Test and influence factors analysis of friction characteristics of millet

Abstract: Millet is one of the most important special crops in arid and semi-arid north region of China. It has the features of strong tolerance to drought and barren soil, high nutritional value, and wide adaptability, and therefore becomes an important part of industry structure development for specialty crop in China. As people adjust their diet structure, millet has increasingly become an essential and high quality food on the dining table. However, the limited development in mechanization of millet production equipment has significantly restricted the development of millet industry. Millet can be mechanically damaged due to the friction in the processes of seeding, harvesting, milling, storage and transportation. So it is important to understand the friction characteristics of millet and use the friction parameters for the design of equipment during the above processes and applications of related technologies. The previous research on grains' friction characteristics was mostly about big grain kernel and small grain kernel like millet has been rarely studied. In this research, a typical millet, Jingu-21, produced in Shanxi Province, was selected as test material with 4 moisture content treatments of 10.7%, 13.7%, 16.0% and 19.8%.. The parameters of friction characteristics of millet including repose angle, coefficient of sliding friction and coefficient of internal friction were measured using a test device of repose angle, a test device of sliding friction and a direct shear apparatus. Variations of the friction characteristics affected by moisture content, contact material and shear velocity were quantified and modeled. The results showed that the moisture content ranging from 10.7% to 19.8% did not significantly affect the size and shape of millet grains, which in turn were not significant factors influencing the friction characteristics of millet. The friction between millet and contacting surfaces was mainly caused by the meshing of concave and convex surfaces and the adsorption of millets. Moisture content had a significant effect on repose angle of millet, which increased from 27.32° to 28.43°. The coefficient of sliding friction linearly increased as the moisture content increased, and the coefficients of static/dynamic sliding friction between millet and steel, iron and acrylic were 0.185-0.290/0.169-0.278, 0.212-0.336/0.190-0.319 and 0.190-0.312/0.182-0.302, respectively. The coefficient of sliding friction of millet against iron plate was the highest, while that against steel plate was the lowest. In the terms of friction reduction, the mechanical parts contacting with millet, such as metering device, should adopt steel material. The internal friction coefficient of millet was not affected by the shear velocity (P>0.05), but linearly increased as the grain moisture content increased, and decreased significantly as the normal stress increased (P<0.01). The coefficient of internal friction of millet decreased quadratically as the normal stress increased, and tended to be stable when the vertical stress was higher than 150 kPa. When the shear velocity was 4.8 mm/min, the normal stress was 50-200 kPa, and the moisture content was 10.7%-19.8%, the coefficient of internal friction of millet ranged from 0.501 to 0.639. In order to improve the flow characteristics of millet in the processes of seeding, storage and transportation, the moisture content should be reduced and the normal stress be increased appropriately. The above results were analyzed using the statistical regression and the mechanical performance parameters of millet on friction were established as test elements respectively. The study can provide a theoretical basis for the design and development of millet mechanization equipment.