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 13 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 11, Ch 2 Shafting Systems and Pt 13 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 formulae in Pt 11, Ch 2, 4.2 Intermediate shafts, Pt 11, Ch 2, 4.4 Screwshafts and tube shafts and Pt 11, Ch 2, 4.5 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
formulae in Pt 13, Ch 1, 3.2 Limiting stress in propulsion shafting. The
influence of reverse bending stresses is addressed by the safety margins
inherent in the formulae from Pt 11, Ch 2, 4.2 Intermediate shafts, Pt 11, Ch 2, 4.4 Screwshafts and tube shafts and Pt 11, Ch 2, 4.5 Hollow shafts.