The extending segment drum - Marchant XL

The drum of the XL is very different in mechanism as well as construction from that of a pinwheel calculator; instead of castings with lots of channels with sliding pins, the drum is built up entirely of stampings. Unfortunately these were blackened steel, so vulnerable to rust. After a lot of oil and working (forcing) some parts, it all started to move. (Fortunate because much is riveted together.)

The drum with all the segments visible in retracted state.

And the drum with the segments extended, teeth out:

This is the early version of the extending segment with sliding geared-sectors. I.e. the segments have a long stem that slides in- and out between studs/pillars. After about 1930 the design was modified with the segments hinged. The segments then rotate around one pillar - making them probably much lighter to actuate. (This lighter-actuation probably also enabled Marchant to do away with the main crank-handle pull-out lock later. The early XL's need to hard lock the setting-dials right when the drum starts rotating. A lighter, later-actuating mechanism could make do with only the hard-lock that kicks in around 5 or 10 degrees into the turn. Then only a detent to 'feel' the rest position is enough for the main crank - easier to use.)

An overall benefit of the extending segment mechanism is that the levers can remain stationary (and can thus have a large grip), and also that the teeth can be properly shaped as involute gear. I.e. driving the register should be much smoother/quieter than with pinwheel pins.

(Yet another benefit, probably the aspect that was the most important for the Marchant company, was that it unlocks the ability to do keyboard-setting of the drum. Actuation is light enough to make that possible - and in the early 1920s Marchant really needed an answer to Monroe's fast keyboard-setting for number entry.)

For now, this particular drum seems functional again and mounted back in the machine it correctly added 5 to the expected position on the carriage register.

There's no revolution counting yet, that whole timing challenge is to come. So best to take the machine apart again:

This XL is one wonderful puzzle! An ingeneous mechanism with history behind it too to discover :)