Having a stiff rotor allows the Astern turbine to be built up and hence allows the bulk of the LP rotor to be forged from a low grade steel whilst only the Astern parts are made from the material necessary to withstand the superheated steam.
If the rotor was flexible and a built up astern turbine wheels fitted then a possibility of fretting exists.
The use of separately machined astern wheels allows the original forging to be more simplistic.
The forging of the higher grade steels required for use in superheat conditions require an increased amount of energy, and hence expense, in the original forging and subsequent machining process.
Another big advantage is that the astern wheels being of smaller mass and free to expand means that they can take more rough treatment then if they formed part of a single mass. The discs are forged from solid ingots and then machined so as to produce a force.shrink fit when heated and hydraulically pressed onto shaft.
The result of thisis that due to the radial and tangential stress the wheel tends to grow in size. The wheel tends to loosen and fretting corrosion can take place
For HP rotors, generally, one wheel per step is allowed with a small clearance between each wheel. The whole is secured by a locknut and each wheel keyed to ensure positive transmission of torque. These keys are displaced by 180' for each step.
For LP turbines 3 wheels per step can be accommodated.
As the combined rotor shaft/wheel hub diameter is about twice that of the gashed rotor the sealing surface is greatly increased
Relative volumes of steam in HP and LP turbines
It can be seen that whereas the increasing volume of the steam in the HP turbine is moderate, The increase in the LP turbine is significant requiring blade height to be increased in successive stages. In the final stages both the height and the angle of the blades have to be altered. See notes on taper/twisting of blades) e