Narrow Results

Aiming at the high-speed friction stir welding deformation and thickness reduction of 1 mm 6061-T6 aluminum alloy sheet, the characteristics of the axial pressure of the welding process and its influence on the joint structure and mechanical properties were analyzed in this paper. Experiments were performed to optimize process parameters. A fuzzy controller with axial pressure error and its change rate as input and welding speed as output had been designed to realize axial pressure control during friction stir welding. The welding axial pressure fluctuates slightly around the target value, and the system was stable with the controller. Besides, the microstructure of the corresponding weld nugget area was uniform, dense, and the transition area was uniform without obvious welding defects. The joint hardness distribution was stable with the classic “W” shape distribution. The mechanical properties of the joint were excellent, and the tensile strength reaches 195 MPa, which was about 70% of the base material (290 MPa).

In the case of high pressure, the size of the multistage pump axial force sometimes reaches hundreds of tons, the seriousness of the problem generated by excessive axial force has surpassed the efficiency, wear and other factors, and has become a decisive factor in the stable operation of multistage pumps. In this paper, the 11-stage double-case multistage pump is taken as the research object. On the basis of studying the clearance internal flow mechanism, according to the pressure difference equation of axial motion of clearance fluid, the main factors affecting the balance force of the balance drum are analyzed, and a new type of balance drum — “double helical balance drum” is proposed, which is compared with the smooth balance drum in the hydraulic performance and axial force performance. The research shows that with the increase of the axial displacement of the balance clearance, the pressure drop between the clearances decreases linearly. Compared with the smooth balance drum, the pressure drop at the inlet and outlet of the helical balance drum is 1.30 times that of the smooth balance drum under the design flow rate condition, the mean velocity, mean velocity curl and velocity coefficient of the double helical balance drum increase, indicating that adding double helix can increase the dynamic pressure ratio of clearance fluid. In addition, the test results show that the head and efficiency of the double helical balance drum of multistage pump is increased by 0.76% and 1.1%, respectively, compared with the smooth balance drum, and the vibration amplitude of the front and rear bearing is reduced by 3.36%, 0.76% and 1.75% on average along the axial, radial and tangential direction. In addition, the temperature of the front and rear bearing is reduced by 3.1% and 8.7%, respectively, indicating that the double helical balance drum can effectively improve the hydraulic performance and axial force performance of the multistage pump at design operating point. The results of the study can provide reference for the long-cycle stable operation of multistage pumps and the optimization of balance drum design.

Beam string structures (BSS) are frequently employed in large-span spatial structures due to their high load-carrying efficiency and elegant appearance. Owing to the neglect of the influence of prestress, the natural frequency of the BSS is often overestimated which could be crucial to the dynamic behavior of BSS. This paper presents an exact matrix stiffness analysis method (MSM) for investigating the relationship between prestress and the natural frequency of the planar BSS. A novel dynamic stiffness matrix of beam–columns considering the natural frequency and axial force is used to develop the MSM. The dynamic analysis of the structural vibration stability of BSS is performed by using the global structural stiffness matrix which is assembled from the element stiffness matrices in the global coordinate system. The proposed MSM with the exact dynamic element stiffness matrix for the dynamic analysis of the BSS is verified by comparing with previous results based on the finite element method. The illustrative examples demonstrate that the prestress in the cable has a negative effect on the natural frequency of the BSS and should be considered in the dynamic analysis. As the axial force increases from zero to the buckling load $P_{cr}$, the natural frequency of the BSS decreases from the maximum vibration frequency to zero. The influence of prestress on the vibration frequencies of BSS is particularly significant when the prestress to balance the loads is large, especially in the case of large dead and live loads (e.g. floor slabs).

The transverse thermal fluctuations of the myosin molecule are significant. This paper explores the contribution of lateral myosin bending to the developed crossbridge force and power stroke. The equipartition theorem is used to calculate the mode amplitudes for myosin bending. Crossbridge axial force $F_x$ and power stroke $\mathrm{\Delta }x$ are developed by transverse in-plane fluctuations along the $y$- and $z$-axes. Practical applications include the effects of temperature on the flexibility of the myosin molecule stiffness and tension, relevant to man-made fabrication of synthetic muscle using micromachines and nanowires. Scaling laws for the $S2$ bending amplitude depend on filament length, mode number, and stiffness, as $n^{-2},L^2$, and (EI)${}^{-1}$. This paper quantifies the effects of thermal motion on the mechanics of miniature molecular motors, including the muscle crossbridge.