FIG. 27
The foundation rings themselves are of dovetail shape in cross-section, and, after receiving the roots of the blades, are inserted in dovetailed grooves in the cylinder and rotor, where they are firmly held in place by keypieces, as may be seen at C in Fig. [27]. Each keypiece, when driven in place, is upset into an undercut groove, indicated by D in Fig. [27], thereby positively locking the whole structure together. Each separate blade is firmly secured by the dovetail shape of the root, which is held between the corresponding dovetailed slot in the foundation ring and the undercut side of the groove.
FIG. 28
Fig. [29], from a photograph of blading fitted in a turbine, illustrates the construction, besides showing the uniform spacing and angles of the blades.
The obviously thin flanges of the shroud rings are purposely made in that way, so that, in case of accidental contact between revolving and stationary parts, they will wear away enough to prevent the blades from being ripped out. This protection, however, is such that to rip them out a whole half ring of blades must be sheared off at the roots. The strength of the blading, therefore, depends not upon the strength of an individual blade, but upon the combined shearing strength of an entire ring of blades.