Shedding is very important, and more so in figured cloth than in plain; for it is by the shedding in its varied character that fancy cloth is principally produced. For plain cloth, a two-leaved or two-plated tappet is fixed on the tappet shaft, acting on a bowl fixed to the end of a lever, as shown in sketches of looms. There is one lever or treadle to each plate, and the heald worked by it is attached at the centre by wire, and a small corded cross-piece of wood, called a lamb. From the sketch it will be seen that, as the tappets revolve, the healds are alternately lifted and depressed, opening the shed in the warp. The shed for one pick will have the front heald at the top, and the back one at the bottom; whilst for the next pick their positions are reversed. The tappets are of rather peculiar form, a common shape for plain being shown at [Fig. 28].
Working on the centre S the eccentricity obtained is employed in depressing the healds, but by an intermittent movement. The shed must be opened quickly to economise time, but must be left open sufficiently long to enable the shuttle being picked through. A reference to the shape of the plate in [Fig. 28] will show this. From N to H is the portion for depressing the heald, from G to H being an arc of a circle and the part by which the heald is held stationary, while G M corresponds inversely with F N, and allows the heald to rise. It is necessary for the eccentricity to be equal at each shed, a point which may be tested by noticing if the sway of the beam is the same at each shedding. To get this, the treadles must be level when the healds are, and the centre of the bowl directly under the centre of the shaft. The bowl should be in contact with the tappet during the whole revolution, and thus give an easy tread—jerky motion in either treading or picking being very detrimental to good weaving. A good shed should be almost as large as the shuttle at the point where it passes through, quite clear from obstructions of any kind, and the lower half of it not too low, or, as it is termed, not “bottoming” too much, for when too low the warp is frayed by the movement of the slay.
A point regarding which weaving technologists often enter into profuse arguments and diagrammatic illustration, may here be summed up in a few words. The line of warp when the healds are level should be below a line drawn from the temple to the back rest, and the lower the healds are in this respect the better is the cover on the cloth, although attention must be given to the remarks in a previous paragraph regarding bottoming. Whether the lowness of the warp line is produced by depressing the healds or raising the back rest is immaterial.
When plain cloth is woven in a twill loom, the tappets act above the centre of the treadle, and of course are of smaller size, the healds being connected to the end of the lever opposite to the fulcrum.
The calculation of the size of the shed from given dimensions of the tappets and treadles forms a good example in leverage. Suppose the stroke of the tappet, or the distance through which it moves the treadle bowl (represented in [Fig. 28]), between the outer and inner circles is 3-1/2 inches. The treadle is 30 inches long, the treadle bowl being 25 inches from the treadle pin, and the healds connected 15 inches from the pin or fulcrum at N. Then the movement of the heald from its highest to its lowest level is equal to the distance moved through by the point N—i.e., 2·1 inches—for if the bowl moves 3-1/2 inches, the point N moves (3-1/2 × 15) / 25 = 2·1 inches. This gives the size of the shed at the healds. Suppose the heald in question is 7 inches from the fell of the cloth, the shuttle passing through the shed 2 inches nearer to the cloth, then the size of the shed at the heald multiplied by 5 and divided by 7 gives its size at the point where the shuttle passes through, or (2·1 × 5) ÷ 7 = 1·5 inches.
There is a slightly different shedding arrangement adopted in some looms, frequently so in Yorkshire, but rarely in Lancashire. This is the Bradford gear shown in [Fig. 23]. The tappets are placed outside of the loom, and the treadles are connected to the healds through jacks at the top of the loom. A leaf or projection on the tappet in this arrangement causes the heald to rise, not depressing it, as in the previously described arrangement.
FIG. 23.—BRADFORD LOOM.