Figure 2.3.1 demonstrates the basic principle of rotor deflection. It represents a simple constant diameter spindle having four integral wheels, each carrying at least one blade row. A single span rotor of length (L) is supported on two bearings (1 and 2), one close to each end of the span. Under the action of its own weight, the rotor will deflect (or "sag") as shown in Figure 2.3.1(b). If the rotor is of uniform section along its length, the form of the deflection will be a true catenary. There are, however, factors that will influence the actual form of the deflection:

• The rotor central portion will not normally be of constant diameter. There will be step changes at many locations, particularly if the rotor has stepped segments at sealing locations. The stiffness of the rotor depends upon the rotor diameter cubed, and the deflection ( ?) is a function of this stiffness

• If the rotor carries shrunk on discs (as shown in Fig. 2.3.2) these discs will normally be of different shapes and therefore, of different mass so the mass distribution along the length of the span will not be constant. Also, the blades will be of varying radial length and of increasing mass towards the exhaust end. These wheel-and-blade masses add weight, depending upon their total volume, but add no stiffness to the rotor section modulus. Deflection is a function of the central spindle diameter and mass distribution, but will be essentially the same as shown in...