Shaft Offset and Angularity

Alignment occurs when two lines that are
superimposed on each other form a single line.
Misalignment is a measure of how far apart the two
lines are away from forming that single line. The
two lines we’re concerned with here are the centerlines
of the pump shaft and the driver shaft. In one
condition, the two lines can be parallel with each
other, but at a constant distance apart. This is
referred to as offset or parallel misalignment. In the
other, one line will be at an angle to the other. This
is referred to as angular misalignment.

Parallel misalignment can be considered as the
distance between the driver shaft centerline and the
pump shaft centerline at any given point along the
length—and it can happen in any plane.
Consequently, it is worthwhile to take the necessary
measurements on the top and on the bottom for
vertical offset, and also on each side for the horizontal
offset.

Angular misalignment refers to the difference
in slope of the two shafts. If the pump, base and
foundation have been properly installed, the shaft
centerline of the pump can be considered as level,
and therefore, as the reference or datum line. The
slope of the driver shaft can be calculated by determining
the offset measurement at two different
points, subtracting one from the other, and dividing
the result by the axial distance between the two
points. (See Figure 2.) This misalignment should be
measured and calculated in both the vertical and
horizontal planes