How can I attach rubber bands to a 2-bar such that they actually help the lift? It seems like anyway I do it, the bands pull along the arm, and not perpendicular to the arm.
You could attach them behind the fulcrum, like a counter weight, thats the only way i can think of based on what your describing
It’s pretty tricky to figure out rubber bands on a 2 bar. If you can’t do it like what @9065_parker has suggested, you need to attach the rubber bands to a point above the pivot point or stretch the rubber bands over this point. However, the rubber bands don’t provide very much force until the lift is half-way up. To compensate we just use a lot of elastics.
Look into using the rubber bands or black latex as a torsion spring. Use the string winch drum (see this: http://www.vexrobotics.com/vexedr/products/accessories/motion/276-1546.html ) on an axle that extends between your towers or exterior to the gearworks, at least. Anchor one end on the drum, wrap the elastic element (rubber or latex) so that it would assist the lift. Tie off the loose end.
Look at a garage door torsion spring for inspiration.
If you put the drum between the arms, you only need one. You can use things other than a winch drum, but it’s actually made for this task.
With this circular rubber band system, you get more stretch out of the rubber bands, and the stretch of the rubber bands also changes as you lift up, which is exactly what you want. The rubber bands are attached to the gear, not to the lift arm itself.