Fig. 37
Effect of Yarn Quality.—In studying the correct size of yarn suitable for any particular gauge of frame, the nature of the yarn and its quality and structure have to be taken into consideration. In regard to material a great point is whether the yarn is made from wool, cotton or silk. Woollen yarns have a considerable proportion of material of open character which adheres but loosely to the core, and are eminently suited for knitted goods, but their character makes it difficult to determine what the exact diameter is. Silk yarns, on the other hand, appear at the opposite end of the scale, for these are fine in diameter and comparatively dense in body and clear in surface, so that they are best accommodated in very fine gauges. Cotton yarns may be said to have intermediate properties in this regard, they are dense in structure and even in diameter with a comparatively small amount of surface material. Woollen textures are usually set more openly and at increased width in the frame to allow of a thickening of the fabric in scouring and milling, but as cotton does not exhibit this tendency to felt in anything like the same degree, it is set much more tightly on the frame, for it requires to receive its necessary consistency when it leaves the machine without depending on augmentation of thickness in the scouring and milling process. All knitted fabrics worked in the plain stitch contract at once on leaving the needles, the rib stitch contracts most. In the case of the tuck varieties there may sometimes be an increase in width if the yarns are heavily worked on the frame, heavy working upsets the equilibrium of the knitted stitch, and this pressure it seeks to relieve by spreading out in the width. In determining the yarns suitable for any gauge, allowance has to be made for the nature of the material and the structure of the yarn. For instance, a woollen yarn spun on a tight principle might appear too thin for a given gauge, whilst another yarn of similar weight but fuller in handle would appear to fill the gauge exactly. It will be seen from these points that it is extremely difficult to lay down rules for setting yarn on any particular gauge of machine, but with practical experience in any branch of manufacture we may arrive at a rule which may be applied with confidence for any particular class of goods. We can also say with fair definiteness what size of yarn will be too light to give a good structure on any particular gauge whilst we may also determine the counts which is too heavy. The frame-worker is a good judge of its suitability by noting the way it works on the needles.
Variation in Loop Length.—In describing the structure of various systems of loop formation it was mentioned that the texture of the fabric worked on any particular gauge of frame could be regulated according to the length of loop drawn on the needles of the machine. This is performed on the flat knitting machine by raising the cams for a shorter loop, and lowering them so as to draw a larger loop. When the loop is shortened it means that a larger number of courses can be inserted per inch and so the weight is increased. When the loop is lengthened it makes the fabric looser in texture, and although the yarn drawn by a course of loops is longer than for any individual course of short loops, the take-up of yarn for the tight fabric is amply compensated for by the increased number of courses per inch which can be inserted. It is a mathematical problem of considerable interest as to what the difference actually is; if we lengthen the loop a greater weight of yarn is necessary to provide a course of stitches, in making the work stiffer we reduce the weight of yarn required for any particular course but increase the number of courses per inch. Variation of yarn tension for making slack or stiff work can only be done to a limited extent in any particular gauge, but it is a very useful method of varying fabric texture. If the tension is not correct, the fabric is lacking in character and this deficiency is at once reflected in the quality.
The point may be better understood by a reference to the diagram given in Fig. 37 where the set of the needles and sinkers with their relative thicknesses is the same as in Fig. 36. In Fig. 36 the yarn varies in thickness according to the various stages illustrated, in Fig. 37 the thickness of yarn remains the same throughout but with a difference in the length of loop drawn. The process is illustrated in five stages beginning with F, where the tension is stiffest and the loop shortest, and culminating at J where the tension is slackest and the loop longest. At first sight these would appear to be reversed in their effect, and Diagram J would seem to give the heaviest fabric seeing that it has a greater length of take-up per course drawn. It must, however, be clearly understood that this shorter take-up enables a greater number of individual courses to be inserted per inch, so that the apparent loss is amply compensated for by the increase in the courses.
In Fig. F the loop drawn is the shortest possible and the sinkers S push the yarn T below the needles N. In Fig. G a slight lengthening of the loop is observable and this stage may be regarded as intermediate between stiff and medium setting of the courses. In stage H the loop is further extended between the needles and may be considered a normal form of loop sinking. In Fig. I a corresponding lengthening takes place, marking the intermediate stage between normal and slack texture, whilst in J the loops are formed for a slack fabric which will give a texture known in the trade under the name of gauze often used for light-weight goods.
Courses and Wales.—A woven fabric consists of two series of threads, one termed the warp runs lengthwise in the cloth, whilst the second series called the weft runs crosswise. The knitted fabric cannot be so exactly described because the plain stitch is in reality a weft fabric having its threads inserted crosswise in the texture and termed courses. At the same time the horizontal courses have vertical features known under the name of wales; the stitches are formed on needles, and from one needle to the next comprises a wale. Wales are measured in the width generally by the number per inch, whilst the courses are similarly measured in the length. In woven goods a normal well-balanced structure known as a square cloth has the number of warp threads per inch equal to the number of weft threads per inch, but this relationship does not hold good for the knitted structure. In the hand-frame times statements were always made out on the assumption that fabrics were worked "to the gauge," that is, a 12-gauge frame Cotton's type would have 12 courses per inch, and an 18-gauge would have 18 courses per inch. If this be analyzed it will be found equivalent to a 50 per cent. increase of the courses per inch over the wales, because the wales per inch are equivalent to needles per inch and, as was mentioned previously, the needles per inch = two-thirds of the gauge. In a fabric worked to the gauge we have 50 per cent. more courses than wales per inch, that is, a fabric with 12 wales per inch would have 18 courses per inch and so on. Present-day practice does not bear out this setting, for the courses per inch usually fall short of a 50 per cent. increase over the stitches per inch. Thus for a normal texture in 18-gauge there are 12 needles per inch, or equivalent to 12 wales per inch. If this were worked to the gauge we should have 18 courses per inch, but in general present-day practice about 15 to 16 is more usual. A 24-gauge fabric has 16 needles or wales per inch, and this worked to the gauge would give 24 courses per inch, but in general practice 20 to 22 would be nearer the figure. This is a point which has to be carefully considered in regard to making out designs for knitted fabrics which have to be ornamented, because if the effect has to be represented on squared paper the horizontal squares should exceed the vertical squares by about 50 per cent., so that the figure may present a true picture of the actual size of design.
Yarns Suitable for Gauge.—The matter of evolving a general rule for finding the yarn which will be suitable for a certain gauge of machine is full of difficulty as will be understood when the elusive elements of the knitted texture are fully realized. In the woollen underwear trade a rule followed in practical working is that for the Cotton's Patent frame, the yarn suitable for any gauge is the worsted yarn number two-ply the same as the gauge. Thus for a 16-gauge 2/16's yarn would be considered suitable, for 2/20's yarn we should select a 20-gauge frame and so on. As has been pointed out, however, each frame has a short series of yarn sizes which can be worked on it, each giving a result which will be suitable for specific purposes. This rule whilst a useful guide for medium gauge machines goes wrong in the extremes, that is, in the extremely coarse and the extremely fine. For example, on 40-gauge it is usual to work yarns much finer than 2/40's worsted, whilst on the 12-gauge frame, 3/12's worsted for stiff military fabrics is often worked.
CHAPTER IX
Various Knitting Yarns
A large variety of yarns other than pure woollens are employed for knitted goods and there is no branch of the textile industry where such free use is made of materials of different type and character. For the plain knitted stitch in particular "the yarn is the fabric" and the qualities of touch and handle as well as colour form the chief features sought after in the yarn.
Cotton Yarns.—Yarns spun from cotton are utilized for a large selection of the trade in knitted articles which perform an indispensable function in clothing. When examined with the naked eye cotton yarns can be distinguished by their dense structure, the fibre is soft when in fibre form, but when spun into yarns takes a firm structure. Cotton yarns used for knitting are usually spun with as slack a twist as is consistent with strength, and an effort is made to throw a considerable portion of fibre to the surface of the thread. When examined under a low-power microscope much useful information may be gained by scrutiny of the lie of the fibres and of the way in which they dispose themselves. A photo-micrograph of a fine counts of cotton yarn used in knitting is given in Fig. 38 where the fibres are noted to be densely packed in the yarn with copious material round the surface to enable the thread to cover up the loop interspaces. From this sample also may be noted the twisted appearance common to all cotton fibres; the single fibres have an irregular twist like a twisted ribbon. This distorted aspect of the fibre is one of the best means of distinguishing this material, and when the feature is noted, it is a sure sign that cotton is present. Another evidence which confirms this is the peculiar manner in which the fibres dispose of themselves, they appear disjointed, form sharp corners and shoot away at steep angles. Wool fibres in a yarn appear curved and dispose of themselves in elegant waves, whilst cotton strands strike out in divergent directions. Fig. 39 gives another view of a type often employed for cotton knitted goods, this being a single thread of long stapled cotton loosely twisted so as to give a soft full handle. This character of thread is often employed for the so-called fleecy underwear where the soft yarn is raised or brushed on the teasles to give an effect extremely pleasant to the touch. In this example the fibres are shown freely scattered from the core of the yarn, and a closer examination of them will show that they have the sharp angles and the spiral twists just referred to as being indicative of the cotton fibre.