Design optimisation and experimental evaluation of a grain vibration screen
Mechanical screening is an important process which is used in a wide range of industries. This study focused on screens in the agricultural industry used for cleaning and classifying of grain. Mathematical models were developed and also implemented in computer programs used for design analysis of a vibration screen. This was regarded as necessary to investigate effective screening of maize in order to remove unwanted larger and smaller particles, and also to provide acceptable service life from a fatigue point of view. A three degree of freedom mathematical model was developed and used for prediction of static and dynamic displacements, static and dynamic forces, and also system natural frequencies. Another mathematical model was formulated and implemented in a computer program and used for fatigue analysis. All the input parameters required for the computer programs were characterised. Different mathematical models were also developed for characterisation of the screen rubber mount vertical and also horizontal stiffness and damping coefficients. Different measured data obtained from different test set-ups were used as input data for these programs respectively. Mount static stiffness coefficients were also experimentally determined. The required amplitude and frequency for a certain layer of maize was also characterised with electrodynamic Shaker tests, and two different feasible sieve apertures identified. Three different design goals for an optimisation approach were identified. Firstly, the criteria for vibration isolation an objective function based on the transmission of dynamic forces to the fixed foundation was formulated. Secondly, two different constraints that also influenced vibration isolation were also formulated. These constraints were regarded as necessary to ensure enough movement for effective sieving, but also to limit the horizontal and vertical mount displacements during transient conditions. Three-dimensional graphical representations and contour plots were constructed in a Matlab environment, and used to determine vertical and horizontal mount stiffness coefficients chosen as design variables, for an optimum design according to the criteria formulated. A Finite Element Analysis (FEA) approach was followed to investigate possible structural resonance of the elastic screen, and also to evaluate the structure’s vertical stiffness coefficient at the point of investigation. An FEA approach was also used to determine static and dynamic material stresses used for fatigue analysis to investigate the screen structure service life. The optimised design parameters were used to build and then test the vibration screen. Effective sieving was evaluated to remove the larger and smaller unwanted particles such as weed seeds, sand, small broken maize kernels, stalks, and maize plant stems as typically present, from harvested maize. Sufficient maize mass flow rates were also evaluated for different screen angles, and with two different sieves simultaneously used. The underlying three degree of freedom mathematical model for the vibration screen was experimentally validated. The predicted responses, dynamic forces, and also the system natural frequencies were compared to the corresponding measured values respectively. This was done for several operational conditions (transient and steady state), at an empty and fully loaded screen respectively. Transient conditions include start-up and shut-down of the screen. The grain vibration screen was designed to mainly sieve maize, but other grain such as sunflower, soybean, canola, groundnuts, wheat, barley, oats and sorghum could also be sieved.
- Engineering