Identifying and keeping yarn tension variation as low as possible is important in textile processes. In weaving process, it is desired to obtain constant uniform warp tension across the width of the warp. Since during fabric formation, low warp tension creates clinging that causes unclear passage for the filling. While high tension increases yarn breaks. Detecting and controlling the optimum warp tension will maximize the weaving efficiency, improves fabric quality, fabric dimensional stability due to better ratio of warp crimp to weft crimp and enhances the uniformity of color shade (Gahide 2001). The running tension should be at a value neither to over-elongate nor to entangle the yarn (Eskew 2006).
Different initial force, which depends on the fabric purpose, weave and weft setting, can be given for warp (Galuszynski 1981; Gu 1984; Hepworth 1984). High initial tension can influence high cyclic deformation of thread and high fluctuation of tension of whole system. When initial warp tension increases, the amplitudes of changes of warp tension and deformations of loom setting system are almost constant (Shih et al. 1995; Zhang and Mohamed 1989; Jeong and Kang 2001; Katunskis 2004).
The backrest or whip roller is one of the most important parts of a weaving machine, and lengthwise yarn of the fabric known as warp yarn passes over the backrest. One of the main functions of the backrest oscillation is to compensate variation in warp tension as they are unwound from the weaver’s beam during each weaving cycle of the loom. This tension can not be kept constant during the process and also during each weaving cycle of the loom. But due to have a very negligible difference among the tensions of each loom cycle, an average value of tension is maintained throughout the weaving process in factory. If this tension is kept at a constant then the fabrics will be woven with minimum number of warp breakages yielding a better loom efficiency and higher quality. The correct selection of the backrest of a loom is vital in producing high quality woven fabric. Adjustments to the positioning of the backrest, its type are inevitable when the performance of the weaving machine can no longer be tolerated with standard settings for low weft density, high weft density, and extra heavy fabrics. Vary in fabric density is a crucial especially in technical textiles and smart textiles (Fernando 2014). Denim fabrics have developed into a part of the garment fashion since the ninetieth century. Most consumers around the world prefer cotton apparel; in particular, they enjoy wearing denim. The success of denim is due to its ability to change with every social and cultural evolution (Card et al. 2006). According to Textile terms and definition published by Textile Institute Denim is traditionally a 3/1 warp faced twill fabric made from yarn dyed warp and undyed weft yarn. As denim is one type of woven fabric so it is obvious that like other woven fabrics, properties of denim is also affected by position of whip roller.
On a loom, warp yarns are divided into two half to make a shed. This division makes up a specific geometry of divided warp yarns called Shed Geometry. Shed geometry plays a vital role in controlling warp yarns tension, elongation and friction between them (Javaid 2011).
Besides the aesthetic properties, mechanical and physical properties of fabric are considered as decisive quality parameters. Fabric structure plays a critical role for predicting the fabric properties. Among other fabric properties, fabric strength is one of the most important properties of woven fabric, especially for technical textiles application (Pavlinic and Geršak 1989; Hearle et al. 1969; Majumdar et al. 2008).
Breaking strength is the maximum tensile force recorded in extending a test piece to breaking point. It is generally referred to as strength. The force at which a specimen breaks is directly proportional to its cross-sectional area, therefore when comparing the strengths of different fibers, yarns and fabrics allowances have to be made for this. The tensile force recorded at the moment of rupture is sometimes referred to as the tensile strength at break (Saville 2002).
The breaking load of a fabric in either the warp or weft direction is primarily determined by the strength of the yarn. The other important fabric variables effecting strength of woven fabrics are fiber properties, yarn properties, warp and weft densities, fabric weave, crimp and finishing process. Fabric strength tends to increase as the thread densities in both directions increase (Greenwood 1975; Adanur 2001; Mohamed and Lord 1973; Lee et al. 1996; Bassett et al. 1999; Cook 2001).
An increase in the weft density or warp density also leads to an increase in crimp. This has detrimental effect on fabric strength because, as the crimp increases, the yarn lies more obliquely in relation to the plane of the fabric. Thus a greater force is required in the yarn to balance a load applied in that plane (Greenwood 1975).
Skewness is a known fabric defect which occurs in both woven and knitting fabrics. Skewness is created when the pattern is distorted across the fabric width. In other words, it is a condition resulting when weft yarns are angularly displaced from a line perpendicular to the fabric selvage, usually, due to uneven distribution of tension (ASTM-International 2006; Lee 1989).
Research on fabric skewness is very scarce, and no studies have been found that investigate the effect of loom settings on fabric skewness. Some studies (Alamdar-Yazdi 2005; Moore et al. 1995) have shown that fabric shear properties and drape are affected by skewness. It was found (Alamdar-Yazdi and Khojasteh 2006) that there is a positive correlation between twist liveliness and fabric skewness, and the skewness is higher when the twist direction of the warp and weft yarns are opposite to each other. Only Nasan and Stylios (2014) analyzed the influence of backrest position on skewness of woven fabric where they found no significant effect of whip roll settings on fabric skewness.
Fabric porosity and air permeability depend on many factors, such as raw material, yarn type, fabric construction, machines working conditions and other parameters. However fabric porosity and air permeability are not constant in the width of the fabric, they are higher in the central part of the fabric than in the border parts. This will limit especially the use of industrial fabric in its whole width (Milašius and Rukuižien 2003).
The waviness or distortion of a yarn that is due to interlacing in the fabric is termed as crimp. In woven fabrics, the crimp is measured by the relation between the length of the fabric sample and the corresponding length of yarn when it is removed there from and straightened under suitable tension. Crimp may be expressed numerically as percentage crimp or crimp ratio. Position of whip roller has significant effect on Crimp% of woven fabric as warp yarn tension is an important player in case of crimp%. It has been found that raising the backrest above the normal height reduces the warp crimp, increasing the weft crimp (Talukdar et al. 1998). In present study, this result was verified both in case of grey and finished state of denim fabric whereas earlier study it was not checked.
Sheikhzadeh et al. (2007) investigated the relationship between the ratio of the force applied on the warp yarn by the whip roller to the warp yarn tension, and the vertical and horizontal position of the whip roller with variations in warp beam radius during the weaving process. Adanur and Qi (2008a) established an on-line tension measurement system by producing denim fabric on an air-jet loom in order to evaluate yarn tensions. Adanur and Qi (2008b) studied the effects of tension on the properties of denim fabrics made on an air-jet weaving loom. The variations in warp yarn tension during the weaving process become smaller by the backrest roller’s swinging motion. At a high speed of the weaving machine, it could be possible to obtain a suitable relationship between movements of the whip roller and warp yarn tension (Kloppels et al. 2002).
Sheikhzadeh et al. (2007) investigated the relationship between the ratio of the force applied on the warp yarn by the whip roller to the warp yarn tension, and the vertical and horizontal position of the whip roller with variations in warp beam radius during the weaving process.
Weinsdorfer et al. (1991) analyzed the distribution of warp tension over the warp width connected with the changes in shed geometry (whip positions). They found that by changing the shed geometry, the warp tension also varies. By lifting the whip, the elongation of warp yarn in the lower shed increases, as a result the warp tension in the lower shed also increases. On the other hand, the warp tension in the upper shed decreases.
Turhan et al. (2007) presented experimental, computational intelligence based, and statistical investigations of warp tensions in different back-rest oscillations. To have different backrest oscillations, springs with different stiffness were used. For each spring, fabrics with various weft densities were woven, and the warp tensions were measured and saved during the weaving process. The empirical data were analyzed by using linear multiple and quadratic multiple regression, and an artificial neural network model. Osthus et al. (1995) reported that the warp end tensions are influenced by changing the height of the backrest roller. They evaluated the fabric appearance using an image processing system. The results for different backrest heights show that in the higher position of the backrest, the colour of the fabric become darker; and the fabric density is greater with an increasing backrest height.
Bilisik and Yolacan (2011) assessed the tensile and tearing properties of large and small structural pattern denim fabrics after an abrasion load and compared them with traditional denim fabrics. They obtained better tensile properties of abraded small structural pattern and traditional denim fabrics than those of large structural pattern denim fabric. They also concluded that with the increment of abrasion cycle, the tensile and tearing properties of all denim fabrics generally decreased.
Turhan and Eren (2012) studied the effect of weaving machine settings on the weave ability limits of air-jet machines. They found that changing the shed adjustment from the zero level of the backrest to higher values increased the maximum weavable weft density slightly; however, increasing the shed asymmetry further (backrest height) has no significant influence on the weave ability limit.
Bilisik and Demir (2010) studied the dimensional and mechanical properties of newly developed denim fabrics based on experimentally determined property and structural pattern. They found significant differences on pilling, tensile and tear properties for small and large structural patterns.
Haghighat et al. (2012) inspected the effect of the backrest roller position on the physical and mechanical properties of worsted fabrics. The results showed that the position of the backrest roller has a significant effect on the breaking strength in the warp direction, weight per area unit, and thickness of fabric.
In this experimental work, which has been done on the factory shop under commercial manufacturing conditions, the effect of changing backrest roller position on some physical properties such as EPcm, PPcm, areal density, tensile strength, crimp%, dimensional stability, air permeability and skewness of denim fabric both in grey and finished state has been investigated.