Fabric Quality

Ellis Developments Limited

Nottinghamshire , United Kingdom

Fabric Quality Systems

These old course notes are incomplete, have not been fully checked since retyping, and do not include the diagrams, but may be of some assistance in home study.

FABRIC AND GARMENT DIMENSIONS -

YARN, FABRIC GEOMETRY AND FINISHING CONSIDERATIONS

INTRODUCTION

There are three dimensions of major importance:-

Dimension

Length

Width

Weight

(mass)

Measured in continuous fabric

Courses per inch or cm pattern repeat lengths

Wales per inch or cm fabric roll width

Wt. Per unit area e.g. ozs/sq.yd. or g/sq metre

Measurements in garments

Length of bodies, sleeves or hose length etc

Width of bodies, sleeves etc.

Weight per unit ;lot e.g. per dozen

Weight is made up from the weight of the total length of yarn in the unit area of fabric or unit lot of garments. Clearly this weight is related to the count of the yarn. The length of yarn involved is related to the mean loop length knitted and total number of loops knitted in this unit area of fabric or unit lot of garments. The total number of loops will depend on the number of loops per unit area of either fabric or garments.

SIMPLE FABRIC GEOMETRY AND THE IMPORTANCE OF LOOP LENGTH

Extensive studies of knitted fabric geometry with yarns not subject to shrinkage on finishing - such as cotton and worsted spun yarns including fully relaxed high bulk acrylics - and with simple fabric structures such as plain knitting or 1x1 rib, have shown that the number of loops per unit area - the stitch density, - is inversely related to the loop length squared - viz. S = k/l2 where S is the stitch density, 1 is the loop length and k is a constant for the particular structure, yarn and state of relaxation. Whilst there is a somewhat different loop shape in these fabrics depending on state of relaxation (e.g. finishing), on the type of yarn, and on the structure (whether rub or plain), there is for a given yarn, structure or finish, a distinct loop shape, so that if loop length and finishing processes are adequately controlled, S will be consistent, and also the courses and wales per unit length (inch or cm). In other words, all three dimensions will be consistent. These dimensions can be calculated from loop length and yarn count. S, and courses or wales per unit length are independent of yarn count over the ranges normally used in apparel, and unless count itself varies, the weight and widths and lengths will be consistent from delivery to delivery, if there is adequate control of loop length and of finishing.

This is not to say that variations in yarn properties do not influence the knitted loop length. However, if machines are correctly adjusted to avoid machine variation, and knitting adequately monitored as it proceeds, loop length variation can be minimised. From fabric geometry considerations, loop length control and finishing control are vital.

Courses and Wales per cm or inch

Courses and wales per cm or inch are measured by placing an inch or centimetre glass on the fabric, and counting the number of courses and wales, which are contained within the area. The values vary if the fabric is distorted as the example shows.

Fabric stretched in direction of wales

Undistorted fabric

Fabric stretched in direction of courses

Variation

Courses per inch, C

Wales per inch, W

Stitch density, S

(C x W)

18.5

296

374

11.5

287

36%

32%

23%

The product of the two, that is the stitch density (represented by S) is less variable than either c or w alone.

Measuring Loop Length with a Course Length Tester

The length of one loop is very small and it is easier to measure the length of yarn in a number of loops. The length of yarn unroved from fabric can be measured on a Course Length Tester.

A weight is suspended on one end of the yarn to remove yarn and knitting crimp of crimped yarns or the knitting crimp of spun yarns.

The appropriate weights or tensions to be used are: -

IN METRIC (S.I. UNITS)

IN TRADITIONAL UNITS

YARN TYPE

Spun

Filament

LINEAR DENSITY

Above 100 tex

Between: 30-100 tex

Below 30 tex

All

TENSION

150mN

100mN

50mN

20mN/tex

COUNT

Below:

9's worsted)

or 6's cotton)

Between:- 30's - 9's)

0r 20's - 6's)

Above:- 30's worsted)

Or 20's cotton)

All

TENSION

15 grams

10 grams

5 grams

0.2g/den.

Notes

The above tensions are quoted in B.S. 5441.

The resultant tex or count of the yarn (i.e. depending on the No. of plies) must be used.

1mN = 0.102 grams tension

therefore 20mN/tex = 0.204g/dtex

IMPORTANCE OF FINISHING AND RELAXATION

Finishing affects the state of relaxation and therefore the loop shape. The ultimate relaxed state is the "fully relaxed" state, in which the loops are in an equilibrium state. When this is achieved - such as by washing and tumbling - no further shrinkage occurs, except felting. However, it is difficult to reach this state

because the last stages of relaxation are progressively more difficult to accomplish,

any tension imposed during subsequent handling will undo some of the relaxation,

achievement of pristine presentation, - i.e. removal of creases and wrinkles, - will involve some degree of tension.

However, this state is likely to be reached if the article is to be washed in service. A feature of this state is that contraction of the loop is always involved owing to the increased three dimensional curvature, (S is at a maximum) so that some area shrinkage always occurs on developing full relaxation.

Therefore the finishing processes should be designed as far as practical costs permit, to provide a relaxation which is as nearly complete as possible, if shrinkage in service (i.e. domestic washing) is to be avoided.

Subsequent processing must be designed to avoid excessive tension on the material especially with single jersey and rib structures relatively sensitive to distortion.

One of the great assets of the knitted structure is its relative extensibility, i.e. movement and distortion of the knitted loop. Whilst a high degree of recovery can be achieved after this distortion, it is not generally complete. If the tension is maintained for a period, a degree of set develops. So in this respect finishing of knitted goods needs perhaps greater control than with woven materials.

FABRIC GEOMETRY OF COMPLEX YARNS AND STRUCTURES

Continuous practical studies of various fibres and yarn types, including continuous filament textured yarns and of knitted structures more complex than plain and 1x1 rib knitting, have led to the following general conclusions: - The dimensions of a fabric knitted from a particular yarn depend mainly upon three factors -

the bulking contraction of the yarn

the loop length (or loop lengths if more than one loop or course length is employed, or average loop length over a pattern repeat of jacquard)

the nature of the finishing treatment.

Only in complex structures and with large changes in yarn count is there any significant influence on dimensions from count variation.

It is therefore apparent that in all knitted fabrics it is of paramount importance that consistency be maintained as far as it is economically possible in these three factors and that the spinner, knitter and finisher have a major contribution each to make to the achievement of consistent dimensions.

IN CONCLUSION

It is difficult to understand how knitted goods behave on finishing and during domestic washing, because of the inter-action of so many factors.

All knitted articles comprise a number of knitted loops. The knitted loops can be regarded as the structural unit and a rectangle can be drawn to contain this unit. Whatever affects the shape and size of this rectangle, and however it is affected, will affect the size of the knitted article correspondingly. If the effect on this rectangle is known or has been determined, and since the number of these in the width of the article and in the length can be determined, the dimension of the article can be predicted.

This unit rectangle containing the knitted loop has relevant two parameters:

its shape

its size

The shape is mainly determined by the treatment history of the article. Knitted goods are produced in the knitting machine under tension. In service they are also subjected to tensions both widthways and lengthways. The great virtue of knitted goods is their extensibility, - their ability to accommodate both shape and movement. On removal of these tensions, the structure in theory, should spring back, to its "natural" shape and dimensions. In practice friction at the various points of contact between loops - the shaded areas in the diagram - and between fibres in the yarn, precludes the complete return to the original state. During movement, scouring, and various relaxation procedures, the fibres and loops are jostled and friction progressively overcomes, enabling the "natural" relaxed shape to be gradually achieved. Even on just standing, some degree of relaxation occurs, the unit rectangle changing shape.

Diagram showing points of contact (shaded) between the dotted loop and its neighbours.

The size of the unit rectangle is mainly determined by the length of yarn in the loop - e.g. the dotted portion in the diagram, i.e. the loop length. If this increases, say, by 10% then the height and width of the rectangle both increase by 10%.

This is why it is important to control the knitted loop length to within reasonable tolerance limits.

Relaxation also affects size. Consider a fully relaxed fabric: if this is stretched width-way by 10%, then the wales per cm will fall by 10%. The compensatory increase in courses per cm is, however, much less, - between 5-8%. Thus the unit rectangle has grown in size. A similar effect occurs if the fabric is stretched length-way. The rectangle containing the loop increases in size on any stretching of the fabric. During relaxation this effect is reversed. The knitted loop is three dimensional and as the fully relaxed state is approached the yarn in the loop is accommodated in a progressively smaller rectangle.

PRACTICAL EXAMPLE

A long roll of knitted fabric was taken from a knitting machine, knitting satisfactorily from one batch of yarn and on positive feed. It was immediately cut into 5 separate lengths which were then treated as described below. A short length was cut from each of these lengths for the determination, by unroving and measuring, of the average loop length of the feeders for that particular separate length of fabric. The courses and wales per cm were determined for each separate length by counting under a piece glass in several places to obtain a representative average. The treatments and results are shown in the following table. The fabrics were handled carefully to minimise errors through distortion or premature relaxation.

Fabric Length

Courses per cm

Wales per cm

Stitch density (per sq. cm)

Loop length (in cm)

A) Off-machine (within the hour

B) Dry static relax. (24 hours relax)

C) Full wet relax

D) Stretched lengthways (from off-machine)

E) Stretched widthways (from off-machine)

12.4

14.1

15.45

11.5

16.5

11.4

10.06

11.85

12.0

7.9

141.4

149.5

183.1

138.0

130.4

0.357

0.358

0.356

0.360

0.359

Although the loop length varies by less than 1% the other measurements vary over a range of 25% or more and have no real meaning until the fabric is in a stable, relaxed condition such as fabric C). These figures will obviously also influence the fabric weight per square metre.

As a guide, we can make the following approximating statements: -

DIAGRAMS go in here

SUMMARY

Structures knitted from stable fibre yarns not subject to yarn shrinkage after knitting - e.g. cotton and worsted spun yarns

The loop shape (i.e. shape of the rectangle):

After distortion the loop shape can usually be restored by washing, steaming, pressing etc. PROVIDED the distortion is not severe enough to have stretched the yarn or fibres.

The finish or history of the article has a dramatic effect on loop shape.

The type of yarn has a significant effect. This is established during sampling and fabric development. Maintenance of consistency of yarn (i.e. avoiding changing from one yarn type to another, - e.g. changing a cotton yarn for a blend) and then consistency of the other factors considered here will control this factor.

The count of the yarn does not have a significant effect with simple structures such as plain knit or 1x1 or 2x2 rib, unless we are considering tightly knitted fabric. With more complex structures such as interlock, cardigan etc. yarn count does have an effect. (Yarn count obviously has a direct effect on weight).

Loop length over a considerable range (say +-10%) does not have a significant effect on loop shape, unless the knitting is already very tight. The loop size:

The relaxation state and type of finishing process affect loop size as well as shape, i.e. the stitch density of the fabric.

The knitted loop length has a direct effect on loop size, and a proportional effect when the other factors above are unchanged, e.g. yarn type, finish and knitted structure are unchanged.

Structures knitted from crimp textured yarns such as false twist and K.D.K.

The above considerations are modified by the yarn relaxation occurring after knitting. The higher the crimp effect (e.g. Shirley Tube Test figure), the smaller the unit rectangle. Also, the denser the structure the less is the effect of crimp variation. The relationship is a very complex one, usually resolved by fabric development trials.

Again, however, control of finishing, knitted loop length, and yarn consistency give the best control of fabric consistency.

RELATIONSHIP OF WEIGHT PER SQUARE METRE IN THE GREY AND STITCH DENSITY IN THE GREY

Frequently weight (mass) per square metre is used as a routine quality control check on grey fabric. On finished fabric this is perfectly meaningful as the fabric should have been relaxed and stabilised in finishing. In the grey, however, the fabric is still relaxing and the figure obtained will vary with the history of the sample of the grey fabric. Wt. of a roll of a stipulated no. of revs. is much more meaningful. If it is not feasible to doff rolls at a stipulated predetermined no. of revs, then weight per square metre can be used if it is in conjunction with the stitch density.

The weight per square metre is determined by cutting circles of a known area from the fabric, at different places across the width. Weighing these and then dividing by the total area of the circles gives the weight per square metre.

The stitch density of these self-same circles, must be determined in the same state as when cut, i.e. at the same time and with careful handling to avoid distortion.

The weight/sq. metre divided by stitch density will equal a constant for a particular fabric so long as it is of correct quality, and this can constitute a Q.C. check.

Example

Data: -

Test fabric: -

Mass of 5 circles - 7.765g

Area of 5 circles - 5 x 100 sq. cm, i.e. 500 sq. cm

Wales per 3 cm 42, 43, 42.5, 41, 43.5 (AV = 42.4)

Courses per 3 cm 54, 56, 55.5, 53.5, 53 (AV =54.4)

Specification: -

Mass/Sq. Metre 145g

Wales/3 cm 43.5

Courses/3 cm 49.5

Wt/sq. metre: stitch density constant 0.6370 - 0.5764

Calculation of the fabric constant for the specification: -

Mass/sq. metre 145g

Stitch density 43.5 x 49.5 = 239.25 /sq. cm

3 3

Constant = 145 239 = 0.6067

Tolerance = + 5%, i.e. + 0.6067 x 5/100 = + 0.0303

Constant limits are 0.6370 - 0.5764

Test fabric figures: -

Mass/sq. metre 7.765 x 100 = 155.3g

If + 5% tolerance is applied here with no regard to the fact that fabric may have relaxed, 5% tolerance on 145 is 152g i.e. fabric is regarded as too heavy.

Stitch density from specification is 43.5 x 49.5 = 239.2

3 3

Stitch density from test fabric 42.2 x 54.4 = 256.2

3 3

which is high

Therefore: - Calculate fabric constant: -

155.3 = 0.6062 which is within tolerance limits.

256.2

Therefore, fabric is satisfactory for weight and not over weight as would otherwise have been deduced. By the same token, fabric can be registered as correct when in fact it is incorrect for weight. Again calculation of the fabric constant will protect you from this.

THE NEED FOR YARN TENSION AND ITS EFFECT ON FABRIC QUALITY AND ON KNITTING PERFORMANCE.

THE NEED FOR YARN TENSION

Because yarns for apparel are highly flexible materials, the only way of keeping them under control is by keeping them taught. In the absence of tension the yarn is uncontrolled and would fail to locate in the needle hook and to knit.

LATCH NEEDLE KNITTING - CIRCULAR MACHINES

Here a low yarn tension before the feeder, - say 3-5g - is used. However, from the yarn package to the point of maximum knock-over depth, the total angle of wrap (i.e. bends in the yarn path) may be of the order of five complete turns even on a single cylinder machine and higher on a dial and cylinder machine.

DIAGRAM

LATCH NEEDLE KNITTING SEQUENCE

At each one of these contact points friction causes a build-up of tension. Also variations in the extensibility (modulus) of the yarn cause variations in tension as the yarn loop is drawn by the needle down to maximum knock-over depth. The tension itself is not critical unless it is in the region of the breaking load of the yarn. It is the degree of variation that matters, and we are depending on the quality of the yarn package.

The effect of tension variation can be understood by reference to needles, P, Q, R and S in the diagram. The former two needles are moving down to maximum knock-over depth and therefore taking yarn into the knitted loop. The latter two needles are moving back to the rest position and therefore spare yarn is available. This yarn is taken up by needles P and Q which therefore, in forming the knitted loop, take yarn partly from the yarn package (feeding) and partly from the loops in needles R and S (robbing-back). An increase in take-down tension reduces the degree of robbing-back and an increase in input tension increases the degree of robbing-back markedly.

The take-down tension should be set to retain the knitting on the verge. The yarn input tension is as light as possible consistent with adequate yarn control. This keeps needle and cam wear to a minimum and reduces the occurrence of yarn breaks. The knitting loop length control is effected by adjusting the knock-over depth. However any significant change in yarn input tension will inevitably change the loop length. Therefore in developing the knitting specification for the fabric, it is important to select yarn for sample knitting that is believed to be as nearly average as possible. The usual 2-3% tolerance will then absorb much if not most of the yarn input tension variation. Positive feed avoids the effect of yarn tension variation on those feeders on which it can be employed. Many types of yarn storage feed, remove off-wind tension variations and reduce variations due to friction.

LATCH NEEDLE KNITTING - FLAT MACHINES

Yarn tension and take-down tension have a similar effect on the knitting, but there is an additional factor. This is the yarn take-up spring, which has to be set up to maintain yarn tension at each carriage direction reversal. The disk tension is set up to prevent the take-up spring drawing yarn from the package in between knitted courses. The yarn input tension is therefore much higher than for circular knitting, e.g. 10-20 grams. To off-set this, it is desirable that the friction of the yarn is low. Also, it is desirable for it to be uniform, since positive feed is not available.

Yarn storage feeds or other devices can be fitted to reduce yarn tension variations.

STRAIGHT-BAR (FULLY FASHIONED) KNITTING

No robbing-back occurs, and therefore the knitting is less variable, and take-down has very much less effect on the course length. Increasing yarn input tension will, however, reduce the course length knitted. This is shown in the diagram. The action of coarse gauge machines (15g and below) is relatively simple and will serve to illustrate the effects of yarn tension.

DIAGRAM

As the yarn tension increases, the pressure of the yarn against the needles is much tighter and the course length knitted shorter. A further increase in tension causes the needle stems to bend towards the plane of advance of the sinkers, again reducing the course length knitted.

EFFECT OF KNITTING MACHINE SETTINGS ON YARN TENSION

Generally with all weft knitting, the longer the loop length knitted, the more tension is generated. Because of the increased number of yarn/knitting element contact points encountered, higher friction is created.

If the yarn breaks, it is the loops in the new course where rupture occurs, resulting in a fabric hole and a local press-off.

Where positive feed is employed, the cams are adjusted to achieve the correct input tension and not stitch length, this being controlled by the positive feed. The correct tension immediately after the positive feed device is one slightly greater than that occurring just before the device. Failure to achieve the proper tension can cause a number of problems through high tensions or inadequate yarn control at the needle.

With double-bed latch needle knitting, delayed timing, - in which the back-bed needles travel to maximum knock-over depth while the front-bed needles are returning to their rest position, - can help to reduce the knitting tension, because yarn is not being drawn into both sets of needles simultaneously. The knock-over depth of those needles first supplied with yarn must be sufficient to furnish yarn for both needles, otherwise tensions will increase rather than diminish with delayed timing. The correct settings of knock-over depth and timing for a given course length and positive feed setting are ascertained by experience with a particular type of machine.

When the course length knitted is very short (tight knitting) the action of drawing yarn though the previous loops also demands more energy and gives higher tensions due to the higher friction generated.

With positive feed, the cams are adjusted to achieve the correct input tension.

When knitting tightly, if the yarn breaks it is generally the loop in the previous course, (the course being knocked-over) which ruptures, resulting in a hole but not a local press-off.

With finer gauge machines, the effect is substantially the same, if the appropriate count of yarn for the gauge of machine is used. However, the yarns generally are weaker at finer counts, and therefore breaks will occur at lower tensions.

SOME IMPORTANT FACTORS AFFECTING LOOP LENGTH

WITH POSITIVE FEED

Even is positive feed is used, problems can sometimes arise in maintaining loop-length control.

Lack of care in controlling yarn tension.

Failure to control this could lead to the development of very high tension and could cause one or both of two things to happen: -

increase in yarn breaks and machine down time

yarn slippage through the positive feed

High yarn tension can arise from: -

incorrect threading up or dirt and fluff in the yarn path - i.e. poor house keeping

ii. tilted cones

iii. poorly wound cones

iv. incorrectly set tensioners

Polished, worn or incorrectly inserted driving tapes.

If the tape surface is too smooth, yarn will slip and positive control ceases. A checking and replacement schedule is needed. The frequency of the schedule will largely depend on the yarn being knitted.

Regular checking of the course length with a yarn length meter, and noting the positive feed setting, will indicate when the feed is incorrect and needs changing.

In a well run installation these checks need not be frequent (say monthly) but must not be overlooked.

Drift of instruments (e.g. yarn length counter) used to check the positive feed.

The instrument used to check the positive feed can be wrong through any of the following causes:

instrument being dropped or mishandled or not used for a long time leading to a malfunction - e.g. by a stiff or bent spindle

Driving pulley becoming polished so that yarn slips

Insufficient contact (i.e. angle of wrap) around the pulley, leading to yarn slippage

Worn pulley.

Looking at each in turn: -

If this happens, spin the pulley and note if stiff or whether there is malfunction in the dial reading. Finally check against the unroved course length as described below.

If the pulley becomes polished, again check against unroved course length, and if necessary clean by carefully rotating against 0000 emery, and rechecking against the unroved course length.

Ensure the jockey pulley is located to give an angle of wrap of the yarn around the driving pulley of at least 180o and preferably 270o. Care should also be taken to ensure the yarn path to and from the instrument is not significantly altered from that existing before inserting the instrument.

d) This can only be checked against the unroved course length.

Checking instrument against unroved course length.

The positive feed checks the knitting; the instrument checks the positive feed; the unroved course length checks the instrument. Thus while no individual check is totally reliable, one can trust the total system of checks. Also, on this basis, the need for checking the instrument by unroved course length checks becomes very infrequent, say, every 3, 6 or possibly 12 months, yet must not be overlooked.

The procedure is as follows: -

Each instrument in use and yarn type and source should be checked at regular intervals. Say there are 2 instruments, 10 different yarn types, on average from 2 spinners then, checking each combination say once a year, would involve 40 checks a year. Experience would teach which type of yarn is likely to give inaccuracies, and so a priority order can be made.

When starting the check, choose an accessible feeder, on a machine with as few a number of feeders as possible, and mark the yarn with a little contrast coloured thread.

Insert the instrument into this yarn path. Run the machine and note the yarn length over a set number of revs. Repeat a few times to ensure the reading is consistent. If not, the instrument must be serviced.

Calculate the course length reading from the average yarn length reading and the number of revs.

Stop the machine and remark the chosen yarn.

Knit the fabric through, and remove fabric including the two marked courses in the fabric cut off.

Unrove, discard the unmarked feeders, and determine the course length as usual. Take a number of readings to ensure the reading is reliable.

Compare the unroved course length with the reading from the instrument.

If virtually the same, no action.

If significantly different (e.g. more than 0.5%) this may be due to factors not affecting the instrument, for example the knitting tension and the tension used in the unroving test being different. So long as the % difference between the instrument reading and the unroved course length remains constant, this is allowed for and there is no problem. Check if the observed % difference is different from that observed in the previous check with this yarn. If not, then no problem. If different get the instrument serviced.

Use of a count testing wrap-reel.

If a wrap-reel is at hand, a much simpler procedure is available as follows: -

Place a cone of yarn being knitted on the count tester's creel, threading-up as for count testing, and adjust the reeling tension to that used in course length tests by adjusting the reeling tensioners and checking the tension with a tensiometer. The reading on the tensiometer will fluctuate quite wildly, but observation of the reading over a period of a few seconds will enable the average figure to be estimated, and the tension adjustment is then made to bring the figure to that required with this yarn. The wrap reel is now zeroed and the yarn length counter inserted into the yarn path at the same place as where the tensiometer has been inserted. Make a known number of wraps (e.g. 100) and note the instrument reading. If the reel has a metre girth, the instrument should read 100 meters if the girth is 1 yard it should read 100 yards. If not, then the instrument is giving incorrect readings.

Repeat the reeling to ensure that there has been no error in reeling.

IN THE ABSENCE OF POSITIVE FEED - LATCH NEEDLES

Because the needles are drawing yarn from the packages into the knitted loop while moving down to maximum knock-over depth, it is possible for yarn to be also drawn into the loop from the previous adjacent needle while this needle is moving back to the rest position. This latter affect is called robbing back.

If the yarn tension increases, the needle moving to maximum knock-over depth will take more of its yarn from the previously formed loop so increasing the degree of robbing back and giving a shorter course length.

If the take-down tension increases, this tension will apply to all the needles, but with the needle moving back to rest, the tension will be mainly on its newly formed loop because it has just knocked-over. With the needles moving down to maximum knock-over, the tension will be mainly on the old loop, until the new loop knocks-over. Therefore the increased take-down tension tends to increase the yarn tension in the loops after reaching maximum knock-over and this reduces the degree of robbing back, giving a longer course length.

Take-down tension should be set to be minimal, to reduce wear, to suit the particular knitting machine, i.e. control the knock-over of the old loop and to hold the knitting plane onto the verge.

Yarn tension should also be minimal to reduce wear and the risk of yarn breakage and hence machine down time, but be sufficient to control the yarn adequately, i.e. to effect correct loop formation.

WITH STRAIGHT-BAR (FULLY FASHIONING) MACHINES

Here an increase in yarn tension can: -

alter the shape of loop while being formed by the sinker and being a tighter loop will give a shorter course length.

flex the needle stems, and so reduce the distance between the stem and the advanced sinker. This will also give a shorter course length.

Because the loop length is, in effect, inserted before knitting, take-down, operated mainly on the knocked over loops and has less significant effects than with latch needle knitting. However, an increase in this tension can still have some effect, as it will also tend to flex the needle stems towards the plane of advance of the sinkers, i.e. reduce the distance between the stem and the advanced sinker. Thus with straight bar knitting, increased takedown tension is likely to reduce the course length. Again take-down should be adjusted to suit the machine.

FACTORS AFFECTING THE YARN TENSION

The above effects leave us to consider factors influencing yarn tension. These can be itemised as below: -

Yarn path irregularity - e.g. incorrect threading up

Yarn path obstructions - e.g. accumulations of lint at guides, feeders, stop motions, tension controllers and positive feeds

Incorrectly aligned cones or wrong positioning of the guide immediately above the cone

Incorrectly wound cones. When irregular tension is suspected, if items 1, 2 and 3 are okay, check visually and by feel, the suspect cones.

Off-wind tension variation from incorrectly wound cones. Inspection and experience will show these up, and they should be returned for re-winding. Yarn storage feed tension controllers will control yarn tension variation from this cause.

Coefficient of Friction. If this varies, even with the use of yarn storage feeding, the yarn input tension will vary, giving rise to increased yarn breakage and knitted quality variation. Preferably, the variation should not be more than 0.04 and the coefficient and tolerances should be agreed and specified with the spinner.

It can be tested for, along with count testing, quite simply.

Yarn modulus. This is the degree of stretch: if too high, the yarn becomes brittle. This is best controlled by purchasing from reputable spinners.

Condition or moisture. This can affect both the friction and the modulus.

An increase tends to increase the friction (but not always) and reduces the modulus. That is why if the yarn is too dry it is difficult to knit.

This is best controlled by buying from a spinners and storing yarn in undamaged, unopened containers in a dry area not subject to vast changes in temperature.

CORRELATION AND CALIBRATION OF CERTAIN INSTRUMENTS FOR KNITTING Q.C.

Yarn tension - tensiometers

Pulley, i.e. mechanical tensiometers, such as Zivi, Mercer, Schmidt are quite suitable for checking yarn tension in machines where there is a reasonably steady run-in of yarn, e.g. in circular knitting.

Whilst experienced knitters and mechanics can level the tension between adjacent feeders by manual assessment ("fingering"), it is found that such assessments are unreliable to level over a suite of machines. One can say the "finger" levels the feeders and the tensiometer keeps the finger level.

However, the tensiometer can become unreliable, through wear, misuse, accident, or lack of use, and this should be checked periodically, - say once a month. This is carried out by: -

checking the free movement of all pulleys and sensor (load detector) arm by gently tweaking and observing the movement for stiffness or uneven action.

hanging a length of knitting yarn from a hook and suspending on the other end a known weight, e.g. 5g. Thread up the tensiometer in the normal way. Do this several times and observe the reading. Ideally, it should be within 5% of the weight, positive difference in one direction and the same negative difference in the other.

2. Yarn Length Counter

Generally the reading of a yarn length counter will not drift. However, once in a while it is as well to confirm it. The positive feed checks the knitting: the instrument checks the positive feed; the unroved course length checks the instrument. While no individual check is totally reliable, one can trust the total system of checks. The need for checking the instrument by unroved course length is very infrequent, say, every 6 or 12 months, MUST NOT BE OVERLOOKED.

The procedure is as follows: -

Each instrument in use and yarn type and source should be checked at the agreed frequency. Say there are 2 instruments, 10 different yarn types, on average from 2 spinners then, checking each combination say once a year, would involve 40 checks a year. Experience would teach which type of yarn etc. is likely to give inaccuracies, and so a priority order can be made.

Choose an accessible feeder, on a machine with as few feeders as possible, and mark the yarn with a little contrast coloured thread.

Insert the instrument into the yarn path. Run the machine and note the yarn length over a number of revs. Repeat a few times to ensure the reading is consistent. If not, the instrument may not be properly in the thread path.

Calculate the course length reading from the average yarn length reading and the number of revs.

Stop the machine and remark the chosen yarn.

Knit the fabric through, and remove fabric including the two marked courses. Unrove, discard the unmarked feeders, and determine the course length as usual. Take a number of readings to ensure the reading is reliable.

Compare the unroved course length with the reading from the instrument.

If virtually the same, no action.

Use of a count testing wrap-reel.

If a wrap-reel is at hand, a much simpler procedure is available as follows: -

Place a cone of yarn being knitted on the count tester's creel, threading-up as for count testing, and adjust the reeling tension to that used in course length tests by adjusting the reeling tensioners and checking the tension with a tensiometer. The reading on the tensiometer will fluctuate quite wildly, but observation of the reading over a period of a few seconds will enable the average figure to be estimated, and the tension adjustment is then made to bring the figure to that required with this yarn. Zero the wrap reel and insert the yarn length counter into the yarn path in place of the tensiometer. Make a known number of wraps (e.g. 100) and note the instrument reading. Thus if the reel has a metre girth, the instrument should read 100 meters. If not, then the instrument is giving incorrect readings.

Repeat to ensure that there has been no error in reeling.

2.1 CALCULATION OF THE CORRELATION FACTOR BETWEEN THE YARN LENGTH COUNTER AND THE COURSE LENGTH TESTER

If in the correlation checks, the instrument is found to be quite consistent, i.e. the readings are consistent within themselves and with the results obtained on the previous checks the instrument can be regarded as satisfactory even if those readings are not identical with the figures obtained on the knitting machine during the course length check.

Some result must be regarded as standard, since the results on the Course Length Tester or Wrap Reel are carried out at a defined tension. On the knitting machine, the tension when forming the loop is not defined, so the latter cannot be an absolute standard. We therefore relate all results to that obtained in the course length test.

To calculate the % correlation factor: -

If L is the average course length figure from the course length or wrap reel test, and L1 that obtained on the knitting machine then: -

The % factor, F, = L x 100

Example: -

Data: Course length from course length tests 6.73 metres

Course length from instrument on knitting machine 6.48 metres

F = 6.73 x 100 = 103.86%

6.48

corrected figure is actual reading x 103.86 - 100

3. Yarn Speed Meter Correlation

Some Yarn Speed Meters can be switched to yarn length counting, and since it is the same pulley drive, these can be checked as yarn length counters as in 2 above.

Otherwise it is difficult to check a yarn speed meter, since yarn speed is a difficult parameter to measure. Yarn speed meters can be checked, but only

a) against yarn length counters (either a separate instrument or by switching to the yarn length counting mode if this facility is available).

b) as course length meters, if the actual speed (no. of revs.) of the machine can be determined.

Example A - Use against yarn length counter.

Insert the yarn length counter in an active feeder and note the yarn length reading over, say, 10 courses. This reading - 10 equals the course length. Immediately insert the yarn speed meter in the same feeder at the same point and with the same configuration of the yarn path. Note the reading. The machine speed will not have changed in this small interval of time. Use this figure of the yarn speed to level all the corresponding feeders around the machine, to within the agreed tolerance (e.g. + 2%).

Data: - Yarn length per 10 revs. 43.8 metres

Yarn speed 1314 cm/min

Tolerance + 2%