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.
Step 2: Central Tower
Pictures only - should be easy to follow or design your own.
The only critical component is the pivot base at the top.
Step 3: Hangers
As in the previous step, pictures only - should be easy enough to follow.
The only critical component is the pivot at the top, the pans should be customised to suit your needs / bits.
Step 4: Main Arm
This beam carries the two hangers, and sits on the central tower.
Step 5: Final Assembly
Combine the arm with the hangers, and add the grey pivot bases.
Mount the arm on the tower.
Hang one pan on the arm carefully, then the other.
If the balance is not perfect, adjust the weighting with small Lego bricks.
Step 6: Weights
For weights, coins can be good. Look up your currency on the internet, it's not that hard to find out what coins weigh.
I find that UK 2 pence coins weigh 7.13g or 7=~50g, UK 1 pence coins weigh 3.56g or 7=~25g. I've also collected some wheel-balancing weights, which are marked: 35, 30, 25, 20, 10g
Under load, 225g on each pan caused a bit of bending in the arm, but no functional problems. This is designed for weighing small quantities up to ~150g, but it's good for 200g.