Technical and Research Bulletin 3-51 - Amazon Web Services

the effective bearing length and the shaft outer diameter), although not all classification
societies compute the pressure load on the bearing in this manner.
(1) Line shaft bearings will run well with pressure limits of 75 psi (c. 500 kPa) for disc
lubricated bearings, and 50 psi (c. 350 kPa) for ring lubricated bearings. It is very
important that the bearings have a uniform load distribution in both the vertical and
horizontal planes since very high-localized pressures can occur on a bearing that is
tipped down or skewed athwartships relative to the shaft. The load distribution on
the line shaft bearings can be checked by applying a strip of bluing axially along the
journal and rotating the shaft through the bearing. All the bluing should be wiped
off; if not, the area with the bluing is not in contact with the bearing. The end of the
bearing should be shimmed until the bluing is evenly removed when the shaft is
rotated. The bearing’s athwartships load distribution can also be estimated by
removing the bearing cap, and measuring the side skew at the four corners with a
feeler gage. A hot-running line shaft bearing usually indicates an overloaded
bearing if the lubrication system is operating satisfactorily [6].
(2) In general, an anti-friction bearing large enough to fit the shaft diameter is capable of
supporting a much higher load than can be applied from a reasonable alignment.
The B10 * life for this type of bearing typically exceeds 100,000 hours. Therefore,
almost any positive, downward load will be acceptable. Although anti-friction
bearings are not nearly as sensitive to angular alignment as sleeve bearings, some
care needs to be taken during installation so that even self-aligning bearings start out
with the proper angular alignment.
(3) Waterborne-shaft bearings may be either oil- or water-lubricated. Oil-lubricated
bearings usually have a length-to-diameter (L/D) ratio of 2. Bearings using synthetic
material are usually longer, with an L/D between 2 and 4. The American Bureau of
Shipping allows oil-lubricated, white-metal lined outboard bearings to be shorter
(but not less than L/D = 1.5) as long as the bearing nominal pressure is not more than
116 psi (800 kPa). Likewise, oil-lubricated synthetic bearings may be less than 2D
in length (limit is L/D = 1.5) if the bearing nominal pressure is not more than 87 psi
(600 kPa).
The following pressures are the maximum allowable limits for each bearing type:
Oil-lubricated synthetic liner: 87 psi (600 kPa)
Oil-lubricated babbitt liner: 116 psi (800 kPa)
Water-lubricated synthetic liner: 70 psi (c. 500 kPa)
Water-lubricated rubber or wood liner: 40 psi (c. 300 kPa)
(4) The propeller bearing is located just forward of the propeller and its primary
function is to support the propeller and shaft weight, and counteract the propeller
induced hydrodynamic forces. The slope (or cant) of the propeller bearing is critical
and the bearing must be aligned with the tail shaft so that the load is distributed over
most of the available bearing area. Otherwise, the overhung weight of the propeller
will deflect the tail shaft and the total propeller weight will be concentrated on a
small area at the aft end of the bearing. A change in the bearing offsets rarely causes
a significant change in the load on the propeller bearing and many more propeller
bearings have failed from a poor load distribution than from an excessive overall
*
B10 is the life in hours that a group of apparently identical bearings will exceed or complete.
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