3IO LOGGING 



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 4I to i to 5 to i; i.e., the tractive force in pounds 

 is from 23 per cent to 20 per cent of the total weight on the 

 drivers. 



The usual formula employed for determining the tractive force 

 of single-expansion rod locomotives with a piston speed not 

 exceeding 200 feet per minute is as follows: 



„ d" X LX .Ssp 

 ^ ^ D ' 



when T represents the tractive force, 



d represents the diameter of the cylinder in inches, 

 L represents the length of piston stroke in inches, 

 .85 p represents 85 per cent of the boiler pressure,^ 

 D represents the diameter of the driving wheel in inches. 



As the speed increases the tractive force decreases because the 

 mean effective pressure in the cylinders falls and friction also 

 increases. 



Resistance to Gravity. — • The resistance to gravity increases in 

 exact proportion to the grade and is always 20 pounds per ton of 

 2000 pounds for each i per cent rise in grade; e.g., for a 0.5 per 

 cent grade it is 10 pounds per ton and for a 4 per cent grade it 

 is 80 pounds per ton. 



^ This has been found by practical test to be the average effective pressure in 

 the cylinder. 



