The effect of the tension of chemical fibres (monofilaments, complex, reinforced) The mechanical characteristics of chemical fibres in twisting to a significant degree determine the further evolution of the technological process and their final properties. The balance and mechanical behavior of the fibre in twisting, winding, and processing are a function of the level of stresses accumulated during twisting. The tendency of a fibre to form loops (snarls) is an important consequence of the presence of torsional stresses in the fibre. Brief slackening of the tension of the twisted fibre can cause the formation of snarls [1,2], which in turn usually increases end breaks, decreases the quality of the fibres [3, 4], and even breakdown of the equipment [5]. However, the existing theoretical criteria of loop formation (Greenhill criterion [6], Ross condition [7]) have been investigated very little for textile fibres, and their applicability for quantitative estimations of their behavior in twisting is subject to doubt [8].The mechanical properties of chemical fibres of different structure with high twist levels were examined; in addition, the boundaries of applicability of the theoretical criteria for determining the conditions of snarl formation were estimated.An experimental setup which was an improved version of the installation described in [9] was constructed to study the stress--strain state and stability of the fibres in twisting (Fig. 1). The setup operated according to the torsion balance principle [10,11], widely used for measuring the torque of fibres [8,12,13].For performing the test, fibre sample 5 was attached in clamps 2 and 3. Weight 8 was added to lower clamp 3, creating a constant tension in the sample. Upper clamp 2 was connected to stepping motor 10. The stepping motor was controlled from PC 11 with specially developed software.Beam 4, connected to torque meter 6 was attached to lower clamp 3 to prevent free rotation of the lower section of the sample in rotation of the upper section. The lower end of the fibre could move freely in the vertical direction, allowing the fibre to shorten during twisting. In this way, sample 5 was twisted in rotation of clamp 2. Torque meter 6 was a calibrated spiral spring whose outer end was rigidly attached, while the inner end was rigidly connected with the axis of the spring and by beam 4 with the lower end of the twisted fibre.The experiments were conducted in normal laboratory conditions at a temperature of 20•176 and relative humidity of 65~:2%. The clamped length of the sample L was set equal to 100 mm. We note that the use of experimental samples with L < 40 mm caused the length to have a significant effect on the torque [ 14]. For L > 100 mm, the effect of the sample length was insignificant [11,15], which was also confirmed by our studies [9]. The twisting rate was 60 rpm. The fibre tension was varied within the limits of 0.4-32.3 cN.
