Engine Top Bracing; Mechanical Top Bracing - Man B&W S80ME-C7 Project Manual

MAN B&W

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 crank-
shaft mechanism. When the piston of a cylinder
is 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 perpen-
dicularly to the axis of the cylinder. Its resultant is
acting on the guide shoe and together they form a
guide force moment.
The moments may excite engine vibrations mov-
ing the engine top athwart ships and causing a
rocking (excited by H-moment) or twisting (excited
by X-moment) movement of the engine. For en-
gines with less than seven cylinders, this guide
force moment tends to rock the engine in the
transverse direction, and for engines with seven
cylinders or more, it tends to twist 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 super-
structure and/or engine room, if proper counter-
measures are not taken.
As a detailed calculation of this system is normally
not available, MAN Diesel recommends that top
bracing is installed between the engine's upper
platform brackets and the casing side.
However, the top bracing is not needed in all
cases. In some cases the vibration level is lower if
the top bracing is not installed. This 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 is 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 is our
experience that especially the 7-cylinder engine
will often have a lower vibration level without top
bracing.
MAN B&W MC/MCC, ME/ME-B/MEC/MEGI engines
Without top bracing, the natural frequency of
the vibrating system comprising engine, ship's
bottom, and ship's side is often so low that reso-
nance with the excitation source (the guide force
moment) can occur close to the normal speed
range, resulting in the risk of vibration.
With top bracing, such a resonance will occur
above the normal speed range, as the natural fre-
quencies of the double bottom/main engine sys-
tem will increase. The impact of vibration is thus
lowered.
The top bracing is normally installed on the ex-
haust side of the engine, but can alternatively be
installed on the manoeuvring side. A combination
of exhaust side and manoeuvring side installation
is also possible.
The top bracing system is installed either as a
mechanical top bracing or a hydraulic top bracing.
Both systems are described below.

Mechanical top bracing

The mechanical top bracing comprises stiff con-
nections between the engine and the hull.
The top bracing stiffener consists of a double
bar tightened with friction shims at each end of
the mounting positions. The friction shims al-
low the top bracing stiffener to move in case of
displacements caused by thermal expansion of
the engine or different loading conditions of the
vessel. Furthermore, the tightening is made with a
well-defined force on the friction shims, using disc
springs, to prevent overloading of the system in
case of an excessive vibration level.
MAN Diesel MAN Diesel
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