Technical and Research Bulletin 3-51 - Amazon Web Services

Practices and Procedures for the Alignment of Marine
Main Propulsion Shafting Systems
SECTION 1.0 INTRODUCTION
1.1 INTRODUCTION AND DEFINITION OF PROPULSION SHAFTING SYSTEM
ALIGNMENT
Common sense suggests that the bearings supporting marine propulsion shafting be aligned
so they hold the shaft in a straight line, since offsetting one bearing with respect to another
would bend the shaft. Moreover, industrial machinery is typically aligned so that the shafts
of the driving and driven machines are collinear [1]. However, this approach is not always
appropriate for marine main propulsion shafting because: a) its bearings move as conditions
change, and b) the shafting is never really straight because it deflects under its own weight
and the weight of the propeller and other attached components. Therefore, the vertical and
athwartships position of the bearings must be aligned so that the limits established to ensure
proper system operation are not exceeded for all normal ship operating conditions. It follows
that shaft alignment could also be called bearing alignment, since the alignment is a direct
result of the position of bearings that support the shafts.
1.2 HISTORICAL PERSPECTIVE
In the early 1960s, it was discovered that serious misalignment of propulsion shafting caused
gear skew on large, steam turbine bull gears, which in turn initiated pitting on the second
reduction gear teeth. At that time shafting systems usually had too many bearings, and a
straight-line alignment was considered to be a good alignment. However, the straight-line
alignment tended to place a large amount of the attached line shaft weight on the aft bull gear
bearing in the cold condition. Then, as the gear heated up, even more shafting weight shifted
onto this bearing, while the forward gear bearing became more unloaded. This unequal
loading of the gear bearings caused gear skew. Computerized shafting analysis was used to
“misalign” the bull gear in the cold condition relative to the propulsion shafting, so that in the
hot condition the vertical loads on the two bull gear bearings would be equal within an
acceptable limit. The SNAME paper “Coordinated Alignment of Line Shaft, Propulsion
Gear and Turbines”, by Andersen and Zrodowski [2] firmly established the relationship
between shaft misalignment and gear problems and offered practical solutions.
As a result of information accumulated since then, today’s shipbuilders produce vessels with
practically no built-in propulsion shafting alignment problems. Nonetheless, alignment
problems still occur and propulsion shaft alignment remains a challenging task for both
shipyards and designers. Some of these alignment problems are caused by: a) worn or failed
bearings, b) extensive hull damage which may have disturbed the bearing settings, and c)
improperly bored stern tubes.
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