I have here a Doppler drift and groundspeed indicator - I believe from a Decca Type 72 Doppler radar installation. It uses a servo-driven needle to display drift angle, and a roller counter to display groundspeed, plus a couple of status flags:

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Doppler drift and groundspeed indicator

These roller-counter displays were everywhere before electronic 7-segment VFD and LED displays displaced them.

These are the components/features I've identified - but do they have specific names?

  1. the drums with the digits printed on them
  2. the pinion gears, that pass rotation from one drum to the next from right to left. They have eight teeth, and half of them are cut down in width so the right part of the gear has effectively four double-spaced teeth
  3. the flange on the left side of the drum, that the four-tooth portion of the pinion usually slides against, locking its rotation
  4. the gap in the flange that allows the pinion to rotate and move the left drum, only when the right drum is moving between 9 and 0
  5. the two drive teeth on the left side of the drum that push the pinion
  6. the complete gearwheel on the right side of the next higher drum which the pinion pushes

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The context here is I want to design and manufacture low-cost replica mechanisms using 3D printing and/or off-the-shelf parts, for flight sim peripherals, not to the same beautifully-engineered standards as the originals but more accessible and sustainable. I'm making slow progress imitating mechanisms like this one by inspection, and no success identifying any component names. I'm hoping with the right terminology I can find a parts catalogue or some more robust design guidelines.

The Doppler example is a simple one, with external pinions and 10 digits on each drum. This radio control unit is more complex: the mechanism is inside the drums (no visible gaps), and the last drum -actually two drums fastened together - has "00-25-50-75-00-25-50-75" (eight sets of digits) and has two places where it advances the next drum rather than one.

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2 Answers 2


I've always seen the entire mechanism called a "counter drum" or "drum counter" in regards to altimeters.

As far as how they are designed and built - I would recommend searching for "mechanical counter" as that seems to be a more generic and broadly used term for the mechanism.


Partial answer: We referred to

the drums with the digits printed on them

as digit wheels

So-named product picked at random, no affliation, no endorsement:


  • 1
    $\begingroup$ Thanks - "digit wheel" gave the necessary clue - I've found this article which is the springboard to everything beyond: ieeexplore.ieee.org/document/8047368 $\endgroup$
    – Jack Deeth
    Commented Apr 12, 2023 at 10:18

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