5.1.1 Moving blades — details and construction
In the impulse stage, invented by Professor Rateau, the majority of the heat drop occurs in the stationary blading and the driving force on the stage arises from the change in momentum of the steam across the moving blades.
The impulse design possesses the advantages of compactness and comparative insen-sitivity to blading clearance since little pressure drop takes place over the moving blading. However, the moving blades are subject to disturbances from the nozzle wakes, so resonance must be avoided and steam bending stresses must be kept low; also, since the momentum change is relatively high, the moving blades have to be robust and tend to be heavy. It is therefore regular practice to attach the individually manufactured blades to a wheel disc, often by straddle roots which engage with corresponding axial shoulders machined on the wheel rim.
One or more protrusions are left at the outer end of the moving blade. These pass through holes in a coverband which may in turn fit into a slot machined in the outside of the blade: when these protrusions or tenons are formed into rivets, they keep the cover-band in place. The coverband acts as a seal and braces the blading to reduce vibration. Each length of cover-band ties a short length of blades together and may butt up to the next length or overlap, so that an extremely robust construction is created. Since a certain degree of reaction builds up towards the outer diameter of all impulse blades, axial sealing fins are formed integral with the moving blade shroud or coverband.