i am trying to make a small, simple 3d printed robot with humanoid characteristics. i am working on its arm at the moment. it is being driven by a small sg90 servo motor which pulls the forearm about a pivot towards the upper arm using a pulley made from fishing wire. this works well enough. however i am having trouble getting it to go the other way. i have tried adding an elastic band from point A to point B on the diagram. this works but it is not very smooth. i have researched into springs and it seems that the torsion variety are exactly what i need but they are jaw dropingly expensive(whats the deal with this? other spring types are quite cheap). could anyone offer an alternative solution to my problem? i am aware that this is a very basic question so hopefully it wont be too difficult for you folks to solve. thanks!