THE ENGINE PROPER 71 



getting hot and causing the brasses to set tight on the pin. 

 When this occurs, the connecting-rod exerts a powerful 

 tendency to twist the pin in the disc. 



The crank-disc on Case engines is pressed onto the 

 shaft with a pressure of fifteen tons or more, and the key 

 is then driven in. It will be seen that, owing to this pre- 

 caution in securing it, the disc is not liable to get loose, and 

 in fact the only strain that can possibly loosen it is the 

 enormous one produced when an engine is started suddenly 

 without allowing the water in the cylinder to escape. 

 Sometimes operators think the disc is loose when it is not. 

 They are deceived by the appearance of oil at the end of 

 the shaft which may have seeped through along the sides 

 of the key, or by the fact that the disc appears to "wobble." 

 The appearance of oil does not indicate looseness and the 

 apparent wobbling may be due to end-play of the shaft. 



The Throttle. The throttle controls the flow of steam 

 from the boiler to the steam chest. It should be left open 

 after the engine is started, allowing the governor to con- 

 trol the speed. The only exception to this rule is when 

 the engine is working hard, as when traveling up a hill, 

 with its boiler showing a tendency to prime. In this case, 

 the engine should be made to run very slowly by means 

 of the throttle. The skill with which some operators 

 handle the throttle enables them to drive an engine up a 

 hill which one less skilled could not make the engine climb. 

 This applies principally to localities in which the water is 



