2Nd Order Moments On 4, 5 And 6-Cylinder Engines; Compensator Solutions - Man B&W S50MC-C8-TII Project Manual

MAN B&W

2nd Order Moments on 4, 5 and 6-cylinder Engines

The 2nd order moment acts only in the vertical
direction. Precautions need only to be considered
for 4, 5 and 6-cylinder engines in general.
Resonance with the 2nd order moment may oc-
cur in the event of hull vibrations with more than
3 nodes. Contrary to the calculation of natural
frequency with 2 and 3 nodes, the calculation of
the 4 and 5-node natural frequencies for the hull
is a rather comprehensive procedure and often
not very accurate, despite advanced calculation
methods.
A 2nd order moment compensator comprises two
counterrotating masses running at twice the en-
gine speed.
Cycles/min. *) Natural frequency
cycles/min.
300
S50MC
250
S60MC
200
S70MC
S80MC
150
S90MC
100
50
20,000
*) Frequency of engine moment
M2V = 2 x engine speed
Fig. 17.02.01: Statistics of vertical hull vibrations in tank-
ers and bulk carriers
MAN B&W S70MC/MC-C, L70MC-C, S65MC-C,
S60MC/MC-C, L60MC-C, S50MC/MC-C, S50ME-B8,
S46MC-C, S46ME-B, S42MC, S35MC, L35MC, S26MC
dwt
60,000 80,000
40,000
178 06 924.2
MAN Diesel

Compensator solutions

Several solutions are available to cope with the
2nd order moment, as shown in Fig. 17.03.02, out
of which the most cost efficient one can be cho-
sen in the individual case, e.g.:
1) No compensators, if considered unnecessary
on the basis of natural frequency, nodal point
and size of the 2nd order moment.
2) A compensator mounted on the aft end of the
engine, driven by the main chain drive, option:
4 31 203.
3) A compensator mounted on the fore end,
driven from the crankshaft through a separate
chain drive, option: 4 31 213.
4) Compensators on both aft and fore ends,
driven from the crankshaft by the main chain
drive and a separate chain drive respectively,
options: 4 31 203 and 4 31 213.
As standard, the compensators reduce the exter-
nal 2nd order moment to a level as for a 7-cylinder
engine or less.
Briefly speaking, solution 1) is applicable if the
node is located far from the engine, or the engine
is positioned more or less between nodes. Solu-
tion 2) or 3) should be considered where one of
the engine ends is positioned in a node or close
to it, since a compensator is inefficient in a node
or close to it and therefore superfluous. Solution
4) should be considered if the engine is positioned
over the node.
A decision regarding the vibrational aspects and
the possible use of compensators must be taken
at the contract stage. If no experience is available
from sister ships, which would be the best basis
for deciding whether compensators are necessary
or not, it is advisable to make calculations to de-
termine which of the solutions should be applied.
17.02
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