X-Y-Z: Indexing Assembly Systems
What do an odometer, a film projector and a rotary indexing table have in common? All three rely on Geneva mechanisms to produce intermittent rotation in one shaft from the continuous rotation of another shaft.
A Geneva mechanism is not the only way to generate the stop-and-go motion of a rotary indexing table. The table can be indexed by a ratchet-and-pawl mechanism driven by a pneumatic cylinder. The table can be indexed with a globoidal cam driven by a motor. Or, the table can be connected directly to a servomotor that is simply programmed to start and stop at the desired intervals. However, compared with these machines, Geneva mechanisms are simpler, less expensive and more robust, says Frank Johnson, president of Tangen Drives (Clearwater, FL).
"You can get some backlash with direct-drive systems," says Johnson. His company's Geneva mechanisms have been used in assembly machines for a variety of products, including batteries and spark plugs.
A Geneva mechanism consists of two wheels: a driver and a follower. The driver is located below the follower. One to four pins or rollers are located along the outer edge of the driver. These pins engage radial slots cut into the follower at regular intervals. As the driver turns, the pin enters a slot and pushes the follower. When the pin leaves the slot, the follower stops.
Thus, for every full rotation of a one-pin driver-or every half rotation of a two-pin driver-the follower will index by the number of its slots divided by 360. If the driver has one pin and the follower has four slots, the follower will index 90 degrees each time the driver rotates 360 degrees.
From a practical standpoint, the follower can have a minimum of three slots and a maximum of 18. Most followers are made with four, five, six or eight slots, which correspond to index lengths of 90, 72, 60 and 45 degrees, respectively.
There are two main types of Geneva mechanism: external and internal. In an external Geneva mechanism, the driver is adjacent to the follower, with one wheel overlapping the other. The driver and the follower move in opposite directions, and the dwell period always exceeds the motion period. In an internal Geneva mechanism, the driver fits within the follower. The outer edge of the driver is almost tangential to the outer edge of the follower. The driver and the follower move in the same directions, and the motion period always exceeds the dwell period.
In both internal or external Geneva mechanisms, the driver and the follower are designed to mesh when a pin is not in a slot. This prevents the follower from rotating during dwell periods.
If the Geneva mechanism is designed correctly, the pin enters and leaves the slot in a direction tangential to the slot surface, so that impact forces are minimized. Nevertheless, one shortcoming of Geneva mechanisms is that the beginning and end of the indexing motion are marked by sharp acceleration and deceleration. However, as the ratio between the diameter of the follower and the diameter of the driver increases, the peak acceleration and velocity of the indexing motion decrease. In other words, the index motion gets gentler as the number of slots increases.
Another limitation of Geneva mechanisms is that they aren't flexible. Once the number of slots has been established, so too are the curves for acceleration, velocity and displacement. The ratio of dwell time to motion time is also set.
On the other hand, dwell time and motion time do not necessarily have to be "regular as clockwork." Engineers can modify the ratio of dwell time to motion time by staggering the driver pins or combining the Geneva mechanism with chain drives or gear trains.