The way I look at it is that although a shaft consists of two half shafts, the half shafts are joined at a rigid coupling so that they behave exactly as if they were a single unit. We do not know the secrets of the coupling itself except that it consists in part of a splined joint transmitting torque from the turbine to the compressor and at least one other element transmitting axial loads between the half shafts (because the splined joint alone cannot transmit axial tension).
The fact that wear rates in the coupling of some engines were found to be higher than expected does not mean that the coupling was anywhere near close to failing. I would assume that at some stage the wear develops to the point where movement in the joint goes out of acceptable limits and has to be removed but this would be well before a hazardous situation is reached.
Last edited by firstfloor; 23rd Dec 2010 at 14:28.