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MAN B&W 5.13
Engine Top Bracing
The so-called guide force moments are caused
by the transverse reaction forces acting on the
crossheads due to the connecting rod and crankshaft
mechan<strong>is</strong>m. When the p<strong>is</strong>ton of a cylinder
<strong>is</strong> not exactly in its top or bottom position the gas
force from the combustion, transferred through
the connecting rod, will have a component acting
on the crosshead and the crankshaft perpendicularly
to the ax<strong>is</strong> of the cylinder. Its resultant <strong>is</strong>
acting on the guide shoe and together they form a
guide force moment.
The moments may excite engine vibrations moving
the engine top athwart ships and causing a
rocking (excited by H-moment) or tw<strong>is</strong>ting (excited
by X-moment) movement of the engine. For engines
with less than seven cylinders, th<strong>is</strong> guide
force moment tends to rock the engine in the
transverse direction, and for engines with seven
cylinders or more, it tends to tw<strong>is</strong>t the engine.
The guide force moments are harmless to the
engine except when resonance vibrations occur
in the engine/double bottom system. They may,
however, cause annoying vibrations in the superstructure
and/or engine room, if proper countermeasures
are not taken.
As a detailed calculation of th<strong>is</strong> system <strong>is</strong> normally
not <strong>available</strong>, MAN Diesel recommends that top
bracing <strong>is</strong> installed between the engine’s upper
platform brackets and the casing side.
However, the top bracing <strong>is</strong> not needed in all
cases. In some cases the vibration level <strong>is</strong> lower if
the top bracing <strong>is</strong> not installed. <strong>Th<strong>is</strong></strong> has normally
to be checked by measurements, i.e. with and
without top bracing.
If a vibration measurement in the first vessel of a
series shows that the vibration level <strong>is</strong> acceptable
without the top bracing, we have no objection to
the top bracing being removed and the rest of
the series produced without top bracing. It <strong>is</strong> our
experience that especially the 7-cylinder engine
will often have a lower vibration level without top
bracing.
MAN Diesel
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Without top bracing, the natural frequency of
the vibrating system compr<strong>is</strong>ing engine, ship’s
bottom, and ship’s side <strong>is</strong> often so low that resonance
with the excitation source (the guide force
moment) can occur close to the normal speed
range, resulting in the r<strong>is</strong>k of vibration.
With top bracing, such a resonance will occur
above the normal speed range, as the natural frequencies
of the double bottom/main engine system
will increase. The impact of vibration <strong>is</strong> thus
lowered.
The top bracing <strong>is</strong> normally installed on the exhaust
side of the engine, but can alternatively be
installed on the manoeuvring side. A combination
of exhaust side and manoeuvring side installation
<strong>is</strong> also possible.
The top bracing system <strong>is</strong> installed either as a
mechanical top bracing or a hydraulic top bracing.
Both systems are described below.
Mechanical top bracing
The mechanical top bracing compr<strong>is</strong>es stiff connections
between the engine and the hull.
The top bracing stiffener cons<strong>is</strong>ts of a double
bar tightened with friction shims at each end of
the mounting positions. The friction shims allow
the top bracing stiffener to move in case of
d<strong>is</strong>placements caused by thermal expansion of
the engine or different loading conditions of the
vessel. Furthermore, the tightening <strong>is</strong> made with a
well-defined force on the friction shims, using d<strong>is</strong>c
springs, to prevent overloading of the system in
case of an excessive vibration level.
MAN B&W MC/MC�C, ME/ME-B/ME�C/ME�GI engines 198 46 72�5.7