Design of Constant-Velocity Transmission Devices Using Parallel Kinematics Principle

This paper presents a new type of constant-velocity transmission devices based on parallel mechanisms with properties of equal-diameter spherical pure rolling. The method we used is essentially an extension of the planar ellipse gear to the spherical one. Both the fixed and moving axodes of a specified parallel mechanism are obtained, as traced by the spatial instant screw axis (ISA) with respect to the fixed and moving coordinate systems. Based on Poinsot’s theorem and achievements, a series of these parallel mechanisms which satisfy constant-velocity condition have been disclosed correspondingly. Their motion range and transmission performances are also explored by taking the 3-4R mechanism as an instance. As the main part of this paper, two important applications for this type of constant-velocity transmission devices are also explored. One is used as a gearless spherical gear, and the other is used as a constant-velocity universal joint (CVJ). Simulations were fulfilled on ADAMS to verify the transmission performance in terms of different applications.</jats:p

Frictional shear-induced nanolamellar oxidation and transformation to oxide nanoparticles during pearlitic steel sliding

The in situ formation of a tribolayer composed of oxide nanoparticles on the wear surface is an important method for suppressing wear in metals with lamellar structures. However, the complex interaction between microstructure evolution and oxidation in a tribolayer obscures the formation mechanisms of nanolamellar oxidation and transformation. In this paper, samples of tribolayers at different oxidation stages were prepared by a focused ion beam (FIB) lift-out method. The dynamic evolution process and distribution of oxides in tribolayers during the dry sliding process were analyzed by characterization and molecular dynamics simulation. The nanolamellar oxidation processes and the effects of oxide nanoparticles on self-lubricating behavior were investigated. The results showed that oxygen diffused into the nanolamellae, resulting in continuous oxidation and the formation of an oxide/metal interface. The nanolamellar oxides were broken and refined in the further friction process and eventually transformed into oxide nanoparticles. The oxidation process was influenced by frictional shear, and the growth of oxides proceeded along the direction parallel to the nanolamellae with high-density dislocations. The above results provide a theoretical basis for the next step to improve the wear resistance and corrosion inhibition of metals by designing lamellar structures

Effect of Traverse Speed on the Defect Characteristic, Microstructure, and Mechanical Property of Friction Stir Welded T-Joints of Dissimilar Mg/Al Alloy

The AZ31 B/2024-T4 T-lap-joint was successfully fabricated by friction stir welding (FSW) with different welding parameters. The defect characteristics and metallurgical structure were observed and analyzed using optical microscope (OM) and scanning electron microscopy (SEM). Besides, the effects of defects and welding parameters on mechanical properties were investigated. The results show that an effective metallurgical reaction zone can be formed between Mg and Al (Mg-Al MRZ) and the island structures and lamellar structures appeared in the Mg-Al MRZ. The T-joints without tunnel defects can be obtained and the excellent mechanical properties of the T-joint were achieved using the welding speed of 50 mm/min. The tensile strength along the skin and the stringer was mainly affected by the kiss bonding defects

Influence of spatial laser beam profiles on thermal-fluid transport during laser-based directed energy deposition

The effects of the two types of typical spatial laser beam profiles (SLBP), including the super-Gaussian profile (SGP) and Gaussian profile (GP), on the thermal-fluid transport during laser-based directed energy deposition (L-DED) were numerically investigated. Considering the laser-powder-pool coupling effect, a comprehensive heat transport model including an input source model, laser-powder coupling model and molten pool thermal-fluid transport model is established. It is found that the SLBP has an important influence on the temperature gradient at the solidification interface and the flow field. Compared with the GP, the temperature gradient at the solidification interface under the SGP is high. Because the Marangoni stress and fluid momentum are in opposite directions, the fluid velocity distribution on the molten pool surface presents an ‘M’ shape. The fluid velocity on the molten pool surface under the SGP is obviously different from that under the GP

Contrasting Regulators of the Onset and End of the Seed Release Phenology of a Temperate Desert Shrub <i>Nitraria tangutorum</i>

Seed release is crucial in the reproductive cycle of many desert plant species, but their responses to precipitation changes are still unclear. To clarify the response patterns, we conducted a long-term in situ water addition experiment with five treatments, including natural precipitation (control) plus an extra 25%, 50%, 75%, and 100% of the local mean annual precipitation (145 mm), in a temperate desert in northwestern China. Both the onset and end of the seed release phenophase of the locally dominant shrub, Nitraria tangutorum, were observed from 2012 to 2018. The results showed that both the onset and end time of seed release, especially the end time, were significantly affected by water addition treatment. On average, the end time of seed release was advanced by 3.9 d, 7.3 d, 10.8 d, and 3.8 d under +25%, +50%, +75%, and +100% water addition treatments, respectively, over the seven-year study, compared with the control. The changes in the onset time were relatively small (only several hours), and the duration of seed release was shortened by 4.0 d, 7.5 d, 10.8 d, and 2.0 d under +25%, +50%, +75%, and +100% water addition treatments, respectively. The onset and end time of seed release varied greatly between the years. Preceding fruit ripening and summer temperature jointly regulated the inter-annual variation of the onset time of seed release, while the cumulative summer precipitation played a key role in driving the inter-annual variation of the end time. The annual mean temperature controlled the inter-annual variation of the seed release duration, and these interactions were all non-linear

Preceding Phenological Events Rather than Climate Drive the Variations in Fruiting Phenology in the Desert Shrub Nitraria tangutorum

Fruit setting and ripening are crucial in the reproductive cycle of many desert plant species, but their response to precipitation changes is still unclear. To clarify the response patterns, a long-term in situ water addition experiment with five treatments, namely natural precipitation (control) plus an extra 25%, 50%, 75%, and 100% of the local mean annual precipitation (145 mm), was conducted in a temperate desert in northwestern China. A whole series of fruiting events including the onset, peak, and end of fruit setting and the onset, peak, and end of fruit ripening of a locally dominant shrub, Nitraria tangutorum, were observed from 2012 to 2018. The results show that (1) water addition treatments had no significant effects on all six fruiting events in almost all years, and the occurrence time of almost all fruiting events remained relatively stable compared with leaf phenology and flowering phenology after the water addition treatments; (2) the occurrence times of all fruiting events were not correlated to the amounts of water added in the treatments; (3) there are significant inter-annual variations in each fruiting event. However, neither temperature nor precipitation play key roles, but the preceding flowering events drive their inter-annual variation