The word of the day is “whiffletree.”
Heck, that’s the word of the month, as far as I’m concerned.
A “whiffletree” is a mechanical digital-to-analog converter. Brilliant science-and-technology documentarian Bill Hammack, professor of Chemical and Biochemical Engineering at the University of Illinois, has produced this fascinating video anatomy of IBM’s classic Selectric typewriter, in which a 7-bit whiffletree is employed to convert keypresses (digital) to precisely coordinated tugs (analog) on the control cables that rotate and tilt the type ball. Doubly awesome is the fact that the video features an appendix (yes, a video appendix) which focuses exclusively on the whiffletree itself, closely illustrating its operation with a simple 2-bit case.
You may remember Bill, aka Engineer Guy, from the office copier anatomy video we hit last July. His videos should be held up as models of how to present complex technical information visually. [Thanks, Bill!]
12 thoughts on “The Mechanical Glory of the IBM Selectric Typewriter”
A whiffletree is the crossbar to which a horse’s harness traces are fixed to even out the pull its gait imposes on the wagon it’s drawing. It’s pivoted in the middle, but that’s about as far as the analogy goes: the IBM is a switching system, working in directly the opposite sense by producing a distinction from the forces applied to its ends: the equine use is aimed at minimising the difference.
I only see 3 bits on the rotation whiffletree, so how does that get the 22 positions he says the print head needs? 2^3 = 8.
Although the tilt whiffletree looks like it’s attached to the rotation one by that spring, so maybe you really have 5 bits somehow? 2^5 = 32 and would be enough.
I’d like to know what logic they used to determine what characters go in which positions on the ball. There must be some optimisation of that to reduce the movement required based on the frequency of use of the letters? Or is it determined by the nature of all those mechanical linkages required and designed to make them simpler?
Whiffletrees are used (with the same name) in aerospace for full-scale structural testing. It allows a single instrumented actuator to evenly apply load to a wing, for instance, to mimick the even distribution of load the airplane will actually see in flight. If you watch the Boeing 777 wing ultimate test video, you can see the whiffletrees flopping around after the wings buckle. http://www.youtube.com/watch?v=WRf395ioJRY
I remember the first time I saw one of those. I wanted to rip it apart to see how it worked. I always thought there must have been tiny motors in the ball that moved it into position. That video solved a question that has been in the back of my brain for years!
Starting in the late 60’s, IBM used a version of the office Selectric as output printers for the 360 series of main frame computers and as printers on early computer terminals such as the 2740/274 models.
I was a field service person specializing in servicing what IBM then called “Teleprocessing” equipment. As such, I serviced many of these modified Selectrics. Later, as my first technical writing assignment for IBM, co-wrote a service manual for them.
I even still have that manual, but I would decline to even open the cover on one today. They were the world’s first totally adjustable mechanism and every adjustment affected a minimum of two other adjustments. Truly a marvel of engineering but a maintenance nightmare.
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