SETTING OF KNITTED FABRICS
By the setting of a fabric is meant the closeness in which the loops are
knitted in the fabric, and this question also bears reference to the
weight of the fabric and its density. The first item which affects the
setting is the gauge of the frame, that is, the closeness with which the
needles are set in the machine and a firm producing a full range of
knitted goods requires to instal a range of machines from fine to
coarse. In machines of the Cotton's Patent type the gauge is given by
the number of leads which occupy 3 in., there being as a rule two
needles cast in the lead. Twice the gauge number gives the needles on 3
in., and to obtain the needles per inch we divide by 3. In other words,
the needles per inch are equivalent to two-thirds of the gauge number of
the machine. From 8 to 12 gauge is considered coarse, from 14 to 20
gauge is medium, whilst the fine numbers range from 20 to 40 gauge. At
the one end of the scale we have gauges on which the coarsest kinds of
military articles are worked, whilst at the other are numbers suitable
for the finest silk-knitted hosiery. Thus the work produced from a
16-gauge frame is termed 16-gauge fabric, that from the 30-gauge is
termed 30-gauge texture. Between these extremes, however, there is a
certain variation of texture possible on each frame according as the
yarn is heavy, medium or light, and within certain limits fabrics of
different weights may be forthcoming from one and the same machine.
_Texture Variation on Individual Frames._—Fig. 36 gives a series of
diagrams which, if examined, will throw light on the interesting
question of variation of set on any one gauge of machine. The diagram
represents the conditions on a frame of the Cotton's Patent type where
the needles are shown in shaded circles marked N and given in end
section, whilst the sinkers are noted to pass between these and are
marked S. The rope-like structure which envelops these indicates the
thread and is marked by T, the whole diagram representing the action of
sinking the loops on any spring needle machine where the sinkers are
pushing the thread between the needles to provide the material for
loops. In all the diagrams marked A to E the dimensions of the
stitch-forming parts such as the sinkers and needles are constant in
size and the variation occurs entirely in the thickness of the yarn. In
diagram A it will be noted that each needle space, that is, from one
needle to the next, is divided thus: needle, space for yarn; sinker,
space for yarn. Between each needle, therefore, we have needle, yarn,
sinker, yarn in sequence, and this order persists throughout the frame.
It will thus be seen that we have various factors of variation, the
first being in the relative spacing of sinkers and needles, this
proportion being determined by the machine builders. Strange to say all
machines of the same type do not have the spacing arranged in the same
proportion, different makers adopting a somewhat varying practice in
this regard. From the point of view of the machine constructor he has to
decide on relative measurements which give the greatest strength to the
parts, the sinkers have to be of the size which will make them
sufficiently stable in working and make wear and tear the lowest
possible.
[Illustration:
FIG. 36
]
_Yarn Variation._—The real foundation of this problem lies in the way in
which the yarn can be increased in size for any given frame gauge, and
the counts can be varied within certain limits to give widely-differing
results. If the spaces marked 1 and 2 of Diagram A in Fig. 36 be noted,
it will be seen that the whole matter depends on the way in which these
are filled by the yarn. If the spaces are but sparsely occupied, then
the setting of the loops will of necessity be slack, and the texture
will suffer owing to the yarn not being bulky enough to fill up the
interspaces. The interspaces are in the first instance created by the
presence of the parts, and for a normally good texture the yarn closes
up to a considerable extent after the fabric has left the needles. If
the space exceeds a certain ratio, the resulting texture is loose and
open in appearance and of little value for ordinary clothing purposes.
True, such texture may be useful in what are termed gauze varieties,
where the thickness of the yarn is diminished out of all proportion to
the gauge, and the fabric has an appearance which is so loose that the
loops cannot be supported in their usual symmetrical form. Diagram B of
Fig. 36 shows a further stage in the yarn thickness where the
relationship between the yarn size and the gauge is approaching normal
and under certain circumstances would give quite satisfactory results in
summer or light-weight goods. The succeeding three diagrams C, D and E
show a graduated thickening of the yarn where the thickness is increased
from normal condition to that in which the yarn is too thick for the
gauge. Diagrams C, D, and E mark the transition from light-weight to
normal and from normal to heavy-weight textures. In Diagram E the yarn
is already occupying all the available space, and to extend the
thickness further in relation to the gauge would give rise to
unsatisfactory results in the texture. Even in Diagram D the yarn is
approaching the stage where it is uncomfortable to work, for if a
certain amount of free space be not allowed, the loops are stiff and
crowded together in too little space. The weight of the fabric is
undoubtedly increased, but owing to the congested state of the loops
little or no interaction is possible amongst them and the cloth becomes
stodgy in character. It may happen that the thickness of yarn is
increased to make stouter fabrics and more durable, but when the
thickness of yarn has exceeded a certain proportion of space, we have
increased weight with a great reduction in elasticity. The net result is
to lower the wearing value of the texture, for what is gained in dead
weight is lost in elasticity. In the wearing properties of the knitted
fabric elasticity is a most important item, and unless there is a
modicum of this property the fabric is seriously reduced in durability.
In addition to this, grave injury is done to the working parts of the
machine by making "full" fabrics, because the extra strain imposed on
sinkers and needles augments the depreciation of the mechanism.
[Illustration:
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.