Cotton Weaving and Designing / 6th Edition

THE three principal movements in weaving are shedding, picking, and beating up the weft. By shedding is meant opening the warp threads to allow the shuttle containing the weft to pass over certain ends and under others. In the common hand loom the shed is made by the weaver operating treadles with his feet. Fig. 35 shows the method of connecting the shafts or staves with the treadles for weaving a plain cloth. There are two treadles, A and B, placed underneath the loom, and centred at C. The stave E is connected to the treadle A through the lever G. The stave F is connected to the same treadle through the “tumbler” T and the lever M. When the treadle A is pressed down it will take the stave E down, and the stave F up. For the second pick, the stave F is connected to the treadle B through the lever H, and the stave E is connected to the same treadle through the “tumbler” R and the lever N. Therefore, when the treadle B is pressed down, it will take the stave F down and stave E up. By alternately pressing first one treadle and then the other, we get each stave up for one pick and down for the next, alternately, as required for weaving plain cloth. The levers M and N are usually called “long lams,” the levers G, H “short lams,” and the top levers R, T “tumblers.” The cords PP connect the long lams and tumblers together at the side of the loom.

In mounting this loom for weaving a three-shaft twill, three treadles are required, one treadle for each pick in the pattern. Supposing one stave to be down and two up for each pick. The stave required to be taken down for the first pick must be connected to the first treadle through a short lam, and the two staves required to be taken up must be connected to the same treadle through their long lams and tumblers. Each pick in the pattern must be gone through in this manner. A separate treadle is required for every pick in the pattern, unless the same pick is repeated, in which case one treadle will do for more than one pick. It is not advisable to break the regularity in the order of treading in order to save a treadle; but in diaper patterns and similar weaves the effect of a point draft is obtained by reversing the order of treading.

Figs. 36 and 37 show the design and cording plan respectively for a twill cloth requiring eight treadles.

The hand loom is practically obsolete in the cotton trade, but it is still extensively used in silk manufacture, where power looms, as at present constructed, are not found advantageous for weaving the finer classes of goods.

The chief shedding motions in power looms are tappets, dobbies, and jacquards.

There are various kinds of tappets, the simplest and best for plain or twill weaving being those shown at Figs. 38 and 39. The former is the more general arrangement. In this the tappets are placed under the loom, inside the framework. In the arrangement shown at Fig. 39 the tappets are placed outside the loom, and thus a larger amount of floor space is taken up by the latter than the former.

Outside tappets are mostly used in the Yorkshire weaving districts, and are commonly made for weaving with about eight shafts. The top levers, with “half moons,” are centred at the cross rods EE (Fig. 39), and the heald is lifted from both sides of the loom. The top levers are very useful for equalizing the shed, as the connection with the upright rod can be altered without difficulty.

In a power loom there are two horizontal shafts, the top shaft A (Fig. 38) and the bottom shaft B. The former is used for working the slay, by means of the crank C, and the connecting rod or “crank arm” D (Fig. 38). The bottom shaft is used for “picking,” and for this purpose it is necessary that the shaft should revolve at one-half the speed of the top or crank shaft. The toothed wheel on the bottom shaft must therefore contain twice the number of teeth in the wheel on the crank shaft which drives it. As a plain cloth contains two picks to the round, and the bottom shaft makes one revolution for two picks, the tappets are fixed to the bottom shaft. Each tappet acts upon treadle bowl E, and therefore the size of the bowl will require to be taken into consideration in shaping the tappets. For weaving plain cloth four staves are usually taken, in order to prevent overcrowding the healds on each stave, the ends being drawn through the staves in the order 1, 3, 2, 4. As the staves are fastened together in pairs, this is the same as two staves.

The kind of movement to be given to the staves is very important, especially in quick-running looms. The staves should be moving quickest when they are level, and their speed should gradually decrease as the shed opens. It is obvious that a movement of this kind will put as little strain as possible on the warp, and therefore cause the fewest breakages. The depth of the shed should only be sufficient to allow the shuttle to pass, therefore the “lift” or stroke of the heald is dependent upon the depth of the shuttle used. The shed when opened should remain open only long enough to allow the shuttle to pass through.

To obtain the proper shape of the tappets for a plain cloth, the lift or stroke of the tappets to give the required lift to the healds must be obtained. If the lift of the heald is required to be 4 inches, and the centre of the treadle bowl E (Fig. 38) is situated 12 inches from the fulcrum of the treadle M, the heald being connected to the treadle at, say, 18 inches from the fulcrum, the lift or stroke of the tappet will be obtained as follows:—

In some makes of looms the staves are connected to the treadles at a point between the fulcrum and the treadle bowl, the fulcrum being at the front of the loom. This necessitates a larger lift of tappet than lift of heald. The tappets in this case are very large, and are preferred by some manufacturers.

To construct a tappet for a plain cloth from the following dimensions.—Lift of tappet, 4 inches. Distance from centre of shaft to nearest point of contact with treadle bowl, 2 inches; dwell one-third of a pick. Diameter of treadle bowl, 2 inches.

At a radius of 2 inches describe the circle A (Fig. 40). This circle represents the distance from the centre of the shaft to the nearest point of contact with the treadle bowl.

At a radius of 3 inches describe the circle B. One inch added for radius of treadle bowl.

At a radius of 7 inches describe the circle C. Four inches added for lift.

The circle B represents the centre of the treadle bowl when the inner circle of the tappet is acting upon the bowl.

The circle C represents the centre of the bowl when pressed down by the tappet.

The pattern being a plain one, the circle must be divided into two equal parts, and each half-circle will then represent one pick. By the line DE divide the circle into two equal parts. Then, as the healds must have a pause or dwell equal to one-third pick when at the top and bottom of their stroke, divide each half-circle into three equal parts by the lines FK, GH. Divide FH and GK each into six equal parts, and divide the space between the circles B and C into the same number of unequal parts, the largest being in the middle, gradually decreasing towards the circles B and C.

From the corners of these unequal spaces, and with the radius of the treadle bowl in the compasses, describe circles representing the position of the treadle bowl at different parts of its movement.

Draw the curved line touching the extremities of the treadle bowl. This gives the outline of the tappet.

As previously stated, the movement of the heald must be quickest when the shed is nearly closed, and must gradually decrease in speed as the shed opens. The unequal spaces into which the lift of the tappet was divided give this eccentric movement to the heald. The curve of the tappet will approach nearer to a radial line as the shed closes, and the heald approaches the centre of its stroke. Referring to Fig. 40, it will be seen that the treadle bowl is at rest from F to G and from H to K, or one-third of a pick at both the top and bottom of the stroke. Therefore the time allowed for change, or for moving the heald from top to bottom, or vice versâ, is equal to two-thirds of a pick. If a dwell equal to half a pick is required, it can be obtained by dividing the pick into four equal parts and taking the middle two parts for dwell. If two-thirds dwell is required, divide the pick into six parts and take four parts for dwell.

It is usual to give the tappet which operates the back heald a slightly larger lift than the tappet which operates the front heald. The difference required can be easily calculated. In looms with the fulcrum of the treadles at the front, and the healds connected to the treadles between the fulcrum and the treadle bowls, some of the required extra lift is obtained by connecting the back heald to the treadle at a point further from the fulcrum than the front heald is connected. In looms with the fulcrum of the treadles at the back of the loom, and the tappets acting between the heald and the fulcrum, there will be a greater difference between the size of tappets in proportion to the lift than in the former case.

Tappets for twills, and other simple weaves, having more than two picks to the round, are usually placed upon a counter-shaft, but outside tappets are usually worked loose upon the bottom shaft.

The following example will illustrate the principle of constructing twill tappets:—

Draw a tappet for a 3 up and 1 down twill. Distance from centre of shaft to nearest point of contact with treadle bowl 3 inches, lift 3 inches, bowl 2 inches diameter, dwell ½ pick.

At a radius of 3 inches describe the circle A (Fig. 41). At a radius of 4 inches describe the circle B (one inch added for treadle bowl). At a radius of 7 inches describe the circle C (3 inches added for lift). There being four picks in the pattern, divide the circles into four equal parts by the lines DE, FG. Then each quarter-circle represents one pick, and the tappets must be made to make one revolution for four revolutions of the crank shaft. As the dwell of the heald (when the shed is open) must be equal to half a pick, or half a revolution of the crank shaft, divide the first pick into four equal parts by the points O, L, M; make DP equal to DO, and FN equal to FM, and rule lines from P, O, M, N to the centre. The distance OM represents the half-pick dwell, and the distances OP and MN represent the half-pick which will be allowed for changing the heald from bottom to top of its stroke, and vice versâ. Divide OP and MN into six equal parts, and the lift of tappet, or the distance between the circles B and C, into six unequal parts, the largest in the middle and gradually decreasing towards the two circles. From the corners of the unequal spaces describe the small circles representing the treadle bowl at different parts of its stroke, and draw the outline of the tappet touching the extremities of these circles.

A tappet of this shape acting upon a treadle bowl two inches in diameter will take the heald down for one pick and allow it to go up for three picks. The heald will be held stationary for exactly half a pick when at the bottom of its stroke, and will begin to rise slowly, and gradually increase in speed as it approaches the centre of its stroke, and will gradually decrease in speed as it approaches the top of its stroke. The downward movement will be an exact counterpart of this. In this kind of tappet it will be noticed that the heald, when it gets to the top (if it is required up for more than one pick), remains stationary until it is required to come down. Thus the heald remains at the top while the circles revolve from N to P.

For this twill there will be four treadles, each treadle being operated by a tappet of the same shape; but the tappet operating each succeeding treadle will be placed one quarter of a revolution later than the previous one.

The size of the treadle bowl has a very appreciable effect upon the shape of the tappet, more especially when there are several picks to the round. The movement imparted to the centre of the treadle bowl will be the exact movement given to the heald as far as regards dwell and eccentricity, and as the tappet acts on the treadle bowl at a distance of 1 or 2 inches from the centre, the required amount of dwell and eccentricity must be given to the centre of the bowl, and the shape of the tappet obtained accordingly. It will be noticed at Fig. 41, that to give a dwell of half a pick to the centre of the treadle bowl, a slightly longer dwell is on the tappet at the inner circle; and as the size of the treadle bowl increases, this hollowing out of the tappet must be increased in order to keep the dwell of the heald the same.

Fig. 42 is a drawing of a tappet for a 3 down, 1 up, 1 down, 1 up (six to the round) twill. Centre of tappet shaft to nearest point of contact with bowl 4 inches, lift of tappet 2 inches, bowl 1½ inch diameter, dwell one-third of a pick.

To construct this tappet:—At a radius of 4 inches describe the circle A. At a radius of 4¾ inches describe the circle B. At a radius of 6¾ inches describe the circle C. As there are six picks to the round, divide the circles into six equal parts by the lines D, E, F, G, H, I. As there is one-third pick dwell, divide each pick into three equal parts, and take the middle one for dwell. Rule the lines L, M, N, O, P, Q, R, S to the centre, and divide the spaces allowed for change into six equal parts, and the distance between the circles B and C into six unequal parts, as in the previous examples. From the corners of the unequal spaces describe the circles representing the movement of the treadle bowl, and obtain the shape of the tappet accordingly. It will be noticed that at point L the treadle bowl begins to dwell, and remains stationary until it reaches the point S, when it begins to go up. The heald will thus be down for the first, second, and third picks, up for the fourth, down for the fifth, and up for the sixth.

Woodcroft’s Section Tappets are much used in weaving heavy goods, such as velveteens and corduroys. They are made with various numbers of sections to the round. A single tappet plate of one twelve picks to the round is given at Fig. 43. Sections are sometimes made in two kinds only. These are termed “risers” and “fallers,” according as they raise or depress a heald respectively. Each heald requires one plate and lever L, and as the tappets revolve, the lever L is moved up and down. When the lever L is lifted, the heald is moved downwards. A difference in the character of the shed produced by these tappets as compared with ordinary tappets will be noticed. When the lever L is lifted for two or more picks in succession, it comes down about half-way each pick. This is unavoidable in section tappets consisting only of “riser” and “faller” sections, which must join together exactly wherever inserted, thereby causing all the healds to come towards the centre of the shed after every pick. If there are twelve sections to the round, any pattern repeating on three, four, six, or twelve picks may be woven.

It is sometimes considered an objectionable feature of section tappets (as represented in Fig. 43) that they cause all healds to be brought level after every pick, thereby producing jerky shedding. This objection, however, has been overcome by the construction of eight distinct varieties of sections, as shown in Fig. 44, whereby healds may remain either up or down for several picks in succession on the “open-shed” principle, as with ordinary box-plate tappets cast in one piece.