Retd. Jt. Director (BTRA) and Consultant
Nipper closing time in Backward Feed and Top comb entry time in Forward feed have critical influence on noil. Theoretical noil% when nipper closes before completion of feed in Backward feed and top comb enters before completion of feed in forward feed have been worked out. Backward feed should be used for high noil levels and super combing and Forward feed is recommended for low noil levels and semi combing.
Sequence in which operations of feeding, combing and detachment take place in comber has significant influence on noil level. Combing can be done either with backward feed or forward feed. Feeding of sheet takes place during backward movement of nipper in backward feed and feeding takes place while nippers move forward towards detaching rollers in forward feed. In most of the modern combers like Rieter E76, Truetschler TCO, LMW LK 64 and Sacolowell Ca comber there is option to work either with backward or forward feed. Essential features in which the two feeds differ are discussed and the comber settings in each type of feed that has critical influence on noil level are indicated. Backward Feed
Sequence of operations in backward feed is combing (by half lap) detachment and feeding.
Fig 3: Fringe length combed by half lap in Backward feed
Fig 1 shows position of nippers in relation to detaching rollers at forward most position. Fig 2 indicates arrangement of fibres in the fringe and line B indicates the position of nippers at forward most position. Line A denotes the detaching roller nip line. BC denotes the feed during each combing cycle. After detachment nippers move backward and feeding takes place and line C in fringe moves to line B . Nipper closes at this point and combing by half lap takes place as shown in Fig 3. Consider fibres in feed sheet whose right hand ends lie in BC. Of these,1. fibres longer than (D + F) will go into detaching roller as they bridge the gap between nipper and detaching roller. 2. As nippers move backward, feeding takes place and fringe moves by an amount F and fringe projecting from nippers will be subject to combing action by half lap as shown in Fig 3. As fibres shorter than D are not able to reach detaching roller nip during detachment, they will be combed out during next combing cycle by half lap as shown in Fig 3. These fibres will go into noil. 3. Fibres with length between D and D + F will go into sliver or noil depending upon where they lie. For example fibres of length D + K will go into sliver if their right hand ends lie in BE, but will go to noil if their right hand ends lie in EC. Assuming uniform fibre end distribution, expected noil will be nearly equal to % of fibres (by weight) with length below D + F/2 with backward feed. Forward Feed
In Forward Feed, sequence of operations is combing followed by feeding (during forward movement of nippers) and detachment. Fig 4 shows arrangement of fibres in fringe in Forward Feed where C indicates nip line of nippers and A that of detaching rollers. Consider fibres with right hand ends in BC. Of these, 1. fibres longer than D will go into detaching roller nip and sliver, 2. Fibres shorter than D - F will not be able to reach detaching roller nip and will be removed as noil during next combing by half lap as shown in Fig 5 and 3. fibres with length between D and D - F will go to sliver or noil depending upon where they lie. Noil % removed will be % of fibres (by weight) below D - F/2. Hence forward feed will result in less noil than Backward feed.
Thus Backward feed is preferred when high noil % is required especially in combers with limitations in maximum detachment settings and when super combing is done with long staple cottons. Forward feed is preferred, on other hand, when noil % has to be kept low and in semi combing. Detachment settings cannot be reduced beyond a certain level in comber and under such conditions forward combing enables reduction in noil. Noil theory by Gegauff also gives similar results. Combination of Backward and Forward Feed
In actual practice, the situation is not as simple as described above, as no comber is fully backward feed or forward feed as shown below. Feed Timing in Backward Feed
If in backward feed, nipper closes before feed is completed, part of the feed will take place after combing by half lap. For this part of feed comber effectively operates as forward feed. Expected noil under such conditions can be arrived at as follows.
Fig 6 Arrangement of fibres in fringe during detachment with backward with nipper closing before completion of feed
Let b denote fraction of feed that takes place before nipper closes. (Fig 6). Then bF indicates the feed during backward movement of nippers and (1 - b)F denotes the feed that takes place during forward movement of nippers Section in the fringe at C moves to B and nipper closes at this point. Since further movement is arrested by nipper, fringe bulges behind nipper as shown in Fig 7. Combing by half lap then takes place as shown in Fig 7 and as nipper opens during forward movement the balance of feed (1 - b)F (shown as a bulge in Fig 7) takes place. Consider fibres with right hand ends in BC. Of these fibres longer than D + bF will go into sliver 2. Fibres shorter than D-(1-b)F will go into noil as they will not be able to reach detaching roller nip and will be combed out during next combing as shown in Fig 7. 3. Fibres with length between D + bF and D - (1 - b)F will go into sliver or noil depending upon where they lie.
Fig 7 : Fringe during combing with backward feed with nipper closing before completion of feed
Expected noil in this case will be nearly equal to % of fibres by weight having length below [D - (1 - b)F + F/2] Top Comb entry timing in Forward Feed
In the case of forward feed, top comb entry timing determines the extent to which comber operates as forward feed. If top comb enters the fringe before completion of feed, feeding will be arrested by top comb and the fringe bulges behind top comb. This part of the feed eventually takes place after top comb lifts from fringe and nippers start moving backwards. This part of the feed therefore acts like backward feed. If C denotes the fraction of feed that takes place before top comb entry, it can be shown that fibres longer than D + (1 - C)F will go into sliver and fibres below D - CF will go into noil. Fringe length during combing by half lap will be D + (1 - C)F. Noil % will be given by % weight of fibres below D - CF +F/2.
Thus feed timing in Backward feed determines the proportion of feed that takes place during backward movement of nippers and critically influences noil %. Early feed timing can be used to reduce noil. Likewise top comb entry time in forward feed determines the proportion of feed that takes place during forward movement and significantly affects noil %. Late top comb entry timing can be used to reduce noil in forward feed. Studies carried out by Balasubramanian and Viswanathan 1 and Sriramulu and Shankaranarayana2 have shown that if noil % is kept same there is no significant difference in quality of combed sliver and yarn properties between forward and backward feed. References
1 N. Balasubramanian and K. Viswanathan, Backward feed and Forward feed in combing, BTRA Scan, 1977, VIII, Sept., p 1
2. V. Sriramulu and K. Shankaranarayana, Relationship between fibre length characteristics and comber settings, Proceedings of 18th Joint Technological Conference, (ATRA, BTRA and SITRA), 1977 Reciprocal links directoryFree BlogFree Directory Submit - SEO Directory Submission ServiceAdd Your Site for FreeFree Directories Submission
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