The attachment of the turbine blades to the rotor is the most critical aspect of steam turbine design. All the forces are transmitted through the attachment to the rotor. Specially, at the low pressure end of turbines of large output, the attachment has to bear a relatively large forces due to high speed, the centrifugal force on the blade is many times its mass. Therefore it becomes necessary to estimate the stresses in the attachment but sometimes it is difficult to get the exact value. There is always the possibility of stress concentration at the sharp corners. Therefore, selection of material is very important which can safeguard from this stress concentration and that is why the calculated stress is kept reasonably low. A careful study of the forms of attachment is also necessary because occasionally it influences the shape of the wheel, rim and stresses in the disc. The form of the attachment should be such that the centrifugal force on the blade is transmitted to the disc in the simplest and most direct manner and it should give the security of attachment.
The various forms are:
The various forms are:
- De-Laval Blade root attachment
- Inverted-T attachment
- Serrated blade root arrangement (Annular fir-tree)
- Attachment for high pressure Crutis wheel
- Straddle attachment
- Modified straddle attachment
- Side entry blades attachment
- Shrouding strip attachment
- Parson's end tightened blading
- Parson's integral blades
Most of the above attachments are also used in gas turbine blading however annular fir-tree or its modified versous are most common.