284 PHILIP DAWSON 



injury. Furthermore, cross compound engines are largely 

 used with only two bearings — the armature and fly wheel, 

 which are next each other, being located between the bear- 

 ings. The slightest wearing of the bottom shells or sinking 

 of foundation might produce very heavy stresses due to mag- 

 netic pull. What these may amount to is shown from the 

 following figures: — 



Pull on shaft (from actual practice) due to 1-32 inch 

 displacement of constant current railway generator armature. 



„7. . , . -, Magnetic pull In 



Kilowatts. Revs, per min. Yu^f f^ ^* J' ^s Po^^^s due to ^ in. 



ture m pounas. displacement. 



100 250 3,000 3,500 



150 225 4,000 5,000 



200 180 8,000 7,000 



250 100 15,000 10,000 



300 160 12,000 8,000 



1,500 75 70,000 40,000 



The clearance all around between the armature and pole 

 pieces would be about | inches. 



In consequence of marine engines having been built so 

 successfully in Great Britain, when direct connected gener- 

 ator sets had first to be considered, the large shafts and bear- 

 ing surface asked for were much ridiculed by many engine 

 builders. To a certain extent the same thing obtained on 

 the continent, owing to storage batteries being nearly uni- 

 versal and heavy overloads necessitating heavy fiy wheels 

 therefore nearly unknown. It was contended that a marine 

 engine driving a propellor had quite as hard work to fulfill 

 as an engine direct connected to a railway generator. Three 

 things, however, were overlooked in this comparison. 



First. A marine engine has a maximum load but cannot 

 ever be instantaneously overloaded as much as 100 per cent, 

 as often happens in traction work. 



Second. The shaft, and consequently the bearings, have 

 not to carry a heavy fly wheel and armature. 



Third. An immense extra stress can be exerted on the 

 shaft should the armature get out of center. 



The pressure on the projected area, that is to say, on 

 the area equal to diameter and length, of a main journal, 



