The Dale C. Maley Family Web Site

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Rotary to Reciprocating hand-cranked wood model

I found a neat computer simulation on YouTube of a unique way of converting rotary to reciprocating motion. You can use this link to see the video.


I used this free drafting program to come up with the basic layout for my hand cranked wood model.

I made an assumption on the partial gears that they were 120 degrees worth of teeth (this turned out to be a bad assumption).

I thought 18 inches wide was too large for a wood model, so I reduced the concept size of the wood model......

GearDXF.EXE Program

I  used this free program to generate a .dxf file of each gear needed, then I imported the DXF file into Sketchup.  I use a 20 degree pressure angle with a 7 diametral pitch for my wood gears.

Vertical Rack Gear

The whole trick in converting this computer simulation into a working wood model is how to guide the vertical rack gear.  The first idea I explored was using 2 wood bearings, 1 at the top and 1 at the bottom of the rack gear.

Packaging size is the issue with this idea. It will make the over-all model height very tall.  Another idea I had was to package the gear directly behind the rack gear, so no model height is added with the bearings.

Material Selection and Colors

I would like to make the model as colorful as possible.  I am going to use maple for the 3 spur gears, then use water dye to change their color to green and blue.  I will use red padauk for the rack gear and yellowheart for the vertical rack gear. I don't know yet, but I will probably dye maple for the drive gears.

3 Spur Gears

1. Print out full-scale gears on paper from Sketchup

2. Elmer's glue paper patterns to wood

3. Bore axle hole

4. Scroll saw

5. Remove excess glue/paper with wet rag

6. Dry gear at 120F in oven for 20 minutes to remove water

Rack Gear

I am not sure how to design the ends, and exactly how long it should be in terms of teeth.  I will make the blank a little longer than I think it needs to be, then adjust the design and length as needed.

Horizontal Gear Bearing 2 boards

I will start from the center axle hole, then add 1/16 to 1/8 inch between centers for the outer 2 gears.  You have to add some tolerance to the theoretical pitch diameter centers because of manufacturing imperfections (axle hole is not perfectly centered, gear teeth not cut perfectly, clearances between gear hole, axle diameter, and axle hole in support piece, etc).

Vertical rack gear

I made a dummy from pine, but I found I really need the bearing to figure out the final design of the vertical rack gear.  I went ahead and made up a 12 inch long bearing for test purposes, from mapel. I will use hot melt glue, which is removable, to hold the bearing temporarily in place.

Trial Assembly

I started to assemble the model, and when I had it far enough along, I used the vise to hold it, and I attached a temporary crankshaft.

Use 2 pieces of wood versus 1 piece to  hold the spur gears really made a smooth running gear-train !!!!!!

I allowed 3/32" extra distance between gear centers, which is between 1/16 and 1/8" tolerance or clearance.

Big Trouble

I had the whole model assembled and was ready to try it out in the vise.  I discovered I did not have enough teeth on the two gear segments!!!!!!!!

I went back to the Youtube computer simulation, and took a screen shot of the model, and imported it into sketchup.  I measured and each gear segment had 139 degrees of gear versus the 120 degrees I assumed when I first designed it !!!!!!!! URRRRRRRRRR, hate it when I make a big mistake !!

Using Sketchup, I modeled the segmented gears and vertical rack, by marking the beginning tooth, then rotate each gear by 1 tooth, and then moving the vertical rack by 39/64" for each spur gear rotation.  I found that I needed 20 teeth on the segmented gears and vertical rack........versus the 14 teeth I had when I assumed 120 degrees of gear.

I was able to add 6 more teeth on the pine prototype vertical rack gear I had...........but I had to scrap the 2 yellowheart segmented gears.  I made 2 new gears using maple because yellowheart is so expensive (I will dye the maple yellow when done).  Boy I hope this design change works.

The Youtube simulation is linked to a warehouse of some type, and one of those view shows pins used for overtravel of the vertical rack, plus a stop at the bottom of the rack???    I guess I will find out what happens with my model after I try it out.

2 Pins are Probably Critical to the Design

I went back and studied the web site linked to the Youtube animation, and I'm starting to think the 2 guide pins are critical to the design.

As I start to think about, the timing would have to be perfect at the end of a stroke of the vertical rack, where 1 segmented gear tooth is leaving the rack at exactly the same time a tooth enters from the opposite rack. The pins could buy some time for the 1 tooth to exit until the other tooth picks it up.

The necessity for the pins is not obvious from the Youtube video, but it is when you click the linked site.

Big Challenge

I thought the linear bearing would be the challenge in converting this simulation to a wood model............boy.........was I wrong !!  The real challenge is the transition as the vertical rack leaves 1 spur gear and engages with the other!!!!!!!!

I got 1 pin in the round gear with no teeth behind the rack, and 1 pin in the rack........for when the rack is in the top position.  The problem I ran into was no way to adjust the angular position of both gear segments, since the mechanism is so sensitive. I was drilling a 1/4 hole in the gear and mating shaft then using a dowel pin.  There is no easy way to change this if it is not right !!

I may put this model aside for a few days, then come back with new ideas on how to set the 2 gears.  The model is pretty much done except for getting the two gear segments and rack to work right.

New idea to try

I need a temporary and adjustable way to connect the gear segments to their shafts, then be able to adjust the angle, without drilling a 1/4" dowel hole and throwing the dowel away each time I adjust the angle.  After a few days away from the project, I came up with a new idea :)

Darn Backlash !!!!!

My idea for the special wood clamp worked fine.

Now, what I find is that when I start the rack gear in the up position, the left hand gear takes over and there is a big motion down on the rack gear, due to the backlash in my gear system!!!   Just guessing, on the hand crank, there is about 30 degrees of backlash.  I am about 3 teeth off on when I try to engage the RH gear.

So, 46 teeth in 360 degrees times 30 degrees gives 3.8 teeth worth of back lash !!!!!!!

Only option I can think of is to remove a couple of teeth on the RH gear to allow for the backlash.  I removed 1 tooth and that was not enough.

Going to let this project sit again for awhile.   I may convert it to driving 2 propellors, instead of running a vertical rack gear, since that requires no timing.