Anyone interested in mechanical engineering should definitely check out the *analog* carry mechanism in these Marchant calculators that relies on planetary gears. These Marchants are arguably the high water mark for electromechanical calculator engineering.
From John Wolff's museum:
"The Marchant "Silent Speed" and its descendants use a complex and unusual continuous-drive mechanism based on proportional and differential gearing. Every column incorporates a ten-speed gearbox with three drive shafts and five selectors. The accumulator tens-carry mechanism is contained within the carriage, using a differential gearing mechanism with two planetary gearsets per digit."
See also Jaap's pages:
@EdS I relied on John Wolff's documentation when I was attempting to repair a previous Silent Speed. Incredibly complex machines. The official service manual just had drawings and it was handy to see high-quality pictures of the mechanisms.