How a Jack Loom Works
A jack is the part of the loom which is attached on a pivot and is used to raise a shaft. There are usually two jacks for each shaft. When you press on a treadle, the jack pivots, one end going down and the other up, raising a shaft. Jacks can be placed under the shafts pushing them up, or can be above the shafts on a castle pulling them up. Shafts only move up and so a jack loom is sometimes called a rising shed loom."The handsomest of all looms is the push-up jack loom with no superstructure. With four or six harnesses this loom is one of the most satisfactory, but for a loom with eight, ten or more harnesses, there should be overhead jacks." Tidball p.8
Shafts are held down on jack looms by their own weight. This weight comes from the shaft frame, metal bars and metal heddles. Do not replace your heddles with string heddles or wire heddles unless you are weaving more than 8 threads per inch per shaft. The manufacturer of your loom has planned on their weight when designing the loom. Each loom manufacturer needs to decide how heavy to make the shafts. If they are too heavy, the weaver will find it difficult to raise them, especially when a treadle is tied to more than 2 shafts. If the shafts are too light, the resting shafts will not stay down when a shed is made, since the warp threads, when tensioned will be pulling them up. The weight of the treadles will also be pulling on the shafts and helping them to move up, especially if many treadles are tied to one lamm as in unbalanced multiharness weaves. Often the shafts need varying amounts of weight when an unbalanced weave is tied up.
"This kind of shed can be very large or smaller depending on the pressure there is on the warp threads and how many threads we have. For example, it will be easier to have a nice shed with a 36" loom with 20 threads per inch than with a 60" loom with 40 threads per inch. Here you have to understand that we may have to raise half of the threads; so the more threads and tension you have the more difficult it will be to raise them." Francois Brassard, www Leclerc home page
If your loom is large, very heavy or very wide, most light weight fabrics can be woven with little difficulty, although more pressure is required to do the treadling, and the foot has less control over how large the shed is. If you do not weave the full width of the loom and you do not have many threads per inch, you will be more successful. On a small loom or one which is less than 40" deep, the most troublesome things to weave are tapestries, rugs, warp faced and weft faced weaves, fuzzy warps and close setts such as with doubleweave, You may also have trouble with unbalanced weaves such as summer and winter and other multiharness weaves where many warp threads are threaded on one harness and many treadles are tied to it. There are a number of things you can do to help alleviate problems when weaving these, and they will be listed.
"Right away you can see that a warp of extreme tension may not be stretched on this type of loom - making it unsuitable for tight weft-faced weaving, especially rugs. Besides this limitation, there is the fact that the rising action of any harness (or harnesses) does not force the opposite action of other harnesses..... For this reason a close-set warp for warp faced weaving may tend to lift all the harnesses when one is raised.... The loom is, however, superlative for light weight multiharness weaving....I suspect the jack loom is losing its popularity somewhat to the ancient counterbalance loom and counter marche loom." Rachel Brown p.120
"A very tightly stretched non-resilient warp (such as for rugs) can cause the shafts to float.....adversely affecting the shed. Additional weights can be added....but unfortunately...that weight also must be lifted.....In general, the action of a jack shed is lighter in a loom which lifts the shafts from above than in a loom which pushes them up.... however, both are considerably heavier than a counterbalanced or counter marche shed, a serious consideration...." Constance LaLena p.31
Rhythm in weaving which comes from smooth, even, fast weaving, can be achieved with most looms with practice. But the loom must have clear sheds, a tight tension and easy treadling.
"He must be able to weave on a narrow warp (12 to 20") plain tabby at a speed of 60 picks per minute. Check it with your watch. If the loom refuses to work that fast, don't buy it.... anybody can weave 2 1/2 yards per hour." S. A. Zielinski vol.2 p.9
"...it would not be unreasonable for a skilled weaver to be able to maintain a sustained rate of 1500 picks per hour over the length of the warp including time to advance the warp, wind shuttle bobbins, and make any necessary repairs in the weaving" Constance LaLena p.30
"Unless you are strictly interested in fairly light-weight multi-shaft weaves, you should not get a jack loom for your only loom. It is not as versatile as the other two types. It is unsuitable for rug weaving and many tightly packed weft-faced weaves, and light-weight jack looms are undesirable for close warp-faced weaving." Rachel Brown p.120
"Present day loom builders, with few exceptions, make the harness motions an integral part of the loom frame and so deny the weaver the facility of changing easily the type of harness motion according to the nature of the job...Unfortunately, this does not usually permit modification except by a complete rebuilding of the loom frame." Fannin p.80
"These looms should be used only when the cost of equipment is of primary importance because a double-tie-up loom although more expensive is much more satisfactory, and if necessary it can be used as a single-tie-up." S.A. Zielinski vol.2 p.13 "A loom is a piece of equipment designed to facilitate the textile process. There is little pleasure and no profit in trying to weave on a loom that will not do its share of the work willingly, accurately and easily." Mary Atwater p.30
Copyright 1998, Joanne Hall, Elkhorn Mountains Weaving Studio. (jah@montana.com) Used by permission of the author. Citation sources.
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