Ever wondered how to convert a continuous rotatory motion to an intermittent one? Well, the Geneva drive, or commonly called the Maltese Cross, is all that you need.
As you can see, for one complete revolution of the driving wheel (left one), the driven wheel (right one) rotates only one-sixth of a revolution, thus making the output intermittent. The rotation frequency of the driven wheel can be changed by changing the number of slots in it.
Even though the official patent of this mechanism credits Thomson Albert R as its inventor in 1940s, uses of this mechanism date back to 1890s in film projectors.
It has a variety of applications:
Interestingly, its usage in watches have attributed the name Geneva, because Switzerland was an important center of watchmaking then. Initially, this was used as a stop work in vintage mechanical watches which use a wound up spring for the watch’s movement. It functioned as a stop work by limiting the torque exerted by the mainspring and essentially provided watches a stable run.
Did you wonder how the date was changed on mechanical watches before the arrival of digital ones? Simple. A variant of Geneva mechanism was used in combination with some gears which changed date once a day!
Interior of “Prim Sport II, calibre 68, vintage diving watch”, the Geneva mechanism (both driven and driving wheel) can be spotted.
The most famous “Iron Ring Clock” made by students of McMaster University, Canada, is a giant clock which uses a Geneva mechanism for the motion of its hour ring i.e. to increment the hour ring once per hour.
Old Movie Projectors
Remember the old projectors, where a wheel containing the film rotates in front of a light source to project movie on screen? As we know, an image can persist in human eye up to one-twenty-fifth of a second and this persistence of vision is taken advantage by the motion pictures by projecting and changing discrete images within a specific time to trick the brain into actually watching a continuous scene. Generally, a theatrical film runs at 24 frames per second.
This is where Geneva Mechanism steps in! The intermittent output of a Geneva drive upon supplying constant rotatory input, will be used to accurately drive the film at the same time rate which is needed. And because of the wheel’s motion in tandem with the movement of the shutter (which blocks the light source at very small time intervals), the motion of the film cannot be seen by the audience.
Modern film projectors, however, are now using stepper motors instead to achieve the required intermittent motion.
Today, this classic mechanism is used in indexing of milling machines with automatic tool changers, changing of tolls in CNC machines, currency note counting machines, indexing in assembly lines, and more.