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. Beneth 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.