KNITTING NOTES

PRINCIPLE OF KNITTED STRUCTURES:

INTRODUCTION:

Knitting Technology comprises all processes that are required to produce knitted fabrics. A knitted fabric is a textile structure made by the interlooping of yarns. A yarn is an assembly of fibres of substantial length and relatively small cross-section, with or without twist. The exact origin of knitting is not known, but it is believed that knitting probably developed from the experience gained by knotting and twisting yarns.

 The present knitting industry is based on the invention of William Lee, who succeeded in the mechanisation of the interlooping of yarns four hundred years ago. A marked development in this technology has taken place over the last fifty years. An immense number of new developments and improvements have continuously changed the state-of-the-art of this technology, and today knitted fabrics have found their way into almost every field of application of textiles. Some areas of their application include socks, stockings, underwear, pullovers, jackets, frocks, blouses, suits, winter overcoats, bed-sheets, pillowcases, baby clothes, curtains, carpets, upholstery materials, table cloths and artificial fur.

 A new and rapidly developing trade in the knitting industry is the production of technical textiles and composites. New products are developed daily; in most cases knitted structures are replacing expensive, heavier and technically inferior constructions traditionally manufactured from other materials. Some interesting industrial applications are warp knitted sacks, high quality warp knitted flags, blood arteries, filter materials, geotextiles for road construction, tear-resistant sails, floor coverings and 3-D knitted preforms for composite components.

General Terms and Definitions

 Knitted Structures and their Properties

 The major difference between knitted and woven structures lies in the way the yarns are interconnected geometrically. In weaving, two sets of parallel yarns are interconnected by interlacing them at right angles. Different woven structures are produced by varying this basic principle.

Weaving is the oldest and the most common method of producing textile fabrics. A woven fabric is a stable construction and has a closed surface. The elastic behaviour of woven structures depends predominantly on the elastic properties of the yarns used to manufacture them.

In knitting, the yarns are initially formed into loops, and then these loops are interconnected in order to produce a textile structure. The term interlooping is used to describe this technique of forming fabrics. Based on this principle, a textile fabric is produced by using only one set of yarns. Thereby, a horizontal set of yarns (weft) could be interlooped to produce a weft knitted fabric, and a vertical set of yarns (warp) could be used to produce a warp knitted fabric.

As a result of this interlooping of yarns, the surface of a weft or a warp knitted fabric is more open when compared to the surface of a woven fabric. Due to this interlooping of yarns a knitted fabric could be stretched more than a woven fabric, even when a small force alone is applied. Once this force is eased the fabric slowly returns to its original dimensions. In fact, weft and warp knitted fabrics have higher elongation values than woven fabrics due to their structure, and their elastic behaviour generally exceed the elastic properties of the yarns used to knit the fabric.

Yarns have poor bending and torsional properties compared to their longitudinal elastic properties, and so once a knitted fabric is stretched and then released, it would slowly go back to its original state. The absolute elongation and the elastic behaviour of the fabric are both determined by the knitted structure and the mechanical properties of the yarns used to knit the fabric.

 Due to the structure and good elastic behaviour of knitted fabrics, garments made of knitted fabrics (knitted garments) are comfortable to wear. The air trapped in the loops of a knitted garment insulates the human body against cold. At the same time the relatively loose and open structure helps the perspiration process of the human body, especially when the knitted fabric is made of yarns spun from natural fibres. Due to the interlooping of yarns, the knitted fabrics also have better crease recovering properties compared to fabrics woven from similar yarns.

 The term binding can be used to describe the connection of one or more yarns in a textile fabric. The structure of a knitted fabric can be evaluated by studying how the yarns in weft and warp knitted fabrics are bound or interconnected, and this can be illustrated using stitch (loop) diagrams (charts). The actual interlooping of yarns in order to produce knitted structures depends on the knitting principle that was adopted to produce the structure, i.e. weft or warp knitting, and on the patterning elements.

Basic Knitted Structures

Each stitch, knitted loop and yarn loop consist of a top arc (head), two legsstitch is bound at the and two bottom half-arcs (feet). A upper and lower ends, i.e. at the head and at the feet. The first loops (yarn loops) are bound only at the head with loosely hanging feet. The knitted loops are bound only at the feet to the heads of the previous stitches.

At the place where the legs transform into feet there are two points of contact with the previous stitch. These are defined as the binding points. Thus a stitch has four binding points, i.e. two binding points at the head and two binding points at the feet of each stitch. Two binding points, therefore, build a binding unit. Thus a stitch has a total of eight contact points, four binding points and two binding units.

A knitted fabric is technically upright when its courses run horizontally and its wales run vertically with the heads of the knitted loops oriented towards the top and the first course at the bottom of the fabric.

For a stitch, depending on the position of the legs at the binding points, a technical back and a technical front side is defined. If the feet of the stitches lie above the binding points, and accordingly the legs below, then this is the technical back of the stitch, and it is called the back stitch, purl stitch, garter stitch or reverse stitch.

The technical back of a stitch and the technical front of a stitch

If on the other hand, the bottom half-arcs are below and the legs above, then this is the technical front of the stitch. This is called the face stitch or plain stitch, stocking stitch, jersey stitch (USA) and flat stitch (USA). A face stitch is produced by intermeshing a yarn loop towards the technical face side of a fabric.

Depending on the geometrical arrangement of the face and reverse stitches in a knitted fabric, i.e. heads, legs and feet of stitches, the following four basic knitted structures are defined:

• plain knitted fabrics;

• rib knitted fabrics;

• purl (links-links) knitted fabrics;

• interlock knitted fabrics.

 Plain knitted fabrics

If a weft or warp knitted fabric has one side consisting only of face stitches, and the opposite side consisting of back stitches, then it is defined as a plain knitted fabric. It is also very frequently referred to as a single jersey fabric (single fabric). Plain knitted fabrics are produced by using one set of needles. As such all the stitches are meshed in one direction. These fabrics tend to roll at their edges. They roll from their technical back towards the technical front at the top and lower edges. They also roll from their technical front towards the technical back at their selvedges. The structure is extensible in both lateral and longitudinal directions, but the lateral extension is twice that of the longitudinal extension. The yarn loop pulled in the longitudinal direction would extend by half its length, while when pulled in the lateral direction it could extend by the entire length. The degree of recovery from stretch depends on the fibres and the construction of the yarn.

 Rib knitted fabrics

If on both sides of a relaxed weft or warp knitted fabric only face stitches, i.e. the legs, are visible, then it is referred to as a rib knitted fabric and has been produced by meshing the stitches in neighbouring wales in opposite directions. This is achieved by knitting with two needle systems which are placed opposite to each other. As such these fabrics are also known as double jersey or double face fabrics. When the fabric is stretched widthwise, both sides of the fabric show alternately face and reverse stitches in each course. Once the fabric is released, it shrinks in its width, thus hiding the reverse stitches between the face stitches. These fabrics do not curl at their edges. The simplest rib structure is 1 x 1 rib.

The longitudinal extensibility of the rib structure equals that of a plain knitted structure. The geometry of the yarn path influences the elastic behaviour of the knitted structures. The change of direction of the interlooping of the stitches of neighbouring wales (cross-over points) results in the wales of a rib knitted structure closing up. This gives rib structures better elastic properties widthwise than other basic knitted structures. With rib structures in the lateral direction, extensions up to 140% can be achieved. Other construction of rib structures include 2 x 2 rib, where two wales of face stitches alternate with two wales of reverse stitches. As the number of wales in each rib increases, the elasticity decreases as the number of changeovers from reverse to front reduces.

 Purl knitted fabrics

If on the both sides of a relaxed weft knitted fabric only reverse stitches are visible, then this is defined as a purl knitted fabric. Generally, weft knitting machines are used to produce these fabrics. Purl fabrics are produced by meshing the stitches in neighbouring courses in opposite directions by using special latch needles with two needle hooks. When the fabric is stretched lengthwise, then the face stitches are visible. The fabric shrinks more in the direction of wales, and once it is released, it relaxes to hide the face stitches between the courses.

The interlooping of the stitches of neighbouring courses in opposite directions results in the courses of a purl knitted structure closing up. The structure, therefore, has a large longitudinal extensibility which is largely elastic.

 Interlock knitted fabrics

These could be considered as a combination of two rib knitted structures. The reverse stitches of one rib knitted structure is covered by the face stitches of the second rib knitted structure. On both sides of the fabric, therefore, only face stitches are visible, and it is difficult to detect the reverse stitches even when the fabric is stretched widthwise.

The geometry of the yarn path influences the elastic behaviour of the knitted fabrics. The change of direction of the meshing of the stitches in neighbouring wales results in the wales of a rib knitted fabric closing up giving it better elastic properties widthwise over other basic knitted structures. The meshing of the stitches in neighbouring courses in opposite directions results in the courses of a purl knitted fabric closing up. Thus they could be stretched lengthwise more than the other knitted structures. The combination of two rib knitted structures in the interlock structure gives very little or no room at all for the wales or courses to close up, and therefore the interlock fabrics show very poor elastic properties in both directions.

Structural Elements

In addition to the basic stitches tuck loops and floats are widely used in weft and warp knitting. In weft flat knitting selvedge stitches are formed.

Tuck loops

This is a loop that is integrated into a knitted structure without actually connecting it with the stitch immediately below it, though it is connected with a succeeding stitch. Tuck loops are formed by the hook of the needle in question receiving a yarn loop in addition to the knitted loop. The knitted loop and the yarn loop are then meshed during the next stitch forming process. A tuck loop is characterised with an upper binding unit and with a missing lower binding unit, i.e. it is bound only at the head. Its legs are, therefore, not restricted at their feet by the head of a stitch so that the legs can open out towards the two neighbouring wales. When tucking occurs across two or more neighbouring wales, the head of the tuck loop will float across the wales. Tuck loops reduce fabric length and longitudinal elasticity because the higher yarn tension on the tuck and held loop(s) causes them to rob yarn from the neighbouring stitches. The fabric width and lateral elasticity are increased.

Tuck loops are employed in weft and warp knitting for patterning and/or to influence its elastic behaviour and to vary the area density and the size of the fabric. In warp knitting, the equivalent of the tuck loop is the fall-plate or Henkel lap. Generally, the tuck loop in warp knitted fabrics has the appearance of diagonally running yarns in which the loops hang in the feet of the stitches.

Floats

A float is a piece of yarn limited by stitches which, in weft knitting, floats over wales. A float is generated when a stitch is missed out of a knitted structure, and does not pass through the stitch below nor connect with the subsequent stitch. The length of yarn that would have formed the stitch lies as a float across the wales. The extensibility of the fabric is reduced. Floats are created during jacquard knitting.

Selvedge stitches

 The selvedge of a weft knitted fabric is made by selvedge stitches. In these the yarn coming out of the last stitch of a course goes back through the same stitch and proceeds to the next course. Thus the stitches at the end of a weft knitted fabric have three legs, and are called the selvedge stitches. A selvedge stitch has nine contact points.

PRINCIPLE OF MACHINE KNITTING:

Introduction
 

A knitted fabric is produced by interlooping yarns of one yarn set. Yarns, which are flexible, are formed into loops and then the newly formed loops are interconnected (meshed) with knitted loops (previous loops). Generally in a weft knitted fabric all the stitches in a course are formed from the same yarn, and therefore in a weft knitted fabric the yarns are in the direction of the courses. On the other hand in a warp knitted fabric the stitches in a course are formed with different yarns. In fact each knitting needle is provided with a separate yarn (this is the minimum requirement, on commercial fabrics each needle is provided with at least two yarn ends). Therefore in warp knitted fabrics the yarns are in the direction of the wales, i.e. parallel to the fabric selvedges.

Weft knitted structure

Warp knitted structure
 

In machine knitting the interlooping of yarns is realised with various mechanical elements termed knitting elements. The knitting elements are manufactured to a very high degree of precision using high quality metals, in order to guarantee the production of quality knitted fabrics.
 

In machine knitting the interlooping of yarns is achieved in steps. These steps are called knitting steps. To form a new stitch all the knitting steps have to be carried out in the correct sequence. The correct sequence of the knitting steps is known as the knitting cycle.
 

In weft knitting a yarn is laid on to the needles individually with a yarn feeder or a yarn carrier, while in warp knitting every needle is provided with a yarn by using a knitting element called yarn guide. Thus in knitting as in weaving the yarn has to be provided to the knitting zone, and this is considered as the first knitting step. This step is known as yarn laying. Loops need to be formed form the newly provided yarn, and then they can be interconnected with knitted loops in the needles. As such another important knitting step is the loop formation, and the final step of a knitting cycle is the linking up step, in which the newly formed yarn loops are drawn through the knitted loop in the needles converting the yarn loops into knitted loops and knitted loops into new stitches. However, before the newly formed yarn loops can be pulled through the knitted loops the needle hooks, which had to be opened to accommodate the new yarn loops in them, must be closed before the yarn loops can be connected to knitted loops. This very important step in forming stitches is called bridge formation. The described steps are the most important fundamental steps of machine knitting, and they are common to all the different knitting techniques available at present, and therefore these are known as the Primary Knitting Steps. The order in which these are carried out will depend on the knitting technique and the knitting needle.

Knitting Needles
 

In machine knitting needles are used to form stitches. Thus the primary function of knitting needles is for interlooping yarns. They perform different functions depending on the knitting technique and the needle type. Linking of new yarn loops with knitted loops and to carry the knitted loops during the early stage of the stitch formation cycle are two important functions of a needle.. This central function of the knitting needle is independent of the knitting process and machine type, i.e. whether its a hand knitting machine or a high production warp knitting machine Needles can also be considered as the primary knitting elements as they are directly in contact with the yarn during the entire stitch formation cycle.. Design and development of a new knitting machine begins with the selection of the knitting needle. Once it has been decided on the type and the geometry of the needle, then the foundation for designing the knitting machine is laid. In order to select the most suitable needle for a new machine design it is necessary to establish all the possibilities for forming stitches with different types of needles and to confirm the findings. The design and construction expenditure would depend on the technological demands set on the knitting process. The following data would explain the above statement. A jacquard knitting machines has 1944 knitting needles with a mass of about 3.5 kg. All the other components necessary to produce a quality jacquard knitted fabric employing the needles would weigh about 1750 kg.
 

A knitting needle has a hook at one end to catch the yarn forwarded to the knitting zone, a stem or a shaft to carry the knitted loop during the early stages of the stitch formation process, and a butt at the other end. The butt is used either to position the needle on a needle bar or to move the needle the stitch formation process. The regularity and finish of the knitting needles influence directly the size and the shape of the stitches formed. On the other hand, they are subjected to intense mechanical forces during the stitch formation, and these would influence their performance. As such for manufacturing needles high quality steels are used and they are hardened using special thermal treatments . During the early stages of the knitting cycle (a knitting cycle consists of all the knitting steps necessary to form a stitch), the hook of a needle is opened to release the retained knitted loop and to receive the new yarn loop which is then enclosed in the hook. Before the new yarn loop can be drawn through the knitted loop (linking up) the hook must be closed (bridge formation) for the knitted loop to slide over the closed hook. All needles must, therefore, have some method of closing and opening the needle hook in order to retain the new yarn loop and exclude the knitted loop. Depending on how the closing of the hook is achieved knitting needles are subdivided into the following three groups:

1. The bridge formation is achieved by applying an external force.

2. The bridge formation is carried out due to the relative movement of the knitted loop and the knitting needle.

3. The bridge formation is accomplished with an additional closing element.
 

Bearded Needles
 

Bearded needle or spring needle was invented by Rev. William Lee, in 1589. Therefore it is the first knitting needle to be invented. It is also the simplest and, therefore, the cheapest needle. Bearded needles are made from steel wire (wire bearded needle or round stem bearded needle) or from punched steel plate (flat stock beaded needle). A bearded needle is shown below:
 

Fig. 1: Bearded needle.
 

By applying an external force on to the needle beard the needle hook is closed, and this is known as beard pressing. In bearded needle knitting machines this is achieved by mounting all the needles on to a needle bar and then by either moving a second metal bar, called the presser bar towards the needle beards or rotating the needle bar towards a stationary presser bar. Such an arrangement limits the ability of pressing the beards of individually, and the patterning potential of bearded needles is thus limited. This arrangement allows the needles to be reciprocated collectively. Knitting machines employing bearded needles are unable to compete in knitting the basic structures and their simple derivatives to other knitting techniques employing latch and compound needles, and their applications are reducing.

The Latch Needles
 

Latch needle was invented by Matthew Townsend's in 1849 and since then it has challenged the application of bearded needles in machine knitting. The latch needle is more expensive to manufacture than the bearded needle and is more prone to making needle marks in knitting, but it has the advantage of being self acting or loop controlled. For this reason, it is the most widely used knitting needle in weft knitting and is sometimes termed the automatic needle. Precisely manufactured latch needles are today knitting very high quality fabrics at very high speeds.

A latch needle has the following important parts:
 

1. the hook which draws and retains the new yarn loop;

2. the latch-blade;

3. the latch-spoon which is an extension of the latch-blade and bridges the gap between the hook and the stem covering the hook when closed;

4. the rivet or axle of the latch needle;

5. the stem which carries the loop in the clearing or rest position;

6. the butt which enables the movement of the needle by using cams.

Fig. 1 Latch Needle
 

The knitted loop is cleared from the hook when the latch needle is lifted because the knitted loop slides down inside the hook and hits the latch. This causes it to pivot open allowing the knitted loop to slide off the latch down on to the stem. The hook is closed automatically as the latch needle is lowered after a new yarn is supplied to it because the knitted loop which was on the stem slides upwards, contacting and pivoting the latch tightly closed.
 

As the latch needle continues with its downward motion the newly supplied yarn is drawn through the knitted loop. Latch needles thus knit automatically. The opening and closing of the hook, i.e. the bridge formation, is carried out by the knitted loop without using additional knitting elements. Such a phenomenon is very rare in processing machines. Except on Raschel machines (warp knitting), latch needles are arranged in the tricks or grooves of a needle bed.
 

To produce purl knitted structures a special needle with a hook and a latch at each end of the needle stem is used. Double-ended latch needles, also called purl needles, can slide through the knitted loop in order to knit from an opposite needle bed, and thus draw a loop from the opposite direction.
 

The latch needles currently being used can be subdivided into the following three groups:

1. wire latch needle; the needle butt is made by bending the end of the needle stem opposite to the needle hook

2. punched latch needle; these are latch needles punched from steel plates

3. double ended latch needles; to produce purl knitted structures, double ended latch needles, also called purl needles, slide through the knitted loops in order to knit from the opposite needle bed and thus draw a loop from the opposite direction.

The wire latch needles are employed in Hand Knitting Machines, in Hand Knitting Machines with motor drive units and in some semi-automated power machines. In the automation of knitting machines and in the development of high speed knitting machines, the wire latch needles have lost their importance to punched steel latch needles. There are about 160 different types of latch needles on offer from knitting needle manufacturers.
 

Punched steel latch needles can be subdivided into two different groups. These are:

1. normal latch needles;

2. loop transfer latch needles.

Loop transfer latch needles are employed in electronic flat bed knitting machines. A normal latch needle consists of three areas of different functions, which are shared by a loop transfer latch needle which also has a fourth area. These are:

1. needle hook area;

2. needle stem;

3. needle butt;

4. loop transfer area.

The needle hook area is of great importance, as its here all the relative motions between the needle and yarn that are necessary for stitch formation take place. The needle stem has a connecting function, i.e. it establishes the connection between the hook and the butt. It also has a guidance function, i.e. to guide the needles in the tricks of the needle bed. The needle butt has the function of reciprocating the latch needle between two dead centres in order to form stitches. The transfer area has the task of transferring the knitted loop to the opposite latch needle. The form and the size of these four important areas will depend on the application of the latch needle.
 

It is a common practise in machine building to design certain parts with weak areas, so that they will break in the event of a malfunction of the machine, thus preventing major damage to more expensive parts. The butt of a coarse gauge latch needle is designed with a weak area in the butt so that it will break if the knitting cam system jams, thus preventing serious damage to the tricks of the needle bed.
 

The latch plays a very important role in the stitch formation process. The latch is fixed to the cheeks or slot walls of the needle in such a way that the latch-spoon can be rotated between two dead points. The cheeks are either punched or riveted to fulcrum the latch. Due to this rotational movement the latch will open the hook in order to release the knitted loop. The latch rotational movement will also close the hook during the latter part of the knitting cycle so that a new loop could be drawn through the previous knitted loop. Although the latch is small during knitting it undergoes tremendous stresses. Modern knitting machines are high production machines, and in these machines the latch needles move in their tricks at very high speeds. The striking action of the latch during the closing of the needle hook by the latch spoon depends on the working speed of the latch needle. It will be very high at higher working speeds. The stresses of the latch will result in very high reaction forces at the fulcrum. Therefore the bearing at the fulcrum is critical, and must satisfy the following conditions:

• a good movement of the latch;

• a stable support of the latch.

The size of the rivet will depend on the size of the latch needle. With fine latch needles the fulcrum point is so small that it is almost invisible to the naked eye. The axle of the latch plays a major role in the function of the latch needle, and several interesting solutions have been developed by needle manufacturers.

Compound Needles

The first patent for a compound needle was awarded in 1856 to Jeacock of Leicester. The patent describes a knitting needle consisting of a needle part (the stem and the hook of the needle) and a tongue part (hook closing element). Both the two parts need to be controlled independently, and thus the new needle was named a compound needle. There are two types of compound needle in current use, the tubular pipe compound needle, where the tongue slides inside the tubular needle part, and the open stem pusher compound needle, where the tongue slides externally along a groove on the flat needle part. The pusher type is cheaper and simpler to manufacture and its two parts are capable of separate replacement. Its dimensions are narrower allowing tighter stitches to be produced. Today, the open stem compound needles are finding most widespread use in warp knitting. The compound needle is expensive to manufacture and each part requires separate and precise control from a drive shaft or cam system. The compound needle has a short, smooth and simple action, without latch or beard inertia problems. The slim construction and short hook makes it particularly suitable for the production of plain, fine warp knitted structures at high manufacturing speeds. Feeding yarn into a compound needle is more critical than for the bearded or latch needle because the yarn has to be laid precisely in the hook of the compound needle, in order to prevent fabric faults. By bearded or latch needle the yarn can be laid across the beard or the open latch, and it will still be taken into the needle hook. On the other hand the positively controlled two parts of the compound needle guarantees a opened hook at the time of yarn in-lay during the knitting cycle.
 

The compound knitting technique was presented on a large diameter circular knitting machine at ITMA 83. The advantage lay in shortening the needle stroke and in increasing speed and the number of knitting systems (feeders). At ITMA 87, Albi in West Germany, presented its model RCU-SN-GT a large diameter single jersey circular knitting machine operating with compound needles, and equipped with 144 knitting systems. Also a single cylinder compound needle knitting machine for producing tubular knitted ladies fine hose with a maximum cylinder speed of 1500 rpm was exhibited. This increase in speed, based on compound needle technique has now led flat bed knitting machine manufacturers too, to test the possibilities of using compound needles on their machines.

Needle Beds
 

Needle beds are employed in latch or compound needle weft knitting machines. Their function is to hold particular knitting elements at exact defined distances and to guide them during the stitch formation process. On an electronic flat bed knitting machine knitting elements such as latch needles or compound needles and needle selection elements are placed in needle tricks. Modern electronic flat bed knitting machines are equipped with holding-down sinkers, and these are positioned at the top edge of the needle beds. As the needle beds are subjected to tremendous stresses due to the movement of the knitting elements they are made from very high quality metals. On one surface of the needle bed grooves (called tricks) of equal width and depth are preciously machined at equal distances. Lasers and numerically controlled cutting machines are used in their manufacture to ensure a tolerance of +30 microns (30 micrometers). The needle manufacturers ensure a tolerance of -30 microns for their needles. The knitting elements are placed inside the tricks and are moved mechanically between two dead centres. The distance between two adjacent needle tricks is called needle spacing (t). The needle tricks are wider at the top, where the needle hook is placed, in order to accommodate the somewhat bigger knitted loop. This top edge also forms the knocking over edge for the stitch formation.
 

Needle beds can be sub-divided into two main different forms:

1. Flat Form: a rectangular thick metal plate is used to manufacture the needle bed, e.g. the needle beds of flat bed knitting machines. In flat needle beds the needle tricks are parallel;

2. Circular Form : a metal cylinder or a metal disc is used to manufacture circular needle beds, e.g. the needle bed(s) circular knitting machines. If the needle bed is made from a metal cylinder, then it is a cylindrical needle bed. If the needle bed is made out of a metal disc, then it is called the dial needle bed. In the metal cylinder the needle tricks are machined parallel to the axis of the cylinder (axial needle tricks), whereas in the dial the needle tricks are not parallel; they are all pointing towards the centre of the metal disc (radial needle tricks).

Industrial flat bed knitting machines are equipped with two flat needle beds arranged in the form of a roof, thus they are also called v-bed knitting machines. The important parts of a flat needle bed are shown in the following figure:

Fig. 1: Cross section of a flat needle bed
 

Usually, all types of needle beds employ all the elements shown in the above diagram except the needle security springs. The needle cover band maintains the knitting needles against the trick base. It also has a braking effect on the knitting needles and prevent them from springing back. The knitting needles move axially between the lower edge of the cover band and the security spring. By moving the security spring back the knitting needles can be brought out of the cam tracks, i.e. the knitting needles will be out of action.
 

The knock-over jack and its front edge with which the yarn comes in contact during knitting is of a special shape. This edge is carefully polished in order to ensure the free sliding of the yarn during stitch formation without damaging the yarn. Also, in order to ensure free downward movement of the knitted fabric between the two needle beds, the under sides of the top edges of the needle beds are machined to a special form.
 

The needle beds are characterised by the following two parameters:

1. the machine gauge; i.e.. the number of needle tricks in a reference length;

2. the maximum width of the fabric that can be knitted, this is known as the maximum knitting width

   • in flat needle beds this is given by the distance between the first and the last needle tricks of the needle bed;

   • the diameter of the needle cylinder in the case of circular weft knitting machines.

One inch is used as the reference length for the determination of the machine gauge of a needle bed. As an example the correct designation of the machine gauge of a flat bed knitting machine having seven needle tricks to an inch is E7. The capital letter E specifies that the reference length is an inch. Sometimes the gauge is also given as 7 npi (needles per inch). The distance between the centre lines of two neighbouring needle tricks is called the pitch (t), and it could be calculated by using the following mathematical relationship:

In the above equation the units of the needle bed pitch is in micrometers
 

The length of the portion of the needle bed, which is present with needle tricks is known as the knitting width (maximum knitting width) of a flat needle bed. The knitting width will depend on the total number of needle tricks on the needle bed and the machine gauge. It could be determined using the following mathematical relationship:

In the above equation the units of knitting width is in centimeters.
 

Other important parameters of a needle bed are:

• the width of the needle trick;

• the height of the needle trick;

• the base of the needle trick;

• the height of the needle bed.

Machine Gauge
 

The distance between two neighbouring needles, called pitch, determines the gauge of the knitting machine. The number of knitting needles contained in a reference length is defined as the machine gauge. Originally, knitting needles were cast in small metal blocks termed leads which were then fitted into the needle bar. In the weft knitting machines with bearded needles (straight bar weft knitting machine), the needles were cast two to a lead and gauged in the number of leads per 3 inches of the needle bar which is equivalent to a gauge of the number of knitting needles in 1.5 inches. In bearded needle warp knitting machines (Tricot machines) the needles were cast three to a lead giving a gauge directly in needles per inch. In the Raschel warp knitting machine the latch needles were cast in 2 inch lead giving a Raschel gauge of needles per 2 inches. In latch needle weft knitting machines the gauge is normally expressed in needle tricks per inch which in the USA is referred to as "cut", being short for the phrase "tricks per cut per inch".
 

Normally all primary knitting elements in the same machine are set to the same machine gauge. The pitch indicates the space available for the yarn. As the diameter of a yarn is proportional to its count, a relationship exists between the range of optimum counts of yarn which may be knitted on a particular knitting machine and its machine gauge. Machine gauge thus influences the choice of yarns and their counts, and affects fabric properties such as the appearance and the fabric weight. For a given needle cylinder diameter or knitting width, finer gauge machines tend to knit a wider fabric as more wales are involved. Coarse gauge knitting machines have latch needles with larger dimensions requiring greater movements. During knitting the width of the knitting cams are correspondingly large so less cam systems can be accommodated around a given needle cylinder diameter, so therefore coarser gauge knitting machines often have fewer knitting systems.
 

There is a number of different machine gauge systems in current use. These are given in the table I.
 

Table 1: Machine gauge systems

Knitting Cams
 

The movement of the latch or compound needles between two dead centres is technically realised by means of inclined metal planes. These operate a defined distance above the needle bed and act on the butts of latch or compound needles. These inclined planes are called knitting cams and usually they are fixed on to a cam plate. The knitting cams can be represented basically by three triangles.

Fig.: The simplest representation of a knitting cam system of a flat bed knitting machine

A central cam raises the knitting needles. This cam is called the raising cam. The functions of the other two cams are to lower the raised knitting needles (lowering or stitch cam) and to prevent the raising needles from overshooting (guiding cam). The stitch cam on the left lowers the knitting needles when the cam plate moves on the needle bed from left to right. Meanwhile the other lowering cam acts as the guiding cam. When the cam plate moves on the needle bed from right to left the raised knitting needles are then lowered by the right stitch cam.

The two elements, the raising cam and the sinking cams, are employed in all type of knitting machines with latch or compound needles, whether they be circular weft knitting machines or flat bed weft knitting machines, hand or automatic.

Characteristics of Raising Cams
 

During the stitch formation process depending on the knitted structure the needles need to be put into action and out of action. This can be easily achieved during knitting by not moving a needle forward during the knitting cycle can not form a stitch, and this is realised by preventing the butt of a needle coming into contact with the raising edge of the raising cam. Thus the raising cams are designed to facilitate this, and there are two popular constructions:

1. hinged or tongued type
   The raising edge can be rotated away from the normal running position of the needle butts;

2. sinkable type
   The raising cam is attached to a mechanism that will allow the cam to be withdrawn into the cam plate, in order to change its position relative to the needle bed surface. When it is fully withdrawn into the cam plate, the raising cam passes above the butts of the needles leaving them idle. Alternatively, in its lowered position the raising cam is down on the needle bed and causes the butts of the knitting needles to ascend. Certain types of weft knitting machines employing high butt and low butt knitting needles are equipped with raising cams that can be set to three different position:

   • in all work position: Engages the butts of all the knitting needles

   • in half position: Engages the high butts only

   • in out of step position : Leaves all the butts idle.

In flat bed knitting the sinkable raising cam is the most popular. Hinged type is more popular in circular knitting.
 

Tuck Cams
 

In order to form a tuck loop the following conditions need to be fulfilled:

1. Forward movement of a latch needle until the closed hook is opened (latch in hook opened position) by the knitted loop in the hook. The knitted loop must remain on the opened latch;

2. A new yarn need to be laid across the needle hook.

The forward movement of the needle is influenced by the height of the raising cam, and, therefore, in order to form tuck loops the raising cam is split into two parts as shown in the following figure:

Fig. 1: Modified raising cam
 

Both parts, ie the knit cam and tuck cam, are mounted on to a cam plate with a cam withdraw mechanism. As such the two cams can be withdrawn independently, in order to form stitches, tuck loops and floats during knitting., and useful combinations are given in the table below:
 

Table 1: Cam positions for producing the binding elements during knitting.

Characteristics of Lowering Cams
 

The primary requirement in the development of lowering cams is the angle of descension. This angle varies, generally, from 50 to 59 degrees, and influence the following:

1. The bouncing of knitting needle butts. This the rapid up and down movement of the needle butts inside the cam track. This is very crucial and is more evident in high speed circular knitting, as the knitting needle bouncing would cause excessive damage to the needles.

2. The number of knitting needles drawing the same yarn simultaneously during the stitch formation.

If the knitting needles descend less rapidly, then the needle bouncing is reduced, but at the same time the number of knitting needles being lowered simultaneously is increased causing the tension in the knitting yarn to increase according to the loop sinking rule in knitting. At present an angel between 45 to 55 degrees in flat bed knitting and 58 to 59 degrees in circular knitting are the standard values used by the knitting machine manufacturers.

In order to knit fabrics of different stitch lengths the lowering cams are designed with a limited mobility, i.e. their vertical position can be altered. In effect, with a lowering cam placed in a high position, the knitting needles make a small descend, ie a shorter length of yarn will be pulled through the previous knitted loops by the needles, and thus smaller stitches will be formed. On the other hand, with a lowering cam placed in a low position, the knitting needles descend further back into the cam track, and form bigger stitches. That is the setting of the vertical position of the lowering cams determines the length of the stitches. For a given machine gauge, bigger stitches will form a slack fabric, where as smaller stitches will make a tighter fabric. This is why, generally, the adjustment of the lowering cams is also known as the stitch length adjustment. How ever, this rule can not be applied in all cases. When positive yarn feeding is used, the stitch length mainly depends on the amount of yarn supplied in to the knitting needles, than on the position of the lowering cams. In this case, the position of the lowering cams will simply influence the yarn tension.

In order to ensure the readjustment of the lowering cams, each lowering cam is connected to a graduated scale. This way the position of the lowering cams can be fixed exactly. Knitting machine manufacturers usually deliver a chart for adjusting lowering cams in relation to the machine type and for machine gauge. This table indicates the average position of the lowering cams for different kinds of fabrics which can be produced on the machine.

It is the flush jack position of the knitting needle that is used as the reference for establishing the settings of the lowering cams. In this position the hook of the knitting needle is exactly aligned, i.e. flushed, with the knocking-over-jack of the needle bed. For a stitch to be formed, the needle must descend lower than the flush jack position, which varies according to the machine gauge, the yarn count and finally the required stitch size. Following emperical standards are accepted when adjusting lowering cams:

1. When knitting rib and rib based structures it is sufficient that the needles are lowered slightly beyond the flush jack position to form stitches.

2. When knitting plain or plain based structures it is necessary to lower the needles well below the flush jack position.

3. The length of a tuck stitch is usually sufficient, when the knitting needle lowered to the flush jack position.

Cams in circular knitting machines

Generally, the relative movement between the needle butts and the cams is uni-directional in circular knitting machines except for hosiery machines (small diameter circular knitting machines used in the manufacture of hosiery products). In circular weft knitting machines, therefore, the needle movement required for stitch formation is realised with a cam system comprising one raising cam and one lowering cam. Such cam systems are also known as unsymmetrical cams.

Technically, the rate of production in weft knitting technique is determined by the velocity of needle butt movement through the different phases during the stitch formation process. In circular knitting the fabric production rate is influenced by the following:

1. the total number of knitting cam systems in the machine;

2. the horizontal velocity of the needle butts, which is equal to the circumferential velocity of the needle cylinder;

3. the knitted structure;

4. the knitting efficiency.

KNITTING CYCLES:

Knitting Cycle of a Latch Needle in Weft Knitting

Step 1: Holding down

The needle begins with its forward movement from its backward dead centre, i.e. from its backward rest position. During the early stage of this movement the knitted loop in the needle hook is also forced to move forward due to the interaction of the fabric take-down tension and the needle movement. This forward movement of the knitted loop is extremely important because this clears the way for the needle to move forward without any obstruction.

The next stage of the step holding-down takes place when the needle hook emerge out of the knocking-over edge of the needle bed. During this period the knitted loop rotates under the influence of the downward oriented fabric take-down force until it is almost at right angles to the needle stem.

The holding-down step is extremely important for forming stitches and tuck loops as well as for automated transferring knitted loops on modern flat bed knitting machines. The correct operation of the holding-down step depends on the fabric take-down tension.

Step 2: Latch opening

Due to the fabric take-down force the knitted loop would remain on the knocking over edge of the needle bed. As a result when the needle continue to move forward the knitted loop would be forced to slide back in the needle hook. Due to these relative movements of the needle and the knitted loop in opposite directions, at a pre-determined time the knitted loop would strike the latch and force it to open. The latch opening time would be determined by the geometry of needle and knocking-over jack. It would also depend on the yarn diameter, i.e. yarn count.

Step 3: Clearing

The forward moving needle would cause the knitted loop to clear the opened latch. The knitted loop would be thus held on the needle stem until later.

Soon after clearing the forward movement of the needle is completed. The needle movement during steps 1 to 3 is referred to as the forward stroke of the needle and it is defined by the geometry of the needle and the raising cam.

Step 4: Yarn delivery

The needle begins to move back. During the early stages of this movement a new yarn is laid across the hook of the needle. Due to the backward movement of the needle the knitted loop is pressed against the knocking-over edge of the needle bed.

Step 5: Latch closing

The needle continues with its backward movement. Due to this movement the knitted loop, which is held on the stem, is forced to move forward towards the needle hook. This results in the knitted loop initially being moved underneath the latch, and then it forces the latch to rotate and close the hook area. The new yarn is trapped in the hook of the needle.

Step 6: Landing

The needle continues with its backward movement, and this forces the knitted loop to move on to the closed latch, and then to continue moving towards the needle hook on the latch.

Step 7: Casting off

Due to the backward movement of the needle the knitted loop is thrown off (casted off) the hook. From this point onwards the needle hook begins to pull the new yarn through the knitted loop. As a result the knitted loop is converted to a stitch and the new yarn pulled by the needle hook becomes the new knitted loop.

Step 8: Knocking-over

The needle reaches the backward dead centre and a new knitted loop is formed in the needle hook.