intends to use. The entire potential force or power of the
locomotive, known as tractive effort and expressed in pounds, cannot
be used solely for hauling freight; some of it is used to move the
locomotive. The tractive effort of a locomotive is generally
included in the data supplied by the manufacturer; however, when it
is not known, military planners compute it by a rule of thumb whereby
25 percent of the locomotive's weight on drivers is its starting
tractive effort.
Starting tractive effort is the power of a locomotive to move
itself and the load it is hauling from a stopped position.
Continuous tractive effort is the effort required to keep a train
rolling after it has been started. No distinction is made between
starting and continuous tractive effort in a steam locomotive because
it can generally continue to pull what it can start. However, a
dieselelectric locomotive cannot continue to exert maximum starting
power without damaging its traction motors. Therefore, the
continuous tractive effort of a dieselelectric locomotive is
approximately onehalf of its starting tractive effort.
In determining tractive effort, an understanding of the Whyte
classification system is essential to the planner because tractive
effort is directly affected by the amount of a locomotive's weight
that rests on its driving wheels. The Whyte system, used by the Army
to classify its locomotives, identifies the wheel arrangements by
symbols and indicates the number of driving wheels.
Certain reductions in the locomotive's power affect the tonnage
capacity of a rail line. The first reduction, caused by the need to
move the locomotive itself, comes when calculating drawbar pull. By
subtracting 20 pounds per ton of total locomotive weight from the
continuous tractive effort you find the actual pulling ability of the
locomotive or its drawbar pull. Other reductions in the locomotive's
power are: rolling, grade, and curve resistance expressed in pounds
per ton of train, and reduced efficiency of the locomotive in bad
weather. The weather factor is expressed in percent.
After making allowances for reductions in the locomotive's
potential power, the next thing a planner must do is to find out the
maximum weight or load that the locomotive can safely pull behind it.
This is known as gross trailing load and is expressed in short tons.
It includes both the weight of the cars and the freight in them.
Remember that when doubleheading trains or using pushers, add the
GTL of each locomotive and then multiply the total by 90 percent for
steam locomotives and 100 percent for diesels.
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