Man B&W L35MC6-TII User Manual

MAN B&W 17.05
Page 1 of 3
MAN Diesel
MAN B&W MC/MCC, ME/MEC/MEGI/ME-B engines 198 42 233.4
Guide Force Moments
Top bracing level
Middle position of guide plane
Crankshaft centre line
Engine seating level
M
x
DistX
Cyl.X
X
L
x
L
L
z
XtypeHtype
L
x
L
Z
M
H
L
z
178 06 816.4
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/crankshaft
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 Fig. 17.05.01.
The guide force moments corresponding to the
MCR rating (L
1
) are stated in Table 17.07.01.
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
limits as stated in Fig. 17.05.02.
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, the above-men-
tioned natural frequency will increase to a level
where resonance will occur above the normal en-
gine 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:
1. A force at crankshaft level
2. Another force at crosshead guide level. The po-
sition of the force changes over one revolution
as the guide shoe reciprocates on the guide.

Man B&W L35MC6-TII Specifications

General

Brand	Man
Model	B&W L35MC6-TII
Category	Engine
Language	English