Dynamic excitation from the gears generates vibration modes in the gearbox which causes radiation of unwanted structure-borne noise. To reduce the noise as well as the vibration, the stiffened plate construction is frequently used for the housing, where the rib stiffener layout is the key to this design. In this paper, the most effective position of stiffeners in order to reduce the vibration and noise radiation is searched and discussed. The analysis considers first a plate fixed at edges and excited at its center, and then a simplified rectangular gearbox excited at the shaft bearing locations. The housing of a simplified single-stage gearbox is modeled by triangular finite shell elements. It is excited by a set of harmonic forces which are applied at the bearing positions. The rib stiffener is modeled by beam elements, and its optimum layout is searched by a genetic algorithm. The vibration energy is adopted as the objective function to be minimized for the design for low vibration. On the other hand, the radiated sound power is the objective function for low noise. Although the noise is caused by the vibration, the optimum stiffener layout to realize low noise is not always identical to the layout that yields low vibration because of the difference in radiation efficiency. This is numerically clarified first from the viewpoint of the vibration mode of a plate. In the case of the design for the gearbox housing concerned, the vibration mode which causes the difference in radiation efficiency is not presented. Consequently, the optimum stiffener is placed along the line from the point of excitation to a fixed point, which reduces the vibration deflection of the faces with bearings as well as the in-plane displacement of the bearings.
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