Scope

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 Vol 2, Pt 5 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 this Chapter and Vol 2, Pt 5 Shaft Vibration and Alignment satisfy the following:

1. 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 Vol 2, Pt 3, Ch 2, 4.2 Intermediate shafts, Vol 2, Pt 3, Ch 2, 4.4 Screwshafts and tube shafts and Vol 2, Pt 3, Ch 2, 4.5 Hollow shafts

2. 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 Vol 2, Pt 5, Ch 1, 3.2 Limiting stress in propulsion shafting. The influence of reverse bending stresses is addressed by the safety margins inherent in the formulas in Vol 2, Pt 3, Ch 2, 4.2 Intermediate shafts, Vol 2, Pt 3, Ch 2, 4.4 Screwshafts and tube shafts and Vol 2, Pt 3, Ch 2, 4.5 Hollow shafts.