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In the automobile industries, weight reduction has been investigated to improve fuel efficiency together with reduction of CO2 emission. In such circumstance, it becomes necessity to make an electric power steering (EPS) more compact and lightweight. In this study, we aimed to have a smaller and lighter EPS gear size by focusing on an impact load caused at steering end. In order to increase the shock absorption energy without increase of stopper bush size, we propose new theory of impact energy absorption by not only spring function but also friction, and a new stopper bush was designed on the basis of the theory. The profile of the new stopper bush is cylinder form with wedge-shaped grooves, and when the new stopper bush is compressed by the end of rack and the gear housing at steering end, it enables to expand the external diameter and produce friction. In this study, we considered the durability in the proposed profile.

This paper will discuss the stress reduction of the worm wheel for an electric power steering (EPS) system. The research discussed in this paper focused on the worm wheel, the EPS component that determines the maximum diameter of the system. If the stress of the worm wheel could be reduced without increasing in size, it would be possible to reduce the size of the worm wheel and EPS system. In order to reduce the stress of the worm wheel, the conventional design method has extended the line-of-action toward outside of the worm wheel to increase the contact ratio of the gears and these method lead to an increase in the outer diameter. In order to address this issue, past research proposes the basic concept to extend line-of-action toward the inside of the worm wheel. And this new meshing theory was named MUB (Meshing Under Base-circle) theory. In this paper, characteristics of meshing of the gear formed by MUB theory are determined in more detail.

This project studied a DC brushless motor for an EPS system that would not cause control performance to decline even when the vehicle was operated with a low steering angle at high vehicle speeds. First, a target for cogging torque was established to ensure that control performance did not decline at high vehicle speed. To set the target, a lane changing simulation using a human-vehicle system model was conducted. A brushless motor structure was designed to achieve this target value, and the magnitude of cogging torque for this design was predicted using FEM analysis. The issues were found when a prototype of this motor was constructed and the knowledge was obtained during the process of analysis were discussed, in addition the design guidelines were proposed to solve these issues. A new prototype brushless motor was constructed on the basis of these design guidelines, and tests showed it to possess suitable cogging torque for an EPS system.