Saturday, March 19, 2011

Tutorial: Knitting Machine Structures

In order to understand the different technical solutions adopted by the different manufacturers for their knitting machines it is necessary to learn (at least schematically), the structural features and the main components of these machines.

The bed
The bed of a knitting machine can be flat or circular, and is made up of a steel body with grooves where the needles slide. In flat machine types, the bed is a steel plate where the needles are arranged parallely one beside the other. In circular models, the needles are accommodated in milled grooves obtained along the generatrices of a cylinder or in radial grooves milled on a metal disk.
Flat-bed machines always incorporate two beds which form a 90° angle with respect to one another and a 45° angle to the horizontal plane. The bed which is nearest to the operator standing in front of the machine is called front bed, while the other is called back bed.

Circular machines can be single-bed, i.e. have a single cylindrical bed, or two-bed, i.e. have two beds placed at 90° with respect to one another called dial and cylinder, respectively, which can be assimilated to the front bed and the back bed of flat machines. There also are two-bed circular machines - the two-cylinder models – which are now produced in small quantities and feature two cylindrical beds placed at 180° with respect to one another. These two beds are called lower cylinder and upper cylinder and correspond to the front and back bed of flat machines.

The needles
The needle type which is most commonly used for flat and circular machine types is the latch needle. This needle does not need any external component to close and open the hook during the loop formation. A variant of the latch needle which was largely used on circular machines especially in the past, is the compound needle, with sliding closing element. In the compound needle, the latch (in the form of a sliding blade) moves along a groove in the needle stem independently of the movement of the stem itself. This technical solution speeds up the needle movement during the loop formation, because it is no longer necessary to wait that the latch is completely closed to continue the working cycle. In addition, the loop aspect is improved as the yarn is not subject to straining. However, the use of this needle requires complicated mechanics for two separate command systems, one for the needle stem and one for the sliding element, respectively. The two-bed circular machines with 180° beds where the loops of purl stitches are formed on both cylinders, require a particular needle type, called the double-latch needle, which incorporates a steel stem of adequate shape and two hooks with their latches at both ends.

It is a technical parameter fundamental for the classification of knitting machines. The gauge is the spacing of the needles, and refers to the number of needles per inch. This unit of measure is indicated with a capital E. The flat and circular machines now available from the different manufacturers are offered in a vast range of gauge sizes. For example, flat bed machines are available in gauge sizes from E 3 to E 18, and large-diameter circular machines from E 4 to E 36. The vast range of gauges meets all knitting needs. Obviously, the most common models are those with middle gauge sizes.

This parameter describes the size of the working area. On circular machines, the width is the operating length of beds as measured from the first to the last groove, and is normally expressed in centimetres. Flat bed machines are available in various widths according to the end product. For example, machines with a width of 20 cm are used in the production of hems and trimmings, whereas those with 250 cm of width produce a large cloth or more cloths at a time. However, the most common models are the so-called short or compact machines with a width of about 100 cm, which are designed for the high-performance manufacturing of shaped knitted goods, as well as the traditional long-bed machines with bed widths of over 200 cm for the production of semi-shaped knitted goods. In circular machines, the width size is the bed diameter in inches. The diameter is measured on two opposite needles. Large-diameter circular machines can have a width
of 60”, however the most common width size is the 30”. Medium-diameter circular machines feature a width of about 15”, and the small-diameter models are about 3” in width. Some large-diameter models incorporate adjustable beds, in which the bed width can be set freely, as a result varying the number of working needles and using a yarn cutting/nipping device at the end of every course.

In knitting machine technology, system is the set of mechanical components which move the needles and allows the formation of the loop. The output rate of a machine is determined by the number of systems it incorporates, as every system corresponds to a lifting or lowering movement of the needles, and therefore, to the formation of a course. The technical progress has led to the  manufacturing of machines with an increased number of systems. However, it is necessary to remember that more systems means a more complicated machine and thus more malfunctions. The system motions are called cams or triangles (lifting or lowering according to the resulting movement of the needles). The systems of flat bed machines are arranged on a machine component called carriage. The carriage slides forward and backward on the bed in a reciprocating motion. The machine models currently available on the market feature 1 to 8 systems distributed and combined in various manners (number of carriages and number of systems per carriage). Circular machines rotate in a single direction, and the various systems are distributed along the bed circumference. By increasing the diameter of the machine, it is then possible to increase the number of systems and therefore the number of courses inserted per each revolution. Today, large-diameter circular machines are available with a number of diameters and systems per inch. For example, simple constructions such as the jersey stitch can have up to 180 systems, however the number of systems incorporated on large-diameter circular machines normally range from 42 to 84.

Yarn feeding
The yarn fed to the needles in order to form the fabric must be conveyed along a predetermined path from the spool to the knitting zone. The various motions along this path guide the yarn (thread guides), adjust the yarn tension (yarn tensioners), and check for eventual yarn breaks. The yarn is taken down from the spool arranged on a special holder called creel if placed beside the machine, or rack if placed above it. The yarn is then guided into the knitting zone through the thread guide, which is typically a small plate with a steel eyelet for the yarn. In order to obtain particular designs such as intarsia and vanisé effects, the machines are equipped with special thread guides. A uniform feeding tension is fundamental for obtaining regular loops. On flat-bed machines, this parameter is controlled through the yarn tensioner, which consists essentially of a metal arm adjusted by means of springs in order to contrast and level off the spool unwinding tension peaks. On circular knitting machines, the yarn feeding control on the various systems is performed with various methods. The most effective is the positive feeding method, which employs small pulleys moved by belts, or gears etc., to exactly control the yarn feeding speed and keep it constant.

Fabric take-down
On knitting machines with latch needles, this motion carries out two distinct functions. At the beginning of the knitting cycle, it opens the latches and, at the same time, creates a tension on the course being formed. During the knitting cycle, the take-down motion winds the fabric with adequate tension. The take-down tension is a fundamental parameter for the knitting process, as it affects the length of yarn used for the loops, and consequently, affects the final geometry of the fabric. On flat bed machines, the take-down motion normally incorporates a main roller which takes up the fabric delivered by the beds, plus a number of counter-rollers arranged by sectors with adjustable pressure in order to guarantee a uniform tension on the whole fabric width. On large-diameter circular machines for the production of continuous fabrics, the fabric is delivered in tubular form which must therefore be flattened for subsequent winding on the take-up roller. This step is particularly delicate because it can cause fabric distortion. In order to avoid these problems, a spreader is added to the take-down motion which by increasing artificially the width of the fabric in the pre-folding zone, equalises the 79 lengthwise and crosswise tension and therefore improves the fabric geometry. The takedown rollers and the take-up roller are arranged below the spreader.


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