220 (#292) ############################################ WOOLLEN AND WORSTED SPINNING 60 arc weight of the top roller is not sufficient, then rather than add a complex weighting system it may be better to add a third roller in which the tension on the sliver will be increased probably according to the formula given on page 188. With front roller control the con- ditions are very different. To obviate wildness of the fibres as they enter the front rollers (the change of speed of fibres being in proportion to the draft and coming on suddenly) the faller or carrier should be able to act as close up to the front roller as possible. But to effect this small front rollers are desirable. But as the speed of the fibres is to be suddenly increased a powerful nip is necessary. Even mechanical weighting fails to control long wool fibres (over 3 in.) unless acting with, say, a 3 in. or 4 in. roller to give the necessary of contact.” Thus a compromise is arrived at-the metal fluted bottom front roller (driven) is made as small as possible, and the leather covered top roller is made sufficiently large (say 6 in. to 8 in.) to assure a reasonably long arc of contact, and weighted by either springs or hung-on weights. Again this problem has never been satisfactorily faced, little better than rule-of-thumb methods, which have been found by practice to give good results, being adopted. The further adjustment of back and front rollers to control drafting will be best studied by reference to Figs. 58 and 69. It will at once be noted that while in gill-boxes the drive is from the back to the front (more speed of front rollers, i.e. more teeth in change wheel giving more draft), in drawing-boxes it is usual to drive from the front to the back (more speed of back rollers, i.e. more teeth in change wheel giving less draft).* The setting of the carriers between the back > * The reason for this should be clearly thought out. 220
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