217 (#289) ############################################ TYPES OF DRAWING MACHINES wool will wear iron. Again those who have had to deal with nicked ” or irregularly worn rollers know to their bitter cost how wool may wear rollers. Of course wrong setting of rollers may make irregular wear of rollers almost inevitable, but with rollers made out of suitable metal it would seem possible to give the worsted drawer easier control of the fibres he has to treat without leaving the fear of the irregular wear of rollers all the time at the back of his mind.* Thus it would seem that here is a field for useful research in which technologist, engineer and metallurgist should combine. Rollers may be made with plain, milled or fluted surfaces. Which of these surfaces is adopted will depend upon the work the roller has to do. As all rollers should nip the wool without rubbing or cutting it, it will be evident that- The nature of the surface of the roller, and The weighting of the roller must between them ensure the required firm nip. Thus, smooth-surface rollers would apparently offer less re- sistance to slipping than rough-surface rollers, rough- surface would control less than, say, one smooth and one fluted roller acting together, one leather-covered smooth roller and one shallow fluted roller would control still better, while two deep-fluted rollers acting together with a leather sheet in between or two leather covered rollers would act better still. Again, hard and soft covered rollers must be carefully studied in view of the work to be done. A careful examination of pairs of rollers in drawing boxes makes it evident that there are three conditions of control exercised :- the • Under certain circumstances it is desirable to traverse sliver to ensure approximately equal wear across the surface of the rollers. 217
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