Page 70 - 39. MRH13-05-May2013
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20th century were of 40, 50, or 70 tons nominal capacity, and
most of what follows will be devoted to trucks of those capaci-
ties. Truck capacity was stated in terms of how much weight
a pair of trucks would support. Thus a single "50-ton" truck
was designed to carry only 25 tons, with the other half of the
50 tons resting on the truck at the opposite end of the car.
Capacity was determined by the size of the journals, as follows:
40 tons nominal capacity, 5" X 9" journals, load limit 136000 lbs.
50 tons nominal capacity, 5-1/2" X 10" journals, load limit
169000 lbs.
70 tons nominal capacity, 6" X 11" journals, load limit 210000 lbs.
Nominal capacities were intended to indicate approximately
how much weight could be loaded into or on the car, so load
limits were more than half again higher than nominal capaci-
ties to account for the empty weight of the car itself.
1
1: A turn-of-the-century arch bar truck with straight
lower chords, cast steel bolster, and steel channel
spring plank. The vertical columns on either side of the
bolster were also steel castings.
Freight Car Trucks - 2
2
2: This typical early 20th century arch bar truck had
symmetrical top and bottom chords, cast steel bolster
and columns, and a steel channel spring plank with
oak spring seats.
Arch bar trucks
The first arch bar trucks began to appear in the 1860s.
Originally the side frames were made of iron bars bolted
together, and bolsters and spring planks were wood. The arch
bar design was adopted almost universally in the years that fol-
lowed, with improvements that included iron and (later) steel
bolsters and spring planks, steel side frame bars, and longer
wheelbases. The arch bar truck had a number of advantages
over other freight car truck designs. It was simple and inexpen-
sive to make, and it rode and tracked well. Moreover, as freight
cars became larger and heavier, the arch bar truck's load-car-
rying capacity could be increased simply by making the com-
ponent parts larger. [For more information on the evolution of
trucks in the nineteenth century, see John H. White, Jr.'s
The
American Railroad Freight Car,
Johns Hopkins University Press,
Baltimore, 1993.]
MRH-May 2013
