The requirements of this Chapter relate, in particular,
to formulae for determining the diameters of shafting for main propulsion
installations, but requirements for couplings, coupling bolts, keys,
keyways, sternbushes and other associated components are also included.
The diameters may require to be modified as a result of alignment
considerations and vibration characteristics, see
Pt 5, Ch 8 Shaft Vibration and Alignment, or the inclusion of stress
raisers, other than those contained in this Chapter.
Alternative calculation methods for determining the diameters
of shafting for main propulsion and their permissible torsional stresses
will be considered by LR. Any alternative calculation method is to
include all relevant loads on the complete dynamic shafting system
under all permissible operating conditions. Consideration is to be
given to the dimensions and arrangements of all shaft connections.
Moreover, an alternative calculation method is to take into account
design criteria for continuous and transient operating loads (dimensioning
for fatigue strength) and for peak operating loads (dimensioning for
yield strength). The fatigue strength analysis may be carried out
separately for different load assumptions, for example as given below.
Shafts complying with the applicable Rules in Pt 5, Ch 6 Main Propulsion Shafting and Pt 5, Ch 8 Shaft Vibration and Alignment satisfy the following:
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Low cycle fatigue criterion (typically <104), i.e. the primary cycles represented by zero to full load
and back to zero, including reversing torque if applicable. This is
addressed by the formulas in Pt 5, Ch 6, 3.1 Intermediate shafts, Pt 5, Ch 6, 3.5 Screwshafts and tube shafts and Pt 5, Ch 6, 3.6 Hollow shafts.
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High cycle fatigue criterion (typically >>107), i.e. torsional vibration stresses permitted for continuous
operation as well as reverse bending stresses and the accumulated
fatigue due to torsional vibration when passing through a barred speed
range or any other transient condition with associated stresses beyond
those permitted for continuous operation. This is addressed by the
formulas in Pt 5, Ch 8, 2.5 Limiting stress in propulsion shafting. The influence
of reverse bending stresses is addressed by the safety margins inherent
in the formulas from Pt 5, Ch 6, 3.1 Intermediate shafts, Pt 5, Ch 6, 3.5 Screwshafts and tube shafts and Pt 5, Ch 6, 3.6 Hollow shafts.