Half ring of blades inserted in the foundation ring before being placed upon the rotor, showing substantial construction. These rings have slots of dove-tail shape cut into them to receive the roots of the blades. These slots are accurately spaced, and inclined so as to give the required pitch and angle to the blades. The foundation rings themselves are dove-tail in cross section, and are inserted in dove-tail grooves cut in the turbine cylinder, and rotor respectively. These rings are firmly held in place by key pieces that are driven into place, and upset into undercut grooves, thus locking the whole structure firmly together. Ques. 791.-How are the tips or outer ends of the blades protected? Ans. By a shroud ring for each row, in which holes. are punched to receive the projections on the tips of the blades. These holes are spaced by special machinery to match the slots in the foundation ring. Ques. 792.-Describe the construction of the shroud rings. Ans. They are channel shaped in cross section, and are made thin, so that in case of accidental contact with an opposing surface no dangerous heating will occur, neither will the rubbing be so liable to rip out the blades, as it is when they are unprotected by a shroud ring. Ques. 793.-Mention another advantage in connection with the use of a shroud ring. Ans.-The blades in each row are stiffened, and held together as a unit by its use, thus permitting smaller clearances, and reducing the leakage loss to a minimum. The channel shape of the shroud ring also forms an effective baffle to the steam leakage. Fig. 211 illustrates blades as fitted in the rotor of Allis Chalmers steam turbine. The shroud ring protecting the tips of the blades is also shown. Ques. 794.-What type of bearings are the Allis Chalmers steam-turbines fitted with? Ans.-Self-adjusting ball and socket bearings especially designed for high speed, shims being provided for proper alignment. Fig. 212 shows a number of rows of stationary blades fitted in the cylinder of an Allis Chalmers steam turbine. In the smaller sizes the bearing shells are made of special bronze, and in the larger sizes white metal is used for bearing surface. Ques. 795.-How are these bearings lubricated? Ans. The oil is supplied freely to the middle of each bearing, and allowed to flow out at the ends, where it is caught, passed through a cooler, and pumped back to the bearings, to be used again and again. Ques. 796.-Does the fact that the oil is supplied to the bearings in large quantities necessarily imply a heavy expenditure for oil? Ans. It does not; for the reason that the bearings practically float on oil films, thus preventing that "wearing out" of the oil which occurs when it is supplied in diminutive doses. Ques. 797.-Can superheated steam be used to advantage in steam-turbines? Ans.-It can; in fact the steam-turbine has solved the problem of superheated steam, owing to the absence of all rubbing parts exposed to the steam. This permits the use of steam of high temperature thus making it possible to realize the advantages of economical operation. Ques. 798.-Is there not danger of distortion of the turbine cylinder being caused by the very high temperatures to which it is exposed by the use of superheated steam? Ans.-There have been numerous instances in the past of unequal expansion of the top, and bottom of the cylinder thereby causing the rotating blades to come in contact with the cylinder walls, and be ripped out, but this difficulty has in a great measure been overcome by certain designers of steam-turbines, who have made a special study of the laws of expansion and contraction of metals, |