The drawing shows the Stepped drum principle. A stepped drum is a cylinder with nine teeth with increment length on it. Above the stepped drum, a pinion can turn and shift on the square shaft.
The function in short: "The pinion convert distance, into an equivalent teeth turning of it. The turning, expressed in teeth, can be zero to nine. The distance of course is generated by the keyboard or sliders, at older machines.
In principle each digit has one pinion. In practice, for instants at the Rheinmetall machines, one Stepped drum is serving two digits / pinion (see later on on this page). The Stepped drum technology appears in more constructions, the original stepped drum (see photo's above). The earlier types had the looks of the one on the drawing. The middle picture shows the one which is used at Rheinmetall. It is a metal and weight saving design. The Monroe and clones, have a Split stepped drum or Geteilte Staffelwalze (DL) see page 3. On the picture to the right the Rheinmetall one and an Madas type is compared. The Madas has the stepped drum divided in two and have therefor two pinion on the same square shaft. They are made of aluminum and much smaller in size. From a calculating speed point of view a good design, it runs faster with less power and it contributes to a lower weight (please see page 4, and at the manufacturer menu: Madas 20AV, the second camera icon and from page 2 on)
One of those photo's is showed below. You recognise the stepped drum with two pinions / gearwheels above on square axis. Those two pinions are shifted by the two keyboard sense plates independently. The combination does the square axis turn. On the same axis is a gear wheel (to the left on the picture) which is feeding the registers at the movable carriage.

Here two times six is added.

Stepped Drum / Staffelwalze (DL) page 1

Nearly the same type as the dismantled version above.

The first video as overview before we jump into details. It add 5 to the first digit, from five to ten, including a tens carry. The counter register (the top display row) is updated from one to two.

The second video is a closer look to the stepped drum from the right. The pinion above is at the lower position, nothing is added. Only the counter register is updated.

Now you see the real form of the drum what I intend with the metal saving model. You will see as well that this manufacturer uses one drum for two pinions, economical for the weight to. I don't know why that is not started with the first two digits, the first has one on it's own.

A closer look from the front side.
In this video I push the pinion to the three position and you'll see an update of the result display.
Now I will ask you to have a closer look to the photo on top in the middle of this page. You see white arias on the teeth. These are the positions where the pinions has made a little wear outs.

Continued on page two with the tens carry mechanism, used in the Stepped Drum machines.




The pocket calculator Curta, has a double interleaved stepped drum on the central axis, serving all digits surrounding.

Back to a simple straightforward stepped drum machine at work. I have dismantled a defect Rheinmetall machine to its basic function. In fact the clear calculation can now executed by hand. For that purpose I have extend the central axis and mount a spare crack on it. Just to give you some aid to memory, an other machine of the same manufacturer in full display on the photo below there.
Back to the Stepped drum photo's. The drum is not covered 360 degrees with teeth. The cover is nearly 160 degrees. This gives the possibility to spread the start of the drums from the first to the last. When the last starts to possibly turn the gearwheel the first gearwheel is ready. This gave the motor less peak current and a smooth calculating sequence.