HAVE A QUESTION?
OUR EDITOR HAS THE ANSWER
I weave with a jack-type loom (rising shed). I have never found a good description of the difference between counterbalance looms, countermarch looms and a sinking-shed lack loom (Louet's David loom is described as a "sinking shed jack loom").
These looms all differ only in the mechanisms that move the shafts to open a shed. Their differences do have consequences for weavers, though.
The shafts on a jack loom are connected to jacks, levers that pivot so that if you pull down on one side of the lever (by stepping on a treadle) the other side of the lever goes up, either pulling the shaft up (if the jack is above the shafts, as with a Macomber), or pushing it up (if it’s below the shafts, as with most jack looms). A jack operates for each shaft on each side of the loom. The warp starts off “down” when the loom is at rest, and when you treadle you raise warp threads to open the shed.
The drawbacks of jack looms are the shaft weight you must move to open a shed, and unevenness of warp tension, because the tension is usually looser on the down threads than on the up threads. To avoid making them very heavy, the shafts are designed not to pull the warp down as much as treadling moves them up. If you tighten the tension to try to overcome this, you raise the shafts. This is less than ideal for rugs that require tight tension and a heavy beat.
Both countermarch and counterbalance looms allow maximum tension and are great for rugs as well as all other fabrics. In both countermarch and counterbalance looms the warp starts off in the center of the shed at rest, and the threads go both up and down to open the shed. The difference between the two loom types is in the way this happens.
On a counterbalance loom, the shafts are connected to each other via pulleys or jacks above the shafts. When you step on a treadle, one shaft is pulled down, and the connected shaft is therefore pulled up. So, for most counterbalance looms, the same number of shafts always goes up as goes down, and the combinations of shafts you can raise or lower depends on which shaft is connected to which. Counterbalance looms are limited as to shaft number (usually four) and possible sheds, hence pattern possibilities.
On a countermarch loom, every treadle is connected to a lamm below the shafts. There are two sets of lamms for every shaft, one that will make it go down, one that will make it go up (via cords and jacks above the shafts). The drawback is that you have to tie every shaft to every treadle, either to make the shaft go up or go down. If you don’t, the shaft won’t move at all and its threads will remain in the middle of the shed. Countermarch looms are time-consuming to tie up, but the advantage is that you have more freedom in the combinations of shafts you can raise or lower than you do with a counterbalance loom.
There is really no such thing as a sinking-shed loom (this is a misnomer) or a rising-shed loom because the sheds don’t sink or rise, but shafts do. Counterbalance looms have been called sinking-shed looms, but really, their shafts also rise; you just connect the treadles to the ones that sink. I think the David, however, only sinks shafts. That is, the warp at rest is in the up shed and you step on a treadle to bring threads down. It's a clever design, but I haven't woven on it enough to know the advantages or disadvantages.
I hope this helps!