STEAM ENGINES 379 



There are several types of this reverse, but probably the 

 Stephenson link is the most popular. It will be described 

 here. In Fig. 260, A is the quadrant over which the 

 reverse lever B works. The reverse lever B, acting through 

 the rocker arm C, raises and lowers the link H. F and G 



are eccentric rods connected 

 at one end with the eccen- 

 trics D and E, respectively, 

 and at the other end with the 

 ends of the link H. I is a 

 block which is attached to 

 this end of the valve rod and 

 is worked over by the link 

 FIG. 26o-pRiNciPLE OF STEPHEN- H With th reve rse lever in 



SON LINK MOTION 



the position in which it 



now is, the eccentric D, through the rod F and block 7, 

 actuates the valve. By throwing the reverse lever to the 

 other end of the quadrant, the link is raised so the eccen- 

 tric E, through the rod G and the block 7, actuates the 

 valve. It will be noticed that the angles of advance of 

 these two eccentrics are practically the same as they 

 were for the two positions of the eccentric in Fig. 259, 

 where the simple engine was reversed. Thus it is seen 

 that the engine has been reversed by simply shifting the 

 motion of the valve from an eccentric which runs the 

 engine under by means of the link 77 and the block 7, to 

 an eccentric which runs it over. If the reverse lever is 

 hooked up in the middle notch of the quadrant, the block 

 7 will be acted upon by both eccentrics, one acting in one 

 direction and the other oppositely ; consequently there is 

 only a very slight movement of the valve. 



Setting the double-eccentric valve. Put the engine on 

 dead center and drop the link down as far as possible and 

 still have clearance between the link and the block ; then 



