STEAM ENGINES 



377 



Moving the valve on the rod increases or diminishes the 

 difference in lead. 



If an engine has a rocker arm pivoted in the center, 

 move the eccentric in the opposite direction. Otherwise 

 proceed in the same manner as without the rocker arm. 



523. Reversing a simple slide valve engine. To set the 

 valve of a simple engine so that the engine will run back- 

 ward, or, as is often termed, under, remove the steam 



chest cover, set the engine 

 on dead center, and ascer- 

 tain the lead. Now loosen 

 the eccentric from the shaft 

 and turn it backward until 

 the lead is again the same 

 as before. The distance 

 which the eccentric is to be 

 turned backward should be 

 180 plus twice the angle 

 FIG - 259 of advance (Fig. 259). 



The valve does not need to be moved on the rod, nor 

 the rod lengthened or shortened. The only caution neces- 

 sary is to be sure that the lead is always on the end the 

 piston is on when the engine is on center. 



An engine running backward or under will do just as 

 much work as one running forward or over, but when it 

 is running over the pressure of the crosshead is always 

 down, while when it is running under the weight of the 

 crosshead and connecting rod is down, but the pressure 

 caused by the steam on the piston and the angle of the 

 connecting rod and piston rods will be up; hence there 

 are two forces working in opposition at the crosshead, 

 and this will cause an up-and-down pound. Not only 

 this, but if an engine runs over, this force will all be ex- 

 erted upon the engine bed and not the frame. 



