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. In particular, the theoretical study was made on the effect due to sliding from the standpoint of the contact pressure and the sliding distance, and durability testing was carried out to check for the durability of the performance after predetermined operation time. After the durability testing, we have examined the force-displacement characteristics, the change in characteristics by thickness reduction agree with the theory. We will report the design method, since the design procedure of the smaller and lighter EPS gear size could be confirmed useful.
