Manual Chapter 5

6-2 Acquiring Alignment Data
When using the 4 point methods, ideally, the sum of the two horizontal
measurements will equal the sum of the two vertical measurements. This
relationship may be used to check the data for validity. Any variance
greater than 10-20% is a cause for concern. When this occurs, repeat the
readings to verify that a variation wasn’t accidentally introduced by the
user. For example, by using the fixtures as levers to turn the shaft or by not
controlling the torsional play in the coupling. Damaged regions in rolling
element bearings and shaft rubs could also cause problems, but are less
common.
Because of this same mathematical relationship, having any 3 of the 4
points would allow the fourth point to be calculated. This can be useful
when obstructions only allow you to rotate the laser fixtures to three of the
four clock positions. Unfortunately, estimating the fourth point like this
removes the ability to check the validity of the data. Abnormal readings at
any one of the three known points could cause error. Therefore, use this
technique with caution and only when absolutely necessary. CSI recom-
mends using a partial sweep method rather than the three 90° clock points
if the arc of rotation is restricted.
Sweep Methods - General Information
The sweep modes use data acquired at several different angular positions.
By using CSI’s patented process, this data is translated into readings for the
four clock positions. There are several advantages to the sweep method
over the 4 point method of acquiring data. Data does not have to be
acquired at 3 or 4 specific angular locations; any angular positions will
work. More data points are used to arrive at the true rotational behavior of
the shaft, therefore a discrepancy in one area of the rotation does not have
as great an effect. If a full rotation of the shaft is not possible, the three point
method still requires at least 180° of shaft rotation, which must begin at one
of the four designated positions; the sweep method can use rotations less
than 90° starting from any position. However, keep in mind that no matter
what method is used, reconstructing a complete picture using partial data is
never as desirable as starting with a complete picture.