436
DOCK ENGINEERING.
of a ring of rollers, which
simply revolve upon their
—.1 -
Section of Roller
brass _
bushed
are either free to travel with the bridge or which
own axes without progression. In other instances
(figs. 419, 420, and 421) a series
of two or more wheels is
attached to the under side of
the bridge and travel over a
circular roller path. The weight
is transmitted through the wheel
axles, and the turning friction is
considerably greater than with
live rollers. Wheels do not
Ö?
.ji4>f;
-inhiiis
Turning
Front Vtew
Scale, i inch = 1 foot.
Figs. 419, 420, and 421. —Balancing Rollers and Roller Path.
Stele Vlew
always run upon the floor of
the bridge pit. In some in-
stances, the ballasting of the
bridge is reduced to a minimum,
and the centre of gravity lies
forward of the pivot. The wheels
are then placed at the extremity
of the tail end and bear upwards
against the under side of a
corbel course or projection in
the circumference of the bridge
pit, which must necessarily be
constructed in heavy blocks of
masonry.
For bridges accurately bal-
anced over their centre of
gravity, no additional support
is required except for steadying
purposes, and that only in the
case of very light bridges, but
it is nearly always provided,
more perhaps as a précaution
than as a necessity. The force
required to disturb the stability
of balanced heavy bridges is
extremely great. M. Barret*
alludes to a bridge at Marseiiles
which, with a length of 247 feet
and a weight of 500 tons, would
allow a two-wheel dray carrying
6 tons to mount one end of it
at the moment of swinging
without disturbing the longitudinal equilibrium, while a force of no less
* Min. Proc. Inst. C.E., vol. Ivii.