I decided to start with my last model of the 2 crank 1 piston mechanism, since it was similar.
I was able to model the Whitworth hand-cranked model using the Linkage software. My process was:
1. Establish rough dimensions from Youtube video of Whitworth mechanism and input into Sketchup
2. Get key dimensions needed from Sketchup to input into Linkage software
3. Run the Linkage model to make sure no interference and what the piston travel was
I decided to go with smaller 5 inch PD gears instead of the larger 6.571 PD I used on the 2 crank 1 piston model to save space of the size of the model.
I used the free GearDXT.exe program to design my gears, then imported the DXF file into Sketchup. You can download GearDXF.exe using this link.
Years ago I made a small Whitworth hand-cranked model, and the driving pin on the linkage had a lot of friction. I think I will try using a bearing on this larger model to reduce friction.
Based upon previous projects, using a 3/8" axle in a 3/4" board, gives less gear movement than a 3/4" axle in a 3/4" bore. I think using an L/D ratio is appropriate here, so the L/D ratio of the 3/8" axles is 2:1 and using a 3/4" axles gives a 1:1 L/D ratio. I am going to try adding a spacer nut on the back, glued to the front piece, to give an L/D ratio of 2:1 on this project.
Since the prices of lumber have doubled, rather than buy more red oak from menards, I decided to use up some white oak boards I have. I will have to resaw the 3x3 by 6 foot boards down to standard 3/4" inch for this project. I think I have enough maple to make the 3 gears and the moving parts.
This was a fun project to build, with no major problems encountered.
Adding the "nut" shaped spacer at the back of the 3/4" axle shafts reduced the friction when cranking the gears.