STEAM ENGINES 213 



piston rod from the crosshead, in case it is fastened by a thread 

 and check-nut. To obviate any danger, the check-nut should 

 be tried frequently. 



Improper Steam Distribution. The primary cause of another 

 source of pounding is the improper setting of the steam valve, 

 or possibly its improper design. In the case of improper 

 setting of the valve, insufficient compression, insufficient lead, 

 cut-off too early, and late release may all cause pounding on 

 the centers. 



Reversal of Pressure. The effect of a reversal of pressure 

 is clearly shown in the accompanying illustration. With the 

 crankpin at a and the engine running in --^^ir-^ 



the direction indicated by the arrow, 

 the connecting-rod is subjected to a 

 pull, but after the crankpin has passed 

 the dead center c, the connecting-rod is 

 subjected to a push, in which case the 

 rear brass, as shown at b, bears against 



the crankpin, while in the former case, ^" *' 



as shown at a, the front brass bears against the crankpin. 

 By giving a sufficient amount of compression, the lost motion 

 in the pins and journals is transferred gently from one side 

 to the other before the crankpin reaches the dead center. If 

 the compression is insufficient, there will be pounding. 



Insufficient Lead. Insufficient lead causes an engine to 

 pound because the piston has then little or no cushion to 

 impinge on as it approaches the end of its stroke, and it is 

 brought to rest with a jerk. A similar effect will be produced 

 by a late release; the pressure is retained too long on the 

 driving side of the piston. The ideal condition is that the 

 pressures shall be equal on both sides of the piston at a point 

 in its travel just in advance of the opening of the steam port. 

 The position of this point varies Vvith the speed of the piston 

 and other conditions that only the indicator card can reveal. 



Pounding at Crosshead. The crosshead is a source of 

 pounding from various causes, of which the loosening of the 

 piston rod is one of the most common. There are several 

 methods of attaching the piston rod to the crosshead. The 

 rod may pass through the crosshead with a shoulder or a 



