TURBINES 237 



should be detachable from the gates. The stem may then be 

 withdrawn from the gate and the latter removed without dis- 

 turbing the crown plate of the turbine. This is a great convenience 

 but, unfortunately, is feasible only in connection with large units, 

 and on smaller work the stems must either be cast or forged inte- 

 gral with the gates. The gate stems must, furthermore, be of 

 ample strength to resist the strain in case an obstruction is caught 

 between two gates and the full power of the governor is con- 

 centrated upon them. The links which connect the gate stem 

 levers to the operating ring should be the weakest element of the 

 gate mechanism, and should be designed to break before the 

 stress reaches the elastic limit of the material of any of the 

 other parts. 



Speed Rings. These were introduced in connection with the 

 large single-runner vertical turbine with volute casings molded 

 directly in the concrete. They consist of a series of curved vanes 

 outside of the turbine guide vanes, forming together with an 

 upper and lower crown (Figs. 113 and 128), a rigid frame to sup- 

 port the weight of the portions of the turbine and of the concrete 

 substructure of the power-house located above the casing, as well 

 as the generator and thrust bearing. The vanes are shaped to 

 suit the free passage of water entering the movable guide vanes, 

 and this arrangement is preferable in every way to round stay 

 bolts, the large, projected area and circular form of which causes 

 considerable hydraulic losses. Besides this, there is a mechanical 

 advantage in the use of a rigid cast-iron connection between the 

 upper and lower speed-ring crowns. 



Casings. The most efficient form of turbine casing in use at 

 present is that of volute or spiral shape, Fig. 119. This type has 

 been in common use under high heads for some years, and is now 

 being adopted with increasing frequency for low heads, partic- 

 ularly where the turbines are of large capacity. The materials 

 most commonly used for medium and high heads are cast iron 

 and cast steel, the choice between them being influenced chiefly 

 by consideration of the stresses imposed. Large casings for high 

 heads are usually made of cast steel. Cast iron, although more 

 suitable for medium heads, may properly be used for high heads 

 if the casings are small and the material is worked at low stress to 

 provide an ample factor of safety against pressure surges which are 

 of more common occurrence in high-head than in low-head plants. 



