Drive mechanism from a <a href="https://en.wikipedia.org/wiki/chart_recorder" class="extiw" title="w:chart recorder">chart recorder</a>.  One <a href="https://en.wikipedia.org/wiki/synchronous_motor" class="extiw" title="w:synchronous motor">synchronous motor</a> drives the annular gear of an <a href="https://en.wikipedia.org/wiki/epicyclic_differential" class="extiw" title="w:epicyclic differential">epicyclic differential</a> while the other drives the <a href="https://en.wikipedia.org/wiki/Sun_and_planet_gear" class="extiw" title="w:Sun and planet gear">sun gear</a>.  The output shaft, driven by the plate holding the pivots for the <a href="https://en.wikipedia.org/wiki/Sun_and_planet_gear" class="extiw" title="w:Sun and planet gear">planet gears</a> turns at one of 3 different speeds depending on the combination of motors currently running.  The mechanism was made by Cramer Controls Corp. no earlier than 1961.  The outside diameter of the drum supporting the annular gear of the differential is 0.95 inches (54 mm).
<a href="//commons.wikimedia.org/wiki/User:Douglas_W._Jones" title="User:Douglas W. Jones">Douglas W. Jones</a>
<span class="int-own-work" lang="en">Own work</span>