yarn count and calculations

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Elements of Weaving

Elements of Weaving

Weaving is carried out on a machine traditionally known as a loom.Fig. illustrates the layout of a weaving loom in the form of a simple schematic diagram.

The sheet of warp yarns, which consists of a number of ends is carried upon the weavers beam A. The warp ends from the beam are then drawn through the healds B1 and B2, threaded through the splits of the reed C and at the point D they become interlaced with the weft supplied by the shuttle E. The cloth is formed at the fell of the cloth, marked F and is wound upon the cloth roller marked G situated at the front of the loom.There are three types of motions identified on a loom.

PRIMARY MOTIONS

Every loom requires three primary motions to produce woven fabrics.

1. Shedding

2. Weft Insertion

3. Beating up

Shedding

It is the name given to the motion which moves the heald frames up and down in order to separate the warp sheet into two layers and create a triangle in front of the reed (referred to as the shed) through which the weft can be passed.

Weft Insertion (Picking)

It is the means by which the weft is projected through the shed. This was traditionally by shuttle, but more recently it is done by projectile, airjet or water jet.

Beating up

It is where the reed pushes the weft into the fell of he cloth to form fabric. This requires considerable force. Hence the term, beating up.

SECONDARY MOTIONS

There are three secondary motions in weaving : let off; take up and weft selection

Let off motion - It ensures that the warp ends are controlled at the optimum tension for the fabric that is being woven.

Take up motion - It withdraws cloth from the fell and stores it at the front of the loom.

Weft selection - It is necessary to change the weft being inserted.

Ancillary or Tertiary motions

These are widely used on modern weaving machinesThese include

1. Warp stop motions

2. Warp protectors

3. Weft stop motions

4. Weft replenishment

Warp stop motions stop the loom at the event of the breakage of an end.

Warp Protectors- These protect the warp in the event of shuttle flying etc.

Weft stop motions halt the loom when the weft yarn break.

Warp protector motions stops the loom before beat up if shuttle fails to reach the other side of the loom.

Weft replenishment assures a continuous supply of weft yarn to the loom wherever a supply package becomes exhausted.

Fabric Parameters

There are four basic parameters that are essential for every woven fabric.
1. Ends per Inch and Picks per inch (EPI and PPI).

2. Yarn count

3. Crimp
4. Weave or Fabric Structure or Design

1. Ends per Inch or Picks per Inch
It is a measure of thread density. The normal method used to determine thread density is to use a pick glass.

2. Yarn count
EPI and PPI affects the compactness of the fabric. It is also known as thread count or cloth count. Thread counts range from as low as 20 threads per inch as used in tobacco cloth to as high as 350 threads per inch, found in type writer ribbon fabrics. Normally EPI and PPI of a fabric are described as EPI×PPI. Thus a fabric of 74×66 means 74 EPI×66 PPI.
Balanced constriction
A fabric is said to be well balanced if the number of warp yarns and weft yarns per inch are almost equal.

3.Crimp
Crimp refers to the amount of bending that is done by thread as it interlaces with the threads that are lying in the opposite direction of the fabric. Crimp is defined as the ratio of difference of length of yarn (Ly) taken from length of fabric (Lf) to the length of fabric (Lf).
Crimp = (Ly-Lf)/Lf
Often it is more convenient and preferable to use percentage values. Thus we can define crimp percentage as:
Crimp% = (Ly-Lf)/LfA crimp will normally give values ranging from 0.01 to 0.14 ie. (1% to 14%).
Crimp is related to many aspects of the fabric. It affects the cover, thickness, softness and hand of the fabric. When it is not balanced it also affects the wear behaviour and balance of the fabric, because the exposed portions tend to wear at a more rapid rate than the fabric. The crimp balance is affected by the tensions in the fabric during and after weaving. If the weft is kept at low tension while the tension in warp directions is high, then there will be considerable crimp in the weft and very little in the warp.

4. Weave
It refers to the arrangement of warp and weft in the fabric.

OTHER FABRIC PROPERTIES
1. Fabric weight (W)
It is the weight of the yarn per square meter in the woven fabric, which is the sum of the weight of the warp (W1) and weight of the weft (W2).
Weight of the warp is calculated as (per square m):
W1= [n1 x 100 (1+c1%)/100] x [N1/1000] gWheren1 = Ends per cmN1 = Warp count in Tex
C1% = Warp crimp percentage.
Similarly weight of the weft is calculated as (per square m)
W2= [n2 x 100 (1+c2%)/100] x [N2/1000] g
Total weight per square meter = W1+W2
weight/piece = (W1+W2) × piece length × piece width in gram.

Example
A fabric 120m long, 1.3 m wide and having 30 ends per cm of 12 tex warp and 24 picks per cm of 15 tex weft. The warp and weft crimp percentages are five percent and eight percent respectively. We describe these fabric particulars as30×24; 12 tex × 15 tex; 5%×8%
Warp weight per square m = [30 x 100 x (1+5)/100] x [12/1000] = 37.8 gms
Weft weight/square m = [24 x 100 x (1+8)/100] x [15/1000] = 38.8 gms
Piece weight = total weight per m × piece length × piece width= 76.68 × 120× 1.3= 11962.08 gm or 11.96 kg.

2. Cover factor
(K) it is defined as the area covered by the yarn when compared with the total area covered by the fabric.The warp cover factor can be found by using the formula.k1= n1 x sqrt(N1)/10Wheren1 = Ends/cmN1 = Count of warp in tex
Similarly the weft cover factor can be found by the formula
k2 = n2 x sqrt(N2) /10So the total cover factor isK = K1 + K2
Thus with fabric (30×24; 12 tex×15 tex) the values are
k1= (30 x sqrt12)/10 = 10.39
k2 = (24 x sqrt15)/10 = 9.30K = K1+K2 = 10.39+9.30 = 19.69

3. Fabric Thickness
For a wide range of fabric, this parameter is not important, but it becomes critical for fabrics that are to be used as belts and felts.