Figs. 123 and 124.J.N.
Fig. 125.
Between the driver and driven roller a series of wheels are interposed, which are used as change wheels, so as to allow of an easy adjustment of the relative speeds of the whole series.
(210) The proper construction of a drawing frame turns largely upon the consideration of a number of small points. In this, as in all other machines used in the preparation of cotton, due regard must be had to the material which is being treated. Especially is this the case in drawing. The “staple” here plays an important part. If the distance between the centres of each line of rollers exceeds that of the length of the “staple” it is quite clear that no drawing worthy of the name will take place. To draw anything one end of it must be firmly held, and if the fibres could lie between two pairs of rollers without being gripped by either it is obvious they would not be drawn. Thus the distance between the centres of the rollers must be regulated to suit the material being treated, and the roller bearings are arranged to permit of that adjustment. This leads to the necessity, where very short slivers are drawn, for the reduction of the diameter of the rollers so as to permit of their being set in together more closely than could otherwise be done. Thus East Indian cotton is best dealt with by rollers of a small diameter set closely; while Sea Island or Egyptian can be drawn by larger rollers set wider apart. The principle underlying this practice has been indicated, but may be formally stated. The necessity exists for the fibre being drawn to be held at one end by one roller, while it is subjected at the other to the pull of a faster running roller. It is therefore essential that the distance between the centres of the rollers shall be such that the fibres are sufficiently drawn without being subjected to overstrain, by which a liability to rupture is incurred.
(211) It is the universal practice to effect the major part of the drawing between the third and fourth rollers, the increase in speed prior to that having been comparatively small. The exact speeds at which the various lines run depend largely upon the material treated, but a common acceleration is as follows: Assuming the first or back roller to revolve at 100 times per minute, the second line would run at 125, the third at 175, and the fourth at 275. The attenuation of the sliver between the first and second line is only 25 per cent; that between the second and third 40 per cent; and between the third and fourth 57 per cent. Putting it in another way, assuming one foot of sliver to have passed through the back rollers, it would become 15 inches long after passing the second roller; 21 inches as it leaves the third roller; and 33 inches as it finally emerges from the front roller. Thus, although the percentage of increase between the third and fourth is not largely in excess of that arising between the second and third rollers, the actual increase is exactly twice as much. These proportions are approximations to those which are actually employed, but, as was said, much depends on the character of the material which is being treated. In dealing with a soft elastic fibre like Sea Island cotton, light weighting and an easy draught is possible, while, if a harsh strong fibre is subjected to the same treatment there would be little effect produced. It is therefore necessary to put additional weight upon the rollers and thus increase their grip. A severe treatment of this character, which would be fatal to a weak or fine cotton, is beneficial to the coarser varieties, which can easily be subjected to a coarser draught. All these are points which require attention in practice, and careful observation will do more than many instructions in giving the knowledge of the right course to pursue.
(212) Another point requiring close attention is the preservation of absolute cleanliness. All cotton in its passage through the machines used, gives off a certain quantity of loose, short fibre, to which the name of “fly” is given. This is found all over the machine, and it adheres to the rollers in considerable quantity. Unless it is removed in some way it is apt to collect into thick pieces, or “slubs,” which attach themselves to the sliver, and thus cause thick places. These, of course, are perpetuated in every subsequent stage, and the removal of the fly, therefore, becomes of great importance. To accomplish this desirable end appliances known as “clearers” are fitted. These are flannel-covered surfaces, either flat or cylindrical, which rest upon the rollers. One common form is a plain, wooden cylinder covered with rough flannel, which is placed upon, and rests between, two of the lines of rollers, the rotation of which causes it to revolve. The rougher surface of the flannel licks up the fly from the rollers, and a periodical stripping is sufficient to keep the clearer effective. A simpler and also a common arrangement consists of a strip of flannel stretched within a cover and resting on the whole of the top rollers. A third modification is known as “Ermen’s revolving clearer,” and is an endless band of flannel passed over two rollers fixed in the cover. The lower part of the band rests upon the top rollers, and the clearer is slowly traversed, so as to remove the fly and convey it to the upper part of the case when the band is turned up. Here an oscillating comb is placed, and scrapes up the fly into small rolls, which can easily be removed periodically. A further form of clearer is made by Messrs. Dobson and Barlow. It consists of two wooden rollers sustained in a frame which is hinged at one end. One of the rollers which presses upon the back rollers is suitably driven so as to take up the fly from the first and second pair. The other roller is loose upon its spindle, which is much smaller than the bore of the roller, so that the revolution of the front rollers causes it to rotate at a speed somewhat below that of the rollers. Both of the wood cylinders are covered with flannel, and thus take up the fly with very great ease. It was found by practice that the positive driving of the back clearer effectually cleaned the first two sets of rollers, while a similar procedure with the front clearer did not lead to the same result. By allowing the latter to be frictionally driven, the rollers can be kept clean without difficulty. To strip the clearers they only require raising, when the fly can be readily removed.