COTTON WEAVING MACHINERY The methods described under Weaving (q.v.) are by far the most widely used for the preparation of cotton warps, especially in Lancashire, but some modifications are used in other centres of the industry, and more productive methods are coming into use, particularly for the preparation of warps for grey or natural coloured cloth.
The beam warping process has been the subject of much experimental work, and in many places high-speed warping machines are in use by means of which the speed of warping has been doubled or more. The chief difficulty in the ordinary system lies in the varying tension on the yarn during unwinding, and particularly when starting and stopping the machine. Neither the flanged warpers' bobbins (fig. 30), which are most commonly used, nor cheeses (fig. 31) are very satisfactory when mounted in the creel and rotated by the drag of the unwinding yarn. The speed of the bobbin or cheese, when nearly empty, is very high; the comparatively heavy bobbins overrun at a stoppage and throw stress on the yarn when restarting; cheeses, on the other hand, become too light and are liable to bounce out of the creel, whilst the quick side-to-side traverse of the yarn on the cheese is an addi tional cause of trouble. In Amer ican beam warping machines the speed of warping is gradually re duced as the beam fills and the bobbins empty, in order to reduce these variations in tension, but this means some loss of produc tion.
In most high-speed warping machines the yarn is unwound from cones (fig. 32), or from bot tle-shaped bobbins (fig. 33) , these packages holding several times as much yarn as the ordinary warper's bobbin, which enables one creeling to suffice for filling several beams. Moreover, these packages are kept stationary in the creel and the yarn withdrawn over the end, which enables it to be unwound at a very high rate. A certain amount of tension is necessary and this is got by applying a small frictional drag to each thread. This tension is readily adjustable, so that all threads can be tensioned alike, and the drag on the yarn remains constant from beginning to end of the package. Also, when the machine stops, there are no bobbins to overrun and slacken the threads. An alternative system is to have the yarn wound on large cheeses, about gin. in diameter, the cheeses rotating in the creel on ball bearing spindles. Overrunning of these cheeses is prevented when the machine has to be stopped suddenly, by electrically controlled brakes which are applied to each cheese at the same time that the beam is braked.
The drying of the yarn after sizing in the slasher sizing machine is often done by passing it through a drying chamber instead of over steam-heated cylinders. The latter are liable to bake the yarn or even to scorch it, and the drying chamber is better for the yarn and enables a higher pro duction to be got.
Preparation of grey weft yarn in cotton weaving is the ex ception rather than the rule, but when automatic looms are used, rewinding of the weft on to pirns is common prac tice. These pirns hold much more yarn than the cops or bob bins from the spinning frames, and this, together with the re moval of faulty places, makes them much more suitable for automatic weaving. Modern pirn winding machines usually have horizontal spindles on whichthe pirns or tubes are held and driven by friction. The spindles rotate at a speed of 2,000-3,000 revolutions per minute, and, as the spindle speed is generally constant, the rate of winding the yarn is continually varying as the guider moves rapidly to and fro across the chase of the pirn (fig. 34).
This varying rate of winding causes a con siderable amount of trouble, due to the variation in yarn tension that it causes, and elaborate tensioning devices have to be used, with varying success, to counteract the changing speed.
Cotton Looms.The cotton weaving trade is divided in its opinion as to the ad vantages of the automatic loom over the ordinary type for the weaving of cotton fabrics. In America, Japan and elsewhere, automatic looms are widely used, but Lan cashire, on the whole, prefers to rely on the older machine. This different attitude is accounted for partly, though not entirely, by the different class of fabrics produced, by differences in the organization of the industry and in the labour conditions in these centres. It is also undoubtedly due, in part, to differences in temperament of both employers and workers.
For the more simple and coarser varieties of cotton cloth, especially when they are produced in large quantities, as they generally are, the automatic loom is unrivalled. But as fabrics become finer and more complicated in structure, the number of causes of damage to the cloth increases, and it becomes more and more difficult to weave the cloth otherwise than under the almost constant supervision of an experienced weaver, always on the alert to stop the loom at the least sign of anything wrong.
For simple cotton fabrics, the essentials for successful auto matic weaving are a satisfactory weft replenishing system, a warp --- -.. .-- - stop motion to stop the loom if a warp thread breaks, and an auto matic let-off motion that will maintain constant tension on the warp. Of these the first has received most attention and the problem of automatic weft supply has been fairly satisfactorily solved. This is done both by systems such as the Northrop, in which a full pirn is forced into the shuttle to replace the empty one, the transfer being effected without any reduction in loom speed, and also by several shuttle-changing systems in which the loom is generally stopped for sufficient time to allow for the change from empty to full shuttle.
A considerable number of weft replenishing attachments are in use, these being fitted to looms of the ordinary type to reduce the work of the weavers by doing away with the necessity for stop ping the loom for weft replenishment. These attachments gen erally consist of a cop or pirn-changing mechanism, but very little saving in labour is effected by their use unless the looms are also fitted with warp stop and automatic let-off motions. Automatic looms so constructed are somewhat cheaper in first cost, but less satisfactory than those of proper design. (For further informa tion and for bibliography, see WEAVING.) (W. A. H.)