Transfer rule of compression and springback stress in compression process of alfalfa

Abstract: This study attempted to investigate the transfer rule of the axial stress and springback models at the different locations of the compression chamber in the compression process of alfalfas, in which the alfalfa was compressed from the loose state into a grass piece. By taking dried high-quality alfalfa as the experimental material and 4 kg as the feeding capacity, with 5 different cross-section sizes (namely 360 mm×460 mm, 385 mm×460 mm, 410 mm×460 mm, 460 mm×460 mm and 510 mm×460 mm) of the compression chamber, the compression experiments were conducted under the conditions of a given compression chamber length, and initial density and moisture content of alfalfa. In order to measure the axial stress, including the compression and springback stress, the 10 moveable pressure sensors were used for the cross-section size of the compression chamber, and 2 pressure sensors and 3 displacement sensors were installed in each section. The compression and springback stress, the location of the grass piece in the compression chamber, the amount of compression, and springback value were obtained and stored in an Excel spreadsheet table by the data acquisition system based on virtual instrument technology, and the online curves of the axial force, the compression displacement, and the thickness of the grass pieces were displayed. All data was imported into the software Matlab and the axial stress curves versus the location of the grass piece in the compression of alfalfa were given. Then curve fitting of the maximum stress and the minimum springback stress were done respectively, the transfer rule of the compression and springback stress and the corresponding mathematic model were obtained. The results showed that the squared multiple correlation coefficient was greater than 0.9091, indicating that the compression and springback stress were closely related to the location of the grass piece and that the model regression effect was better. The compression and springback stresses had the similar transfer rules for different cross-section sizes of the compression chamber, and the cross-section size of the compression chamber had effects on the maximum compression stress and maximum springback stress of alfalfa. The compression stress required from loose alfalfa into the grass piece was highest, which was a fundamental parameter in designing the baler. The compression stress of 4-5 the grass piece was higher in the front of the compression chamber but lower in the rear parts of the compression chamber. It suggested that: 1) the suitable length for the compression chamber was 900-1 000 mm; and 2) the reinforcement structures should be added in the front part of the compression chamber to satisfy the strength and stiffness and to decrease baler weight. The study revealed that the envelop line of minimum spingback stress increased with increasing compression displacement, and the trend was opposite when it reached a certain location. Eventually, the difference between the axial compression and springback force of the grass piece was decreased and then became stable. It indicated that the damping plate should be set at the front part and the rear of the compression chamber, which would lead to the maximum springback force occurring in the compression process. As such, the baling efficiency and qualified products were improved. The results here provide valuable information for parameters optimization in the compression process and power choice.