Allowable Static Runout
0.002 X Length of Shaft
Diameter of Thrust Runner
To start, the maximum allowable runout diameter should be calculated by the formula:
All dimensions are in inches. In Method I, the runout is going to be calculated at the fourth
reading elevation, the "Length of Shaft" would be, based on the dimension labels on figure 14,
equal to A+E, or the distance from the thrust runner to the first elevation plus the distance from
the first to the fourth elevation. In Method II, the "Length of Shaft" is simply the distance
between the dial indicators.
The static runout is not a measure of the dynamic runout that will occur when the unit is
operating because the guide bearings will hold the shaft in place to some extent. Most of the
movement caused by the nonperpendicularity will be seen at the thrust bearing. The formula
above limits the up and down movement of the thrust bearings to 0.001 inch, assuming that the
shaft is held in position with the guide bearings.
6.4 Static Runout Method I
For this method, the form in figure 16 should be used. The form is like the one discussed above
for four reading elevations, except readings are taken only at the first and fourth elevations.
After all the readings have been taken and the plumb calculations made, the runout calculations
can be made.
Once the allowable runout is calculated, the points for the runout plot are determined. The
values in column 5 are the values the shaft is out of plumb from the first to the fourth readings
and will be transcribed to Column B of figure 17. To correct these values to reflect the out of
plumb from the thrust bearing to the fourth elevation, a multiplication factor must be calculated.
This factor, based on similar triangles, is (A+E)/E. Each of the values in Column 5 are
multiplied by this factor and entered into the appropriate spot on the table. These values can then
be plotted to show the runout diameter and its relative location to the thrust bearing.
An example of this plot is shown in figure 17. This plot is a top view. The origin of the plot
(point 0,0) is the shaft centerline at the thrust runner elevation, or the center of the thrust runner.
The points at the 0, 90, 180, and 270 degrees are the positions of the shaft at the fourth reading
elevation as the shaft is rotated. The intersection of the lines drawn from 0 to 180 and from 90 to
270 is considered the center of runout. A line drawn from the origin to the center or runout
would be the axis of rotation for that unit. As mentioned before, the primary objective is to make
the center of runout or axis of rotation, plumb. In this plot, the center of runout will be plumb
when it is located directly under the center of the thrust runner (point 0,0). How this is done will
depend on the design of the thrust bearing. These specific procedures will be discussed in the
next section. The runout diameter, the distance from 0 to 180 and from 90 to 270, should be
checked at this point. The diameter can be checked graphically by simply measuring the distance
on the plot.