MOTIVE POWER AND ROLLING STOCK 309 



weight with three cyhnders, placed side by side and directly 

 connected, 120 degrees apart, to a driving rod which is supported 

 on a heavy bearing attached to the boiler. The driving rod is 

 broken both with universal joints and also with two slip joints 

 to permit either an increase, or a decrease, in the length when 

 passing around curves. 



The right-hand wheels on each truck are fitted with gear rims 

 into which mesh the pinions which furnish the driving power 

 for the locomotive. 



A 50-ton Shay locomotive, f.o.b. factory, costs about $7500. 



Fig. 88. — A Shay Geared Locomotive. 

 HAULING ABILITY OF LOCOMOTIVES 



The hauling ability of a given locomotive depends largely on 

 (i) the tractive force, (2) the resistance of the load to gravity, 

 and (3) the frictional resistance. 



Tractive Force. — The tractive force of a locomotive, some- 

 times improperly called the "draw-bar pull," is the power 

 possessed by a locomotive for pulling a train, including the weight 

 of the locomotive itself. If one end of a rope is passed over a 

 pulley and fastened to a weight hanging in a pit, and the other 

 end is attached to a locomotive running on a straight level track 

 without regard to speed, the tractive force of the locomotive will 

 be represented approximately by the amount of weight the 

 locomotive can lift. Tractive force increases in direct propor- 

 tion to the area of piston heads, length of stroke and steam 

 pressure in the cylinders, and decreases directly as the diameter 

 of the driving wheels increases. 



