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Techno Cases |
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Techno
Fysica BV is a
Dutch-based engineering
company, highly specialised in troubleshooting and
solving all kinds of problems related to the dynamic loading of machinery
and installations. For this, several measuring
(vibration measurements) and analysis techniques,
are
used. In addition, the use of advanced
calculations and calculations programs enables the predicting of vibration
behaviour in an early stage of a design.
In a series of cases,
we will give examples of what can go wrong in the
engineering and operating of vessels, and what errors lie at the root of the
damages observed. |
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Example: Reluctance of shipbuilders to involve third parties
During start up of the main engines of a 50.000 kW RoRo ferry at the quay
side, rattling noises were emitted by both gearboxes.
The
manufacturer of the gearbox identified the noise as gear hammer and stated
immediately that he does not accept gear hammer under any operational
condition and that warranty was pending until the gear hammer was solved.
The
ship, however, had to be commissioned and handed over to the owner due to
a substantial penalty clause in the building contract.
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General arrangement of a
50.000 kW
RoRo Ferry’s engine room
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Measurements of torque and vibrations showed that the gear hammer emanated
from the gear teeth of the PTO shaft.
Because this installation can be operated at two different engine speeds,
i.e. 345 rpm and 414 rpm, the PTO drive is equipped with two different
gear ratio’s and two clutches which means one gear is always unloaded.
The torsional vibration mode that
caused the gear hammer was the mode where the secondary part of the elastic
coupling of the forward engine vibrates against the gearbox at a frequency
of 18 Hz.
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Gearbox with two speed PTO drive |
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The
vibratory torque showed amplitudes up to 22% of nominal alternator torque
under zero mean load whereas Class allows maximum levels of 20% for
unloaded gears (apart from the fact that the OEM does not allow gear
hammer at all).
The
most simple way to solve this problem was to shift the natural frequency
for torsion that caused the gear hammer by installing a disc with an
inertia of 630 kgm² to the secondary part of the elastic coupling. This
shifted the resonance speed to an engine speed below the minimum of 345
rpm.
Vibratory torque was reduced to a level of 6% of alternator torque which
lead to such low energy of the impacts due to gear hammer that the OEM
finally judged the situation as acceptable.
However, the fact is that this problem could have been prevented because
from the torsional vibration calculation, that was submitted to the
shipyard and the equipment suppliers, it could clearly be seen that gear
hammer in the PTO drive was calculated.
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Added inertia disc of 630
kgm² |
Torsional
vibration calculation data
showing gear
hammer with unloaded PTO |
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If
the supplier or manufacturer
of the gearbox would have studied the
torsional
vibration calculation
in the design stage, he could have warned the shipyard that gear hammer
was calculated, which would not be accepted and
that it was necessary to perform modifications in the design stage.
In
this case, these conclusions were drawn when the ship was already finished
and ready to sail for it’s sea trials.
This caused a lot of additional cost, stress and endless discussions
and friction
between parties about their individual responsibility.
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For
more info, or question regarding what Techno
Fysica can mean for you in solving your mechanical
problems,
please contact techno
Fysica BV in the Netherlands at:
Techno
Fysica BV
P.O.
box 351
2990 AJ Barendrecht
The
Netherlands
Tel. +31 180 620211
Fax. +31 180 620705
info@technofysica.nl
www.technofysica.nl
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