Synthesis of conjugate Geneva mechanisms with curved slots

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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].