The frame of the _Pioneer_ defies an exact classification but it more
closely resembles the riveted- or sandwich-type frame than any other
(figs. 18, 27). While the simple bar frame enjoyed the greatest
popularity in the last century, riveted frames were widely used in this
country, particularly by the New England builders between about 1840 and
1860. The riveted frame was fabricated from two plates of iron, about
5/8-inch thick, cut to the shape of the top rail and the pedestal. A bar
about 2 inches square was riveted between the two plates. A careful
study of photographs of Hinkley and other New England-built engines of
the period will reveal this style of construction. The frame of the
_Pioneer_ differs from the usual riveted frame in that the top rail is
1-3/4 inches thick by 4-1/8 inches deep and runs the length of the
locomotive. The pedestals are made of two 3/8-inch plates flush-riveted
to each side of the top rail. The cast-iron shoes which serve as guides
for the journal boxes also act as spacers between the pedestal plates.
The bottom rail of the frame is a 1-1/8-inch diameter rod which is
forged square at the pedestals and forms the pedestal cap. The frame is
further stiffened by two diagonal rods running from the top of each
truck-wheel pedestal to the base of the driving-wheel pedestal, forming
a truss. Six rods, riveted to the boiler shell and bolted to the frame's
top rail, strengthen the frame laterally. Four of these rods can be seen
easily as they run from the frame to the middle of the boiler; the other
two are riveted to the underside of the boiler. The attachment of these
rods to the boiler was an undesirable practice, for the boiler shell
was thus subjected to the additional strain of the locomotive's
vibrations as it passed over the road. In later years, as locomotives
grew in size, this practice was avoided and frames were made
sufficiently strong to hold the engine's machinery in line without using
the boiler shell.
The front and rear frame beams are of flat iron plate bolted to the
frame. The rear beam had been pushed in during an accident, and instead
of its being replaced, another plate was riveted on and bent out in the
opposite direction to form a pocket for the rear coupling pin. Note that
there is no drawbar and that the coupler is merely bolted to the beams.
Since the engine only pulled light trains, the arrangement was
sufficiently strong.