Impressive Mechanisms, Impressive Explanations

Blog Post created by benz on Jun 30, 2017

  Engineers that exemplify creativity, and the ability to explain it

School is out and some are on holiday. It’s a good time to briefly widen this blog’s technology focus a bit with one of my occasional off-topic wanderings. This time we’ll look at impressive achievements of some engineers of yore, and a couple of enlightening explanations of their creations.

These days we combine our electrical skill with processors, software, and myriad actuator types to generate virtually any kind of complex mechanical action—wherever we need to connect electrons with the physical world. It’s easy to forget how sophisticated tasks were accomplished in the past, without computers or stepper motors, and how even advanced techniques such as perceptual coding were implemented with physical mechanisms.

All these elements were brought together for me recently in an impressive YouTube explanation by “engineerguy” Bill Hammack of the University of Illinois. In just four minutes, Bill explains several poorly understood aspects of film projectors that evolved in the century between their invention (c.1894) and their replacement by digital cinema technology (c.1999).

Bill uses slow-motion footage and animated diagrams to do a great job of explaining how a projector keeps the film going smoothly across the sound sensor while intermittently starting and stopping the film between the lamp and lens. This precisely executed start-stop motion, projecting the film image only when it isn’t moving, coaxes our vision system into seeing a series of stills as fluid motion.

Bill shows how the motion is produced using a synchronized cam, shuttle, and wobble plate. As I dug deeper, further research showed that some projectors instead use an equally innovative mechanism called a Geneva drive (or Geneva stop), a mechanism that was already old when the first crude projectors were created in the late 19th century. Seeing the shape of the Geneva mechanism sent me to my reproduction of the very old book Five Hundred & Seven Mechanical Movements.

Scanned image of Geneva mechanism or Geneva stop from 1889 book Five Hundred & Seven Mechanical Movements

This composite figure shows two examples of Geneva drives from the mechanisms in Henry T. Brown’s 1896 book Five Hundred & Seven Mechanical Movements. These convert continuous motion to intermittent motion with smooth starts and stops, and have built in limits or “stops.”

I figured I was nearly alone in my interest in in the old book, but that is not the case. Another quick search revealed that these manifold fruits of the Industrial Revolution have been brought into the internet age, with hyperlinks and animation at The animations are addictive!

The book is a potent antidote to the tendency to forget how clever and imaginative the engineers of the past actually were, though they were often self-taught and worked with limited materials. And, if we take Edison and the Wright Brothers as examples, they were tireless experimenters.

From an 1896 book to the joys of YouTube, there is cleverness in both the engineering and the explaining. If you’re looking for something closer to our RF home, check out Bill’s demonstration of performing Fourier analysis with a mechanical device. You may never think of FFTs in quite the same way again.