PROGRESS IN TURBINE SHIP PROPULSION. 63 



nient, resulting in a concentration of the tooth load and often failure of the teeth. . 

 it is obvious, then, that merely to limit the unit tooth pressure to some fixed em- 

 pirical figure for all gears in no way insures the safety of the teeth. On the other 

 hand, anything which tends to maintain the alignment of the teeth and to prevent 

 concentration of pressure is to be welcomed as a source of safety in the operation 

 of the gearing. It is for this reason that the Melville-Macalpine invention of the 

 I-beam support for the pinion bearings is so thoroughly justifying the claims made 

 for it. The closer this mechanically correct device is investigated, the more appa- 

 rent becomes its most important function, namely, to maintain automatically a very 

 uniform distribution of tooth pressure under the most severe distortions of the gear 

 case that can occur in the frailest hull. Its value in permitting higher unit loads 

 than are customary on what have been termed "rigid bearing" gears is graphi- 

 cally illustrated in Fig. 4, Plate 31. 



The curves of the rigid bearing dimensions have been compiled from recent ma- 

 rine practice in Great Britain. The floating frame figures are those used by the 

 Westinghouse Electric and Manufacturing Company for marine work, and there are 

 points indicated on this curve showing gears actually in service. 



As previously referred to, the gear, by permitting high turbine speeds, much 

 simplifies the turbine design. It enables a speed to be selected for the turbine appro- 

 priate for best economy. Furthermore, it inherently permits a design of increased 

 reliability by virtue of sensibly reduced dimensions. 



The small high-speed geared turbines require few of the elaborate precautions in 

 warming up and getting in service that must be practiced with the large direct-con- 

 nected turbines. 



Gears may readily be arranged with two pinions for even the smallest single 

 screw steamer, permitting the employment of what are known as cross compound 

 turbines ; that is, a high-pressure and a low-pressure turbine, the steam passing them 

 in series. Piping is arranged that by the manipulation of valves the high-pressure 

 turbine may exhaust direct to the condenser, or the high-pressure turbine be isolated 

 and the low-pressure turbine receive high-pressure steam direct. Thus, generally 

 speaking, no turbine or gear accident is likely to so incapacitate the machinery that 

 the vessel cannot reach port at reduced speeds. There is one record of a lO'^A knot 

 steamer, which, on account of breakage of one pinion, crossed the Atlantic with the 

 low-pressure turbine alone, obtaining speeds as high as 9 knots. 



The general arrangement of such an installation is shown in Fig. 5, Plate 32. 

 Its reliability has much to commend it. This is not only due to the flexibility above 

 referred to, but further due to the simplification of the turbines themselves. Each 

 turbine only expands the steam corresponding to half the heat drop, and so has a 

 much reduced number of turbine elements, smaller temperature range, and reduced 

 length between bearings as compared with a complete expansion turbine, having 

 similar economy. Both high and low-pressure turbines are provided with reversing 

 elements arranged in a similarly compounded manner to the ahead elements, thus 

 rendering maneuvering possibilities as complete with one turbine element operating 



