STEAM ENGINES 375 



521. Dead center. An engine is on dead center when 

 a straight line passing through the centers of the cross- 

 head and crank shaft will pass through the crank pin. If 

 an engine is on dead center it will not start, although the 

 ports may be open. Locomotives and often traction en- 

 gines have two cylinders with their cranks at right an- 

 gles, so that one or the other will always be off center, 

 and consequently will start without turning the wheel by 

 hand. 



Locating dead center. When the crank is passing dead 

 center the piston moves so slowly that a movement of 

 2 or 3 inches of the crank is hardly perceptible on the 

 piston. This, however, is not true of the valve, for when 

 the crank is passing dead center the valve is moving its 

 fastest, consequently it is essential that dead center be 

 definitely determined. About the simplest and most ac- 

 curate method for putting an engine on dead center is 

 by means of a tram (Fig. 255). At some convenient 

 place in the engine frame make a clear, sharp-cut center- 

 punch mark, and with the flywheel about one-eighth 



F 



FIG. 255 TRAM FIG. 256 



revolution off center make another center-punch mark in 

 the wheel. Set the tram in the center-punch marks as 

 shown in Fig. 256. Now with a sharp knife make a mark 

 C across the intersection of the crosshead an dthe guide. 

 Turn the wheel down until the mark on the crosshead 

 and the guide come together again, then make another 

 mark in the wheel so that the tram will drop into it as 



