Technical and Research Bulletin 3-51 - Amazon Web Services

Although the strain gage method has significant advantages, its complexity can be a
drawback. For example, it is sometimes difficult for the novice to recognize improper strain
gage readings, and on-site calibration and determination of the results are required to help
eliminate this problem. It is recommended that the results for a sample of the bearings be
determined before leaving the ship so that any errors can be determined and corrected. The
strain gage results can also be compared with bearing reactions measured by the hydraulic
jack method. The results should agree within 15% when the measurements have been taken
under the same conditions. Errors in the strain gage method are caused by improper location
of the strain gages, insufficient gage bonding, improper wiring techniques, damaged or
defective strain gages, poor surface condition underneath the gage, or sign-convention errors.
Any suspect gages should be replaced.
Another disadvantage of the strain gage method is that it cannot easily be used to measure the
shaft runout, as can be done with the hydraulic jack method. An indication of the shaft
runout can be determined from the strain gage measurements as follows, where a strain
runout value of less than four microstrain (strain expressed as parts per million) is usually
acceptable:
Strain runout = (strain90° + strain270°) – (strain0° + strain180°) (7-2)
The strain gage report should include the following: the strain gage locations and bridge
configuration, strain readings, the calculated bearing reactions, the calculated strain-to-load
conversion factors, the computer model input for the analysis of the strain gage results, the
table of influence coefficients, the evaluation of results, and any realignment
recommendations.
7.4 GAP AND SAG METHOD
The gap and sag method is a traditional technique primarily used to determine the
preliminary alignment of inboard bearings and to establish the alignment of the machinery
relative to the stern tube bearings during new construction prior to the final shaftline
assembly. Figure 7 illustrates the gap and sag. The gap is the difference in opening between
the top and bottom edges of the unconnected flange pair. The sag is the vertical distance
between the centers of the unconnected flanges. The gap and sag are normally measured
with two dial indicators making simultaneous readings on two mutually perpendicular
surfaces of the coupling flanges as the shaft is rotated through 360°.
This method, once referred to as “drop and gap,” is based on the theory that the alignment
can be estimated by the relative position of the mating pairs of shaft flanges. The theoretical
distance between the face and periphery of the unconnected coupling flanges determined
from the alignment software can be compared with the actual measurements. Since the free
end position of a shaft flange depends on the elevation and athwartship position of the
adjacent bearings, this method can be used to establish and adjust the bearings’ alignment.
The alignment software can be used to determine a family of sag and gap combinations for a
cold condition that will give acceptable bearing reactions in the hot condition.
The alignment must be checked for at least every 90° of shaft rotation through 360°. The
tolerance on the flange measurements should be 0.002 – 0.004 inches (0.05 – 0.10 mm),
depending on the size of the components and the length of the bearing spans, and the overall
accuracy required for the particular shaftline.
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