Papers by Author: Yong Yun Nam

Abstract: Most wind turbine gearboxes and pitch/yaw reducers consist of several planetary geartrains. Planetary geartrains allow gearboxes to be small, light, low noise, and efficient. The most important factors in the planetary geartrain design are load sharing among the planet gears and load distribution on the gear tooth flank. This study was conducted to analyze the load sharing and distribution in reducers with and without a slewing bearing. Load sharing and distribution of a pitch reducer without a slewing bearing were very uniform and even. However, in the other case, they were non-uniform and uneven, and the gear meshes were formed on the right or left. These results show that heavy loads act on the right or left flank of a meshed gear pair and reduce the gear life. Therefore, the slewing bearing must be considered when pitch/yaw reducers are designed and analyzed.

Abstract: Mechanical post treatments for welded structures have been applied in various industrial fields and, in most cases, have been found to cause substantial increase in their fatigue strength. These methods, generally, consist of the modification of weld toe geometry and the introduction of compressive residual stresses. In hammer peening, the weld profile is modified due to removal or reduction of minute crack-like flaws; compressive residual stresses are also induced by repeated hammering of the weld toe region with blunt-nosed chisel. In this study, a hammer peening procedure, using commercial pneumatic chipping hammer, was developed; a quantitative measure of fatigue strength improvement was performed. The fatigue life of hammer-peened specimen was prolonged by approximately 10 times in
S=240MPa, and was doubled for the as-welded specimen.

Abstract: The hot spot stress or the notch strain alleviates the welding detail dependency of S-N curve to some extent. This paper suggests a new stress model which alleviates the dependency further, thus the fatigue strengths of several welding details of same material can be evaluated with the S-N curve of the base material. A stress at the hot spot of a weld joint is decomposed into two components; linear rising one, and rapid rising one which is inversely proportional to the distance from the hot spot. For the stress decomposition, a formula is proposed with which the configuration of stress distribution near a hot spot is fitted exactly. The new stress model makes use of a geometric characteristic of the stress distribution curve by the formula. The stress model is applied to five different weld joints. As the result, the experimental fatigue data are plotted very closely to the S-N curve of the base material