Jack type looms, (often referred to as rising shed looms) create the top shed of the design for the shuttle to pass through by raising the warp end on the selected Shafts the design calls for.
The warp ends of the bottom shed remain stationary
To enhance a Jack loom to produce a shed sufficient to allow the proper Shuttle to pass through cleanly, when the loom is at rest, the warp yarns angle slightly downward when leaving the Back Breast Beam to the eye of the Heddle and then up slightly kissing the Race Plate(or Reed bottom if no Race is present) to the Front Breast Beam.
Determining the correct warp tension for weaving any fabric construction is arrived at by the weaver.
There are no magic formulas that cover all the variables, Re:,
type fiber, type yarn, yarn size, yarn twist, thread count, temp., humidity, etc.
The weaver acquires this expertise through experience with the tools of the trade.
Much like playing a musical instrument, you can become an expert at reading and writing music, but you need to put time into playing the instrument, to produce a sweet tune.
There are guides for the inexperienced weaver to start with to build your expertise;
Focus on properly dressing the loom prior to weaving. Learn how to correctly wind your yarn and warp the loom. It is said that 90% of good venerability starts with a properly wound warp. From my experience that is a good rule.
Always weave a plain weave header before starting your pattern. The purpose of the header is to check your pattern for errors and to equalize the tension in the warp ends.
To arrive at a good starting point for warp tension, advance the warp to the point where you see the Shafts starting to lift off their resting position. Then back off slightly.
Open the shed in a plain weave pattern and check that the bottom shed is kissing the Race(or bottom of the Reed) in a clean even profile across the width.
If the bottom shed is slack(too much tension), back off on the tension until the bottom shed is uniformly even.
Many weavers weave with too high a tension. Remember, the warp yarns actually do the weaving by their rising and lowering action wrapping the filling. For the wrapping to produce the desired locking of picks per inch the warp yarns must be allowed to wrap the filling at the fell of the cloth at beat up.
When weaving with too high a tension, the pics tend to bounce out of position before the wrap starts to take place. There are techniques of shed timing to address that phenomena, but you do not have to go there for routine weaving.
I spent many (many) years in weaving mills world wide, weaving many(many) types of fabric. All were unique with respect to warp tension on looms to produce the desired fabric. But for the hand weaver weaving common household type fabric, the above overview provides a good starting point.
For those wishing to go into greater detail on any aspect of greige mill manufacturing, or finishing mill if you wish
You can contact me at www.TRBeaudet.com