Fooling Physicists since 1669. All instincts (mine too) tell us to treat this like a simple see-saw, but we quickly realize that no matter how far out we slide the weight on the right we can not balance the weight on the left. Sliding the weight out does increase the reaction forces acting on the ends of the straight links, but does not increase torque acting on middle pivot points.
Why would Gilles Personne de Roberval create such a thing? There are several reasons, but the one that we are interested in is illustrated in the picture of a simple pan balance. Notice how the weight does not have to be placed in the center of the pan to get it to balance. The arms in my design represent the pan. Flip a simple pan balance over and you will see the second straight link hidden below.
This balance is designed to be snapped together, therefore is a more challenging print. Since every printer is going to print parts with slightly different tolerances and quality, results may vary (read "disclaimer"). I broke quite a few fingers trying to dial in the design, so hopefully you will not need to print additional links.
Three files are necessary to print this balance; vertical link, horizontal link, and tee link. I printed a tee and a horizontal link at the same time, then ran the print again. The vertical link was printed on its side. Since I designed the model standing upright, the models will have to be rotated to lie flat. Make sure fingers are pointing up (Remember to check the scale too). I used PLA but ABS would probably snap together better. Total print time at medium speed and 35% fill was about 4 hrs.
Before assembly, run a 3/8" drill through the holes (I did it by hand). Because of the layering, holes tend to print a little smaller, and the holes on the vertical link are printed vertically so they tend to be a little D-shaped. Oil the fingers with light oil before snapping together to reduce the chances of the fingers breaking and to reduce friction at the joints once assembled. Mount on a piece of wood as shown.
Pan Balance Bottom View