8 THE STEAM TURBINE
By the "double-flow" arrangement in this design by which the steam is passed from the center to the exhaust at both ends there can be very little axial thrust on the shaft. Any thrust that does occur, however, is balanced by the pressure of the exhaust steam in the chambers E, E at the ends of the casing. A slight movement of the shaft toward either end checks the flow of the exhaust steam and increases the back pressure at that end. This increased pressure then moves the shaft back to its normal position. Usually it is not possible to balance the parts of a rotating mass to make its center of gravity coincide exactly with the geometric center about which it revolves. In any machine like a steam turbine, when these two centers do not coincide excessive vibra- tions of the shaft are produced which at certain speeds * are sufficient to break it. To overcome this difficulty, Parsons in- geniously allowed a little lateral play, or "elasticity," as he called E FIG. 6.- ― Screw Type of Steam Turbine. it, for the shaft by means of a series of rings of two different diameters, in principle very much the same as the present con- struction of the main bearings of Parsons turbines (see Fig. 100), so that it was permitted to move laterally a certain amount, say a hundredth of an inch, to allow the proper adjustment in passing from rest to the normal speed of running. Among his early experiments Parsons also tried a purely reaction steam turbine, following almost exactly the published designs of Hero. This turbine, running with 100 pounds per
- This phenomenon occurs at very definite speeds, called "critical," for every
rotating mass. Fuller discussion, with a method for calculating “critical" speeds, is given on page 338. I
