Roy Inman et al --
Small drive wheels have a greater mechanican advantage thatn large drive
wheels. The radius is the lever arm. A given torque (at the axle) delivered
through a lever arm (the wheel radius) produces a force at the end of the lever
arm (at the railhead) that is inversely proportional to the length of the lever
arm. In other words, a given torque applied to a 72" drive wheel produces only
half the tractive effort of the same torque applied to a 36" wheel. This is
why steam freight locos have smaller drivers than passenger locos, and why
switchers have even smaller drivers.
Then there is the matter of the propulsion plant. Electric motors deliver
their highest torque at starting, whereas external combustion engines (like
steam engines) deliver their highest torque at speed. Additionally, electric
motors can be forced to deliver as much torque as one wishes, subject of course
to the availability of sufficient amperage (from the diesel motor-generator),
and the ability of the insulation on the motor windings to withstand the heat.
There are stories out there of overzealous engineers who towed monstrous loads
at extremely low speeds, leaving a trail of molten insulation, then puddles of
melted wiring, and finally fried traction motors.
Dick Karnes
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