Shaker Screen
The shaker screen is a machine used to sort
gravel into different size aggregates. This is necessary in
the placer mining process to remove the large rocks and to sort
the remaining material into different sizes so that it can be
directed into sluices that are appropriate to catch gold that
would be found at that size range. The principle behind the
shaker screen is simple; an inclined screen is caused to vibrate
so that the material larger then the screen size moves along the
screen to one location while material smaller then the screen
falls through the screen and therefore ends up in a different
location. A shaker screen is usually made to have more than
one screen deck so that one machine can sort the aggregate into
three or four different size intervals. The image
on
the right is a CAD drawing put out by CMB International Limited, a
manufacturer of shaker screens. There are two
basic concepts at work in these screen both explainable with
elementary physics. The first is the the inclined plane used
to translate the force of gravity into lateral motion so that the
material can be moved from one end of the screen deck to the
other. The other basic physics concept is what
provides the vibration. Beneath the cover labeled 12 in the
drawing of the screen is an eccentric shaft or a flywheel with an
unequal weight distribution. The drawing on the
right provides a simple line drawing of such a flywheel.
This flywheel is connected to a motor by a drive belt and is spun
at high RPMs. 
Because the center of mass of the flywheel or shaft
is not the axis of rotation a constant angular acceleration is
produced (Knight 328). This acceleration is in the direction
from the center of mass to the axis giving rise to the force
acting in this same direction. As you can see in the drawing
on the left the whole screen assembly sits on springs, this allows
for movement of the screen decks up and down. The force
produced by the rotating mass works in conjunction with the
springs to setup vertical vibration and the whole thing starts
shaking.
www.cmb.uk.com/screens_feeders.html