Strona 2_redak - Instytut Agrofizyki im. Bohdana Dobrzańskiego ...

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Molenda et al. [107], for wheat sliding against steel surface, also found the relationship
between friction force and moisture content with a minimum at approximately 15%, as
shown in figure 9.3.
Fig. 9.3. Force of friction versus moisture content for wheat sliding on smooth steel surface.
Experimental results and estimated curve [107]
9.3.2. Surface roughness
Researchers were usually interested in friction of grain against materials that
were commonly used in the construction of agricultural equipment and storage
facilities. In earlier investigations wood, steel and rubber were tested, later investigations
on galvanized steel, concrete, aluminium, polyethylene or teflon have been
conducted. The state of the surface of investigated material was not precisely defined
in a majority of published results because of the complexity of a methods of
determination of surface properties. Usually the commercial name of material was
given, sometimes with a parameter of surface roughness added. As an example,
Cyrus [40] reported an increase in the coefficient of friction of wheat kernel against
steel with an increase in parameter R a LQ D UDQJH IURP WR P $ VSHFLILF
difficulty of investigation of friction is the changing state of surface with prolonging
time of measurements. Richter [142] observed that the coefficient of friction
decreased with increasing number of performed tests. He suggested polishing sliding
surface till the moment of stabilization of frictional force. Thompson and Ross [161]
observed considerable variations of coefficient of friction between samples of