How to build a balance with Lego, for weighing things.
Some basic Technic pieces
A few other basic Lego pieces
(optional) plastic containers
Step 1: Key features
For smooth operation the balance requires low-friction pivots. I hope the pictures show this well enough - I used the edge of a brick, balanced in the gap between two lugless pieces.
The pans for weighing / weights used to hold Canon B95 FAXPHONE inkjet cartridges. (Note that fax machine failed this week: it was a nasty, noisy, slow, weedy-plastic, cheap thing. While the old Brother Thermal-Fax is still working after at least 7 years. Having been disappointed with the Canon L160 in another office, "we" decided to replace the B95 with a Brother 1460 - so far pretty good.)
The pans are PP and lift in and out easily.
The pans are supported by hangers, resting on pivots as described above
The hangers rest on an arm, it's self resting on the same type of pivot on top of a tower.