Page 210 - Machinery Component Maintenance

192 Machinery Component Maintenance and Repair

interpretation and Data Recording

Due to sag as well as geometry of the machine installation, it is diffi-
cult and deceptive to try second-guessing the adequacy of alignment
solely from the “raw” indicator readings. It is necessary to correct for
sag, then note the “interpreted” readings, then plot or calculate these to
see the overall picture-including equivalent face misalignment if pri-
mary readings were reverse-indicator on rims only. Sometimes thermal
offsets must be included, which further complicates the overall picture.
As a way to systematically consider these factors and arrive at a solu-
tion, it is helpful to use prepared data forms and stepwise calculation.
Suppose we are using the two-indicator face-rim method shown in Fig-
ure 5-3; let’s call it “Setup #1.” To start, prepare a data sheet as shown in
Figure 5-15. Next, measure and fill in the “basic dimensions’’ at the top.
Then, fill in the orientation direction, which is north in our example.
Next, take a series of readings, zeroing at the top, and returning for final
readings which should also be zero or nearly so. Now do a further check:
Add the top and bottom readings algebraically (T + B), and add the side
readings (S + S). The two sums should be equal, or nearly so. If the
checks are poor, takc a new set of readings. Do the checks before ac-
counting for bracket sag. Now, fill in the known or assumed bracket sag.
If the bracket does not sag (optimist!), fill in zero. Combine the sag alge-
braically with the vertical rim reading as shown, and get the net reading
using (+) or (-) as appropriate to accomplish the sag correction. A well-
prepared form will have this sign printed on it. If it does not, mentally
figure out what must be done to “un-sag” the bracket in the final posi-
tion, and what sign would apply when doing so.
Now we are ready to interpret our data in the space provided on the
form. To do this, first take half of our net rim reading:

- .011
-- - -.0055
2

This is because we are looking for centerline rather than rim offset.
Since its sign is minus, we can see from the indicator arrangement sketch
that the machine element to be adjusted is higher than the stationary ele-
ment, at the plane of measurement. This assumes the use of a conven-
tional American dial indicator, in which a positive reading indicates con-
tact point movement into the indicator.
By the same reasoning, we can see that the bottom face distance is .007
in. wider than the top face distance.
Going now to the horizontal readings, we make the north rim reading
zero by adding - .007 in. to it. To preserve the equality of our algebra,