346 LOGGING 
weight with three cylinders, 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 the pinions mesh which furnish 
the driving power for the locomotive. 
HAULING ABILITY OF LOCOMOTIVES 
The hauling ability of a given locomotive depends largely on 
(1) 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 pos- 
sessed by a locomotive for pulling a train, including the weight 
of the locomotive itself and its tender. 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 loco- 
motive 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. 
Tractive force is dependent on the weight of the locomotive on 
its driving wheels because it adheres to the rail only by the 
friction developed between these wheels and the rail head, and 
the resistance to slipping increases with the weight on the driving 
wheels. The weight on wheels other than drivers has no effect 
on the tractive force. If the engine is too light in proportion 
to its power it will be unable to hold itself to the rail and exert a 
strong pull, while on the other hand if the weight of the locomo- 
tive is too great in comparison to its power, it will not haul 
maximum loads because of the excess weight in itself that must 
be moved. In industrial locomotives the economical ratio 
between the weight on the drivers and the tractive force ranges 
from 4j to 1 to 5 to 1; i.e., the tractive force in pounds is from 
23 to 20 per cent of the total weight on the drivers. 
The usual formula employed for determining the tractive force 
