Abstract — There are many applications where non-uniform rotation mechanisms are required. Noncircular gears can be used for this purpose, having some advantages when compared with linkages. This paper presents an analysis and comparison of the driving torque fluctuations and the forces transmitted to the frame by a linkage and a noncircular gear mechanism with the same displacement law and operating conditions. In addition, to define the displacement law, and hence the pitch curves of the gears, the use of Bezier curves is suggested and an optimisation procedure based on genetic algorithms is developed. The results demonstrate that the forces exerted by noncircular gears on the frame tend to be lower than those by linkages. Moreover, the variability of these forces is much lower. Therefore, noncircular gears tend to produce less vibration and fatigue in the system. 1234 Keywords: Noncircular gears, driving torque fluctuation, frame transmitted force I. Introduction In many applications, it is necessary to have mechanisms with variable transmission ratio. This can be achieved by different mechanisms, including noncircular gears, which offer certain advantages. For example, noncircular gears are more compact and easily balanced than linkages [1]. Noncircular gears enable to obtain any variable displacement law, which is the ratio between the angles of rotation of a gear pair, provided that the periodicity, continuity, and curvature conditions are satisfied by the pitch curves. As there is much freedom to select the pitch curves, as occurs with cams, a continuous transmission law can be defined without being limited by a few precision points, as in the case of linkages. Research on noncircular gears has been very limited; a review has been presented in previous works [2,3]. The design of a mechanism cannot be completed without focusing on the interface between the mechanism and its mounting frame and on the driving torque. As a linkage moves, it transmits forces to its surroundings, and these forces result in vibration, noise, and wear and cause fatigue problems. When the mechanism is completely