187 (#249) ############################################ ING PRINCIPLES IN WORSTED DRAWING in. to s un other rollers either by fallers or carriers. Fallers naturally eliminate the necessity for or the possibility of twist support; but carriers offer little or no obstruction to twist support and twist-fibre-control. Thus it comes about that it is customary to add such twist to slivers coming from any given box as will best ensure perfect delivery from the box, and more particularly ensure the control of these slivers during entry into and drafting in the succeeding box. In open drawing the twist added is designed to ensure strength of sliver for winding on to the bobbin and yet not to interfere with the fibres sliding upon one another. Usually the twist will be decided to ensure efficient control during drafting, this being ample to ensure efficient "winding on" and "drawing off.” It is much to be regretted that so little is known about the frictional co-efficients of fibres, since the amount of twist may obviously be varied in accordance with the friction of fibre on fibre. Thus to satisfactorily control Alpaca slivers much more twist must be inserted than in drawing wool slivers of a similar thickness, while for Ovibos -apparently a very slippery fibre--less twist is needed.* From recent researches carried out in the University of Leeds two results obtained have a bearing on this present problem. It has been found that to obtain the maximum strength for any yarn or sliver an ex- terior fibre angle of about 25° is necessary. The discovery also has been made that unless an angle approximating to 25° is obtained, under con- tinuous strain such as during weaving, there may be slipping of the fibres one upon the other. It will, therefore, be obvious that in drafting, an exterior fibre angle under 25° must be given, and that * The “spring” of Alpaca and the “ plasticity" of the Ovibos fibre is probably the explanation. ) 187
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