Determining optimal kinematic parameters of a screen shoe used for separating breeding seed samples

. In the post-harvest processing of breeding material, the absence of a sieve machine of appropriate productivity that meets all the requirements for breeding machines is relevant today. For research, a mock stand of the sieve machine was used, on which the kinematic parameters of the sieve mill and the tilt angles of the sieves were determined.


Introduction
To perform cleaning and sorting of seed samples grown on breeding plots, usу is made of complex machines from domestic and foreign producers [1][2][3]. Domestic machines feature the performance rate less than 150...200 kg/h. Processing lots of seeds weighing more than 15 kg with these machines is economically unfeasible. In this respect, there is a need to have a set of simple machines: an aspiration column, a screen cleaner and a grain sifter, which could be used depending on the content of impurities in the source material, either independently or as a production line. The most vivid drawback in post-harvest processing of breeding material today is the lack of a screen cleaner of an appropriate performance that could meet all the requirements for breeding machines [4][5][6][7].
The research purpose -to develop a screen cleaner for breeding enterprises and determine the kinematic parameters of its screen shoe, to study the influence of the inclination angle of the screen shoe on the performance.

Material and methods
The authors considered materials of theoretical and experimental studies on the cleaning of breeding seeds. For the experiments, use was made of a model bench of a screen cleaner shown in Fig. 1.
Kinematic parameters of the screen shoe. As the source material for the experiments, use was made of wheat with a seed variety purity of 97.9%, pre-treated in a vertical air flow, after which all the light impurities were removed. The grain material contained 1.76% of fine and 0.34% of large impurities. Fig. 1. Model bench of a seed cleaner:1 -loading hopper; 2 -vibratory feeder; 3 -primary screen; 4 -cleaning screen; 5 -separating screen; I -large impurities; II -fine impurities (dunst); III -cleaned material; IV -second grade.
The screen cleaner bench included: an upper screen with round holes of ∅4.5 mm in diameter and a lower screen with oblong holes of ∅2.0 mm. The screens were of the same size -790×400 mm. The upper screen featured a slightly smaller size of the holes for the purpose of a more visual and a precise study of the effect of loading and kinematic parameters of the screen shoe on the quality of grain material separation.
The material was divided into three fractions at a capacity of 500±5 kg/h: the siftings from the upper screen, the siftings from the lower screen and the pass through the lower screen. All fractions were weighed on AND GF600 scales and analyzed. Upon the analysis results, the authors determined the extraction coefficient for small impurities using a formula where Qim is the amount of impurities passing from the lower screen, %; Qо -the amount of fine impurities in the initial material, %. The authors determined the losses of seeds in the total amount of material siftings from the upper screen, and the weight of 1000 seeds in the outcomes of the upper and lower screens.
The clogging of screens was determined after passing through them an amount of seeds weighing 10 kg. After the processed material passed through the machine, the latter continued to work idle for another minute. Then the screens were removed, turned over 180° upside down, to let off all the unseized grain seeds. Further on, the seized grains were separately removed from the holes of the upper and lower screens, collected, and weighed with scales.
The clogging of screens was determined with a formula: where m1 is the mass of seeds seized in the holes, g; m -the mass of seeds passing onto the screen, g.
One of the main parameters of grain separation mode are the number of vibrations per minute n and the amplitude A, which are called kinematic parameters. The acceleration of the screen determined with the expression ω²А (ω is the angular velocity of the eccentric, rad/s) is referred to as a kinematic indicator of the screen. The experiments were carried out at five vibration amplitude variants of the screen shoe (7, 10, 14, 18 and 22 mm) when the vibration frequency changed from 169 min-1 at А = 22 mm to 470 min-1 at А = 7 mm. The kinematic indicator of the screen ranged between 6.9 and 19.3 m/s2. The inclination angle of the screen shoe. The experiments were carried out on the same initial material at two inclination angles of the screen shoe of 6 and 90. The influence of the screen shoe angle inclination on the machine performance and cleaning quality of wheat seeds was determined at a constant value of the kinematic indicator of the screen ω2А = 12.9 m/s2. Only the area of the hopper's exhaust window changed in the range of 26.4...39.6 cm².

Results and discussion
The experimental results on the determination of the kinematic parameters of a screen shoe are presented in Table. 1 and Figure. 2.
An increase in the kinematic indicator of the screens is accompanied by more intensive clogging of their holes. For example, at a change of this indicator from 6.9 to 19.3 m/s2, clogging of a screen with oblong holes decreased from 0.34 to 0.02%, and that of a screen with round holes, -respectively from 0.019 to 0%. We should note that screens with round holes clogged about 10 times less frequently than those with oblong holes in all operation modes. The sifting performance of the screen with round holes was the best at kinematic parameters of screens equaling 11.0...13.0 m/sec2. Higher results were obtained at amplitudes of 7...10 mm and screen vibration frequencies of 410 and 340 min-1, respectively. Optimal values of the kinematic indicator of the screens with oblong holes, depending on the vibration amplitude, are within 11.0...15.0 m/sec2. The extraction coefficient of small impurities in this case ranges between 84.6 and 89.6%. The lower the amplitude is, the greater value of the kinematic indicator of a screen should be observed. Thus, for an amplitude A of 7 mm, the optimal value of the kinematic indicator of a screen ω²А equals 14.85 m/sec2, and at A of 22 mm, ω²А is equal to 11.0 m/sec2. It should be noted that for screens with oblong holes, optimal values of the kinematic indicator (except for data obtained at A = 22 mm) are more important than for screens with round holes. So, if at an amplitude A of 7 mm, the optimal value ω²А for screens with oblong holes is 14.85 m/sec2, and for screens with round holes ω²А = 12.9 m/sec2, but at an amplitude A of 14 mm, respectively, ω²А = 13.5 m/sec2 for screens with round holes.
At larger values of the kinematic indicator of screens (ω2A> 15.5 m/sec2), the grain mass is strongly loosened, the process of sifting sharply deteriorates, and the mass (up to 25.5%) overtails from the upper screen.
At small values of the kinematic indicator (ω2A < 8.0 m/sec2), the grain mass on the screen is not loosened, but it is poorly transported, and the quality of sifting sharply deteriorates.
The extraction coefficient of small impurities drops to 44.5%, and the capacity of the screen shoe cannot be raised above 300...325 kg/h, though in all experiments, the authors have tried to set the performance within 500±5% kg/h. At small values of the screen vibration amplitude (A = 3 mm), the grain separation process is not working at all tested vibration frequencies of a screen (up to 450 min-1).
On the basis of the conducted research it is possible to recommend the following kinematic modes for the separation of wheat seeds: that will be close to the optimal values for the performance of a screen shoe, equal to 500±5 kg/h, for both screens with round holes and those with oblong ones.
When installing screens with oblong holes only in the screen shoe, the following operating modes can be recommended: А = 7 mm, n = 440 min-1 (ω2А = 14.85 m/sec2); А = 10 mm, n = 360 min-1 (ω2А = 14.2 m/sec2); А = 14 mm, n = 300 min-1 (ω2А = 13.5 m/sec2); А = 18 mm, n = 240 min-1 (ω2А = 11.1 m/sec2); А = 22 mm, n = 214 min-1 (ω2А = 11.0 m/sec2). In these modes, the separation efficiency of small impurities is equal to 84.6...89.6% and clogging of screens does not exceed 0.15%. When installing screens with round holes only in a screen shoe, the following operating modes can be recommended: А = 7 mm, n = 410 min -1 (ω2А = 12.9 m/sec2); А = 10 mm, n = 340 min -1 (ω2А = 12.7 m/sec2); А = 14 mm, n = 254 м min-1 (ω2А = 9.9 m/sec2); А = 18 mm, n = 212 min -1 (ω2А = 8.9 m/sec2); А = 22 mm, n = 214 min-1 (ω2А = 11.0 m/sec2). These modes will ensure maximum sifting and clogging of the screens will not exceed 0.02%. The experimental results to determine the inclination angle of a screen shoe are shown in Tab. 2 and Fig. 3. At an increase in the hopper outlet window area, the machine performance grows as well at all inclination angles of the screens. And with higher inclination angles, the performance was greater by about 40 kg/h at all operating modes.  The coefficient of extraction of small impurities decreases as the performance grows by a dependence that is close to linear. So, at an inclination angle of the screen shoe ƛ=0°, the coefficient decreased from 89.12 to 75.2% at the performance rate, respectively equal to 360.1 and 691.9 kg/. At an inclination angle ƛ = 9°, the coefficient decreased from 81.91 to 73.51% with an increase in performance from 400.5 to 721.3 kg/h. It should be noted that at the performance rate above 600 kg/h, the values of the extraction coefficients of small impurities converge and range within 72.4...76.86%, i.e. the inclination angle of screens does not have almost any effect on this indicator. The clogging of the screens is almost the same for all performance values. At a large inclination angle of the screens (ƛ = 9°), the penetration rate of the upper screen (with round holes) is 3...10 times higher, and that of the lower screen (with rectangular holes) is about 2 times higher as compared with screen clogging at an inclination angle of 6°. The siftings from the upper screen at the performance rate of up to 450 kg/h was less (up to 1%) at an inclination angle of the screen shoe ƛ=9°, and the screen shoe performance rate of 593.2...674.5 kg/h, it increased up to 7.6...17.59% and became 2...3 times higher than the amount of siftings for the inclination angle of the screens of 6°. At 691.9...721.3 kg/h, the screen value remained the same.
The carried out research has allowed to determine the operating modes of a screen shoe, which are close to optimal for the separation of grain seeds. However, the developed screen seed cleaner should process the seed material of small-seed crops (grass, etc.), which is usually heavily clogged with various impurities. According to V. Lampeter, for their effective removal, the kinematic indicators of the screen should not be less than 14.4 m/sec2 and the inclination angle of the screen shoe should equal 7°.

Conclusion
In order for a screen seed cleaner to operate in modes that are close to optimal when processing small-seed and grain crops, the vibration amplitude of the screen shoe should be equal to 9 mm, and its vibration frequency should be adjustable in the range of 350...450 min-1 at an inclination angle of the screens of 7°. In this case, a screen seed cleaner can operate with an adjustable value of the kinematic indicator of screens in the range of 12.2...20.0 m/sec2, which will allow processing seed material of practically all crops with different content of impurities.