A Study of Tribological Processes During the Milling of Rice

This thesis is concerned with the wear of machinery during the rice milling process. Two machines have been studied in detail (which combined constitute “milling”). Background literature has been studied and further added to such that machine improvements can be made relating to both capacity and component wear life.
The first part of the work undertaken considers material properties of rice grains in their various forms and machine components with which contact is made. Dimension measurements and SEM imagery have highlighted the organic nature of rice grains as an abrasive, and observations of the change in properties from pre to post process made. Mass change, grain profiles, coefficient of friction, grain hardness and compressive strength have all been measured.
Wear observations of husking and polishing components has revealed similarities with known wear mechanisms. Microscopy and profilometry have shown the areas which are susceptible to increased wear rates whilst measurement of scratch angles has illustrated the motion of grains through the polisher.
Small scale testing has been a key part of the research due to the impractically high capacity of full scale machines. Husking tests have established that the shear stress required to achieve a given husked ratio is constant suggesting that a harder rubber with the same coefficient of friction as a softer rubber will husk more efficiently. Fast capture camera footage of husking experiments has shown that grains rotate between the rubber counterfaces before husking which effectively creates a region of high pressure at either end of the grain. It has also revealed that grains maintain their initial contact with the slow roller whilst sliding against the fast roller once husked. Probable mechanisms for this effect have been determined and experiments have been conducted showing that the uneven wear created by the effect could be minimised by axial vibration of the slow roller.
Small scale polishing experiments have been used to reveal the locations of the highest wear (through paint test) which have a pattern consistent with an uneven fill by the grains entering the polishing chamber or a resonance of the screens caused by cage spacing. High wear was noted at inlet thought to be a result of rapid scattering of grains on entry from the feed screw. Pressure sensitive paper has been used to determine contact pressure profiles for various stages and conditions in the polishing chamber and laser vibrometry has been used to observe the effect of loading on machine vibration. Computer simulations have been developed to simulate grain flow and have shown good compatibility with the physical experiments. This leads to the potential to use such methods for future machine design.
Finally, this thesis covers the potential for further work along with how the work is best suited for industrial implementation.