The Dale Maley Family Web Site

Subtitle

Double-Crank Mechanism to Hand-Cranked Wood Toy

I found a video of this mechanism on Youtube.  You can see it using this link.


Modeling  the Design

Because I had so much trouble with the last 2 mechanisms that I built, I decided that I would like to model the to crank one piston mechanism before I built it in would.


I think the model shown on YouTube was originally drawn in something like pro E. This software allows you to do motion simulations like shown in the YouTube video.


Pro E software is not free and I am not paying please to get it. I checked and it is over $2,000 for a 1 year license fee. All I have is free Google sketch up.  My Version 7 has no modeling capability, maybe a later version does, I don't know.


Couple years ago I found a free simulation package called Linkage. I downloaded the most recent version of the software and I tried to model the to crank one piston mechanism. I tried for over 4 hours to get that software to work with no success.  Boy, was that frustrating !!  If it worked, it would be great for testing the impact of changing variables and see the impact on piston stroke, plus check for interferences.



 

Sketchup Simulation


I finally decided to use the old fashioned method with sketch up that I have used a few times before.


With this method, you save an image with the mechanism at a certain angle of rotation, then you increment the mechanism say by a 45゚ and take another image and keep doing this until you achieve full rotation.


I decided to use 45゚ increments and when I modeled my 1st concept I discovered I had an interference problem between the top linkage arm and the round driving gear.


I moved the piston further away from the driving gear to eliminate the interference.


Because it is so time consuming to do this in sketch up, I decided to just build a simple wood model which would allow me to vary some of the key dimensions.


You only took me a few minutes to build the crude would model. I found that my concept designed worked OK but it only gave half of an inch travel of the piston.


I don't think half of an inch of travel is very good for a model. The person cranking the model does not get to see much impact from their work.


Using the crude would model I figured out how to increase the stroke from half of an inch clear out to 3” of stroke. I could make the stroke longer but I would have to make the a new driving gear and then it would be too big for this bottle.

Experimental Data from Wood Prototype Model

#1 Original Set-up

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Top link was 7 inches.

Bottom link was 8.125 inches

Piston stroke was 1/2"


#2 Set-up

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Changed bottom link to 7.25 inches

Piston Stroke was 3/4"


#3 Set-up

=======

-changed 2 holes in driver gear so they were 180 degrees from each other.  Youtube model looked to be about 120 degrees

      between the driving pins

-top link was 7 inches

-bottom link was 7.25 inches

-piston stroke went up to 3 inches !!!!!


#4 Set-up

========

-left pins at 180 degrees from each other, but reduced the throw from 2.25 inches to 1.5 inches

-left 2 links the same

-piston stroke went down to just 1.5 instead of 5 inches

-could not increase the throw above 2.25 inches because I would have had to make a new wood driver gear.

-3 inches should be fine for my hand-cranked model.

-make both upper and lower linkages about 7.25 long



Main driver gear

I used GearDXF to design my gears.  This is the main driver gear, and is also the crank gear in the final design.


3rd Design Iteration

Material and Colors

I will  use maple for the gears,linkages, and piston........the rest will be red oak.  I will used water based dyes on the maple to add color to the model.  The crank handle will be red enamel.


Updated Design

Build Issues

Everything went pretty smooth.  


When I got the mechanism together, it seemed like the 3/4" holes I drilled with my old Forstner  bit for the shafts, the holes wandered and were not perpendicular to the face of the link.  I got out my 3/4" auger bit, put it in the drill press, and carefully rebored the holes. This seemed to straighten them out.  Since a Forstner bit does not guide on all of the hole drilled already, it is conceivable it could wander going through 3/4" oak or maple.


Had 1 minor safety issue. To make the wood washers I needed, I used a hole cutter bit in my drill press to cut the OD with a 1/4" pilot hole.  Then I had to Forstner bit drill the 3/4" shaft hole.........I was holding the disc with my fingers, and it slipped and rotated and gave me a small cut on 2 fingers.  This has happened before, and I should have used the water pump pliers to hold the discs instead of my fingers.


Dyes

It has been years since I have mixed up the dyes.  I used baby bottles to keep old water/dye mixed, and most of them have survived for years. I bought the dyes from Rockler back in 2013.  The powder is still ok in the 7 year old bottles.


They offer no blue color, just an aqua blue, which is ugly.  Maybe I can change the aqua blue to blue by mixing in another color?


On the yellow dye, my test sample gave bright yellow.   I ran out after the 1st coat on the parts, so I mixed some more, but it gave more of a   yellow-gold color??  I am going to go with it, but I'm guessing I mixed it too strong, so if I added more water, it might go to more of a pure yellow.


I apparently bought a bottle of blue dye from a different company in liquid form.  I still don't care for that blue color either.


The golden yellow color actually came out nice after I put on a coat of polyurethane :)




Drying parts in oven

I set my oven to about 120F to quickly dry the dyed parts, and the polyurethane coated parts.


Final Build

The 3 gears had almost zero friction, and when I hooked up the piston to link driven from the front, still no friction.


But when I added the last link, the one driven by the back into the piston, I had a ton of friction.  I ended up using the Dremel with a drum sander to increase the bores of both front and back links.  Eventually, I got the whole mechanism down to very little friction.  Causes for the friction on the links included:


1. I think the Forstner bit is getting dull, the 3/4" one for the bore holes, and wandered such it was not perpedicular to the link. I tried to fix using an old fashioned auger bit.

2. the 2 upright main pieces are not perfectly perpendicular to the base.


Having extra clearance in the link connections does not bother the performance at all, so no big deal.


Final Design

Sketchup Warehouse

I uploaded my design to the warehouse, and you can access it using this link.


Finished Model

Youtube Video

You can see a Youtube video of the model in action using this link.


Closing Thoughts

I wasted a lot of time trying to make the Linkage software to work........I should have just built a crude prototype and verified the model geometry......which is what I eventually did.


I like how colorful the model turned out. They dyed maple parts add a lot of color. The kids should have fun with it.