an angularity of 0.012″/6 inches, or 0.002″/inch (2 mil per inch). Since the coupling faces and the shaft centerlines are perpendicular, exactly 90 degrees, the slope change across the coupling faces is the same as the slope change along the lengths of the shaft centerlines. In other words, if the coupling faces have a slope of 2 mil per inch, the shafts also have a slope of 2 mil per inch.
This simply means the slope from the coupling center to the movable machine feet slope at 2 mil per inch. If I measure from the coupling center to the inboard feet, say 18 inches, then the inboard feet need 36 mils of shim (2 mil per inch x 18″). If it’s an additional 24 inches to the outboard feet (42 inches total), I need 84 mils of shim at the outboard feet.
This will correct the vertical angularity quite nicely. Horizontal angularity corrections work the same way. We still have the offset to correct, but that is easy to measure once the angularity is gone. Most quality laser alignment measuring tools will show both angularity and offset simultaneously.