The main objective in setting and maintaining rotor alignment is to achieve satisfactory dynamic behaviour of the running shaft line.
Excessive misalignment can affect the vibration behaviour of the multi-bearing shaft line. It imposes an effective bending moment at the couplings which acts like a rotating out-of-balance. Changes between hot and cold running vibration behaviour may be indicative of misalignment, with the effect showing as a once-per-revolution vibration. Very drastic misalignment may have the capacity to cause bearing unloading and hence alter the shaft vibration behaviour.
A long shaft bends naturally under its own weight to form a catenary, but nevertheless revolves around its curved centreline during rotation. The alignment is arranged so that the shaft system has minimum bending moments at the shaft couplings. The catenary depends solely on the mass and stiffness of the shaft system. Figure 1.120 shows a typical shaft catenary for a large turbine-generator.
During initial erection, bearings are set at appropriate heights relative to one another to form the catenary shape.
The final bearing height must ensure that adjacent coupling faces are square and true to one another, so that there is no transmission of bending moment through the coupling. Final adjustment of bearings must be made with all the cylinders fully assembled and all pipes, which may exert a strain on any turbine cylinder, fully assembled and attached to that cylinder.