It’s amazing what you can learn sometimes.

Junior #2, now in grade four, is mired in the dreaded “gears and pulleys” science unit. Dreaded, I say, because I’m still scarred from Junior #1’s project, now two years past. In that one, we (and by “we”, I mean “mostly I”) built a Kid-Powered Wooden Lifting Machine, designed to lift toys from the ground floor to the second floor, up the stairwell. To his credit, he designed it, and created a rather spiffy descriptive advertisement. That’s “media literacy”, in twenty-first-century elementary school ed-speak. Or so I’m told by the teacher in the family.

That creation was built from first principles, using discs of medium density fibreboard liberally laced with nails as gear teeth, as well as pulleys fashioned from ribbon spools, pie plates, and the like. There are no photographs of that unholy mess, and it has since gone the way of all good school projects.

So, with some trepidation, we embarked on Jr. #2’s project. Which turned out to be a bit easier – pick an object that uses gears, and create a poster about it. And that’s where the adult remedial learning comes in.

She chose, for reasons best known to herself, the pencil sharpener. Not the manual kind (which is a simple machine, by the way – the knife blade is a wedge). No, I’m talking about the school classroom, crank-handle kind, a design surprisingly unaltered since the days that I was in grade four, and it turns out, long before that.

Some investigation revealed that the “pencil pointer” in its more-or-less modern form has been around since at least the late 1800’s. Various online sources suggest that it became rather popular in the 1910’s, displacing other designs. And it really hasn’t changed a whole lot since.

But here’s the learning bit – I had no idea, no idea whatsoever, that these things operate through the use of planetary gears. You crank the handle, which turns a shaft. That shaft is attached to a barrel carved with spiral blades, which is itself tipped with a small toothed gear – a planetary gear, in fact. That gear makes its way around the inside of a ring gear, and in so doing rotates the barrel. Tilted at a jaunty angle, the rotating blades chew the pencil to bits through a gap in the shaft, which is hollow. Fancier designs use two barrels and two planetary gears, but the principle is the same.

It's got a planetary gear. Amazing.

Perhaps I’m over-impressed with little things, but this seems like a tremendously elegant design. It has few moving parts, works with nice mechanical advantage provided by the crank handle (that’s a lever, folks – there’s another one of those simple machines), and requires absolutely no electrical power or fossil fuels to operate.

Planetary gears and nineteenth-century technology – who knew?


About Richard Wintle

I am Canadian by heritage, and a molecular biologist and human geneticist by training. My day job is Assistant Director of a large genome centre, where I do various things along the lines of "keeping the wheels on". In my spare time, I tend to run around with a camera, often chasing horses, race cars, musicians, and occasionally, wildlife.
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5 Responses to Planetary

  1. Stephen says:

    Neat! An inspired choice by junior #2.

  2. Grant says:

    Reminds me of a book I recently read about the Antikythera mechanism, which also had planetary gears; the pencil sharpener is a nice example of one.

  3. Dawn says:

    And that’s why the design hasn’t changed since about 1910. Elegant, simple. It works! Any idea who designed it?

    Ha! More homework!

    • Here’s a website from someone who is fond of pencil sharpeners:

      The “planetary” ones seem to start with one from the A.B. Dick company, said to be patented in 1896. It used planetary gear(s?), but not the cutting cylinder of the modern one. I wasn’t able to find the patent (old patents from the USPTO website not being searchable, unfortunately) so don’t know who actually invented it.

      The design above with a cutting cylinder is said to have started with the “Olcott Climax Pencil Sharpener” design in 1904. Again, no idea if J.M. Olcott designed it, but it was sold by that company.

      The only sensible references I could find are on that website and are in Scientific American from 1913, and a journal called ToolTalk, neither of which I can access. Ah well.

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