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

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9.3.3. Normal pressure
Normal pressure was shown to be another important factor influencing friction.
Thompson and Ross [161] applied normal pressure in a range from 7 to 172 kPa and
observed a decrease in the coefficient of friction of wheat against steel with an increase
in pressure. The authors suggest that with an increase in normal pressure contact
stress increased less than proportionally. Hertz’s theory [104] postulated that with
an increase in normal force N normal contact stress increases as N 1/3 . This way,
according to Thompson and Ross [161], frictional forces in the contact area that are
proportional to normal loads are weaker than proportional to normal stress. In such
a way Hertz’s theory would explain the decrease in the coefficient of friction with
an increase in normal pressure. A similar tendency was reported by Fiala [52] for
different agricultural materials, Lobotka [97] in the case of corn, and Zhang et al.
[174] for wheat. Moore et al. [113] stated an opposite tendency, one of increase in the
coefficient of friction with an increase in normal wall load in a silo made of corrugated
steel. Typical results of friction of wheat against smooth and corrugated galvanized
steel are shown in figure 9.5 after Molenda et al [111]. The tests were performed using
the tilting table method and soft red winter wheat of 11.5% moisture content.
Fig. 9.5. Coefficient of friction of wheat on smooth and corrugated galvanized steel determined using
tilting table method. Mean values and 95% confidence intervals for five levels of normal pressure [111]