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Synthesis of conjugate Geneva mechanisms with curved slots - naval - 10-06-2017 Synthesis of conjugate Geneva mechanisms with curved slots [attachment=16737] Introduction Indexing mechanisms find several applications in instruments, watches, projectors, ordinance machine tools, as well as in printing, packaging, and automatic machinery, transfer lines, etc. [1 3]. Mechanisms of this kind are ratchet gears, escapements, indexing cams, intermittent gears, star wheels and Geneva mechanism [4]. Dating back to the old Colt system, ratchet gears have been used in weaponry, while escapements find several applications in mechanical watches, electric meters, and odometers. Indexing cam mechanisms can be designed for high-indexing numbers; Conventional Geneva mechanism Referring to Fig. 2, a conventional Geneva mechanism is composed of a driving crank, which meshes through a circular pin or roller with a straight radial slot of the Geneva wheel. The number of slots n can be equal to four. The driving crank, with radius a, rotates about centre O1 counterclockwise (ccw) through an angle a, which is measured with respect to the X-axis of the fixed frame OX Y . Kinematic synthesis of conjugate Geneva mechanisms with curved slots Conjugate Geneva mechanisms give a better kinematic and dynamic performance with respect to that of conventional Geneva mechanisms, but a suitable kinematic synthesis must be formulated in order to ensure the required indexing motion program. In particular, a cycloidal motion program is adopted for designing a conjugate Geneva mechanism, even if this approach is very general. However, using the cycloidal motion of Eq. (5), the first consequence is that slots are not straight and radial as those of Fig. 2, since a different indexing motion must be given, that requires curved slots. The centerline and profiles of the curved slot can be obtained by using the methods of cam design, where the cam is fixed and the line-of-centers rotates upon kinematic inversion. Pure-rollingcam-equivale nt The proposed algorithm for the kinematic synthesis of conjugate Geneva mechanisms with curved slots gives an improved performance with respect to the conventional mechanism of Fig. 2. However, the same indexing motion with cycloidal motion program can be obtained by indexing cams and in particular by cams with pure-rolling contact, as proposed in [7,8]. |