This is a remix of Eckard College's Roverbal Balance. See the description in the linked source file.
This has been redesigned to use a 608 ball bearing rather than a joint using the printer plastic. Such bearings can be found in many places. Here is one listing:
Here is another:
The unshielded bearing has much lower friction. But that's not really necessary for this application.
Three modifications to the balance are also possible with the extra parts.
The first uses a bent crossbar. This gives a slightly lower energy to the configuration when the balance is in the center position. This is more like the construction of real two-pan balances. The balance is still insensitive the the placement of the weights. But the center position gives you a clear indication when the mass on each side is equal.
The second modification replaces one side of the balance with a lever. This side acts more like a beam balance. Notice that the side with the "tee" connector is still constrained to move vertically making in insensitive to the location of the mass. On the "beam" side the placement of the mass precisely determines when the device will be in balance. This side acts like a seesaw.
The third modification is a combination of the other two. It has both the bent-arm designs and the beam-balance side. This version can act like a beam balance and also will self-center when the device is in balance.