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This paper discusses the size reduction and performance enhancement for worm gears of a pinion assist type EPS system. In conjunction with recent trends in vehicles getting larger and tires gaining larger footprints, the EPS motor assist torque has increased considerably. This leads to the issue involving enlargement of the external diameter of the worm gear and the worm wheel gear, which are reduction gears that boost motor torque. In order to address this issue, the authors examined single-threaded worm gears, and fundamental factors that cause the enlargement of the worm gear external diameter from the standpoint of the working of the gear. After the details are explained, a new worm gear that addressed this issue is proposed. This paper describes this worm gear, tooth form shape, and its construction. Moreover, this worm gear was built, installed into an EPS system, and tested.

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.

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.