STEAM ENGINES 381 



most common, Avill be discussed here. This reverse gear 

 (Fig. 261) has few parts to wear and get out of order 

 and may be set so that steam can be used on expansion. 



It will be noticed that in this reverse gear (Fig. 262) 

 the throw of the eccentric is set opposite to the crank 

 instead of about at right angles to it, as shown with other 

 gears. By moving the lever from one end of the quadrant 

 to the other the guide A, which carries the roller B, 

 changes position as shown by dotted lines. This causes 

 the valve to move in the opposite direction. All types 

 of reversing gears have some mechanical means of oper- 

 ating the throw of the valve. This is equivalent to 

 changing the position of the eccentric in the shaft, and 

 if one method of setting the valve is mastered all others 

 will be easily picked up. 



528. Angularity of connecting rod. Due to the angu- 

 larity of the connectfng rod, the piston of an engine 

 travels faster and farther while the crank is passing 

 through the half of its rotation nearer the cylinder than 



it does while the crank 

 travels the opposite half of 

 its rotation. By reference 

 to Fig. 263 it will be no- 

 ticed that the crank has 

 traveled only half its dis- 

 tance and the piston has 



passed over more than half its stroke. As the crank 

 passes through the other half of its revolution, which 

 it does in the same time as it did the first half, the 

 piston travels as much less than half its stroke as it trav- 

 eled more than half during the first revolution of the 

 crank, consequently does not travel nearly as fast during 

 this half of the time as it does during the other half. 

 Because of this unequal travel of the piston one end of 



