INTRODUCTION

There are three dimensions of major importance:-

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 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.

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: -

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.

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.

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

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

Specification: -

Mass/Sq. Metre 145g

Wales/3cm 43.5

Courses/3cm 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

5

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 180^o and preferably 270^o. 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.

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.

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. 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 L1that obtained on the knitting machine then: -

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).

or

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%

Upper limit of yarn speed = 1314 + 1314 x 2 = 1340 cm/sec.

100

Lower limit of yarn speed = 1314 - 1314 x 2 = 1288 cm/sec.

100

Example B - When yarn length is required from a yarn speed meter reading.

Note the time for 100 revs. or no. of revs. in 1/2 minute. preferably 1 min. Repeat till a consistent figure is obtained. From the specified course length and tolerances, calculate the upper and lower limits for the yarn speed.

Data:- Specified course length 4.38 metres = 438 cm

No. of revs./minute 29.5

Tolerance + 2%

Yarn speed = 4.38 x 29.5 cm/min = 129.1 cm/min.

Upper tol. 129.1 + 129.1 x 2 = 131.7 cm/sec

100

Lower tol. 129.1 - 129.1 x 2 = 126.5 cm/sec

100

 

4. Correlation of Readings From Hatra Course Length Tester and the Shirley Crimp Tester

There are two main reasons why these two instruments could give slightly different readings even in the hands of skilled operatives.

The different method of gripping the ends of the yarn.

The different speed with which the test load is applied.

The resultant discrepancy could vary from yarn type to yarn type, and from yarn unroved from grey, steamed, scoured or set fabrics, but within any one of these categories should be constant. Therefore even in situations where there is a discrepancy between readings from these two instruments, this can be resolved by correlation.

Example: -

Data: 22g interlock knitted from 150 dtex or 1/67's metric cotton spun polyester.

Av. course length - Hatra Course Length Tester 9.14 m

Av. Yarn length/100 m - Shirley Crimp Tester 38.05 cm

Machine diam. 36", no. of feeders in cyl. 2484

Course length from Shirley Crimp Tester = 38.05 x 2484 = 945.2 cm

100

% correlation of Hatra figures/Shirley figures is 9.14 x 100/9.452 = 96.7%

Shirley figures = Hatra figures x 100/96.70.

4.1 Calibration of the cursor on the Shirley Crimp Tester

Place the calibration gauge with one end in each clamp so that each notch fits over the respective jaws, and allow the clamps to close.

Slacken the four corner screws holding the cursor. With the eye over the two red index lines aligned to avoid parallax, the cursor setting is correct if it registers exactly 6.00 inches. Gently tap the cursor until this is so, and retighten the four screws.

5. Calibration of the dial height gauge

The method often used to zero the instrument is to stand the instrument on a flat surface on the machine and rotate the scale until the pointer reads zero. Although the foot be pressed firmly on the surface slight rocking of the instrument causes significantly different readings to be displayed.

To avoid this, a calibration jig is made. It comprises a horizontal and a vertical plane arranged like a capital "L". The back of the instrument is pressed against the vertical face of the jig, and the foot of the instrument is pressed against the horizontal face. The scale is rotated to give a readout of zero.

To avoid any possible skewness of the dial of the knitting machine causing inconsistent readings to be obtained, the instrument is always inserted at the same dial segment on any given machine.

6. Correlation of knock-over depth gauge reading and course length knitted in order to set tolerances.

One of the main reasons for using the Knock-over Depth Gauge is to level the needle movement over every feed, between dial and cylinder (or back and front beds) of every machine knitting to a particular specification.

Normally the tolerance on knock-over depth is + 0.1 mm. However, to set realistic tolerances, the stitch length is determined at a particular knock-over depth with the yarn and machine gauge concerned. The cam settings on both beds are then readjusted by a uniform amount until a stitch length just out of tolerance results, and the new knock-over depth is noted. The difference between this and the original knock-over depth gives the maximum limit of tolerance.

In order to reduce the cumulative effect of variables, it is a wise precaution to endeavour to apply closer tolerances than that determined above, say, half.

Example: -

Data: Original course length 244 cm, + 2.0%

Original knock-over depth 4.2 mm

Final course length 250 cm

Final knock-over depth 4.4 mm

Advised tolerance is (4.4 - 4.2) - 2 = 0.1 mm.

 

ESTABLISHING CHECKS ON YARN DELIVERY WEIGHTS

Before materials control is secured, yarn delivery weights need to be continuously monitored. In the absence of checks serious losses may be incurred without their magnitude being realised. Accurate costings are impossible in these circumstances.

Weighing-in

All yarn entering the factory should do so over a weigh bridge or scales. Thus the gross weight of every case of yarn is known and is entered on a tally sheet by the Stormed and attached to the Delivery Note.

Example Tally Sheet

Spinner Yarn Description Entry No. Advice No. Date

---------------------------------------------------------------------------------------------------------------

Case No. Gross Wt. Case No. Gross Wt. Case No. Gross Wt.

Tare Weights

The tare weights given by the spinner on the Advice Note are rarely incorrect and can be used to obtain the next weight as received of each case of yarn. If any tare weight is abnormal a Tare Check is undertaken. The contents of the case concerned are knitted, all wrappings, bags, core formers, case liners etc. being reserved and weighed. This overall weight is the correct tare weight.

Nett Weight as received

The tare wt. of each case is deducted from each gross weight to give the nett weight of each case, as received.

This is checked against the Spinner's declared nett weight.

If, over the delivery, there is a discrepancy above an agreed amount, a claim is made. The discrepancies are recorded as a trend graph, to keep a cumulative check as the contract continues.

Condition - Oil content

If the yarn is hygroscopic (e.g. has a regain of 5% or over) such as wool or cotton and their blends with other fibres, before a claim is made, the tally, and advice note with yarn discrepancy note are sent to the lab.

Those cases in the delivery showing the greatest discrepancies are sampled - say, 2 cones per case - and the % correct weight is determined of each cone. If the results are in poor agreement 2 further cones are tested and the overall average taken. The four cones coming from the top, middle and bottom of the cone. If there is poor agreement between the cases, further cases are sampled. From this the % correct weight figure is calculated by which the received nett weight of each case is adjusted to give the nett weight at correct condition.

A conciliation for the whole delivery is made between the case nett weights so found and as declared by the Spinner. From this the final discrepancy is calculated, and claims are based on this.

If the yarn is oil spun, the samples taken for condition testing are secured free of oil and then oven dried to get the clean over dry weight. This is adjusted by an agreed figure based on this weight, representing the adjustment for both oil and water. In securing, the liquor is poured off through a fine gauze filter and any lint recovered.

Example Condition Test Form

-----------------------------------------------------------------------------------------------------------------

CONDITION TEST

Spinner Yarn Description Entry No. Advice No. Date

------------------------------------------------------------------------------------------------------------------

Case Sample Weight Agreed % correct Nett Weight

No. as recd. clean/ "Regain" weight as recd. at correct oven dry * cond.**

----------------------------------------------------------------------------------------------------------------

 

 

-------------------------------------------------------------------------------------------------------------------

Total Nett Weight at Correct Condition (kg)**

Invoice Weight (kg)

Discrepancy (kg)

-------------------------------------------------------------------------------------------------------------------

% Correct Weight or "wt. adjustment factor" is calculated as follows: -

Sample wt. oven dry x (100 + % regain)

Sample wt. as received

** Nett weight at correct condition is calculated as follows: -

Nett wt. as received x % weight adjustment factor - 100

The regain to be used in the above calculations is the standard regain or the "% addition to the dry clean mass or wt." as given in and BS 4784: 1973 depending on the fibre and method cleaning and if cleaned. With fibre blends the resultant % regain allowance, or % addition to the dry mass, is determined from the figures for each component fibre and the declared or determined composition of the yarn.

Control and Recording

These checks, together with count tests (on staple fibre yarns) check that the correct amount of pure fibre at the right count is being received.

Both weighing in and count testing are necessary before this is secure.

The count testing should be carried out under the same conditions as that employed in weighing in - viz.

The results should be recorded on a trend graph or on a frequency distribution chart (i.e. a histogram), as count and yarn discrecpancy respectively.

The state of affairs can then be assessed quickly at any time as the contract proceeds. The overall or cumulative average is the most important single check in this respect, because the deviation of this from the costed average gives the picture of any overall loss or gain developing during the contract, so avoiding contracts and knitting programmes terminating with deficiencies in yarn. This cumulative average is readily determined from a histogram because the peak occurs sufficiently near to the average for routing purposes.

DIAGRAM

DIAGRAM

General Comments

The above scheme operates successfully in the firms where it has been installed. It should be appreciated, however, that agreement with the spinner would be required before installing such a scheme.

INTRODUCTION

All products are subject to variation militating against consistency, due to variations in raw materials processes, operatives, conditions, measuring etc. The likelihood, therefore, of a product always being spot-on, is very remote. However, the variations are just as likely to occur giving results too high as too low, - the results will vary in a random manner. This variation is not chaotic; it obeys a definite law of nature which we can exploit. This law is so fundamental to an understanding of what Q.C. is all about, that there is wisdom in a brief study, even when one is less heavily involved with measurements and their tolerances than with faults and their assessment or comparison against standards.

LAW OF RANDOM DISTRIBUTION

This is the law to which we are eluding. If repeated measurements of any particular factor are taken as production proceeds, the results will be found to cluster around a figure we call the average, with a spread of results on either side with diminishing frequency as we move away from the average. In fact if we draw a graph for a particular set of measurements, - i.e. plot the results - with the range of spread (from well below the average to well above) shown horizontally, and the frequency or number of occasions of a particular figure occurring as a result, we get a curve like that shown below, to which the vertical height or frequency increases rapidly to a maximum as we approach the average and then falls off again as we pass beyond this. It is for this reason, as we shall see later, that the specified tolerances ought to bear some relation to this peak in the curve.

DIAGRAM

TOLERANCES

Now if we fix limits within which we will accept a product as satisfactory, then clearly, the further apart and on each side of the average we set these limits or "tolerances", the greater will be the proportion of production that is accepted.

From the above curve, a position can be ascertained at which we can set the tolerances, so that the seriously wrong articles can be rejected while retaining the minimum reject rate. If tolerances are planned so as to occur at the positions equivalent to c and c on the graph, it can be seen that the curve flattens out quickly at this point as we move further away from the average and steepens quickly if we move towards the average from c. Here generally is the optimum position for the tolerance limits, and here 95-96% of the production becomes acceptable, 2% rejected with measurements regarded as too low and 2% rejected with measurements too high.

Conversely, if tolerances are fixed, e.g. by agreeing on a level of acceptance, then the reject rate ensuing can be calculated and accurately costed into a process.

STANDARD DEVIATION

The spread of results will affect the curve, - from being a tall narrow hump, the ideal situation - to being a low flat lump, an undesirable situation. The Law of Random Distribution enables us to define what governs particular curve or distribution of results. There are, in fact, only two factors, - viz. the average and a factor called Standard Deviation. This factor can be calculated from the curve or from a set of results, and is important because many decisions about measurement, such as location of tolerance limits, stem from a knowledge of standard deviation.

The higher the figure the greater the spread of results, and it is calculated in the same units as the results or measurements to which it refers, or as a percentage of the average of the measurements. It therefore represents a certain zone of spread or deviation area, in which a certain proportion of the results will always occur. In the curve on the previous page zones have been drawn at a, b, c and d, each side of the average equal to 1 x SD, 2 x SD 3 x SD and 4 x SD and the zones marked 1 to 4. The proportion of the results always occurring between the limits bounded by these zones is also shown. It can be seen that where we discussed locating the tolerances was, in fact, at a position equal to 2 x SD.

Example to show the principle behind calculations of SD - I the SD of a set of results from a sample of measurements.

Consider two deliveries of yarn of nominal count of 2/32's: -

A

Test results Deviation Square of deviation

of count (x) (x-x) (x-x) 2

32.3 max 0.3 0.09

32.0 0.0 0.00

31.9 -0.1 0.01

32.2 0.2 0.04 Standard deviation

32.0 0.0 0.00 of test results. 31.9 -0.1 0.01

31.9 -0.1 0.01 = 0.028

32.2 0.2 0.04

31.8 min -0.2 0.04 = 0.17

31.8 -0.2 0.04

Mean 32.0 = x Total 0.28

Mean 0.028

 

B.

Test results Deviation Square of deviation

of count (x-x) (x-x) 2

32.4 0.4 0.16

32.1 0.1 0.01

31.5 min -0.5 0.25

32.2 0.2 0.04 Standard deviation

31.6 -0.4 0.16 of test results.

31.9 -0.1 0.01

32.6 max 0.6 0.36 = 0.134

31.5 -0.5 0.25

32.3 0.3 0.09 = 0.37

31.9 -0.1 0.01

Mean 32.0 = x Total 1.34

Mean 0.134

II the SD over a total delivery or contract

The best estimate of the standard deviation of total delivery (total population) is obtained by dividing by one less than the number of test results. This number is 9 in the above two examples, since the total number of tests were 10 for each delivery. Therefore the standard deviation of the deliveries are: -

A 0.28 = 0.033 = 0.18

9

B 1.34 = 0.149 = 0.39

9

Standard Deviation

The calculation of standard deviation can be expressed for any measurements and any number of test results.

Notation

x an individual measurement

x average or mean of a set of measurements

total

n number of measurements in the set

sd standard deviation

Calculation

Tolerances can be used to decide courses of action.

If the machine average drifts, the machine will be producing work with an abnormally high reject rate. Tolerances will indicate when adjustment is needed and by how much.

DIAGRAM

The right hand curve represents work from a machine whose average has drifted. The increased work out of tolerance can be seen.

In order to contain and check any such drift without the dangers of over correction, the required action can be summarised as follows:

1. If work is within tolerance: -

normally no action (see under 3 below).

2. If work on a check is out of tolerance: -

Repeat check. If now within - no action. If still out, adjust machine by an amount expected to bring work just within tolerance. *

b. Continue checks until work is within tolerance.

This avoids over-correction.

*Before making any actual adjustment, however, it is wise to check that no obvious fault has occurred - e.g.: yarn path dislodged, dirt or lint caught -, and to rectify this and recheck. If still out, then make the appropriate adjustment.

3. If work on regular checking is consistently within tolerance but near one of the limits: -

Adjust by an amount expected to bring average right back to the specified figure.

Continue regular checking.

 

Although a certain percentage of work is always going to be outside the tolerances, - depending on their location compared with the spread of results, - the whole point of a Q.C. scheme is to prevent, as far as is practical, any drift in the average what so ever. In other words, there is no tolerance on the overall average.

EXPLOITATION OF STATISTICS IN THE DEVELOPMENT OF THE SPECIFICATION

In the above section we have seen how keeping the average of the knitting machines under control can effectively reduce the volume of work out of tolerance. We are in effect reducing inter-machine and inter-feeder variation.

This can be taken a step further and we can come closer to the adage "being right first time" with its implied message of giving a further reduction in waste through work being out of tolerance.

If we could select yarn for a machine in use for sampling that is "average" in its properties, and again do this when first setting up knitting machines on a production run, we would have all the machines knitting correct work, so long as the yarn was average. This would be marvellous!

We know however that yarn has a degree of variability, even when bought from a reliable source, and that this variability can be frequent (e.g. dye lot changes). Therefore machines known to be correctly set up can suddenly knit incorrect fabric and that the changes can swing rapidly. In this situation, the yarn can change by the time you have correctly adjusted the machine so you are "chasing your tail", and yet if you don't attempt machine adjustment you can be in trouble. So what should you do?

Yarn bought from a reliable source, though variable, will have a reliable average. Long term variation is likely to be under control.

General experience, coupled with tests and pilot knitting trials will indicate what yarn (blend or colour) knits "average" i.e. neither tight nor slack. It is a good plan to liase with the spinner here and to tell him of your plans, and he will respond because it is in his interests also to maintain satisfaction.

This "average" yarn should be selected for sampling and for setting up production machines.

The yarn properties (e.g. count, coefficient of friction, twist) should be ascertained and incorporated into the yarn specification, i.e. the properties of this "average" yarn.

The sample machine settings should be ascertained and noted and these incorporated into the knitting specification.

If production machines are of a different type from the sample machine, from experience and pilot knitting any variation in the settings from those of the sample machine are ascertained and entered into the knitting production specification.

Now from our study of random distribution and fixing of tolerances for yarn and for knitting, 95-6% of the work WILL BE correct from all the machines and all the yarn covered by these specifications provided and only provided we have taken the trouble to set up machines to be "spot-on" with average yarn.

Only 4-5% of the work will be found to be out of tolerance and even here there is a choice, because knitting machines, kept in good order, with the yarn path clear, do not change. If out of tolerance we can: -

change the machine yarn load, so concentrating "wrong" yarn onto one or two machines, so concentrating problems to one corner and where the work may be planned for an easier market.

Adjust the machine to compensate.

All such adjustments should be noted, so that the machine can be adjusted back to the average settings as soon as this yarn is knitted, and the trouble taken in getting rid of inter-machine variation will not have been lost.

Replace and reject this yarn.

Accept the situation and cost for it.

The principle of this concept is that huge amounts of time, "admin", "aggro", yarn waste and lost production are involved through incorrect knitting due to inter-machine variation and yarn variation, and yet this could be reduced to the statistical 5% simply by sampling and setting-up on "average" yarn.

The trouble and cost of specifying and selecting "average" yarn is likely to be more than off set, by the reduction in waste during production. The attraction is that the extra time in sampling and setting-up is compensated by time saving in the reduction of production problems, so that the knitting order is completed on time but with less waste. Personnel take more time and trouble in sampling and setting-up to save more time and trouble in production, and the firm saves money on raw materials.

This is the attraction, and therefore there is a strong argument to put in such schemes on a trial basis.

RESULTANT STANDARD DEVIATION OF SEVERAL VARIABLES

The resultant standard deviation, although increased, is not the sum of the standard deviation of each variable. It is less, and is given by the equation:

S.D. = = [_sd ² = _sd1² + _sd2² +_sd3 ² + ---

Example:

3 variables of S.D. 1, 1½ and 2 respectively.

Resultant S.D. = 1½² + 1+ 2² = 7.25 = 2.77

and not the straight sum, viz.:

1 + 1½ + 2 which = 4.5

This principle is important in calculating the resultant S.D. and hence tolerance limits on such parameters as weight per doz. and fabric roll weight per set no. of machine revs.

PRINCIPLES OF LOOP LENGTH CONTROL

The action/no action sequence of routine checking in a control scheme will follow the flow diagram below.

Routine

Measurement

And Record

Inside limit outside limit

Just outside well outside

No action Take second Inspect yarns

Measurement and machines

And record

No abnormality abnormality

Inside outside

Limits limits

Rectify,

recheck

and record

Next adjust machine

routine and recheck

check until within outside

and limits, and limits

recording record

Inside

limits

Experience has shown that limits set at +- 2% of the average are generally satisfactory. However, in the absence of pre-fixed limits (e.g. commercial directories or customer stipulation), when experience of a control scheme has been gained, a good guide is to set limits so that about 4-8% of routine checks come out of tolerance on the initial measurement of each routine check.

When an Electronic Course Length Meter or other accurate instrument is available for checking knitting quality during knitting - i.e. on the knitting machine, a modification to the above scheme can give much better control with the consequent likelihood of a reduction in the number of times a machine needs adjustment.

Routine

Measurement

And Record

Inside limit outside limit

Just outside well outside

Take second Inspect yarns,

No Measurement machines and

Action And record fabric

No abnormality abnormality

Inside outside rectify,

Limits limits recheck

And record

Next Adjust machine and record:- outside

Routine 1. If average of all feeders limits

Is outside limits, adjust

All feeders by same amount

To bring average inside limits inside

Limits

2. If only certain feeders are

outside limits, change packages,

and recheck. Retain such

packages for rejection or knitting

speciality.

By this technique, machine adjustments are kept to a minimum consistent with maintaining knitting quality. Rogue yarns can be identified by exchange of positions with yarns on other machines knitting satisfactorily and noting effect on fabric and course length at the feeder with the rogue yarn. They can be collected for knitting especially at the end of the order or for rejection.

Experience will give guidance on the frequency of routine checks and when to increase vigilance, e.g. with yarns of known difficulty. Close attention immediately after setting up machines, and especially at yarn changes will enable machines gradually to be corrected until they knit correctly with average yarn. The number of times a machine needs adjustment will fail.

As a consequence of the machines knitting correctly with average yarn, those packages of yarn which now fall on the upper or lower tolerance limit will produce fabric within tolerance, whereas previously there was a much greater probability of the fabric being out of tolerance with those packages.

Loop length control together with machine links checking, are used to control the production of half-hose machines.

The step-by-step procedure for this on these machines is described for: -

Yarn length counter only.

Lateral stretch device only.

Yarn length counter and lateral stretch device.

Electronic course length meter alone or in conjunction with lateral stretch device.

To set up machine

Select specification for the garment to be produced.

Check that correct studs and plain links are set for each part of the sock, to give the requisite number of courses in each part.

Check that tension at each feed in turn is within the specified tolerances, by inserting tensiometer in yarn path at the corresponding point in each feeder while machine is running and adjusting tensioners as necessary.

Locate some easily visible feature or mark on the rotating parts of the machine, and sight against a fixed part so as to count machine revs.

e. In order to check the balance, or yarn distribution, between upper and lower cylinders as well as to check levelness of stitch length, set machine to knit on bottom cylinder only, in the leg part of the sock, and select a feeder. In this feed check yarn length over the appropriate number of revs., and adjust cams, as follows:-

Insert Yarn Length Counter so that angle of wrap around pulley is over 90 deg and yarn path in knitting is not disturbed.

Switch into off position. Set zero of instrument.

Start instrument and counting revs. (Omitting one rev. is avoided by a count down, 3, 2, 1, and pressing start lever on zero.)

Stop instrument on completing appropriate number of revs, and read.

Remove counter from path of yarn.

Check that reading is within required tolerance.

If necessary, adjust cam, and repeat i. to vi. above until reading comes within tolerance.

Repeat i. - vii. For each feeder.

Carry out the exercise e. & f. in each appropriate part of the sock.

Set machine to knit normally, on both cylinders. Select any feeder, and for each part of the sock, in turn as it knits down, check yarn length over the appropriate number of revs, in top, leg and foot, and adjust upper cylinder cams by carrying out exercise under e.i. - vii above.

Continue knitting. After the appropriate number of courses to reach the next part of the sock, make a yarn length measurement on the same feeder, by the method described in e.i. to vi.

If necessary, adjust cam and retest until correct reading obtained.

Repeat d., e. and f. for the other feeders.

l. Check goods coming off machine for wt/doz. pr. off machine.

Routine control

Work through items e. - h. as frequently as necessary to keep the production within required limits. Item l., used regularly, provides a valuable routine check on overall stitch length, correct links and correct yarn count, in combination.

LATERAL STRETCH DEVICE ONLY

To set up machine

Select specification for garment to be produced.

Check that correct studs and plain links are set for each part of the sock, to give the requisite no. of courses in each part.

Check that tension at each feed in turn is within specified tolerances by the method in section 1c.

Set machine by experience to required quality.

Knit down sock.

Check on Lateral Stretch Device in rib, top, leg and foot, and weigh.

Compare measurements in each section and weight with specification.

If outside limits, adjust machine as necessary and re-check.

Repeat until measurements are within required tolerances.

Check goods coming off for wt./doz. pr. off machine.

Routine control

Work through items e. - i. as frequently as necessary to keep the production within the required tolerances. Again item j. provides a valuable routine check if used regularly.

YARN LENGTH COUNTER AND LATERAL STRETCH DEVICE

To set up machine

As for a. - k. in 1. Yarn Length Counter only.

Routine control

Work through items e. - g. for 2. Lateral Stretch Device, as frequently as necessary to keep the production within the required tolerances. Again, item 2.j. provides a valuable routine check if used regularly.

If measurement is outside the tolerances, check on machine using the Yarn Length Counter, and proceed as already described for setting up.

Specification must include figures with tolerance ranges determined by both instruments. Make routine checks as frequently as necessary to keep the production within the required tolerances.

To set up machine

As for a. - c. in 1. Yarn Length Counter only.

Set machine to knit on bottom cylinder only in the leg part of the sock, and select a feeder. In this feed check course length with the Brionic C.L.M.

Adjust cam as necessary and recheck.

Repeat until course length is within tolerance.

Repeat b. - d. for the other feeders.

Carry out the exercise b. - e. in each appropriate part of the sock.

Set machine to knit normally on both cylinders. Select and feeder and for each part of the sock, in turn as it knits down, check course length in top, leg and foot respectively, with the Brionic C.L.M.

Adjust upper cylinder cam as necessary and recheck.

Repeat until course length is within tolerance.

Repeat g. - i. for the other feeders.

Check goods coming off machine for wt./doz. pr. off machine.

Routine Control

Work through items b. - e. as frequently as needed to keep production within the required limits. Item k., used regularly, again provides a valuable routine overall check on stitch length, correct links and yarn count.

4B. ELECTRONIC COURSE LENGTH METER AND LATERAL STRETCH DEVICE

To set up machine

As for a. - j. in 4A. Electronic Course Length Meter only.

Routine control

Work through items e. - g. for 2. Lateral Stretch Device, as frequently as necessary to keep the production within the required tolerances. Again item 4A.k, provides a valuable routine check if used regularly.

If measurement is outside the tolerances, check on machine using the Electronic C.L.M. and proceed as already described for setting up, items a. - e.

Specification must include figures with tolerance ranges determined by both instruments.

Make routine checks as frequently as necessary to keep the production within the required tolerances.

Notes

If the machine being set up is cold, re-check when warm. Routine checks must be made on warm machines.

The stud and plain links on each machine must also be checked as a routine. Weekly or perhaps even monthly checks may suffice.

For jacquard machines the machine may be set up with plain knitting using the Yarn Length Counter, or, by taking care to measure in the same pattern area, a satisfactory scheme may be developed using the Lateral Stretch Device.

There is an optimum frequency at which routine checks should be made. This will be determined by the conditions existing on a given plant. However, wt./doz. pr. provides a simple valuable routine check.

This optimum frequency of checks can be assessed by the process of Analysis of Results.

LOOP LENGTH CONTROL ON JACQUARD MACHINES

Loop length cannot be used as the normal parameter for knitting quality because the jacquard patterning gives varying lengths of the cross ties, i.e. that part of the yarn interlooping between the dial and cylinder, due to the patterning on the face of the fabric.

Loop length can be used in setting up a machine, by bluffing out the jacquard motion during the check. The actual yarns required in the ultimate fabric must be used although the fabric produced during the test will be a waste. Thus the method is not suitable for normal routine checks. It is used, however, in setting up a machine.

For routine tests, recourse is made to weight per square yard (or square metre).

Commencement

The procedure recommended is described with illustrations in Knit Knacks April 1971, Vol. 16 No. 1.

Select specification for the fabric to be produced, and run machine with waste yarn to warm up.

Set up machine with correct pattern and yarn and check visually that it is knitting correctly. Check that the yarn tensions are within tolerance.

Bluff out the pattern so that all cylinder needles are knitting.

Adjust dial height to the specified setting using the Dial Height Gauge.

Level all the dial feeders to the specified figure with the Knock-over Depth Gauge.

Adjust the cylinder cam on one feeder to the required yarn length per 5 (or 10) courses with the Yarn Length Counter inserted in the yarn path of that feeder.

Check the yarn speed on the same feeder with the Yarn Speed Meter, and then adjust the cylinder cams of the remaining feeders to give a yarn speed within the acceptable tolerance (usually +-2%).

If machine is rotating cam box type, f. and g. will be effected (i.e. levelling cylinder cams to give correct loop length at each feeder) by the technique detailed in QK18 Rot. C/B M/c part 2, d. to j.

h. Restore the jacquard pattern and commence knitting.

2. Routine Control Scheme

a. Check wt/sq. yd. (or metre) off machine against specified limits for off machine results, or preferably weight of fabric length or roll of set number of revolutions. This latter actually checks stitch quality and yarn count consistency in combination.

b. Examine fabric for bariness.

c. Check length and width of a suitable multiple of pattern repeats. This must be done with caution because of various distorting effects, which can arise.

a. and b. are better checks.

The frequency of these checks is ascertained from analysis of results over a period.

If any machine goes slightly out of tolerance, check yarn path and tensions. Rectify as necessary, followed by a weight check of a set number of revs. approximating to a yard or metre length of fabric. If still out, make a small adjustment to dial height, followed by a weight check as above. Repeat until weight checks come within tolerance.

If any machine is grossly out, the above procedure, under "Commencement" c. to h., must be undergone.

LOOP LENGTH CONTROL ON STRAIGHT BAR MACHINES

Loop length control is the technique used to control the production of knitting machines.

The step by step procedure for this on the above machines is as follows: -

Commencement

a. Select specification for the garment to be produced.

b. Check that corrects cards and trims, correctly doubled on transfer bars, are set up.

c. Thread up with the correct yarn and the tensioner slightly overtight. Tie a suitable weight to the yarn just after the tensioner. If the yarn tension is to be 5 grams a 5g crimp rigidity weight could be used, or a "S" - hook made from brass rod (a 5g hook can be made from 1/8" rod 3" long, nearest metric equivalent 10 gauge <3.25mm> 75m long).

Set up a convenient number of needles (e.g. 50 or 100 leads). Run down some fabric and check stitch length by unroving courses and measuring the yarn length on a course length tester, measuring yarn from both directions of traverse.

e. Adjust needle bar until the correct stitch length is obtained, and then set machine onto knitting programme.

Control Schemes

The following techniques are used in running a Q.C. scheme: -

a. Inspect fabric for faults and unlevelness.

b. Check weight per doz. blanks. This gives a check on correct loop length, yarn count and needles and courses in combination.

c. Check one or two garment blanks for stitch length in each traverse by unroving full courses, or a set number of needles, away from fashioning areas.

Check a. and b. above are the most important routine checks. c. should also be carried out periodically.

If measurements come out of tolerance, check any or all of the following as appropriate: -

Stitch length as under c. above.

Yarn tension, yarn path for obstructions.

Yarn count etc. No. of courses and wales in each part.

Adjust as needed and recheck, until correct figures are obtained.

If the machine being set up is cold, re-check when warm. Routine checks must be made on warm machines.

There is an optimum frequency at which routine checks should be made. This will be determined by the conditions existing on a given plant.

This can be assessed by the process of analysis of results.

3. Hatra Thorn Automation Stitch Length Meter

With this instrument stitch length measurements are simplified, being done while knitting, instead of unroving and measuring course length after knitting.

The procedures under headings 1 and 2 can now be simplified as follows: -

1A. Commencement

- c. as for 1., a. - c.

d. Put machine into knitting programme and start up.

e. Insert Stitch Length Meter detector head into yarn path and arrange readings to occur: -

i. when needles knitting are in excess of 100 if possible or at least in excess of 50 and press appropriate switch.

ii. on a course when fashioning does not occur or in a part of the blank where fashioning has not occurred over the 2 previous courses. Watch pilot light on instrument to ensure this.

f. Note stitch length reading and adjust needle bar until correct.

2A. Control

Item C. in 2 above is simplified to -

Periodically check for stitch length while knitting by carrying out 1A. e, and f above as appropriate.

LOOP LENGTH CONTROL ON POWER FLAT MACHINES

Loop length control is the technique used to control the production of knitting machines. The step by step procedure for this on the above machine is as follows: -

Commencement

Select specification for the garment to be produced.

Check that correct needles are set up in each bed.

Use the following procedure in the rib section (welt section) and body section respectively.

Stop machine with carriage at end of a traverse.

Set wheel or drum into knitting structure to be checked.

Check yarn tension by adjusting tensioners so that yarn loaded to the correct tension with a suitable weight just starts to slip through.* (Ref. Knit Knacks Vol. 15 No. 3 Sept. 1970)

Check calibration of cam indicator dials on back and front beds with the K.O.D. Gauge. Set the cams to the correct K.O. on both front and back beds. The figures required should be quoted in the specification so that machine is then already set up to knit approximately the correct stitch length at the correct yarn tension. Do this for each direction of carriage traverse and for each feeder. **

Experiment to note best place to mark both yarns so that marked yarn will appear on the needles (both marks) in each direction of the carriage.

Mark one yarn using jig and crayon (and 10g scissors grip if yarn marked prior to tensioning device on machine.)

Run machine on power.

Stop when carriage has made appropriate traverses.

Count needles between marks using the needle gauge, and note figure.

*For running tensions of 10-20g, a 10g weight is suitable. This is achieved using a 6" (15.2cm) length of 1/8" (0.32cm) diameter brass rod bent into a ring or other compact shape. The yarn is correctly threaded-up and the weight hung on after a guide as near to the feeder as possible.

** In the absence of a K.O.D.G., the only accurate method of balancing loop lengths on both beds is to follow c. by t