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MAN B&W 17.05
Guide Force Moments
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Fig. 17.05.01: H�type and X�type guide force moments
The so�called guide force moments are caused
by the transverse reaction forces acting on the
crossheads due to the connecting rod/cranskahft
mechan<strong>is</strong>m. These moments may excite engine
vibrations, moving the engine top athwartships
and causing a rocking (excited by H�moment) or
tw<strong>is</strong>ting (excited by X�moment) movement of the
engine as illustrated in the above figure.
The guide force moments corresponding to the
MCR rating (L ) are stated in Table 7.07.0 .
Top bracing
The guide force moments are harmless except
when resonance vibrations occur in the engine/
double bottom system.
As th<strong>is</strong> system <strong>is</strong> very difficult to calculate with the
necessary accuracy MAN Diesel strongly recommend,
as standard, that top bracing <strong>is</strong> installed
between the engine’s upper platform brackets
and the casing side.
The vibration level on the engine when installed in
the vessel must comply with MAN Diesel vibration
units as stated in Fig. 7.05.02.
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MAN B&W MC/MC�C, ME/ME-B/ME�C/ME�GI engines 198 42 23�3.3
MAN Diesel
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178 06 81�6.3
We recommend using the hydraulic top bracing
which allow adjustment to the loading conditions
of the ship. Mechanical top bracings with stiff
connections are <strong>available</strong> on request.
With both types of top bracing above�mentioned
natural frequency will increase to a level where resonance
will occur above the normal engine speed.
Details of the top bracings are shown in Chapter 05.
Definition of Guide Force Moments
Over the years it has been d<strong>is</strong>cussed how to define
the guide force moments. Especially now that
complete FEM�models are made to predict hull/
engine interaction, the propeller definition of these
moments has become increasingly important.
H�type Guide Force Moment (M H )
Each cylinder unit produces a force couple cons<strong>is</strong>ting
of:
. A force at crankshaft level.
2. Another force at crosshead guide level. The
position of the force changes over one revolution
as the guide shoe reciprocates on the
guide.