Patent Publication Number: US-2006005525-A1

Title: Method and false twist spindle for false twist texturing

Description:
BACKGROUND OF THE INVENTION  
      The invention relates to a yarn processing method for manufacturing a fancy yarn from at least one filament yarn comprising the process steps of supplying the filament yarn into a false twist texturing unit and producing twists of the filament yarn with a twist density by transferring an axial torque to the filament yarn by means of a false twist spindle of the false twist texturing unit as well as a false twist spindle and a yarn processing machine for implementing the method.  
      The texturing of filament yarns, especially multifilament yarns, is known in the prior art. The texturing is used to produce a crimped and structured yarn from a plastic-like, flat and smooth multifilament yarn which, as a result of its voluminous and bulky structure (bulk), has a textile character. For this purpose, the multifilament yarn (thread) is generally unwound from a spool, passed through first feeder rolls, then heated in a heater (primary heater), cooled on a cooling rail, passed through a false twist spindle and second feeder rolls arranged thereafter, and so-called take-off feeder rolls before being finally wound onto a yarn spool. The false twist spindle is used to highly twist the multifilament yarn temporarily in one working process, i.e., to produce a twist of the multifilament yarn or the individual filament yarns by transferring an axial torque to the filament yarns. This temporary twist (torque state) is designated as false twist (FD). As a result of the twisting, a rotation back pressure is formed which extends back into the heater (twist zone) whereby it is possible for the torque state of the filament yarn to be thermally fixed by heating and cooling before the false twist spindle. After the false twist spindle, the twisting is then released again. As a result of the thermal fixing accomplished in the torque state, the yarn has the desired crimped structure.  
      Very high production speeds can be achieved by using a friction false twist spindle as the false twist spindle. In these false twist spindles the filament yarn is indirectly driven using friction surfaces. As a result of the smaller diameter of the thread compared to the spindle, i.e., to a disk of a disk friction unit, for example, a high transmission ratio is achieved between the revolution of the disk and the twisting of the filament yarn. A triaxial disk friction unit is especially suitable for this purpose. Thus, predominantly friction false twist spindles, especially triaxial disk friction units and also so-called nip twisters which transfer a torque to the filament yarns by means of crossed belts are used as false twist spindles. Such a disk friction unit is disclosed in DE 3743708 A1 for example. A nip twister is disclosed in JP 06184848. The imparting of twist by means of friction makes it possible to achieve very high rotation speeds and therefore also high production speeds. If the friction relationships between the filament yarns and the false twist spindle vary, that is if process fluctuations or instabilities occur, this then results in a non-uniform yarn structure or defects in the yarn and thus in loss of quality in the yarn produced. Such defects or disturbances can, for example, result from disturbances in the spinning mill, from non-uniform application or non-uniform adjustment of the spinning preparation on the thread surface, from temperature fluctuations during texturing or from contamination e.g. in the heater and/or in the cooling rail. The disturbances can bring about a so-called ballooning of the yarn which occurs particularly at high rotation speeds. Ballooning of the yarn results in an uncontrolled run of thread and fluctuations in the thread tension. As a result, the thread can, for example, jump over the disk surface of the false twist spindle. This twist slippage leads to a twist deficit inside the twist zone, i.e., the twist density, that is the number of twists per unit length of filament yarn fluctuates. The thread to be processed can thus pass in sections through the false twist spindle without twisting. This results in short closed yarn sections, so-called “tight spots” and long non-uniformly textured yarn sections which is called surging. During surging the thread tension increases abruptly whereby the equilibrium of forces in the false twist spindle is destroyed. Zones are formed in the thread without twist. In addition, the stretching values fluctuate and the dyeing is unsatisfactory.  
      Texturing speeds of over 300 m/min can be achieved with friction false twist spindles. The lengths of the heating and cooling zones in the texturing zone are adapted to these texturing speeds in order to ensure sufficient thermal fixing of the crimping. If the total length of the texturing zone is 5-6 m, the phenomenon of surging occurs particularly frequently in conjunction with the friction false twist spindles which operate by force locking. In force-locking false twist spindles according to the prior art, the twist density produced cannot be controlled very exactly, and this results in process-technology production limitation of the surging and therefore ballooning of the thread in the twist zone with associated thread tension fluctuations which in turn results in twist fluctuations. The stability limit of the process is influenced on the one hand by the geometry of the texturing zone, e.g., its length, deflection points, thread support etc. and on the other hand by the quality of the feed material, i.e., its uniformity, preparation etc. that is by process fluctuations which occur.  
      It is the object of the invention to provide a yarn processing method and a false twist spindle to produce a fancy yarn by means of false twist texturing, which avoids the disadvantages of the prior art and especially avoids the phenomenon of surging.  
     SUMMARY OF THE INVENTION  
      This object is solved by the yarn processing and the false twist spindle of the independent claims. The dependent claims represent preferred embodiments of the invention.  
      In the yarn processing method according to the invention for manufacturing a fancy yarn from at least one filament yarn, after supplying the filament yarn into a false twist texturing unit, twists of the filament yarn with a twist density are produced by transferring an axial torque to the filament yarn by means of a false twist spindle of the false twist texturing unit. According to the invention, the process fluctuations influencing the twist density are compensated by keeping the transferred torque constant.  
      Friction false twist spindles according to the prior art are operated at a constant speed as far as possible which is predetermined as a desired value relative to the production speed. Fluctuations in the friction or in the thread tension thus always result in fluctuations of the torque transferred to the filament yarn to be processed. Since the false twist spindles are force-locking elements, the introduction of a constant torque is thus considerably more appropriate.  
      The torque (D) is defined as the vector product of the force (K) acting tangentially to the filament multiplied by the radius (R) of the filament yarn: 
 
 D=K×R  
 
      A perpendicularly acting force thus results in a torque in the axial direction of the filament yarn. According to the physical principle of “action equals reaction”, the force required to drive, for example, the disk at constant speed of rotation fluctuates in the event of a fluctuation of the radius of the filament yarn or in cases of poor force transmission on account of a fluctuating friction between the false twist spindle, e.g. a disk of a disk friction unit, and the filament yarn. The rotation of the disk is more or less strongly braked. If the transferred torque is now kept constant rather than the speed of rotation of the false twist spindle, the same number of twists of the filament yarn is continuously produced per unit length of the filament yarn i.e., the twist density remains constant. Process fluctuations such as fluctuating thread tension, different thread radius, alternating surface roughness of the thread etc. are compensated.  
      By introducing a constant torque, the twist density of the yarn remains constant and the manufacturing process is significantly stabilised. A further advantage is that by introducing a constant torque, the manufacturing process can be maintained at the same twist density which is not possible when the speed of the false twist spindle is controlled depending on the process speed.  
      The transferred torque is preferably implemented by controlling the power consumption of an electric motor driving the false twist spindle wherein the electric current intensity of the electric current flowing through the electric motor at constant voltage is kept constant.  
      The electrical power taken up by an electric motor corresponds to the force which the electric motor must expend to turn a disk. The power P is defined as the product of the applied electric voltage U and the electric current I flowing through the electric motor. If a constant electric voltage is applied, the power consumption and thus a generated and transferred torque can be adjusted and kept constant by controlling the electric current. This is a proven and reliable method which can be implemented using known electronic components.  
      In a further embodiment of the invention the transferred torque is measured by means of a torque sensor and the speed of rotation of the false twist spindle is controlled such that the torque is kept constant. It is thereby possible to directly control the torque transfer.  
      A false twist spindle according to the invention for a false twisting texturing unit is set up to produce twists of a filament yarn with a twist density by transferring an axial torque to the filament yarn. According to the invention constant holding means are provided and set up to compensate for process fluctuations influencing the twist density by keeping the transferred torque constant. The method according to the invention can be implemented by means of the false twist spindle according to the invention. The false twist spindle according to the invention thus makes available the advantages of the method according to the invention.  
      The introduction of constant torque can be implemented using any friction false twist spindle, wherein preferably the triaxial disk friction unit and the nip twister are used. The false twist spindle according to the invention thus preferably comprises a disk friction unit or a nip twister. These false twist spindles are force-locking false twist spindles with which high process speeds, i.e. yarn production speeds, can be achieved.  
      A false twist spindle according to the invention can be implemented, for example, by using a drive motor having a flat characteristic. The power of such a drive motor automatically follows any braking which is present since its power cannot be quickly adapted. However, such a drive motor has the disadvantage that the torque cannot be transferred.  
      A false twist spindle according to the invention preferably has an electric motor set up for its drive wherein the constant holding means are set up to keep constant the power consumption of the electric motor preferably by keeping constant the electric current flowing through the electric motor at constant voltage. This embodiment of the constant holding means can be manufactured inexpensively and is reliable to use. The false twist spindle can naturally also have a non-electrical drive such as a water or air turbine or an electrostatic drive or a gravitational drive.  
      In a further preferred embodiment the constant holding means of the false twist spindle according to the invention comprises a torque sensor wherein the constant holding means are set up to measure the transferred torque by means of the torque sensor and to control the speed of rotation of the false twist spindle so that the torque is kept constant. The torque transfer can be controlled directly by means of the torque sensor.  
      A yarn processing machine according to the invention comprises at least one false twist texturing unit comprises a false twist spindle according to the invention.  
      The invention is explained in detail subsequently using an exemplary embodiment with reference to the drawings.  
      The figures in the drawings show the subject matter of the invention highly schematically and should be understood as not being to scale. The individual components of the subject matter according to the invention are represented so that their structure can be clearly shown.  
    
    
     DESCRIPTION OF THE DRAWING  
       FIG. 1  shows the run of thread in a yarn processing machine according to the invention. The filament yarn  1 , e.g. a partially oriented yarn (POY) is taken off from a supply package behind a bobbin creel  2  with first feeder rolls  3 . The filament yarn  1  is stretched in the twisted state by means of the texturing unit  5  positioned between the first feeder rolls  3  and the second feeder rolls  4 , heated by means of the primary heater  6 , cooled again by means of the cooling rail  7  and then the twist produced by the false twist spindle  9  by transferring an axial torque to the filament yarn  1 . The thread temperature should reach about 200° C. for good fixing of the crimping. For a uniform yarn quality it is also important to cool the thread under the glass transition temperature of about 80° C. before it enters the false twist spindle  9 . The thread tension can be measured and evaluated after the false twist spindle  9  for online quality monitoring. The intermingling  8  is generally carried out before a set zone using a set heater  10  since this is fixed in the set heater  10  and the knots thus become more stable. In order to decouple the intermingling  8  from the set process, further feeder rolls  11  are placed between the intermingling  8  and the set heater  10 . 
    
    
      In the set zone some of the crimping is removed again. In addition, the tendency of the textured yarn to shrink and snarl is reduced. Since with increasing texturing speeds a simple heating process is no longer sufficient to eliminate the snarling tendency of the yarn, a slight counter-rotation is frequently applied in the set zone using a twist nozzle (detorque)  14  positioned before the take-up rolls  12 .  
      In the spooling zone spooling oil is applied to the filament yarn  1  by means of a spooling device  16  and the yarn is spooled on a yarn spool  18 . Here the purpose can be very different. In the standard process the largest possible spools, having a weight of at least five kilograms, are simply to be wound on paper tubes.  
      In false twist texturing the desired bulk is achieved by thermo-fixing the yarn in the twisted state. In order to achieve sufficient crimping, very high twist densities are required, lying between 2500 and 7000 m −1 . As a result, fluctuations in the transfer of torque from the false twist spindle  9  to the filament yarn  1  caused by process fluctuations have a particularly strong effect. According to the invention, the transferred torque is thus kept constant. For this purpose the false twist spindle  9  comprises an electric motor  20  whose power is controlled. An electric current flows through the electric motor  20  which is kept constant by means of constant holding means  21 . Since the electric motor  20  is connected to a voltage source  22  which provides a constant electric voltage, the electric current is also kept constant by means of the constant holding means which results in a constant electric power of the electric motor. The electric motor  20  drives a rotating disk  24  which is in force-locking contact with the filament yarn  1 . A torque is thereby transferred to the filament yarn. The torque is constant since the power of the electric motor is kept constant. The constant-holding means  21  are also set up to compensate for process fluctuations influencing the twist density by keeping the transferred torque constant.  
      A yarn processing method is proposed for manufacturing a fancy yarn from at least one filament yarn comprising the process steps of supplying the filament yarn into a false twist texturing unit and producing twists of the filament yarn with a twist density by transferring an axial torque to the filament yarn by means of a false twist spindle of the false twist texturing unit as well as a false twist spindle and a yarn processing machine for implementing the method. According to the invention, the process fluctuations influencing the twist density are compensated by keeping the transferred torque constant.  
      The invention is not restricted to the exemplary embodiments specified previously. Rather it is possible to have a number of variants which make use of the features of the invention in a fundamentally different type of design.  
      A yarn processing method is proposed for manufacturing a fancy yarn from at least one filament yarn comprising the process steps of supplying the filament yarn into a false twist texturing unit and producing twists of the filament yarn with a twist density by transferring an axial torque to the filament yarn by means of a false twist spindle of the false twist texturing unit as well as a false twist spindle and a yarn processing machine for implementing the method. According to the invention, the process fluctuations influencing the twist density are compensated by keeping the transferred torque constant.