Crushers and
Screens
A portable rock crusher from the early 20th century in the Buskerud
Vegvesens Museum in Kongsberg, Norway (2005).
machine comprises a box-like frame; a centrally
located, horizontal shaft-rotating element on
which hammers are mounted; and a set of
circumferentially arranged grates in the lower
part of the housing. The shaft rotates at high
velocity, which break the stone by impact. From
the 1920s through the 1950s, the hammer
mills were modified to include impactors that
A Bucyrus steam shovel from the early 20th century.
broke the rock with fixed breaking bars and to
eliminate the grates on certain machines.
It is here, in the early 1900s, that we see
the first Trommel Screener introduced to the
crushing process, where the crushed rock is
passed directly through the drum. The Trommel
Screener is a cylindrical drum that is elevated at
an angle at the feed end. Physical size separation
is achieved as the feed material spirals down the
rotating drum, where the undersized material
(smaller than the screen apertures) passes
through the screen, while the oversized material
exits at the other end of the drum.
In the 1930s, gyratory screening was
introduced, which created a huge change in the
machinery industry due to the much greater