Design and test of screw cleaning mechanism for corn

Screening is the most important part of post-harvest processing of grain, and it is mainly vibration screening. Traditional methods of vibration screening have many problems, such as large noise and low screening efficiency. The aim of this article was to solve the problem mentioned above in the traditional screening process, and the screw cleaning mechanism was designed based on the principle of screw conveying. The device was mainly composed of conveying auger, inlet charge hopper, frame, collection box, semicircular screen, discharge port, gear motor, variable-frequency drive, and so on. In terms of mechanism parameters, the inner diameter of semicircular screen was 130 mm, the external diameter of the conveying auger was 100 mm, the screw pitch was 100 mm, the working length was 2 000 mm, and the diameter of screw shaft was 20 mm. There were 2 types of semicircular screens, the first one was 6 mm circular aperture with the open porosity of about 40%, and the other one was 16 mm circular aperture with the open porosity of about 35%. The critical screw rotational speed was 205 r/min, and the maximum theoretical cleaning ability of the device was 6.8 t/h. In order to determine the screening effect of the device, the screening efficiency and the breakage rate increment were used as the test indices in this study, and the corn with 14% moisture was adopted in the test, which included the separation test of big-sized impurities and the separation test of small-sized impurities. In terms of separation test of big-sized impurities, it was a single-factor test. Appropriately sized corn cobs and stalks were selected as big impurities. Test samples were prepared according to the relevant standards, and corn samples were obtained, of which big-sized impurities content was 2%. The conveying inclination was 0°, the initial filling factor was 40%, the working length was 960 mm, and 30 kg corn samples were tested each time. The results of the separation test of big-sized impurities showed that screening separation occurred mainly at the front portion of the screen, and the main function of the latter portion of the screen was to separate corn entrained by corn cobs and stalks; and the breakage rate increased with the increase of the screw rotational speed. Although lower screw rotational speed reduced cleaning ability, it could protect corn from break, and higher screw rotational speed might increase the working length of the screen surface. For the separation test of small-sized impurities, the clay granules which had passed through 3 mm circular aperture grain sieve were selected as small-sized impurities and the test samples were also prepared according to the relevant standards; corn samples were obtained, of which small-sized impurities content was 5%, and 30 kg corn samples were tested each time. The single-factor test showed that screening efficiency and breakage rate increment increased with the increase of the conveying auger speed; with the increase of the initial filling factor, screening efficiency decreased slowly, but breaking rate increment increased gradually; screening efficiency increased initially and decreased afterwards as the conveying inclination increased, and breaking rate increment increased gradually. The orthogonal test selected conveying inclination (-10°, 0°, 10°), screw rotational speed (150, 200, 250 r/min) and initial filling factor (20%, 30%, 40%) as the test factors, and 9 groups of experiments were conducted. The orthogonal test results showed the optimal test levels of the 3 experimental factors: Initial filling factor was 20%, conveying inclination was 0°, and screw rotational speed was 500 r/min. The ANOVA (analysis of variance) results showed that initial filling factor had significant (P<0.05) influence on screening efficiency, but it didn't have significant (P>0.05) influence on breakage rate increment; screw rotational speed had significant (P<0.05) influence on both screening efficiency and breakage rate increment; but conveying inclination didn't have significant (P>0.05) influence on either screening efficiency or breakage rate increment. The device had low noise and reliable operation in working process, and screening efficiency of the device reached 98.5%. This study can provide reference for the design of the screw cleaning equipment in the future.