Yuvarlak Örme Makinesi Elemanlarının Aşınma Ve Dinamik Davranışının Modellenmesi

Abstract

Yuvarlak örme makinelerinde çalışan iğneler, kumaş üretimi esnasında değişken ve tekrarlı yüklere maruz kalmaktadırlar. Etki eden bu kuvvetlerin yarattığı gerilme değerleri malzemenin karakteristik değerleri olan akma ve kopma dayanımından küçüktür. Ancak dinamik yüklemeler altında malzemeler bir süre sonra maruz kaldıkları bu yükleri taşıyamazlar ve kırılırlar. Bu durum, iğnelerin yorulması olarak değerlendirilir ve bu yorulma karşılığında meydana gelen maliyet kayıplarının önüne geçebilmek için iğne yorulma ömrünün tayinini zorunlu kılar. Bu noktadan çıkışla, yapılan doktora çalışmasında; yuvarlak örme makinesi iğnesine etki eden kuvvetler ve bu kuvvetler doğrultusunda iğne ömrünün hesaplanmasına yönelik bir çalışma kurgulanmıştır. Doktora çalışması ana hatları ile iki kısımdan oluşmaktadır. Birinci kısmı İstanbul Teknik Üniversitesi Tekstil Örme Laboratuvarı’nda laboratuvar tipi yuvarlak örme makinesinde gerçekleştirilmiştir. İğneler, Ne 4/1 % 100 telefli open-end pamuk ipliği ile kumaş örerek ilk iğne kırılıncaya kadar aşındırılmışlardır. Daha sonra iğne üzerine gerinim ölçer (strain gage) bağlanmış; 100 dev/dak makine hızı ve 100 Hz’de birim şekil değişimi değerleri ölçülerek Catman yazılımı ile bilgisayara kaydedilmiştir. Daha sonra elde edilen bu değerler, nCode yorulma programına aktarılarak sanal yorulma analizi yapılmış ve bu programdan elde edilen değerler ile gerçek yorulma sonuçları karşılaştırılmış; aradaki farkın % 9,04 olduğu tespit edilmiştir. Bunun devamında, 3-boyutlu olarak modellenen örme sisteminin sonlu elemanlar modeli oluşturulmuştur. Bu modele, uygun sınır şartları ve kuvvetlerin uygulanması ile sonlu elemanlar analizleri tamamlanmıştır. Analiz sonucu elde edilen gerilme değerleri kullanılarak nCode programında sanal yorulma testleri yaptırılmış ve elde edilen sonuçlar gerçek kırılma zamanı ile karşılaştırmalı olarak değerlendirilmiştir. Çalışma sırasında, iğnenin yedi farklı yüzeyinden dört farklı tekrar sayısına ait farklı büyütmelerde SEM çekimleri yapılmış ve iğne profilinin üretim esnasında nasıl değiştiği de irdelenmiştir. Çalışmanın ikinci kısmı ise RWTH Aachen Üniversitesi ITA Enstitüsü’nde 90 beslemeli E24 30” sanayi tipi yuvarlak örme makinesinde gerçekleştirilmiştir. Bu aşamada, iğne hareketine makine hızı, iplik tipi, iplik gerginliği ve elastanın etkisi irdelenmiştir. Makine üretim halindeyken OLYMPUS i-SPEED 3 yüksek hızlı kamera yardımıyla alınan görüntüler yoluyla iğnelerin hareket eğrileri irdelenmiştir. Yuvarlak örme iğnelerinde karşılaşılan yorulma hasarlarının tespiti amacıyla gerçekleştirilen sonlu elemanlar analizi üzerinde farklı üretim verileri anlamında değişikliklere gidilerek değişen koşullar için iğne ömürlerinin hızlı bir şekilde belirlenebildiği gösterilmiştir. Diğer bir deyişle, çalışma kapsamında geliştirilen model kullanılarak; düşük maliyetlerde ve çok hızlı sürelerde iğne yorulmasına, dolayısıyla hatalı kumaş üretimine yol açabilecek verilerin belirlenebileceği gösterilmiştir. Buna ilave olarak, iğnelerin değişen örme şartlarına bağlı olarak yüzey topolojilerinin nasıl değiştiği de irdelenmiştir. Son olarak, üretim şartlarına bağlı olarak iğne hareketinin değişimi analiz edilmek suretiyle, kumaş üretimi esnasında iğne sıçrama miktarını en aza indirebilecek ve dolayısıyla iğne ömrünün uzatılmasını sağlayabilecek bir optimizasyona gidilmesinin de mümkün olacağı gösterilmiştir.

Needles are the most important stitch formation elements of the knitting process and they are exposed to variable and repetitive loads and forces during fabric production. Stresses that are obtained due to these forces are smaller than the material yield and tensile strength. However, after a while the material cannot carry these dynamic loading they are exposed and finally broken. This is regarded as the fatigue of the needles. Any abraded or fracture of these needles cause unwanted and irrecoverable faults on fabric like uneven loop structure, holes and dropped stitches, which cause huge irresistible loss costs. Meanwhile, majority of the manufacturers would like to produce their products with high speed but without any faults in the process. This is achieved to some extent with the advance in many industrial areas, especially machinery and computer technology. However, it has been realized that it is impossible to increase the production speed infinitely without considering textile materials using on the machines due to the material related problems occurring in the production. Increasing the speeds of weft knitting machines will consequently enlarge the reaction forces between knitting elements and then cause their attrition. From this point of view, the doctoral thesis under discussion was conducted with the aim of obtaining the forces acting on the circular knitting machine needle, and then of determining the fatigue life of the needle in accordance with these forces. The study consisted of two parts. The first part was performed at Istanbul Technical University, Knitting Laboratory. With the aim of determining the life of the needles, the laboratory type knitting machine, which helps to save material and energy, was employed. The machine was 3.25” in diameter and had fifty six needles. During the production process, all the settings were kept constant in order to investigate the lifetime of the needle at standard conditions. The machine had the same equipment such as automatic oil, take up and feeding system, as an industrial one. Before starting the experiment, all of the needles of the machine were replaced with the new ones and the knitting period was ended when a needle breakage occurred. For the study, second quality Ne 4/1 100 % cotton open-end yarn was used. In the beginning of the study the machine velocity was adjusted to 100 rpm, but then increased to 250 rpm and finally the velocity was adjusted to 450 rpm. A positive feeding system was used to supply yarn from side creel to the knitting zone under four grams of tension. The periodic machine maintenance was also applied such that the needles, sinkers, slots and cams were cleaned in detail. Up to needle fracture, three maintenance processes were made. At first section of the first part of the study, the surface change during fabric production was examined by scanning electron microscopy (SEM) due to its good resolution, depth of focus and its ability to provide an understanding of the surface change during fabric production. To do so, seven different surfaces of the needle at four different cycles were analyzed at different magnifications. The SEM examination was conducted using a 10-20 kV electron beam and with a magnification range of 10X to 3000X. In our study, the first fracture occurred at rivet part of the needle after knitting 10.042.072 courses. In our study there was only one fatigue source propagating from the surface. Heterogeneities, micro cracks and mechanical striations were observed when lateral surface of the unused needle rivet part was examined in detail under electron microscope. This could be an additional factor that had given rise to crack initiation as a result of stress concentration. The fracture surfaces of the particles are mostly perpendicular to the uniaxial loading direction, and the cracks propagate from the free boundary to inside, which is corresponding to a maximum principle stress direction. From the SEM photos, it appears that wear of the latch hole and rivet had perpendicular visual sight to the latch stem. This wear might be the result of abrasive particles in the open-end yarn etching the metal from the surface. As the yarn comes into contact with the throat area of the needle, these fiber particles which contain certain levels of micro dust generate wear on the latch blade and saw slot walls. The SEM photographs also showed that there was a wear on both upper and bottom parts of the needle butt. The wear mostly occurred in the areas where needle butt follows the cam path and the wear at these points grew gradually when the total revolution of the needle was increased. Moreover, the amount of wear on each side of the needle butts was dependent on direction of the machine revolution as the right sides of the needle butts tended to wear more, when compared to the left sides. Also, it was found that the wearing off in the hook part moved in more by increasing the number of knitting courses especially at inner contour of the hook. Furthermore, the grooves occurred due to the heavy yarn friction between the yarn and the needle hook became more deepened and extended as the number of knitted courses increased. At second section of the first part, strain gage was glued onto the closed position to the fracture occurred point and strain values were recorded at 100 Hz and 100 rpm conditions. These strain values were evaluated at Catman program and then imported to nCode Glypworks program in order to perform virtual fatigue analyze. nCode Glypworks results showed that there was a 9,04% difference between the real and virtual fatigue lives of the needle. In the following section, 3-D models of the cam, needle as well as cylinder of the knitting system were drawn by Solidworks program and then a finite element solver ABAQUS was used to determine the forces acting on the needles during knitting production process. The stress values obtained were then imported to nCode Designlife program in order to comparatively study the fatigue lives of the experimental and finite element results and with a 14,17 % difference the lives of the needles could be estimated. Also, the damage images obtained by nCode Designlife program were similar to SEM images. Second part of the study was performed at RWTH Aachen University Institut für Textiltechnik (ITA). A E 24 30” circular knitting machine, equipped with Groz-Beckert needles, with 90 positive feeding systems was used in an attempt to investigate real time needle movement on an industrial machine using a OLYMPUS i-SPEED 3 high speed camera, which was a novel approach for such studies. The measurements were recorded at two different machine speeds, which were 15 and 20 rpm, and Ne 50/1 ring cotton and 110 dtex textured polyester yarns were used at three different yarn tension values in order to see their visual effects on the needle movement. Also, elasthane and its three different tensions were also added to the experimental set. By doing so, fourty eight varied situations were investigated. The important results of the study under discussion can be summarized as follows: 1. The needle displacement in both x and y coordination did tend to increase as the yarn tension raised, irrespective of machine speed. 2. For all the relevant cases discussed, the presence of elastane yarn was a significant parameter on needle movement. However, its input tension was not as influential as its presence. 3. The data obtained for knitting with textured polyester yarn showed that the needle movement was highly dependent on yarn tension. 4. Producing with cotton ring yarn made the needle bounce more than knitting with the textured polyester yarn, especially for low yarn tension and machine speed. 5. For all situations, the velocity of the needle was higher at robbing back, which in turn caused a greater displacement. Accordingly, it may be suggested that for a longer needle life; • Manufacturing at optimized yarn would decrease the force inside needle hook. • Working at lower machine speeds causes needles to bounce less, which in turn results in decreasing impact force. • Working with yarns of low frictional properties would allow needles to follow cam path thoroughly, which results in less abrasion on cam and needle butt. The finite element model developed to determine the fatigue damage encountered in circular knitting needle showed that by changing production data, the lifetime of the needle can be determined very quickly at low cost. In addition to that, the surface topology of the needle during fabric production was investigated. Finally, it was showed that with the help of an optimization process based on analyzing needle movement at various production conditions, not only needle life could be extended, but also the production conditions causing possible needle damage could be avoided.