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Designer's
Checklist No.1
Azimuth Stern Drive Tugs
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INTERMEDIATE
SHAFTS
Shaft
Arrangements
The power train between the prime mover and
the propulsor is by an intermediate shaft. Depending
on application and installation there are several
different possibilities for the intermediate shaft
layout. In the shortest case the shaft is only a
tooth coupling and a flexible coupling, between
the engine flywheel and the propulsor input flange.
In
order to prevent the forces from the propulsor and
the shafts to damage the prime mover a flexible
coupling has always to be installed between the
engine flywheel and the intermediate shaft package.
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Using only a tooth coupling and a flexible coupling
results in the shortest possible intermediate shaft
line arrangement |
Tooth Coupling and Flexible Coupling
This is the shortest possible intermediate shaft installation,
where the shaft is a mere tooth coupling connected to
the propulsor flange at one end and to the flexible
coupling on the prime mover flywheel. |
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The stub shaft arrangement is another very compact
installation method
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Stub
Shaft
The next shortest intermediate shaft is a short, so
called stub shaft. The shaft should be equipped with
a pair of bearings to take the gravity load of the shaft
and thus the shortest possible length is one that provides
for space for the bearings. The stub shaft is usually
fixed by flanges to the propulsor input flange and the
flexible coupling on the prime mover flywheel. |
The
"flexible" shaft line arrangement - with no bearings
- enables a fast and easy installation of the intermediate
shaft |
Flexible Shaft
If a short shaft without bearings is preferred a
possibility is to use a so-called flexible shaft, e.g.
Centalink, where the shaft (normally of tube type) is
integrated to torsionally stiff flexible elements at
each end of the shaft. |
Despite a rather high initial cost, the lack of
bearings, bearing supports, alignment etc. makes
the carbon fibre shaft a good choice also on workboats
and tugs
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Carbon
Fibre Shaft
A novel solution for straight shaft installations is
to use a large diameter hollow carbon fibre shaft, which
requires no bearings. The use of a carbon fibre shaft
is feasible in stern drive tugs, where a conventional
straight shaft otherwise would be used. The carbon fibre
shaft will save weight and installation work compared
to a conventional shaft with bearings and bearing foundations.
The carbon fibre shaft is using the same flexible elements
as in the Centalink solution above. Maximum length of
a single piece shaft is approx. 8 m. |
Sometimes it is feasible to use the single cardan
shaft solution. Also this installation will require
a stub shaft and flexible coupling.
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Single Cardan Shaft
If the engine is installed close to the same level as
the propulsor and rather close to each other a single
cardan shaft can be used as the intermediate shaft.
In order not to damage the engine a stub shaft is required,
at least for power ratings above, say 600 kW. The stub
shaft requires a pair of bearings capable of taking
axial forces and to take the shaft weight. Note that
the cardan shaft cannot be used with very small angles,
a minimum of 1º is normally required to ensure that
the joint bearings are moving. |
The typical intermediate shaft on stern drive tugs
comprise two cardan shafts with a long straight shaft
in between. At the prime mover end a stub shaft and
a flexible coupling wil be required |
Typical ASD Intermediate Shaft
The typical intermediate shaft arrangement on a stern
drive tug comprises cardan shafts and a rather long
straight shaft. Also this arrangement will require a
stub shaft at the prime mover end as well as a flexible
coupling to save the prime mover from damage. |
Shaft
Angle
The height difference between prime mover flywheel
and azimuth propulsor input flange should be kept
as small as possible. Maximum allowed height is
dependent on distance between the prime mover and
cardan shafts chosen.
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The
angle of the intermediate shaft should be minimized
- maximum recommended angle is usually 15º
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Maximum
usable angle is approximately 7.5º per joint, depending
on cardan shaft rpm, bearing lifetime, and vibration.
Use of the maximum angle has to be separately checked.
In order to avoid later problems, maximum recommended
angle is 6 ...6.5º per joint.
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