The Dale C. Maley Family Web Site

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Rotary to reciprocating linear motion model

A friend of mine that knows I build hand-cranked wood models sent me a link to a Youtube Video with cool motion.


I imported a screen shot of the youtube video into my Sketchup drafting program.  I decided a nice size for the small gear would be 3 inches.  Scaling the Youtube image gave a 5.5 inch height of the rack gear, and it is about 14.5 inches wide.

Gear Design for 3 inch internal gear

I had an old GearDXF.exe program on my computer.  I downloaded the new version which is better than the old one, it does internal and rack gears, and exports a dxf file.

The DXF file is not the right scale, so I usually use the 3/4" diameter bore to scale it correct in Sketchup. This works fine for the external and internal spur gears, but does not work for the straight rack gear!!  I could not figure out any way to scale it correctly in Sketchup.

So, I went back to the online gear generator web site I had used before.   You can generate 2 gears at the same time, so I generated one gear and the rack together, so I can scale both in Sketchup using the 3/4" bore diameter.  Unfortunately, it does not generate a DXF file, so I had to draw 1 tooth then copy and rotate for the spur gears, and for the rack draw one tooth and copy.

So, on future projects, if I don't have a rack gear, I should use the GearDXF.exe program on my computer because I can scale it using the shaft diameter and it generates a DXF file I can import into Sketchup.

Starting to design internal gear in Sketchup

Frame that holds internal gear

I need to know how much the frame has to allow for vertical stroking of the internal gear.  Pitch diameter of small gear is 3.0 inches.

Pitch diameter of 38 tooth ends is 5.429 inches.

So 5.429 minus 3.0 = 2.429 inches which is total travel, stroke is half of that or 1.2145 inches.

Frame has to allow the internal gear to move up 1.2145 inches and down 1.2145 inches from centerline of internal gear.

I then thought, what keeps the internal gear from falling down when the small gear is in the lower position?  I went back to the Youtube video, and the vertical pointer is also a follower, that holds the internal gear up while the small gear is at the bottom of the rack gear :)

So, allow maybe 1/2" clearance at top and bottom so internal gear does not hit the top of the frame or the bottom, so make it about 1.25 plus 1/2" clearance = 1.50 inches for frame.

Design Concept #1 too big

I try to keep my hand-cranked table top models within an envelope about 16 inches side, 10 inches deep, and less than 16 tall.  Below is size I have so far on this project...

I am already 20 inches long.........then I will have to add about 10 more inches to allows for the stroke.

How best to downsize it?

-reduce 3" gear to a 2" gear, which is 14 teeth. I have made 2 inch gears on other projects, that is about as small as I want to go.

-internal gear was about a 5.5 pitch diameter with 38 teeth, if I ratio it.......5.5/3 x new 2 = 3.7 pitch dia.  So 25 teeth is a 3.571 pitch diameter

-reduce straight section length.  Watching the video, most of the exciting thing to watch is when the small gear goes around both ends, not in the middle. I have about 8 inches of straight section now, so reduce it about 5 inches.

So new width will be about 3.571 + 5 = 8.571 inches. This leaves me room to get both linear bearings installed on each side.

It would save me a lot of time if I can use my gear DXF program since it exports a DXF file I can import into Sketchup. Problem is scaling the rack gear.  I checked some more and I think the height number is the overall height, try that.

changed 25 teeth to 26, so I can align teeth in vertical position, can't do this with odd number of teeth.

Using the rack did not work, it was not the right size..........I finally just stole a tooth from the end curved section, rotated it to horizontal, and then duplicated it.

Design # 2 (smaller size)

Making frame to hold sliding rack gear

You need to try to get all 4 pieces to have the same thicknes, where the rack gear fits into..........then plane the rack gear until it slides properly.


Boy, this model has a lot of pieces !!!!!

I built it without any gears in back, just to see if I could get the concept to work.

When I got it built, I decided to make a wood roller bearing on the dowel that runs around the horizontal piece on top.  There is a lot of friction on the ends, and I think the wood bearing helps with this.

The Sketchup size of the top horizontal guide piece is wrong, I  had to do some trial & error on the length on each end to get it to work.  The top piece is also not centered exactly because I made the rack by adding the same distance from the semi-circle round gear outwards.........and it turns out this is about  0.4 inches different than if I had measured from the center of the straight teeth section.   I am not going to change it now since all the parts are built.

Basic concept working

I made a video of the basic concept can use this link to see the video.

Just Add the back drive gears, right?

Since I had made stacked gears like this before, on the propellor fan hand-cranked toy, I thought this would be no problem.

I needed about 12 inches of height, so I went with PD's of 2, 5, 5, and 2 at the bottom.  I scroll sawed the gears and assembled them to the vertical oak board.  I allowed between 1/16 and 1/8" extra between the gear centers.

I got it all together and tried to crank it......and it took too much force to crank!!!!!   I disconnected the mechanism from the gear train, a just the gears alone were too hard to crank!!

I went back to the the propellor model and found I used 3/8" dowels instead of 3/4" dowels for gear shafts..........which allows too much slop or tilting of the a couple of the propellor gears have mating gears on the other side of the board.  My L/D ratio for the difference in shaft sizes worked against me.

I tried making a 2nd board identical to the main vertical so the gears were no longer cantilevered.........and friction at all in the gear train !!!!!!

Original Cantilevered Gear Set

Gear Support added

Crank handle not big enough

My standard crank handle only has about 2 inches of "throw".   This turned out to not be enough to run this high friction model.  I made a temporary 4 inch throw handle, and it worked fine.  Now I need to play around and see how I get enough throw, yet enough clearance for your hand.

I ended up wanting to keep the 4 inch throw, so I raised the bottom horizontal boards up about 1.5 give enough clearance for your fingers around the handle. I cut a large dowel to get the risers, and screwed them to the oak base.  I made a few holes too deep and the brass screws poked up above the oak boards, so I got some SS washers to space them away from Ace hardware.

Model working - only need to add base

You can use this link to see the model working again.

Model Done and ready to dis-assemble and stain/varnish


I  usually sand to 220 grit, then apply stain.......and then 1 to 3 rounds of polyurethane and 220 grit sanding.  On this model, I may stain some parts, with no poly, and on the big pieces try to do just 1 coat of poly.

Johnson paste wax

I usually lubricate my models with this wax to reduce friction and noise.  It gets hard, but melts with your fingers........which is good since my can is more than 5 years old !!

Updated Design in Sketchup

Rendering of the updated design

Finished Model

YouTube Video of finished model in action

Here is a link to the Youtube video.......

Sketchup Warehouse

I put my model in the Sketchup Warehouse and you can access it using this link.

Closing Thoughts on This Project

I wasn't sure if I could make this model work or not..........but thank goodness it works fine !!!!!!

After I applied Johnson Wax to all the moving elements, the model ran the smoothest, and it was a considerable improvement.

If I had it to do over again, I would raise the crankshaft axle higher so no round legs were needed.  Of course, I would probably have to use 3 gears in back instead of 4. Also measure from the center of the gear rack to set the RH and LH borders of this piece, versus going a set distance from each side.

I used Kreg pocket screws to attach the "wings" to the base, and they worked very well. I also used a Kreg pocket screw to hold the rack gear against its top follower, so I could still dis-assemble the unit.