February 07, 2011

20 Geneva Mechanisms

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As we know that GENEVA Mechanisms are very important in Automated machine tools, today we look towards 20 different types of Geneva mechanisms. Geneva mechanisms are nothing but the Indexing mechanisms like Dividing head. We can adjust the Geneva mechanism so as to rotate or transfer work around the working stations within some fixed Intervals
1) Locking-arm Geneva drive: The driving follower on the rotating input crank of this Geneva enters a slot and rapidly indexes the output. In this version, the roller of the locking-arm (shown leaving the slot) enters the slot to prevent the Geneva from shifting when it is not indexing.
2) Planetary gear Geneva drive: The output link remains stationary while the input gear drives the planet gear with single tooth on the locking disk. The disk is part of the planet gear, and it meshes with the ring-gear Geneva to index the output link one position.

3) Four bar Geneva drive: A four-bar Geneva produces a long-dwell motion from an oscillating output. The rotation of the input wheel causes a driving roller to reciprocate in and out of the slot of the output link. The two disk surfaces keep the output in the position shown during the dwell period.
4) Twin Geneva drive: The driven member of the first Geneva acts as the driver for the second Geneva. This produces a wide variety of output motions including very long dwells between rapid indexes.
5) Groove cam Geneva drive: When a Geneva is driven by a roller rotating at a constant speed, it tends to have very high acceleration and deceleration characteristics. In this modification, the input link, which contains the driving roller, can move radially while being rotated by the groove cam. Thus, as the driving roller enters the Geneva slot, it moves radially inward. This action reduces the Geneva acceleration force.
6) Locking slide Geneva drive: One pin locks and unlocks the Geneva; the second pin rotates the Geneva during the unlocked phase.  In the position shown, the drive pin is about to enter the slot to index the Geneva.  Simultaneously, the locking pin is just clearing the slot.
7) Rapid transfer Geneva drive: The coupler point at the extension of the connecting link of the four-bar mechanism describes a curve with two approximately straight lines, 90° apart. This provides a favorable entry situation because there is no motion in the Geneva while the driving pin moves deeply into the slot. Then there is an extremely rapid index. A locking cam, which prevents the Geneva from shifting when it is not indexing, is connected to the input shaft through gears.
8) Dual-track Geneva drive: The key consideration in the design of Geneva's is to have the input roller enter and leave the Geneva slots tangentially (as the crank rapidly indexes the output). This is accomplished in the novel mechanism shown with two tracks. The roller enters one track, indexes the Geneva 90° (in a four-stage Geneva), and then automatically follows the exit slot to leave the Geneva. The associated linkage mechanism locks the Geneva when it is not indexing. In the position shown, the locking roller is just about to exit from the Geneva.

9) Long-dwell Geneva drive: This Geneva arrangement has a chain with an extended pin in combination with a standard Geneva. This permits a long dwell between each 90° shift in the position of the Geneva. The spacing between the sprockets determines the length of dwell. Some of the links have special extensions to lock the Geneva in place between stations.
10) Modified motion Geneva drive: The input link of a normal Geneva drive rotates at constant velocity, which restricts flexibility in design. That is, for given dimensions and number of stations, the dwell period is determined by the speed of the input shaft. Elliptical gears produce a varying crank rotation that permits either extending or reducing the dwell period.
11) Internal groove Geneva drive: This arrangement permits the roller to exit and enter the driving slots tangentially. In the position shown, the driving roller has just completed indexing the Geneva, and it is about to coast for 90° as it goes around the curve. (During this time, a separate locking device might be necessary to prevent an external torque from reversing the Geneva.)
12) Progressive oscillating drive: A crank attached to the planet gear can make point P describe the double loop curve illustrated. The slotted output crank oscillates briefly at the vertical positions.
13) Sinusoidal reciprocator drive: This reciprocator transforms rotary motion into a reciprocating motion in which the oscillating output member is in the same plane as the input shaft. The output member has two arms with rollers which contact the surface of the truncated sphere. The rotation of the sphere causes the output to oscillate.
14) Controlled output escapement: The output in this simple mechanism is prevented from turning in either direction unless it is actuated by the input motion. In operation, the drive lever indexes the output disk by bearing on the pin. The escapement is cammed out of the way during indexing because the slot in the input disk is positioned to permit the escapement tip to enter it. But as the lever leaves the pin, the input disk forces the escapement tip out of its slot and into the notch. That locks the output in both directions.

15) Parallel guidance drive: The input crank contains two planet gears. The center sun gear is fixed. By making the three gears equal in diameter and having gear 2 serve as an idler, any member fixed to gear  3 will remain parallel to its previous positions throughout the rotation of the input ring crank.
16) Rotating-cam reciprocator  drive: The high-volume 2500-ton press is designed to shape such parts as connecting rods, tractor track links, and wheel hubs. A simple automatic-feed mechanism makes it possible to produce 2400 forgings per hour.
17) An external Geneva drive. The driver grooves lock the driven wheel pins during dwell. During movement, the driver pin mates with the driven-wheel slot.
18) An internal Geneva drive. The driver and driven wheel rotate in same direction. The duration of dwell is more than 180º of driver rotation.
19) A spherical Geneva drive. The driver and driven wheel are on perpendicular shafts. The duration of dwell is exactly 180° of driver rotation.
20) An intermittent counter drive. One revolution of the driver advances the driven wheel 120°. The driven-wheel rear teeth are locked on the cam surface during dwell.
Look For Other Geneva Drives 
Advanced Modified Geneva Drives

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