Man B&W S80ME-C7, Project Manual

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MAN B&W 17.05
Page of 3
MAN Diesel
MAN B&W MC/MCC, ME/ME-B/MEC/MEGI engines 198 42 233.3
Guide Force Moments
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178 06 816.3
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
mechanism. These moments may excite engine
vibrations, moving the engine top athwartships
and causing a rocking (excited by Hmoment) or
twisting (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 this system is very difficult to calculate with the
necessary accuracy MAN Diesel strongly recom-
mend, as standard, that top bracing is 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.
We recommend using the hydraulic top bracing
which allow adjustment to the loading conditions
of the ship. Mechanical top bracings with stiff
connections are available on request.
With both types of top bracing abovementioned
natural frequency will increase to a level where res-
onance 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 discussed how to de-
fine 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 con-
sisting of:
. A force at crankshaft level.
2. Another force at crosshead guide level. The
position of the force changes over one revo-
lution as the guide shoe reciprocates on the
guide.