A Guide To Shaft Alignment
A Guide To Shaft Alignment
A Guide To Shaft Alignment
Modern risk assessments and technical reviews alignment a critical one for owners, operators and
often highlight the propulsion shafting system of a yards.
ship as one of its most important assemblies,
especially for single screw ships. Methods
Powerful modern engines often feature large-
diameter propulsion shafts and relatively stiff The four most popular methods of controlling shaft
shafting systems which can adversely affect main alignment are:
engines and gearboxes. • the gap and sag method
On the other hand, the more flexible shafting • optical or laser sighting
systems used to mitigate these effects can suffer • jacking
from whirling (lateral bending vibration modes). If • the strain gauge (bending moment) method.
whirling frequencies coincide with rotational These methods can be used either singly or in
speeds, catastrophic resonant response results can combination.
occur.
It is clear that a delicate balance must be struck The gap and sag method uses precalculated
during the processes of shaft design, installation flange measurements to establish the alignment,
to be carried out whilst the flange couplings are
disconnected. The advantages of this method
include the simplicity of the measuring equipment
and the ease of control in both the horizontal and
vertical directions. The disadvantages are limited
accuracy and the lack of applicability in the
service condition. The latter is significant, as it is
advisable to check the alignment of the shafting
system whilst the machinery is hot.
After the grounding of
the Keymar in heavy
weather, its owners The optical sighting or laser sighting method
engaged Lloyd’s is more accurate than the gap and sag method. It
Register to check the uses high-quality sighting equipment and is
vessels shaft alignment generally used before the shafts are installed and
during each of the
major stages of the coupled up. Optical sighting is often used to
vessels’s repair establish the reference line and the position of the
engine supports and for determining how to bore
the stern tube. Alignment and verification follows
and alignment in order to avoid costly repairs and by means of the gap and sag method and final
delays later in the ship’s life. An unexpected alignment control by the jacking and/or strain
shafting system problem, for example, failure of a gauge method.
stern tube bearing due to misalignment, leading to
an emergency dry-docking and consequent loss of
availability, can cost well in excess of $250,000
Optical sighting is
per incident for a small vessel.
one of four popular
More serious problems, for example, involving a tried-and-tested
bent shaft and replacement for a larger vessel, can methods used,
cost in excess of $1 million. either singly or
together, to align
The risk of lost earnings and the potential threat shafts.
to the safety of the ship make the issue of shaft
The jacking method lifts the shaft line clear of a grounding, for instance, small movements at
each bearing in turn by means of a hydraulic jack the shaft bearings can impact bearing loads,
and calibrated load cell. The shaft is lifted in leading to premature alignment failure. One
steps, whilst deflections are recorded on a dial owner who recognised the importance of
gauge and plotted against the applied load. This monitoring shaft alignment was Stelmar,
method uses simple equipment and is employed following the grounding of its aframax tanker
when the shaft line is coupled up, ready for Keymar.
operation.
However, it is not suitable for the control of In early 2003, the vessel was blown onto the
horizontal alignment and the yard may need to North African coast, along with three other
arrange for special supports for the jacks. Finally, tankers, by severe force 11 winds.
as the jacks have to be positioned beside the
bearings, adjustments have to be made to the The grounding severely damaged 11 of the 16
relevant calculations to evaluate the true bearing double-bottom tanks in the cargo region, leaving
load. the ship effectively floating on its tank tops. The
damage eventually resulted in the replacement of
Lloyd’s Register is over 1,500 tons of steel, and a total repair bill in
able to provide excess of $10 million. In view of the significant
owners, operators
and yards with
size of both the ship and the machinery it was
effective advice and considered prudent by the owners to engage
guidance on shaft Lloyd’s Register EMEA’s Technical Investigations
alignment. to check the alignment at several stages during
the repair process.