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February 08, 2011

6 Modified Geneva Drives


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The mechanisms shown here add a varying velocity component to conventional Geneva motion.
1) With a conventional external geneva drive, a constant-velocity input produces an output consisting of a varying velocity period plus a dwell. The motion period of the modified geneva shown has a constant-velocity interval which can be varied within limits.When spring-loaded driving roller a enters the fixed cam b, the output-shaft velocity is zero. As the roller travels along the cam path, the output velocity rises to some constant value, which is less than the maximum output of an unmodified geneva with the same number of slots. The duration of constant-velocity output is arbitrary within limits.When the roller leaves the cam, the output velocity is zero. Then the output shaft dwells until the roller re-enters the cam. The spring produces a variable radial distance of the driving roller from the input shaft, which accounts for the described motions. The locus of the roller’s path during the constant-velocity output is based on the velocity-ratio desired.
2) This design incorporates a planet gear in the drive mechanism. The motion period of the output shaft is decreased, and the maximum angular velocity is increased over that of an unmodified Geneva with the same number of slots. Crank wheel a drives the unit composed of planet gear b and driving roller c. The axis of the driving roller coincides with a point on the pitch circle of the planet gear. Because the planet gear rolls around the fixed sun gear d, the axis of roller c describes a cardioid e. To prevent the roller from interfering with the locking disk f, the clearance arc g must be larger than is required for unmodified genevas.
3) A motion curve similar to that of in 2 can be derived by driving a geneva wheel with a two-crank linkage. Input crank a drives crank b through link c. The variable angular velocity of driving roller d, mounted on b, depends on the center distance L, and on the radii M and N of the crank arms. This velocity is about equivalent to what would be produced if the input shaft were driven by elliptical gears.
4) The duration of the dwell periods is changed by arranging the driving rollers unsymmetrically around the input shaft. This does not affect the duration of the motion periods. If unequal motion periods and unequal dwell periods are desired, the roller crank-arms must be unequal in length and the star must be suitably modified. This mechanism is called an irregular Geneva drive.
5) In this intermittent drive, the two rollers drive the output shaft and lock it during dwell periods. For each revolution of the input shaft, the output shaft has two motion periods. The output displacement Φ is determined by the number of teeth. The driving angle, Ψ , can be chosen within limits. Gear a is driven intermittently by two driving rollers mounted on input wheel b, which is bearing-mounted on frame c. During the dwell period the rollers circle around the top of a tooth. During the motion period, a roller’s path d, relative to the driven gear, is a straight line inclined towards the output shaft. The tooth profile is a curve parallel to path d. The top land of a tooth becomes the arc of a circle of radius R, and the arc approximates part of the path of a roller.
6) An intermittent drive with a cylindrical lock. Shortly before and after the engagement of two teeth with driving pin d at the end of the dwell period, the inner cylinder f is unable to cause positive locking of the driven gear. Consequently, a concentric auxiliary cylinder e is added. Only two segments are necessary to obtain positive locking. Their length is determined by the circular pitch of the driven gear.
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