7.4.3 Rigid couplings
On large turbines, the high torque to be transmitted renders the use of flexible couplings impracticable. Consequently, it is now normal practice for rigid couplings to be employed between the turbine cylinders, so that the turbine shaft behaves as one continuous rotor.
The long shaft that is formed naturally bends under its own weight to form a natural catenary (see section 7.5 of this chapter). Because of this, the use of rigid couplings means that the shaft alignment must be set to ensure that the coupling bending moment forces are minimised.
Rigid couplings are either integral with shaft forging (monobloc) or are shrunk on to the shaft. It is now common practice for the turbine rotors to have monobloc half couplings. Shrunk-on couplings have been used on turbines and still are used on the generator. They allow the couplings to be taken off for the removal of turbine rotor discs, or the generator rotor endwinding retaining rings (end bells). To minimise stress concentration in the coupling region, the monobloc design of half coupling has a large transitional fillet radius. Figure 1.118 shows a typical monobloc coupling.
Half couplings, which are forged separately from the shaft, are assembled by heating and shrinking them on to the shaft. Dowels are then inserted between the bore of the coupling and the diameter of the shaft. For additional security against axial movement of the coupling, a screwed ring, secured by a locking screw, is fitted to the shaft end outboard of the flange. An oil injection point may be provided on the coupling through which oil can be supplied under pressure to one or two annular grooves in the coupling. This allows the coupling to be 'floated' to ensure that it is seated correctly following the shrink-on procedure and also assists removal. Figure 1.119 shows a typical arrangement of a shrunk-on type half coupling with oil injection facility, bolted to a monobloc half coupling.
Earlier designs of shrink-on couplings employed a tapered seat, with a corresponding shaft taper. With this type, an oil injection system was used to remove the coupling from the shaft.
A forged steel spacer plate is normally fitted in each coupling to provide a means of axial adjustment of the rotor relative to the cylinder. This ensures that operational design clearances between the stage diaphragms and the rotor blades are always maintained and also facilitates rotor interchangeability. The spacer plate is annular in shape and dowelled to one half coupling and secured by screws with countersunk heads. The spacer plates must be sufficiently robust to avoid distortion.
The couplings are connected by fitted bolts which are arranged so that torque is transmitted by a combination of axial clamp load (resulting in friction at the coupling faces) and radial grip (resulting in shear load on the cross-sectional area of the bolt). The bolts are tightened in a controlled manner to a predetermined extension. The use of fitted bolts means that some final on-site reaming of the bolt holes is necessary at the assembly stage, which can make this a prolonged operation. In an attempt to reduce assembly time, it is now normal practice only to 'fit' sufficient bolts to enable system fault torques to be transmitted without damage to the bolts. The remaining bolts are installed with larger clearances to permit speedier assembly.
Couplings should be designed to withstand a close up three-phase fault or out-of-phase synchronising without damage. The highest torque under these conditions (typically 4-5 times full load torque) occurs at the generator/turbine coupling and would almost certainly cause 'stepping' of the fitted bolts.
To further reduce the assembly time of couplings, hydraulic bolt fitting techniques have been developed. These have the advantage that bolts can be fitted quickly and also removed quickly without damage to the bolt hole or the bolt.
The standard coupling bolts have cylindrical heads, with an internal hexagon for tightening, and are recessed into the coupling to cut down windage losses and noise. Each coupling is normally surrounded by a stationary steel guard to reduce windage heating of the adjacent pedestal and the creation of oil mist.