THE EFFECT OF YARN FRICTION ON YARN TENSION IN KNITTING &

LOOP FORMATION.

THARANGA A D (061054 E)SOMASIRI M R P (061051 R)SADARUWAN W M M (061046 G)THARMARAJAH E J (061055 H)SANJE M (061047 K)

KNITTABILITY AND FRICTION

Knittability is practicaly measured by machine efficiency rates

and indicates unscheduled machine down time and defect

levels.

The knittability of yarn depend on the strength and the

frictional properties.

Friction affects the knitting of yarn by two ways.

1. It affects the tension of the yarn supplied to the knitting elements.

2. A high friction increases the abrasion of yarn with knitting elements

and guides.

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THE MAIN EFFECTS OF USING HIGH FRICTION YARN

The range of possible loop lengths on any machine is more

limited.

High knitting tension prevails undue strain on the yarn

resulting fabric weaknesses and mechanical difficulties.

“Stick-rings” formation.

Variation in loop lengths.

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THE EFFECT OF YARN FRICTION ON LOOP LENGTH.

The experimental data shows that;

The relationship is approximately linear except the tension is less than

8g (~ 8g) for the unwaxed yarns.

Always unwaxed yarns exihibit a higher loop length compared to

waxed yarns when the tension is below 2g and it is about 10 %.

In between 5 – 10 g the relation is reversed up to 5 %.

In other words due to yarn friction the loop length may vary between

+/- 10% 4

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THE EFFECT OF YARN FRICTION ON KNITTING PROCESS.

Increasing the input tension of high friction yarn will;

Holes begin to appear.

“Press-off” may occur.

Much larger forces will imposed on the needles and the

machine could not be rotated by hand.

Earlier considered that press-off occurs when loop

lengths are become smaller due to the friction forces.

But it is not the case. A unwaxed yarn have a bigger

loop length at the press-off always. So the reason is

yarn brakes due to high tension. 6

Continued…

This statement is proven that the waxed yarns do nothing till

reach to the 40g of tension. Input tension and loop length of low friction yarn

-low input tension knitting points robs yarns from only one needle -high input tension knitting point robs from two

needles until yarn will break. Input tension and loop length of high friction yarns.

-low input tensions knitting point robs yarn from one needle.-high input tensions knitting point robs yarn from the package.

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Continued…

The result is that ;

The small loop-length values of the high friction yarns, as

compared with the low friction yarn.

High friction yarns have more linear relationship between

input tension and loop length.

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WHAT IS ROBBING BACK ?• When needle is lowering to form a loop, some amount of yarn

length is drawn by the previously formed loop.• Yarn may be robbed from either one or two needles to the right of

the knitting zone• The tension around these needles must be less than that of yarn at

knitting point.• At the knitting point, generally the tension of both side yarns are

equal.• So no yarn movement over that surface will occur• So with the law value of input tension yarn can be robbed only

from right hand side needle (N6)• Percentage of robbing back =( lt - lu ) *100 lt

Where, lt - Theoretical length of loop lu - Un roved loop length

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COMPARISON OF INPUT & OUTPUT YARN TENSIONS.

Both waxed and unwaxed yarns show a linear relationship

with all warp angles.

But yarns do not pass through the origin.

Amonton’s low state that;

“ The Loge of the ratio of the output to input tension will be

linear (through the origin ) when plotted against wrap angle.”

- In this relationship, the slope of the graph will be equal to

the coefficient of the friction of the respective yarn. 10

For waxed yarns

Although there is a qualitative difference the appearance of the unwaxed yarn is also same. 11

Angle of wrap yarn

For the unwaxed yarnSlope = 0.365

For the waxed yarnSlope = 0.242

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Continued…

The slope values shows the friction properties of the two

types of yarns.

“Moss” assumptions;

- For the yarns of µ<0.3, tolerable tension during the

knitting process would occur and “press-off” would be

very infrequent under usual commercial input

tensions.

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TENSION VARIATION IN THE YARN PATH.

Under the dynamic conditions of loop formation, as the yarn

passes over each metallic knitting element surface, the

tension in the yarn increases according to Amenton’s low.

Then the tensions for each angles can be calculated by the

equation of T n =T1e µ(Σθn)

The graphs for two yarns can be shows as this.

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Predicted yarn tension in the knitting zone for the waxed yarn. 16

Predicted yarn tension in the knitting zone for the unwaxed yarn. 17

Waxed yarn knitted a fabric up to and beyond T1 = 25g and at

no time during the knitting of this yarn did holes appear in the

fabric or press off

Unwaxed yarns appear small holes at as a low value of T1 = 6g

At T1 = 10g more holes were appear but still possible to knit

fabric

T1 >12g the fabric then press-off the machine

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POSITION OF THE TK WITHIN THE KNITTING ZONE

Tk -Maximum tension of the yarn at the knitting zone

For the waxed yarn, Tk is move away from the knitting point

with the increase of the input tension towards the yarn input

point (A)

Ex - when input tension is 20g, Tk lies near to S3

when input tension is 2g, Tk lies between N3 & S4

For the un-waxed yarn, Tk is lie within small range close to N3

with the change of input tension (2-20g).

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Continued…

Always the Tk point lie to the left side of the knitting point. (towards the yarn input side)So robbing back is always occurs.In waxed yarn the range of variation of Tk point is larger than that of un-waxed yarn with the change of input tension.The Tk point has more effect on determining the robbing back and hence Loop length variation. So waxed yarns shows more variation in Loop length to given input tension than un-waxed yarn.Hence fabrics produced by waxed yarn may show larger variation in loop length than that of un waxed yarn.

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AFFECT OF FRICTION TO PRODUCTIVITY.

Yarn breakages

Fabrics produced from the high-friction yarns give maximum

tension during knitting and it may exceeds the yarn breaking

load, so the yarn breaking limit may high.

When high-friction yarns use in knitting cause more difficult

to rotate the machine by hand and it causes difficulties in

doing adjustments.

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Continued … Fabric faults may occur,

• Tension Variation – Barre, Stick Rings• Courses per inch in the fabric may change• Variations in Loop length

Fluff Formation Main reason for fluff formation is friction between yarn and

knitting elements that occurs where the moving yarn passes over machine parts.

Because of the increasing of yarn friction can cause to increase the fluff formation in spun yarns like Cotton. Also fluff can effect on,

Yarn Tension Yarn Feeding Speed Yarn Feeding Angle

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