In traditional assembly dimensional chain theory, actual geometrical features are obtained by translating or rotating from the nominal geometrical features. Thus, the deviation stack-up function is irrelevant with the machining stroke. In order to solve this problem, a model describing the variation in the dimensional chain of stroke-related mechanical assemblies is proposed. The geometrical features of assembly parts are discretized as a set of coordinate points, and the traditional geometrical dimensional chain is equivalently replaced by several linear dimensional chains. For every linear dimensional chain, the assembly datum of two parts is deformed due to gravity or thermal effect and so on, resulting in the non-closure of dimensional chain. In this article, the closure of the dimensional chain is guaranteed by replacing the assembly datum by the plane determined by three highest points. Finally, an experiment was carried out to verify the necessity of constructing the variable dimensional chain and its accuracy. Combining this new model with Monte Carlo simulation method, the influence of gravity on tolerance analysis was analyzed, and the results show that the variation in the dimensional chain induced by working conditions cannot be ignored in the tolerance design stage.
Soil enzymes are involved in the process of mineralization of soil organic matters. The close-to-nature transformation (CNT) of plantations changes the soil enzyme activities by changing the composition of stand vegetation, which in turn affects the change process of soil organic carbon. We therefore selected two typical coniferous plantations in southwest China, Pinus massoniana and Cunninghamia lanceolate, to explore the effects of CNT on soil enzyme activities and soil organic carbon, and the relationship between them is explored through comparative study. Compared with control stands (CCK and PCK), CNT enhanced soil organic carbon; the content of water-soluble organic carbon in the 0–10 cm soil layer of the transformed C. lanceolata plantations (CCN) is 81.29% higher than those in the control stands (p < 0.05); the contents of particulate organic carbon and water-soluble organic carbon in the 10–30 cm soil layer are 95.42% and 48.68% higher than those in the control stands (p < 0.05), respectively; after the CNT, the protease, urease, and acid phosphatase in C. lanceolata plantations were higher than control stands, while protease and catalase in P. massoniana plantations were higher than control stands. Correlation analysis showed that catalase and protease were more closely related to organic carbon components than other organic enzymes; redundancy analysis (RDA) results show that pH and total nitrogen are key factors that cause changes in carbon fractions after the CNT. In general, CNT enhanced soil organic carbon in coniferous plantations, which was more conducive to soil organic carbon accumulation but had a negative effect on soil organic carbon stability to a certain extent. Therefore, the effect of tree species configuration on soil carbon stability components should be considered in the forest management practice.
A geometric model of rotors is established on the basis of the structure of vertical shaft impact crusher. The dynamic simulation of the rotor is carried out by using the discrete element software EDEM. At the same time, the correlation between the angle of the guide plate and the crushing performance of the crusher is studied systematically. The optimal angle of the guide plate is obtained by theoretical analysis of the particle motion. The simulation method is used to analyze the velocity of particles, so the influence law of the angle of the guide plate on the acceleration effect of particles is obtained. The results of this paper show that the peak value of the maximum velocity of particles is the highest when the angle of the guide plate is
35
°
, which is consistent with the theoretical results. At this time, the average velocity of particle population is the largest and the proportion of particles with high velocity is the highest. It means that the particles obtain the largest crushing kinetic energy and the best crushing performance.
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