Technical and Research Bulletin 3-51 - Amazon Web Services

looking aft. The offsets of all bearing centers are measured relative to an optical line-of-sight
and compared to the output from the alignment computer model.
Optical methods can use a micro-alignment telescope (a borescope with a cross-hair lens) or
a more elaborate computerized laser sensor and targets, depending on the shaft configuration,
length, and alignment scheme. The laser method is based on the same technique as the
optical scope method, except that the laser beam replaces the visual line-of-sight. The main
advantage of laser alignment is that it can measure large distances with minimal loss of
accuracy.
The scope or laser is attached to the flange of a reference shaft (bull gear, line shaft, directcoupled
engine drive flange, or propulsion motor) so that the line-of-sight or laser beam is
perpendicular to the flange face. This should be checked with a target at the after end of the
aftermost bearing. The flange with the scope or laser attached is then rotated through 360°.
The line-of-sight or laser will trace a small circle on the target, and the mounting is corrected
until the circle is reduced to a point. Once the correct mounting has been established, targets
should be installed at the forward and aft ends of each bearing. Each bearing is set in place
and adjusted until the light-of-sight or laser beam hits the target. As with all alignment
measurements, the optical work must be performed at night when the ship’s hull is least
affected by temperature variations.
The optical line should be repeatable with good accuracy, and must be most accurate at the
inboard end. It must be an extension of the shafting continuous beam to permit the
mathematical analysis of the alignment data. The optical line must be a true projection of the
center of rotation of the freely suspended after end of the existing portion of the propulsion
system, i.e. reduction gear flange, direct-connected engine or propulsion motor drive flange,
or line shaft. The deflection and slope must be calculated from the end of the shaft from
which the optical line is projected. These theoretical data are used to compensate for the
differences between the projected optical line and the calculated straight-line.
7.6 WIRE METHOD
The bearing alignment can be checked by anchoring a wire above the shaft at one end and
tensioning it with a weight over a pulley at the other end of the bearing line to be examined.
The location of the bearing in relation to the wire can then be measured. In most cases, the
gear output flange or direct-connected engine drive flange is a good support for the wire.
The wire method is based on the same concepts as the optical method, although there are
some drawbacks because the wire sags and vibrates, and must not be moved during the
measurements. During installation of a bearing, a target can be installed in the bearing with
the wire running through it. The hole in the target is sized to the allowable tolerance in the
alignment procedure, which reduces the need to continually measure the bearing location
relative to the wire [13]. The sag in the wire must be accounted for according to the
following equation:
S = w·x 2 /(2·T) (7-3)
where: S = wire sag
w = weight of wire per unit length
x = one-half the distance between wire supports
T = wire tension, i.e., the weight of the suspended mass
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