This paper presents the features of yarn structure formation on spinning machine, i.e. yarn twist change when winding. It was considered that the twist distribution was one of the reasons for its decrease along the formed yarn. In this paper, based on analysis of changes in thickness and twist due to axial deformation, we consider a yarn moving at constant speed. Moving dynamics of yarn are studied here by using Euler variables. The correspondences of forward and reverse twist waves’ distribution speeds on presented frequency at various vibration forms are obtained. The parameters of Doppler effect for the waves distributed along the yarn are determined.

#### Keywords

- Yarn structure
- yarn twist
- twist distribution
- thickness
- twist waves

Analysis of research literature indicates that the structure of yarn depends on technological and kinematic factors. Structure control, i.e., ways to reduce twist loss and twist distribution along the yarn, and the methods of density regulation are not considered. It is a big problem as characteristics and quality of yarn depend on it. We consider the change in torsion quantity on a ring spinning machine. Formed yarn gets one twist per one turn of the runner. The number of coiled yarn twists changes depending on coil diameter. So, with an increase in diameter of winding, a larger twist occurred. Practice has shown that when twisting the yarn into a coil with a larger and smaller diameters, the difference is about 1% [1]. Twist unevenness also occurs due to vertical movement of the ring bar at a certain distance. So, for example, when lowering the ring strip, a shorter yarn is wound on the package because a part of its length is not wound due to increasing cylinder height. Therefore, in practice, it is used the average twist obtained when the number of torsions in any section of yarn divided by its length. In this case, appearance of twist irregularities is not taken into account, as it is very important for uniform yarn structure. In well-known papers [2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17], the roughness in twist is not analyzed, i.e., the conditions for formation of structure, on which the quality indicators of yarn obtained by the spinning machine are largely dependent, are not taken into account. As noted above, the difference in the number of yarn twist segments leads to appearance of twists in short sections when the annular strip moves up and down. Uniformity of twist distribution in this case depends on the spinning method, i.e., a twist along the yarn is distributed differently. Investigations of these issues are described in papers [2, 5,6,7,8,9,10]. On yarn twist distribution at different spinning methods, the positive results were obtained. A number of works are about the study of twist distribution in OE yarn [11,12,13,14,15]. The issue of twist distribution in the modified yarn was also studied [7]. In all papers, the process of twist wave distribution along the yarn is considered. In this case, the distribution of shear deformations occurs both along the axis of the yarn and along the radial direction. For twist distribution along the yarn, the structural unevenness is important. Twist distribution has been sufficiently investigated, but, as a rule, a structurally uniform and homogeneous product is considered, while the structure of the product is not considered. Therefore, twist distribution with such shortcomings is investigated in this paper.

In paper [5, 6] considered twist distribution of the yarn with structural unevenness where applied the generalized law for Hooke's anisotropic medium, according to which the correspondences between the components of the stress and strain tensors in ax symmetric coordinates are represented by five elastic constants [7].
_{rr}_{zz}_{θθ}_{rz}_{zθ}_{rθ}_{rr}_{zz}_{θθ}_{rz}_{zθ}_{rθ}_{1} and _{2} correspond the Young's modulus along axial directions 0_{12} and _{32} are corresponding Poisson's ratios, characterized by a change in yarn thickness and twist due to axial deformation

We denote the angular displacement of yarn arbitrary section by _{θ}

We denote the angular displacement of yarn arbitrary section by

In Eq. (2), we see that deformation shifts _{θz}_{rθ}_{zθ}_{rθ}

Eq. (4) describes the distribution of twisting waves along the yarn with a structurally heterogeneous property. In this case, the twist changes both along the axis of the yarn and along the radial direction. We consider a yarn with radius _{0} and final length

For yarn axis, the displacement roundedness condition is applied:

Yarn initial cross section

Full angular speed of the yarn in a cross section _{*} is the angular speed of the twisting part.

The solution for Eq. (5) satisfying the boundary conditions of Eqs (6) and (7) can be represented as follows:
_{n}

The solution of Eq. (11) is represented through the first-order Bessel function, _{n}_{1}(_{n}x_{n}_{0} (

Using the condition of orthogonality

Eq. (12) for known values of characteristic numbers _{1} = 0 is considered in the paper [11].

The solution of Eq. (14) is represented as follows:

Function _{z}

We consider the case

_{n}_{1n} + _{2n} is a complex constant, the component of which is determined from conditions of Eq. (18)

Thus, the angular movement of the yarn between sections

As seen in Eq. (19), the values _{1n} and _{2n} are positive (_{1n} > 0) and negative (_{2n} < 0) numbers accordingly. They are the wave numbers corresponding to various forms of fluctuations on variable _{1n} = − _{2n} (_{2n} = − _{1n} (

If we do not take into account the twist changes in radial direction, then from Eq. (14), we obtain the expressions for the wave numbers as given in the study by Xu et al [10].

Moreover, the wave numbers _{1} and _{2} indicate the wave distribution along positive and negative directions of axis 0_{n}_{1n} (_{2n} (_{0} /

Figure 2 presented the curves of _{1n} and _{2n} speed dependence (referred to shear wave speed _{0} /

The analysis of curves shows that Doppler effect for waves distributing along the yarns is held for each fluctuation form of the yarn by its thickness. In this case, the Doppler effect for the backward wave is stronger than that for the direct one. This is due to the processes of wave's reflection from the axis of the yarn.

The conditions for distribution of twist waves for a moving yarn with a constant speed are studied, and the parameters of Doppler effect for waves that are distributed along the yarn are determined. It was found that the Doppler effect for the backward wave is stronger than that for the direct one for 3.5 times. This is due to the processes of wave's reflection from yarn axis.

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