Weaving machines used for manufacturing terry fabrics, double velvets and narrow fabrics are categorized in special weaving machines. Some of the examples of such machines are terry weaving machines, double velvet weaving machines, ribbon weaving machines etc.
Special Weaving Machines
This definition identifies the weaving machines equipped with particular devices to permit the production of particular types of fabrics. We shall handle here terry fabrics, double velvets and narrow fabrics weaving machines.
Terry weaving machines
For the production of terry fabrics, following machines are used: rapier, projectile or even air jet machines. These machines differ from the standard machines for plain fabric production owing to following features:
- double bearing, to house two beams: the ground warp beam, generally situated down, and the loop yarn beam, in upper position in order not to increas floor space requirement, even when using large diameter beams;
- double warp let-off electronic motions with auxiliary motors electrically connected to the driving motor of the cloth take-up roller and controlled by the machine’s PLC;
- loop formation mechanism: this device permits to close up the wefts to the cloth formation edge during the first two or three insertions, however without tightening them completely (short stroke or pre-strokes), and to produce their definite tightening against the cloth together with the third or fourth weft (long or loop stroke), with subsequent formation of the loop by the effect warp. This occurs because the group of wefts slides on the stretched ground threads, while the less tensioned threads of the pile warp, being tied up within the group of wefts, bend to form loops. The distance of the wefts closed up to the cloth formation edge (pre-stroke length) gives rise to the loop height. This last can present maximum values ranging, depending on the manufacturer, from 19 to 25 mm, corresponding to maximum loop height between 9 and 12.5 mm;
- electronic microprocessor controlled device: this permits to program the formation of the various effects (high loop, low loop, absence of loop, fringes) by means of an automatic control of the related mechanisms. Moreover it permits to program the manufacture of single towels or of towel sets in a prearranged number, and to execute on each of them a sequence of motifs. The sequence can be programmed at will, according to one’s own needs, or can be chosen out of a series of programmed sequences.
The loop forming mechanisms mostly in use are based on the principle of causing a shifting of the fabric and of the warp, so as to modify the position between the stroking reed and the fabric formation edge.
The reed maintains a regular movement, whereas the fabric edge is displaced periodically from the stroke point through the horizontal shogging of the temples, of the cloth diverting beam and of the back rest roller. Thus two or three pre-strokes followed by a loop stroke are produced, with the consequent formation of the loops.
Fig. 63 shows the cross-section through the warp of a double terry (loops on both sides) with loop repeat every 3 wefts (two pre-strokes and one loop stroke). The ground weave is an irregular warp rib (2/1), an ideal weave to attain a good anchorage of the pile threads.
Fig. 64 shows a general view of a teddy weaving machine, while Fig. 65 illustrates schematically the loop formation mechanism. The cam 1 which is mounted on a tertiary or quaternary shaft causes − with its maximum diameter, through the roll lever 2, the pull hook 3, the lever with regulation window for the high loop 5 and the lever 6 with fulcrum on the summit − the shift of rail 7, which moves the fabric formation edge away from the stroke line during the insertion of 2 or 3 wefts, depending on the loop repeat. This way the reed can only close up the wefts (pre−strokes) to the fabric and their distance from the fabric gives rise to the loop height. The temple 8 follows the movements of the fabric, being mounted on a lever with a pendular movement.
Double velvet weaving machines
While loop pile velvets are produced with wire weaving technique, which is quite labour intensive, cut pile velvets are produced on particular machines which permit to obtain two pieces of fabric at the same time.
These machines are equipped with a 3 position shedding device (dobby or Jacquard machine), so as to form two overlapped and properly spaced out sheds and to permit to the pile warp to tie up the two fabrics together. Into each of the two shed a weft is inserted, usually by means of a pair of superimposed rods driven by the same gear.
The pile warp is subsequently cut directly on the machine through a blade with horizontal traverse motion, thus forming the pile on both fabrics, which are then wound up separately. This system is largely used today owing to its high performance.
Ribbon weaving machines
The name ribbons identifies fabrics usually with a minimum width between 5 and 20 mm. They are produced today on particular multi-head machines with 2 to 12 heads, which are interchangeable at will and thus permit weaving several ribbons at the same time. The warp threads can be fed by beams or by a bobbin creel. The shed is formed by frames driven by cams (in case of small weft repeats) or by disks bearing linear cams composed of of glieder chains (these are small cams linked together to form closed rings) in case of larger weft repeats. When weaving figured ribbons, the threads are controlled by an electronic Jacquard machine. The weft is inserted into the shed by a particular mechanism. The thread taken up from bobbin 1 passes through a weft feeder which adjusts its quantity and tension, then enters the eyelet of a weft inserting element 2 composed of a bent traveling arm which, while penetrating into the shed 3, drags the weft until it protrudes properly from the opposite shed side 4, where a driven latch needle holds it when the weft feeder returns to its starting position; this way a double weft is laid down in every shed.
The latch needle will form with the subsequent wefts a mesh chain originating the selvedge. In order to get a firmer selvedge, an additional resistant yarn (supporting yarn) can be knitted separately or together with the wefts. Figure shows the different binding systems. On the other fabric side the selvedge is formed in the normal way by interlacing the warp threads.