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<strong>MAN</strong> B&W 17.06<br />
Axial Vibrati<strong>on</strong>s<br />
When the crank throw <str<strong>on</strong>g>is</str<strong>on</strong>g> loaded by the gas pressure<br />
through the c<strong>on</strong>necting rod mechan<str<strong>on</strong>g>is</str<strong>on</strong>g>m, the<br />
arms of the crank throw deflect in the axial directi<strong>on</strong><br />
of the crankshaft, exciting axial vibrati<strong>on</strong>s. Through<br />
the thrust bearing, the system <str<strong>on</strong>g>is</str<strong>on</strong>g> c<strong>on</strong>nected to the<br />
ship’s hull.<br />
Generally, <strong>on</strong>ly zero�node axial vibrati<strong>on</strong>s are of<br />
interest. Thus the effect of the additi<strong>on</strong>al bending<br />
stresses in the crankshaft and possible vibrati<strong>on</strong>s<br />
of the ship`s structure due to the reacti<strong>on</strong> force in<br />
the thrust bearing are to be c<strong>on</strong>sidered.<br />
An axial damper <str<strong>on</strong>g>is</str<strong>on</strong>g> fitted as standard <strong>on</strong> all engines,<br />
minim<str<strong>on</strong>g>is</str<strong>on</strong>g>ing the effects of the axial vibrati<strong>on</strong>s (4 3 ).<br />
Torsi<strong>on</strong>al Vibrati<strong>on</strong>s<br />
The reciprocating and rotating masses of the engine<br />
including the crankshaft, the thrust shaft, the<br />
intermediate shaft(s), the propeller shaft and the<br />
propeller are for calculati<strong>on</strong> purposes c<strong>on</strong>sidered<br />
as a system of rotating masses (inertias) interc<strong>on</strong>nected<br />
by torsi<strong>on</strong>al springs. The gas pressure of<br />
the engine acts through the c<strong>on</strong>necting rod mechan<str<strong>on</strong>g>is</str<strong>on</strong>g>m<br />
with a varying torque <strong>on</strong> each crank throw,<br />
exciting torsi<strong>on</strong>al vibrati<strong>on</strong> in the system with different<br />
frequencies.<br />
In general, <strong>on</strong>ly torsi<strong>on</strong>al vibrati<strong>on</strong>s with <strong>on</strong>e and<br />
two nodes need to be c<strong>on</strong>sidered. The main<br />
critical order, causing the largest extra stresses<br />
in the shaft line, <str<strong>on</strong>g>is</str<strong>on</strong>g> normally the vibrati<strong>on</strong> with<br />
order equal to the number of cylinders, i.e., six<br />
cycles per revoluti<strong>on</strong> <strong>on</strong> a six cylinder engine.<br />
<str<strong>on</strong>g>Th<str<strong>on</strong>g>is</str<strong>on</strong>g></str<strong>on</strong>g> res<strong>on</strong>ance <str<strong>on</strong>g>is</str<strong>on</strong>g> positi<strong>on</strong>ed at the engine speed<br />
corresp<strong>on</strong>ding to the natural torsi<strong>on</strong>al frequency<br />
divided by the number of cylinders.<br />
The torsi<strong>on</strong>al vibrati<strong>on</strong> c<strong>on</strong>diti<strong>on</strong>s may, for certain<br />
installati<strong>on</strong>s require a torsi<strong>on</strong>al vibrati<strong>on</strong> damper,<br />
opti<strong>on</strong>: 4 3 05.<br />
Based <strong>on</strong> our stat<str<strong>on</strong>g>is</str<strong>on</strong>g>tics, th<str<strong>on</strong>g>is</str<strong>on</strong>g> need may ar<str<strong>on</strong>g>is</str<strong>on</strong>g>e for<br />
the following types of installati<strong>on</strong>:<br />
• Plants with c<strong>on</strong>trollable pitch propeller<br />
• Plants with unusual shafting layout and for special<br />
owner/yard requirements<br />
• Plants with 8�cylinder engines.<br />
<strong>MAN</strong> B&W S90MC-C/ME�C, S80MC-C/ME�C, S70MC/MC-C/ME�C/ME�GI,<br />
L70MC-C/ME�C, S65ME�C/ME�GI, S60MC/MC-C/ME�C/ME�GI, L60MC-C/<br />
ME�C, S50MC/MC�C/ME-B/ME-C, S46MC-C, S42MC, S40ME-B, S35MC/<br />
ME-B, L35MC, S26MC<br />
<strong>MAN</strong> <strong>Diesel</strong><br />
Page of 2<br />
The so�called QPT (Quick Passage of a barred<br />
speed range Technique), <str<strong>on</strong>g>is</str<strong>on</strong>g> an alternative to a<br />
torsi<strong>on</strong>al vibrati<strong>on</strong> damper, <strong>on</strong> a plant equipped<br />
with a c<strong>on</strong>trollable pitch propeller. The QPT could<br />
be implemented in the governor in order to limit<br />
the vibratory stresses during the passage of the<br />
barred speed range.<br />
The applicati<strong>on</strong> of the QPT, opti<strong>on</strong>: 4 3 08, has to<br />
be decided by the engine maker and <strong>MAN</strong> <strong>Diesel</strong><br />
based <strong>on</strong> final torsi<strong>on</strong>al vibrati<strong>on</strong> calculati<strong>on</strong>s.<br />
Six�cylinder engines, require special attenti<strong>on</strong>.<br />
On account of the heavy excitati<strong>on</strong>, the natural<br />
frequency of the system with <strong>on</strong>e-node vibrati<strong>on</strong><br />
should be situated away from the normal operating<br />
speed range, to avoid its effect. <str<strong>on</strong>g>Th<str<strong>on</strong>g>is</str<strong>on</strong>g></str<strong>on</strong>g> can be<br />
achieved by changing the masses and/or the stiffness<br />
of the system so as to give a much higher, or<br />
much lower, natural frequency, called undercritical<br />
or overcritical running, respectively.<br />
Owing to the very large variety of possible shafting<br />
arrangements that may be used in combinati<strong>on</strong><br />
with a specific engine, <strong>on</strong>ly detailed torsi<strong>on</strong>al<br />
vibrati<strong>on</strong> calculati<strong>on</strong>s of the specific plant can<br />
determine whether or not a torsi<strong>on</strong>al vibrati<strong>on</strong><br />
damper <str<strong>on</strong>g>is</str<strong>on</strong>g> necessary.<br />
Undercritical running<br />
The natural frequency of the <strong>on</strong>e-node vibrati<strong>on</strong><br />
<str<strong>on</strong>g>is</str<strong>on</strong>g> so adjusted that res<strong>on</strong>ance with the main critical<br />
order occurs about 35�45% above the engine<br />
speed at specified MCR.<br />
Such undercritical c<strong>on</strong>diti<strong>on</strong>s can be real<str<strong>on</strong>g>is</str<strong>on</strong>g>ed by<br />
choosing a rigid shaft system, leading to a relatively<br />
high natural frequency.<br />
The character<str<strong>on</strong>g>is</str<strong>on</strong>g>tics of an undercritical system are<br />
normally:<br />
• Relatively short shafting system<br />
• Probably no tuning wheel<br />
• Turning wheel with relatively low inertia<br />
• Large diameters of shafting, enabling the use of<br />
shafting material with a moderate ultimate tensile<br />
strength, but requiring careful shaft alignment,<br />
(due to relatively high bending stiffness)<br />
• Without barred speed range<br />
198 42 25�7.4