CHAPTER 3 ‘THE GENERAL PRINGIPI OF ROLLER DRAFTING get PERFECT DRAFTING In Chapter 1 it was stated that drafting consists in making slivers and rovings longer and finer by causing the fibres to slide past one another, and in Chapter 2 that drafting increases the irregularity of the material. It follows that the increase in irregularity is due to the way in which the fibres move during this sliding. In this chapter the movement of fibres during per- fect drafting, i.e., “drafting that does not cause any irregularity, will be described. This kind of drafting is easy to describe on Paper and simple to understand, but it cannot be realized in practice. It is important, however, because it is not possible to understand the various kinds of irregularity introduced during drafting without first understanding perfect drafting. A sectional view.of a set of drafting rollers is shown at the top. of Fig. 17; the rest of the figure consists of five plan yiews repre- _ senting five stages during the drafting. In, these the rollers are, not shown, but the roller nips, which are the lines along which the top and bottom rollers make contact and grip the fibres, are shown as thick verticallines. The dotted vertical lines merely mark out equal distances, The fibres are shown as horizontal lines, Those to the left of the back nip represent the sliver entering the back rollers; it will be seen that they have been made all of the same Iength, all straight and parallel, and have been ar- ranged in echelon formation. This has been done,to make the diagram clearer; if the fibres were mixed up across the sliver without being moved lengthways, their arrangement would be more like that in an actual sliver, but the explanation of perfect drafting which follows would not be affected by the rearrangement. The fibres have also been sketched so that where one fibre ends another begins. Consequently,’ the number of fibres which cross any one of the vottical det yes’ is always at f/ of ' LIBRARY { 1 \ \7 MANUAL OF COTTON SPINNING 4 four (except near the ends of the sliver). The sliver therefore ha the same thickness all along its length; it is in fact an ideal uni form sliver, but the sliver pictured is only four fibres thick whereas actual slivers are many thousands of fibres thick. re al sliver may now be ‘drafted’. Suppose that the Uftance apart of the roller nips—the réller setting—is equal to the length of the fibres (it is shown a little greater in the dia- Fic, 17 Perfect drafting. Draft = 2 grams for the sake of clarity), and suppose also that the front roller. speed is is twice that of the | back k rollers, so that the draft is 2. In diagram 1 the lead ing fibre, A, has just reached the fro1 front nip, and the fibre is is therefore moving at the the speed of the front _ rollers. All the other fibres are either held ir in the back nip or belong to the sliver being fed to “the back rc rollers, and are there- fore moving at back-roller speed. Diagram 2 shows the position a moment later when the fibre B has moved one unit, has just released by the back nip, and has reached the front nip.THE GENERAL PRINGIPLES OF ROLLER DRAFTING 43 In the meantime, fibre A, which has been moving at twice the see ela has moved two units, and consequently the leading ends of fibres A and B are two units apart, instead of one unit as they were in the original sliver. Siitce they are now both moving at the same speed they retain this spacing. The drafting proceeds according to diagrams 3 and 4, and the state of affairs ater it has continued for some time is shown in the bottom iagram, with the drafted sliver to the right of the front-roller nip. The effect of the drafting is to space the left-hand ends of the fibres twice as far apart as they were in the original sliver. The drafting has also apparently opened up diagonal gaps in the sliver, but this is merely a consequence of the fact that fibres ~ are arranged in echelon and would not happen in an actual sliver, Ignoring these gaps, and following each dotted line across the sliver, it can be scen that the drafted sliver is everywhere two fibres thick. It is again a uniform sliver, but has half the weight per unit Jength of the entering’sliver, as of course it Should because the draft is 2. The drafted sliver is uniform be- cause the fibre ends are equally spaced just as they were in the original sliver; referring back to diagrams t and 2 of Fig. 17, we see that this is so because the fibres change from back- to front- roller speed only ‘when their leading ends reach the front-roller nip. This is perfect drafting, and does not cause irregularity. It is evident that the above argument holds whatever the draft. For example, if the front-roller speed is four times the back- roller speed, the fibre A in Fig. 17 moves four units between diagrams 1 and 2 and the spacing between the fibres is increased four times. In general, the effect of a perfect draft is to multiply the disthnces between the leading fibre ends by the draft. Diagrams like those in Fig. 17 will be used in later chapters to explain more complicated effects of drafting, and three things should be noted about them. First, in order to give a set of fibres sie ideal draft it is necessary only to sketch them again with the aTstances apart of their leading ends multiplied Py the drafts second, a wider _spacing of the fibre ends_corresponds car thinner sliver; and third, in order to judge the thickness of @ on one of these diagrams the number of fibres crossing 4 Pion of the diagram must be counted. poco— a MANUAL OF GOTTON SPINNING 44 3.2 REAL DRAFTING | Real drafting differs from the perfect drafting described in the above section in four zt (4) the fibres in the sliver or roving are not all the same length 7 and are not perfectly straight and parallel, (8) the roller nips do not always remain in fixed positions, _{3) the rollers do not always run at steady speeds, and the rollers sometimes slip, so that the fires under them do Y not move at the speed of the rollers. ways: . All of these cause irregularity. The first of these differences is largely a property of the cotton, and causes a wave-like irregularity, called the drafting wave, whose wavelength and amplitude are both very variable. , The drafting wave is at present unavoidable, and is the main cause of irregularity on machines which are in good mechanical condition. The other three differences between actual and per- | fect drafting are due to mechanical faults in the machinery, and usually cause periodic irregularities. They can be avoided by proper design and maintenance of the machines. The next four chapters will deal in greater detail with these four causes of irregularity. \