| Variable lead screw drive is proposed as a new drive way with the development of the mechanical manufacturing industry. There are kinds of variable lead mechanics, such as cylindrical cam, variable lead worm and other institutions. For cylindrical cam, the transmission part takes circular motion in the cam groove. For variable lead worm, the worm can only moves a short distance relative to the turbine changes. This article analyses the variable transmission mechanism structural, and proposes the variable lead screw drive mechanism model. According to segmentation theory of the TRIZ technology innovation theory, the variable lead screw drive mechanism derives from the ball screw and cylindrical groove cam by abstracting and combining the rotation and lead property together. After the institutional innovation, the lead screw drive mechanism has more characteristics than traditional transmission mechanism, such as self-adaptivity, self-locking, to variable lead characteristic and hydrodynamic lubrication, which are significant in the actual applications. Considering of the parameters and characteristics of transmission, the ideal model of the screw drive mechanism is built, and the ideal degrees of sliding screw, scroll screw and variable lead screw are calculated respectively, which quantifies the comparison method of structure innovation.The mathematical model is built for the spiral groove surface of variable-lead variable-radius screw and roller contour surface. The meshing characteristics is analyzed, and then the gap function is established. Under the condition of constant radius, the geometric characteristics of variable lead helix are researched in cylindrical surface to obtain the expression of the curvature and torsion. By arc transition and polynomial transition, the helix of the cylindrical surface is optimized. In accordance with the extremum condition, the transition curve functional is established, which is solved by variational method.The kinematics modeling of variable lead screw drive mechanics is established by using the coordinate transformation theory and spiral theory. The relative sliding velocity of both the roller and the helical groove is calculated during the transmission and the conditions for pure rolling are then certified. Through multi-objective optimization and fuzzy theory, the generalized roller motion parameters are designed and the transmission characteristics of optimized roller are compared with the result before optimization.According to the Hertz elastic contact theory, the simplified model of variable lead screw pair rolling contact is proposed and the three-dimensional rolling contact stress distribution is calculated. Considering of the actual situation and the processing conditions, the geometrical conditions of the point contact and line contact between the roller and the helical groove is given, and the point contact is the contact type of the screw pair. According to the relationship of the minimum film thickness, roughness and the elliptical contact region, all the contact points of the state of lubrication is judged. Maple mathematical software is used to simulate the minimum film thickness in the contact process, and the minimum film thickness in different variable lead along the screw axis displacement is obtained.The friction and wear mechanisms of variable lead screw pair are researched. The specific conditions of contact between the roller and the spiral groove are analyzed, and the load spectrum is prepared by rain flow counting method. Then HyperMesh pre-processing software and FE-Fatigue software is used to analyze the fatigue life for variable lead screw pair. Finally, the fatigue life affected by the variable lead function is researched. |
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