I am thinking of turning my 4-shaft counterbalance loom into eight shafts by adding a dräll pulley system. I am told that this would mean I can only weave on opposites. Is this correct and what does this mean? Will I be restricted in my choice of patterns/designs?
I am assuming you have a loom with an upper framework that holds your counterbalance horses or pulleys. If your loom is a standard Scandinavian loom, such as a Glimakra or Toika, I would suggest you get the countermarch system instead. That would consist of upper jacks (the mechanism that raises and lowers the shafts) and a set of lower lamms (bars that pivot below the shafts as part of the countermarch system and would be added to your counterbalance lamms. This would be much more versatile than using the dräll pulleys.
The dräll “pulleys,” are not actual pulleys (though pulleys were used historically); see the photo and diagram here. They are suspended on a round rod above the shafts, one on each side. Cords attach pairs of shafts to each other on each side. The pulley in the diagram is set up for eight shafts. A cord attaches shaft 4 to shaft 5, shaft 3 to shaft 6, shaft 2 to shaft 7, and shaft 1 to shaft 8. To treadle, if a shaft is pulled down, the shaft attached to it rises. This means that four shafts must always go down, four shafts must always go up (which is probably what your advisor meant by weaving “on opposites”). This setup was most often used in Scandinavia for weaving two blocks of turned twill, since its tie-up fits the requirement. There are other structures you can weave and some ways to manipulate the treadling, but on the whole, the restrictions are limiting enough that I’d opt for the countermarch system.