In figuring the drawbar pull for a steam locomotive, remember
that the starting and continuous tractive efforts are the same.
Once you have determined how much a locomotive is capable of
pulling, the next step is to find out how much resistance is offered
in opposition to that pull. Three forms of resistance that oppose
the pull of a train, caused by the train and the track it runs on,
are rolling, grade, and curve. A fourth, referred to as the weather
factor, is a measure of the decreased efficiency of a locomotive in
cold weather. Each of these forms of resistance is discussed
separately in the next four main paragraphs.
2.6. ROLLING RESISTANCE
The forces acting upon a train in a direction parallel to the
track and tending to hold back its movement are called rolling
resistance (RR). This resistance takes many forms: the friction
between the heads of the rails and the flanges and treads of the
wheels, undulation of the track beneath a moving train, internal
friction of the rolling stock, and friction and pressure of the air
through which the train is moving. An absolute figure to represent
rolling resistance is unknown, but experience with train movements in
theaters of operations has produced some safe average values, as
shown in table II. This table shows what value to use for rolling
resistance expressed in pounds per ton of train. The value chosen
depends on the overall quality of the track. Items such as ballast,
ties, rails, and tie plates must be evaluated in determining the
track's quality. No set policies or rules are available that
indicate which track features cause which condition of track; the
choice depends solely on experience. For planning purposes, however,
this may be obtained from the detailed railway construction or
rehabilitation plan of the engineers.
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