Cranks are bends in the axle of any part of a machine by which an
up-and-down motion is converted into a circular or rotatory one,
as in the common knife grinder’s machine; in this arrangement
a fly-wheel is necessary to continue by its momentum (tendency
to go on) the motion begun by the upward and downward action of
the treadle, piston-rod, &c., as the case may be. The cranks of
steam-vessels are among the heaviest pieces of forging that are
wrought by Nasmyth’s steam hammer, cast-iron being too brittle to
be used for the purpose.