Experimental study on friction characteristics of gorgon nuts (Euryale ferox) in mechanical shelling

Abstract: Gorgon nut is a local special agricultural and sideline product rich in nutrients and with high economic value. However, the lack of machinery related to production, processing of gorgon nut has seriously held back the development of the gorgon nut industry. Experimental study on gorgon nut friction characteristics can provide a theoretical basis for its peel, separation and other related machinery researches and development. Parameters like inner friction coefficient, static roll stable angle and angle of repose are measured on gorgon nuts with equipments including shear apparatus, self-made bevel instrument and angle of repose measuring device. Friction characteristics are analyzed on gorgon nuts of different maturity levels, particle diameters and surface roughness. It is demonstrated that, smaller particle diameter results in more particles per unit area of inner contact and thus larger mosaic force of engagement between the concave and convex surface of particles, which also leads to higher friction resistance and increased inner friction coefficient. It is also showed that, under the same measurement conditions, inner friction coefficient was decreased with the increase of maturity level of gorgon nuts.When measuring static roll stable angles using bevel instrument, put a single gorgon nut on the inclined plane, then turn the handle slowly to gradually increase the plane inclination. When the gorgon nut just starts to roll down, measure the inclination immediately using protractor instrument. The measured value is then used as the static roll stable angle. Static roll stable angles of gorgon shells are significantly bigger than gorgon jens and gorgon nuts with shell. But the difference is smaller between gorgon jens and gorgon nuts with shell (difference of 0.1°-2.4° for medium maturity gorgon nuts). Gorgon shells can be separated from gorgon jens by bevel instrument with inclined plane of 14.7°-21.5° gradient, which cannot be used for efficient separation of gorgon nuts with shell and gorgon jens.It is also observed that using different plates (e.g rubber plate, unpolished wood and stainless steel plate) can result in different efficiency in separation of gorgon shells, gorgon jens and gorgon nuts with shell. For gorgon nuts of medium maturity, rubber plate is the best way for separation of gorgon shells and gorgon jens due to the biggest difference (16.9°-18.5°) in static roll stable angles. On the other hand, with a difference of 12.3°-14.1° in static roll stable angles between shells and jens, stainless steel plate is least effective in separation of the two. For all the three plates, there is no significant difference (0.1°-2.4°) in static roll stable angles between jens and nuts with shell; thus none of the plates can be used for efficient separation of jens and nuts with shell.For all the three plates, repose angle of gorgon nuts was decreased with the increase of particle diameter. Under the same measurement conditions, low-maturity gorgon nuts have the biggest repose angle while high maturity gorgon nuts have the smallest and medium-maturity gorgon nuts in the middle. In comparison of repose angle on different plates, for gorgon nuts of different particle size and maturity levels, rubber plate always has the biggest repose angle while stainless steel plate has the smallest.