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Class Room Odometer Answered

I want to build a large odometer with a hand crank to help my students understand place value, does anyone have any ideas on how i can build a mechanical odometer? of course i am not using it to measure any given distance, i just want numbers that turn.


Is the display near the wheel, or the handle? If its near the wheel, you can just chop off the handle. Otherwise you might need to mod it.

To make a real simple one, make three strips of paper with the digits 0-9 on them, like this: 0 1 2 3 4 5 6 7 8 9 Put each one on a doughnut shaped piece of wood. Get a wooden rod or dowel, and place on the rod so they spin easily. Then just spin with your hand to change the numbers.

For an individual wheel, study a ratchet and pawl mechanism. The gearwheel goes against the number wheel to make one unit. For the application, the pawl can simply be a short piece of thin stiff steel like is used for palette strapping. And of course the gearwheel has ten teeth.

Now study the feed mechanism of an old post drill. The eccentric cam moved a lever, which moved a pawl that pivoted on the end, which pushed forward a gearwheel by one tooth (indexed the gearwheel). The pawl then moved backwards, riding over the next tooth to be pushed, the end then dropping into pushing position.

You will have one pawl that will hold a wheel unit in place. Then out of the side of a wheel will be a single pin near the outside that will do the same thing as an eccentric cam's lobe. When completing a revolution, the pin will bear against a lever that will move a different pawl that will index the next wheel.

So, your smallest digit will have a holding pawl and a pin out the side facing the next digit. That next digit will have a holding pawl, an indexing pawl above that, and a pin. Repeat 2-pawl-1-pin until the greatest digit which will have no pin. Then there will be the indexing levers between wheels (5 digits 4 levers, 8 digits 7 levers, etc).

In operation, rotate smallest digit. After one rotation (ten increments) the 2nd digit will index one increment. After one hundred rotations of the first digit, the 2nd digit will complete ten increments (one rotation) and index the 3rd digit. Etc. Note if you watch your odometer while driving you will see the next digit starting to move while the smaller digit is approaching zero (one revolution), that's the indexing in operation.

Like a vehicle odometer, there is no reset. Unlike a vehicle odometer, you can simply spin the digits back to zero by hand.

If you missed it so far, you'll have to figure out the "timing" of getting multiple digits (going from 0999 to 1000 for example) to "click" at the same instant, but precise duplication of parts will help and might be enough. You'll also have to figure out where to put the numbers, can't have the second digit indexing while the first one is coming up on 6. Well, now that you know the mechanism, the rest of that should be easy.

Maybe you could get an actual odometer from a scrap yard and mod it to work for your purposes.

. A lot of hour-meters have the same mechanism. Check at places like diesel repair shops, equipment rental stores, &c; for broken ones.