Rotary to linear motion with wooden gears

Build a robust, accurate rotary-to-linear motion transfer box within a weekend, without spending any money.

Some projects need an element that moves in a straight line, but this is not always as easy as getting rotary motion straight from a motor, pedals, or crank.

To accomplish this, I read online about mechanical linkages and selected a hypocycloid linkage. It has a very elegant and simple design, with a long linear travel for a small size. Rotary input (like a hand crank or drill) applied to the central shaft is converted to straight line motion across the diameter of the ring gear. The linear motion will be as long as the ring gear is wide.

I hand-cut two plywood gears on the bandsaw and built a frame to run them in. The ring or internal gear has 32 teeth, the round gear has 16 teeth. Using powers of two ensures both good support from the spirit world, and harmony with the geek nature.

My gears were cut from a paper pattern generated with the free online gear calculator at http://woodgears.ca/gear_cutting/template.html , which I printed out and glued right to the plywood.

According to the site's gear-making recommendations, this is sometimes a problem with laser printers -- though not with inkjets, since they have tighter registration requirements. I used a laser printer and measured the gear patterns in several places to ensure that the laser printer didn't stretch the image in any direction.

All parts were cut by hand on a bandsaw, trimmed by hand (very little) with a wood file, and finished and lubed with paste wax. 1/2" bolts and washers were used throughout.

Since the gears are hand cut, and since this is the first time I have made gears like this, there are small variances despite the saw being quite accurate and maneuverable. The dimensions aren't exactly perfect -- they are very close, however, and well within working tolerances.

I clamped all the various layers and spacers together, rotated the movement by hand and tested the path, tuning with a small hammer to tap the pieces into their best fit.

I aligned the teeth to find a combination that gave smooth travel without binding.
I rubbed some wax into the teeth to ease their meshing up during the initial tests.

Then I chucked up a drill on the main shaft and gave a test run, repeating my tuning process and eventually screwing all the layers together when I was satisfied. I then ran it for a few minutes to work in the gears.

The time spent aligning the parts by hand provided a much better end result -- this was time well spent. I haven't finalized this as a working tool, just a prototype, so it's not painted. But it is sanded smooth and finished on all 6 sides, square, and generally nicely worked.

The gearbox is quite stable, and it goes way faster than I would need. I estimate the main shaft got going at 200-240rpm, at the drill's fastest speed in high gear.

This was a successful project and consumed about 10 hours of time to plan and complete. (This includes the time spent taking the laser printer completely apart due to paper slippage problems, servicing the paper feed rollers, and reassembling it.)

This project was purposefully set up with very rough measurements throughout. I used it to improve my skills at eyeballing, guessing correctly without actually measuring anything, and improvising solutions precisely the first time.

I used a divider to copy one measurement --I needed to rebuild the center arm, but with a 1mm increase in length between the main shaft hole and the gear hole.

Everything else was strictly eyeballed, even measuring the center of the main axis hole was an eyeball measurement. This was probably not the best or easiest choice. But that was the point, to do something using basic hand techniques, and have the project come out working as planned despite the lack of laser cutters, CNC tools, or even exact measurements.

Don't be afraid to extend the boundaries of your ability and sharpen your techniques for making good guesses and improvising in your projects. It improves your work when you have more practice at deciding confidently off the cuff, and fixing as you go.

* Add an adjustable counterweight to the main shaft to counter a minor wobble.
- Apply a more attractive, durable finish like battleship grey spray paint.
- Consider making parts in metal or acrylic. Heavier = more stable.
- Add proper bearings for smoother and faster travel. These could be shop built or salvaged.
- Add a better driver, like a crank or built-in electric motor.

1 comment:

  1. here's a free book on archive.org:


    "Five hundred and seven mechanical movements"

    I don't think it contains this specific movement, but can't be bothered to locate my original reference after searching for an hour. Lots to see, move along.