Marine Installation Manual
2021-08
6-12
6 Engine Dynamics
6.4 Torsional vibration
X72DF
6.4
Torsional vibration
Torsional vibrations are generated by gas and inertia forces as well as by the ir
-
regularity of the propeller torque. It does not cause hull vibration (except in very
rare cases) and is not perceptible in service, but produces additional dynamic
stresses in the shafting system.
The shafting system comprises of the crankshaft, propulsion shafting, propeller,
engine running gear, flexible couplings, and power take-off (PTO). The complete
assembly of the shafting system must be considered when determining the tor
-
sional loads in the system components.
Torsional vibration
calculation (TVC)
The torsional loads in the system components are determined by performing a
torsional vibration calculation (TVC). The TVC must be done in the early stage
for every project.
Across the engine's speed range, all system components must remain within their
corresponding torsional vibration load limits. If in a component, the torsional
loads exceed the corresponding limit, appropriate countermeasures have to be
applied.
The calculation normally requires approval by the relevant classification society
and may require verification by measurement on board ship during sea trials. All
data required for torsional vibration calculations should be made available to the
engine supplier in an early design stage (see section
).
Barred speed range (BSR)
At a certain speed range the torsional vibration stresses in the shafting may ex
-
ceed the limits for continuous operation. If this occurs, a barred speed range
(BSR) must be defined. The width of the BSR is defined by the classification so
-
ciety. The BSR must be passed through rapidly and some classification societies
have defined rules about the maximum permissible passage time through the
BSR. In general, the target is to have a maximum passage time of 30 seconds.
In order to guarantee a rapid passage of the BSR, a minimum power margin of
10% at the upper boundary of the BSR must be present. The power margin is de
-
fined as the margin between the bollard pull curve and the engine torque limit.
In any case, within the BSR the torsional shaft stress must not exceed the tran
-
sient limit, otherwise other appropriate countermeasures have to be taken.
6.4.1
Reduction of torsional vibration
Excessive torsional vibration can be reduced by optimising the shaft diameters,
selecting a different (heavier) flywheel, adding a front disc (tuning wheel) to the
free end of the crankshaft or adding a torsional vibration damper to the free end
of the crankshaft. A torsional vibration damper reduces the torsional stresses by
absorbing part of the vibration energy.
