Steam Turbines / A Book of Instruction for the Adjustment and Operation of the Principal Types of this Class of Prime Movers

by Hubert E. Collins

Detail of Blade Construction

In this construction the blades are cut from drawn stock, so that at its root it is of angular dovetail shape, while at its tip there is a projection. To hold the roots of the blades firmly, a foundation ring is provided, as shown at A in Fig. [27]. This foundation ring is first formed to a circle of the proper diameter, and then slots are cut in it. These slots are accurately spaced and inclined to give the right pitch and angle to the blades (Fig. [28]), and are of dovetail shape to receive the roots of the blades. The tips of the blades are substantially bound together and protected by means of a channel-shaped shroud ring, illustrated in Fig. [31] and at B in Fig. [27]. Fig. [31] shows the cylinder blading separate, and Fig. [27] shows both with the shrouding. In these, holes are punched to receive the projections on the tips of the blades, which are rivetted over pneumatically.

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.

The blading is made up and inserted in half rings, and Fig. [30] shows two rings of different sizes ready to be put in place. Fig. [31] shows a number of rows of blading inserted in the cylinder of an Allis-Chalmers steam turbine, and Fig. [32] gives view of blading in the same turbine after nearly three years' running.