Grains characteristics

Knowing the physical characteristics of grains helps determine how best to separate out impurities.
Depending on their final use, grains of main crops, other crops or vegetables and forage seeds, will need a corresponding level of cleaning to remove inert materials, weed seed, grains of other cultivated species and even small, shrunken or damaged grains.

Aspiration achieves a separation by density and screens achieve a separation according to size and shape.

Density and aspiration

An airflow passing through a bed of grain can pick up and separate light grains and impurities. Increasing the air speed removes more light particles.
There are however some limitations:
- Only light trash is removed
- Some small grains may be removed before larger trash particles are lifted up.
- The grain flow needs to be even and regular for efficient operation.
- If the airflow is turbulent efficiency is decreased.

Grain size and shape in relation to screens

Perforations in screens allow separation in two dimensions:
A kernel passes through a round hole depending on the width of the kernel and through a slot depending on the thickness of the kernel.
One chooses a round hole or a slot according to the kind of grain and impurity to be separated:
- Large perforations allow good grain to pass through whilst retaining large impurities. This is called scalping.
- Small perforations allow small impurities through and retain good grain. This is called screening.

On a Cleaner Separator (flat bed), two screens are superimposed with the "big" scalping screen on top and "small" screening screen under. It's a compact design.

On a Rotary Cleaner (rotary drum), each screen on the drum can be used for screening or for scalping. It's a flexible design.

Other features

Round holes or slots cannot achieve a proper separation based on the length of the kernel, but separation by length may be achieved with an indented cylinder, where a grain enters an indentation based on its length.
It is used in seed cleaning or for fine cleaning (e.g. organic grains).
Some other machines allow complementary separations or use other features of the grains (e.g. colour, shape, hairiness, texture, etc..) for separation.