Man B&W S50ME-B9.3-TII, Project Manual

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MAN B&W 17.06
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MAN Diesel
198 42 245.4 MAN B&W MC/MC-C, ME/ME-C/ME-B/GI engines
When the crank throw is loaded by the gas pressure
through the connecting rod mechanism, the arms of
the crank throw deflect in the axial direction of the
crankshaft, exciting axial vibrations. Through the thrust
bearing, the system is connected to the ship’s hull.
Generally, only zeronode axial vibrations are of
interest. Thus the effect of the additional bending
stresses in the crankshaft and possible vibrations of
the ship`s structure due to the reaction force in the
thrust bearing are to be consideraed.
An axial damper is fitted as standard on all engines, min-
imising the effects of the axial vibrations, EoD: 4 31 111.
Torsional Vibrations
The reciprocating and rotating masses of the engine
including the crankshaft, the thrust shaft, the inter-
mediate shaft(s), the propeller shaft and the propeller
are for calculation purposes considered a system of
rotating masses (inertias) interconnected by torsional
springs. The gas pressure of the engine acts through
the connecting rod mechanism with a varying torque
on each crank throw, exciting torsional vibration in
the system with different frequencies.
In general, only torsional vibrations with one and
two nodes need to be considered. The main critical
order, causing the largest extra stresses in the shaft
line, is normally the vibration with order equal to the
number of cylinders, i.e., six cycles per revolution on
a six cylinder engine. This resonance is positioned at
the engine speed corresponding to the natural tor-
sional frequency divided by the number of cylinders.
The torsional vibration conditions may, for certain
installations require a torsional vibration damper, op-
tion: 4 31 105.
Plants with 11 or 12-cylinder engines type 98-80 re-
quire a torsional vibration damper.
Based on our statistics, this need
may arise
for the
following types of installation:
• Plants with controllable pitch propeller
• Plants with unusual shafting layout and for special
owner/yard requirements
• Plants with 8cylinder engines.
The socalled QPT (Quick Passage of a barred speed
range Technique), is an alternative to a torsional
vibration damper, on a plant equipped with a control-
lable pitch propeller. The QPT could be implemented
in the governor in order to limit the vibratory stresses
during the passage of the barred speed range.
The application of the QPT, option: 4 31 108, has to
be decided by the engine maker and MAN Diesel &
Turbo based on final torsional vibration calculations.
Sixcylinder engines, require special attention. On
account of the heavy excitation, the natural frequen-
cy of the system with one-node vibration should
be situated away from the normal operating speed
range, to avoid its effect. This can be achieved by
changing the masses and/or the stiffness of the
system so as to give a much higher, or much lower,
natural frequency, called undercritical or overcritical
running, respectively.
Owing to the very large variety of possible shafting
arrangements that may be used in combination with
a specific engine, only detailed torsional vibration cal-
culations of the specific plant can determine whether
or not a torsional vibration damper is necessary.
Undercritical running
The natural frequency of the one-node vibration is
so adjusted that resonance with the main critical
order occurs about 3545% above the engine speed
at specified MCR.
Such undercritical conditions can be realised by
choosing a rigid shaft system, leading to a relatively
high natural frequency.
The characteristics of an undercritical system are
normally:
• Relatively short shafting system
• Probably no tuning wheel
• Turning wheel with relatively low inertia
• Large diameters of shafting, enabling the use of
shafting material with a moderate ultimate tensile
strength, but requiring careful shaft alignment,
(due to relatively high bending stiffness)
• Without barred speed range.
Axial Vibrations