Patent Publication Number: US-6904745-B2

Title: Thread-guiding device for open-end spinning frames

Description:
BACKGROUND OF THE INVENTION 
   The present invention relates to collecting spun yarn produced or worked by textile machines to be wound on bobbins. In the industrial production of spun yarns it is common practice for them to be collected on an idle tube carried by a bobbin-carrying arm, which rests on a rotating driving roller and takes up the spun yarn coming from a feed element to wind it onto itself. The bobbin is thus formed by pulling and winding the spun yarn on its surface, it being drawn in rotation by the roller underneath on which the bobbin being formed rests. This practice allows the spun yarn to be wound at a substantially constant linear speed, irrespective of the increasing dimensions of the bobbin and depending only on the rotation speed of said driving roller. The spun yarn is wound in spirals onto the rotating bobbin as the pick-up unit is provided with a thread-guiding device which distributes the spun yarn on the outer surface of the bobbin with backward and forward axial motion. In industry, the bobbins may be shaped like a truncated cone or a straight cylinder with substantially flat bases, with the exception of a few specific cases in which the terminal parts of the bobbins are shaped with a pronounced flare. 
   In the prevalent industrial use of spun yarn in bobbins, downstream working requires the bobbin to be conical in shape, for example when the spun yarn is unwound in an axial direction from the bobbin fixed on creels. This conicity is however slight and restricted to a few degrees of inclination of the generatrix of the cone in relation to its axis, generally between 2° and 6°, except for some specific uses for which “superconic” bobbins are required. 
   In the case of winding on a winder the most widespread device for distribution of the spun yarn on the surface of the bobbin with axial backward and forward movement consists of a spiral backward and forward groove cut into the surface of the driving roller which causes the spun yarn to perform an axial excursion of a pre-established length, for a pre-established number of turns of the roller and with a pre-established wind ratio. In other words, the yarn winding and spun yarn distribution elements operate according to a fixed speed ratio. 
   However, in other cases the device for distribution of the yarn on the bobbin is produced with an independent thread-guiding device, moved by its own driving element, with which the frequency of the backward and forward movement, its travel, the length of the spiral wound and the wind ratio, etc. may be modulated time by time and according to need. 
   Typically, distribution of the spun yarn on the bobbin with modulatable thread-guide is required in open-end spinning frames, for which distribution of the spun yarn on the bobbin with grooved cylinder does not meet the conditions required for efficacious winding on a bobbin of the desired quality. These winding conditions in particular include its wind ratio, the speed and excursion travel, which cannot be maintained at a single pre-established value, as is the case with the grooved cylinder, but must be adapted time by time to the spun yarn being produced and also modulated during production of the bobbin. There are also other impediments to the use of the grooved cylinder, both due to the geometry of the system and to the overall open-end spinning procedure. 
   In open-end spinning there is a further limiting condition in that the spun yarn is produced at constant linear speed and therefore must be picked up at a speed corresponding to the speed at which it is made available, substantially equal and constant, maintaining it at a moderate tension, while when forming both straight cylindrical and conical bobbins the pick-up speed typically has a pulsating trend. 
   It must also be borne in mind that, to compensate these pulsations of adjusting tension and path length, the elasticity of the spun yarn could be taken into account only within the limits of a few per cent, also because the yarn is already stressed considerably at the operating speed of current open-end spinning frames. 
   To explain more clearly the problems dealt with and the technical solutions proposed with the present invention reference is made, in the description below, to pick-up of “open-end” spun yarns on bobbins, provided purely as a non-limiting example, it being explicitly specified that it may be used advantageously to wind spun yarns produced with different spinning technologies on bobbins. 
     FIG. 1  show the layout of an open-end spinning station  1  with its most significant components.  FIG. 1A  shows a front view of it while  FIG. 1B  shows a side view of it. 
   Proceeding from the bottom upward, it is first encountered the spinning unit  2  and then the pick-up unit  3 , the main components of which used to transform staple parallel fibres into the bobbin of wound spun yarn are illustrated briefly below. 
   The feed strip or staple S is contained in a cylindrical vessel  4  where it is deposited in a double spiral. The staple S is taken up from this and fed to the unit by a feed roller  5  passing through the condenser/funnel conveyor  6 . The strip S then passes to the so-called card  7 , a small roller equipped with a toothed seal which rotates at high speed to separate and select the fibres of the staple S and convey then by suction to the spinning rotor  8 . In this path the short fibres and impurities are separated, so that only the long and cleanest fibres reach the rotor. The impurities are unloaded in to a suction outlet common to all the spinning units. 
   In the spinning rotor  8 , which rotates at a speed ω R  which reaches 150,000 rpm and over, the fibres are deposited in its peripheral groove through centrifugal force; they are then collected and taken up from here in the form of yarn F. 
   The fibres are delivered axially from the rotor  8  through the opening of the extractor funnel  9 , receiving torsions from rotation of said rotor during the path stretching between its inner groove and said extractor  9 , to create the plied yarn F. 
   The yarn is taken up with an extraction system comprising the extraction roller  11  opposite which is an idle pressure roller  12 , generally in elastomeric material and pressed with controlled force to grip the yarn F. This extraction roller  11  is operated at controlled speed and determines the spinning speed or the linear production of spun yard in relation to time. The ratio V F /V S  between the linear speeds, generally expressed in metres per minute and respectively of yarn extraction and of staple feed with roller  5 , determines the drawing ratio between the rotor rotation speed W R  and the yarn extraction speed V F  in metres per minute determines the number of torsions per metre imparted in the spinning rotor. 
   To prevent uneven wear, the spun yarn extraction system is equipped with a weft-moving control  13 , consisting of an auxiliary thread-guide  14  mounted on a longitudinal rod  15  in common with the other spinning units which moves longitudinally on the front of the machine. The motion of the auxiliary thread-guide  14  is a backward and forward movement with the so-called pilgrim step, for reduced travel, generally below 10 mm, and moves the yarn F crosswise to obtain uniform wear on the pressure roller  12 , preventing grooves from forming rapidly on its surface. 
   The yarn F thus produced is fed to the pick-up unit  3 , still moving upwards, and encounters a compensator  16 , consisting of a straight or barrel-shaped profile onto which the yarn is diverted to compensate or at least decrease the variations in length of the path stretching between the spinning unit  2  and the point in which the yarn F is deposited on the bobbin, due to the axial motion of the thread-guiding device  20  it follows. 
   The yarn F therefore reaches the thread-guiding device  20 , which distributes the yarn on the bobbin being formed moving crosswise with backward and forward motion. This consists essentially of a main thread-guide  21  mounted on a longitudinal rod  22  common with the other spinning units which moves with alternate motion longitudinally on the front of the machine, with an excursion corresponding to the winding travel on the base tube, generally between 120 and 160 mm. 
   The excursion frequency required is of 100 to 250 forward and backwards strokes per minute, with position precisions in the order of tenths of mm with regard to the axial coordinate of the inversion points. 
   In prior art different devices are provided to create, adjust and modulate this alternate motion, in frequency, width and axial shift, in order to obtain bobbins that are stable and good quality. These devices use kinematic systems of the connecting rod/crank, four-bar linkage type and so on. In devices of more recent conception, the rod  22  is moved by a large cylinder cam, not shown in the figure for simplicity, driven to rotate at the controlled speed. 
   Regulation of the cylinder cam rotation speed allows modification of the frequency of the strokes of the thread-guiding devices  20  and the wind ratio of the spun yarn on the bobbin. A further possibility is also provided of adding a second movement of axial modulation to move the motion inversion point of the thread-guides  21  to decrease phenomena of unevenness at the two ends of the bobbin, distributing them over a greater axial extension. 
   The thread-guide  21  is extremely near the surface of the bobbin being formed. The bobbin  25  is held by the bobbin-carrying arm  26  provided with two openable idle tailstocks  27  which come into contact with the base tube  28  of the bobbin. The bobbin being formed  25  rests on its driving roller or pick-up roller  29 . This pick-up roller is provided with one or more drawing bands  30  in a material with a high friction coefficient, generally rubber. In the case of pick-up on conical bobbins these bands make it possible to establish the drive ratio between bobbin and roller, while in the case of cylindrical bobbins they allow a balanced driving torque to be transmitted to the bobbin  25 . The bobbin  25  being formed increases progressively in size and weight. The contact pressure of the bobbin on its pick-up roller  29  has a considerable influence on the density of said bobbin. The contact pressure is therefore controlled with a counterweighing system which acts to keep the contact pressure at a determined value, compensating the effects of its increase. 
   The use of thread-guiding devices  20  with independent action has noteworthy advantages, such as being able to operate with the exact wind ratio required by the production in progress, to control and avoid ribboning on the bobbin, to obtain stable and well-formed bobbins, but still does not solve all winding problems. 
   Further problems still encountered in winding spun yarn on a bobbin with distribution by means of independent thread-guiding devices  20  are essentially caused by phenomena, the details of which shall be described below. 
   One of these concerns distribution of the spun yarn on the generatrix of the bobbin—whether conical or cylindrical—with a thread-guiding means with alternate excursion between the two ends of the winding. This excursion periodically lengthens and shortens the length of the stretch of yarn running between the spinning unit  2  and the point of pick-up on the bobbin  25 . This is minimum when the thread-guide is halfway through its travel, and maximum when the thread-guide is at the ends of its travel. This variation therefore causes a first pulsation in the take-up speed of the yarn, as at all times it is necessary to attain from below the algebraic sum of the length of yarn wound on the bobbin with the periodic variation in length—positive and negative—of the path that joins the spinning unit which feeds the yarn F at constant speed and the pick-up element which takes it up a pulsed speed and hence with pulsed tension. 
   If the bobbin  25  is conical, the phenomena of pick-up speed pulsation is combined with an additional speed pulsation and tension caused by the conical shape of the bobbin. When it is wound on the part of the bobbin  25  with the largest diameter, the yarn F is taken up at a higher speed than the speed at which the yarn F is fed from the spinning unit  2  and is therefore stretched more; instead, when the yarn F is wound on the part of the bobbin with the smallest diameter the situation is reversed: the yarn F is slack as it is taken up at a lower speed than the feed speed of the spinning unit  2 . 
   The average pick-up speed is maintained coinciding with the speed at which the yarn is fed by the rotor  8 , or just above this to obtain moderate additional draw and ensure the yarn is always stretched. The overall effect deriving from these tension and speed pulsations on the yarn F being wound is essentially of greater density and compactness of the bobbin  25  in its end points, where tension is greater. The variation in the tension of the yarn F and the consequent more or less dense and compact zones is only partially compensated by the compensator  16 , which alleviates the phenomenon but does not eliminate it, and in any case with a certain shift due to the friction of the yarn which runs crosswise on its diverter profile. 
   A second problem in pick-up of the spun yarn F with the independent thread-guiding device  20  derives from the mass of the thread-guides  21 , their rod  22  and their alternate motion control device. The assembly has a significant mass, therefore causing limits to the admissible braking and acceleration values, as well as the minimum braking and acceleration times and spaces. The times and spaces for inversion of motion must in any case come within the order of milliseconds and millimeters respectively, to provide the bobbin with the quality required for the subsequent use, as regards shape and mechanical stability. 
   In prior art devices which employ the control with cylinder cam and, due to these limits, in the two ends of the stroke where motion is inverted, the profile of the cylinder cam have a smooth radius to avoid impacts, vibrations and damage to the overall system. Consequently, the speed of the thread-guide  21  also has a radiused trend—generally sinusoidal—compared with the axial coordinate while in the remaining part of its stroke the thread-guide is controlled at a constant speed, or at a constant wind ratio. In the end stretches they therefore have a lower average speed and a longer stay time, compared with the intermediate stretch of travel. 
   The first effect of this longer stay time is that a greater quantity of yarn F is deposited at the two bases of the bobbin, where the yarn being wound is also stretched more tightly. This causes increased compactness at the ends and a further increase in unevenness forming protrusions where the bobbin is already denser. Two harder terminal bulges are formed giving the bobbin an M-shaped profile; this uneven winding causes noteworthy drawbacks in the use and further working of the spun yarn wound on said uneven bobbin. 
   This winding defect is not permitted for some uses of the spun yarn; in some cases, therefore, the bobbin produced in the open-end spinning frame must be unwound and rewound more evenly with an additional operation and cost. 
   BRIEF SUMMARY OF THE INVENTION 
   The object of the present invention is to produce a device to distribute the spun yarn on bobbins being wound which overcomes the drawbacks of thread-guiding devices available at the state of the art and makes it possible to obtain bobbins with more regular density, shape and stability. 
   The device according to the invention is defined, in its essential components, in the first claim while its variants and preferred embodiments are specified and defined in the dependent claims. 

   
     To illustrate the characteristics and advantages of the present invention in greater detail, it shall now be described with reference to some typical embodiments indicated in  FIGS. 1  to  4 , purely as a non-limiting example. 
     Said figures relate to an embodiment of the thread-guiding device according to the invention to distribute yarn on the bobbin being wound, showing only the system to move the longitudinal bar  22  which carries the thread-guides  21  supplying the spinning units aligned along a front of the spinning frame, to illustrate the characteristics and benefits of the present invention. 
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       FIGS. 1A and 1B  schematically show, in a front and side view respectively, a typical embodiment of an open-end spinning unit essentially to illustrate the technical problem underlying the present invention. 
     The  FIG. 2  show an embodiment of the thread-guiding device driven by a linear motor, for example of the magnetic type.  FIG. 2A  shows its front view,  FIG. 2B  shows its sectional side view, while  FIGS. 2C ,  2 D and  2 E show constructional details of the device. 
       FIG. 3  shows an alternative embodiment of the directional guide and support system for the slider  41  of the linear motor: in particular,  FIG. 3A  shows the front view, while  FIG. 3B  shows the sectional side view. 
       FIG. 4  shows an alternative embodiment of the linear driving motor, in which there are no substantial variations in the elements of the mechanical guide and support layout of the slider. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIGS. 2A , B show the thread-guiding rod  22 , with thread-guide  21  with span corresponding to the span of the spinning unit  2  in  FIGS. 1A and 1B . The moving equipment assembly of the thread-guiding device is produced with maximum savings in weight, directing choices of structures and materials which guarantee high rigidity and precision of movement with the lowest possible weight, for example employing composite materials and ceramics for the rods and thread-guides. The precision of movement and slenderness of the structure therefore require guides  40  aligned along the front of the machine, positioned for example every two spinning units, only one of which is shown to simplify the drawing. These guides  40  are provided with low friction and wear-resistant surfaces or facings in contact with the rod  22 . The rod or bar  22  is made to move alternately to translate its thread-guides  21  in the front of the bobbins being formed by the alternate motion of a slider  41  connected integrally to the thread-guide rod  22  with a connection per se known, such as with through pins  42 , which connect the protrusions  43  of the slider  41  with corresponding holes in the ends of the rod  22 . 
   The slider  41  contains internally, in a corresponding cavity  44  with longitudinal trend, a magnetic bar body  45 , for example a permanent magnet, integral with it, positioned in an axial direction and provided with a plurality of alternate positive and negative polarities which are currently indicated as north and south, N and S. This magnetic bar body  45  is illustrated below with reference to FIG.  2 C. The slider  41  is also provided with projections and directional bearings to control its alternate horizontal motion in relation to the stator  60  which drives and supports it. The body of the slider  41  has an enlarged T section and rests with its outer wings  46  on the two side projections  61  of the stator  60  which act as a track for motion of the slider  41 . In the embodiment shown in  FIG. 2 , the slider  41  is equipped with a series of rollers with horizontal axis, or equivalent rolling bodies  47 , supported by a suitable retaining frame, which allows mutual horizontal translation of the slider  41  and stator  60  with rolling friction, maintaining the facing surfaces of slider and stator at a pre-established and rigorously controlled distance. The space between the two elements is currently called “gap” and, generally, the distance which separates them is around 0.5 to 1 mm. 
   On the two opposite parts  48  of the T-shaped stem, the slider body  41  is similarly equipped with a series of directional bearings  49  with vertical rollers, to maintain the body of the slider centred in relation to the supporting tracks  61 , and to keep the thread-guide rod  22  correctly aligned with its guides  40  during its backward and forward motion. 
   As illustrated as an example in the perspective view in  FIG. 2C , the magnetic bar body  45  is electrically constituted by a magnetic body with a rectangular section, obtained by connecting a plurality of permanent magnets  52   1 ,  52   2 , . . . facing one another with their side faces, alternating the N and S polarities on their upper and lower faces. The direction of alternate motion in the two directions which moves the slider is indicated by the arrow  53 . The magnetic field of the single magnets constituting the bar  45  of the slider  41  is closes through the stator. The number of magnets  52  which are used to constitute the magnetic bar  45  varies as a function of the characteristics of force exerted, speed, and so on, and is generally between 2 and 10. The layout of the pack of magnets  52  may be either orthogonal or inclined in relation to the direction of motion, thus regulating the precision and so-called roughness of its operation. 
   The body of the stator  60  is connected integrally with the structure of the pick-up unit  3  wit means per se known, for example by screw fixing. A longitudinal groove  63  is produced in its innermost part in which the electromagnetic body  64  is housed axially, energized to move the slider  41 , according to the base layout of the linear motor adapted to the specific purpose attained with the present invention. An example of the structure of said electromagnetic body is illustrated with reference to FIG.  2 D. 
   The energized electromagnetic body  64  is constituted by a bar core  65  of ferromagnetic material with a polygonal section, for example rectangular, around which a plurality of windings energized electromagnetically are positioned between  65   d  and  65   s , to provide the surface of the bar  65  facing the slider  41  with a north and south polarity sequence, indicated with N and S, or alternately positive or negative polarity which move axially forward and backward so as to attract or repel the facing polarities of the slider  41  and thus make it move to the right or to the left, according to the direction and the sequence of the energizing electrical current supplied to the windings. 
   In the example of embodiment in  FIG. 2D  the bar  65  of the stator  60  is shown in a perspective view. The bar  65  is electrically constituted by a lamination stack with slots and windings which correspond fully with those of a normal rotary electric motor, with its lamination winding stack open and extended. In  FIG. 2D  the bar  65  comprises a lamination stack  70  the length of the entire bar  65  with repetitive profile constituted by a series of Ts  71 , side by side and joined at their base, to insert the windings  72  in their slots  73  corresponding with the stems of the Ts of the lamination stack. 
   The number of slots  73  for the energizing windings  72  of the bar core  65  is greater than the number of magnets  52  and, in general, is between 6 and 30. 
   The bar  65  of the lamination stack may also be overturned to place the openings of the slots  73  in the lower face and with the smooth and continuous face of the lamination stack  70  facing the slider  41 . 
   The windings  72  are therefore toroidal in shape extending horizontally and are energized in sequence with currents that change in direction with regulated frequency, to produce a series of N and S polarities which move axially and with them draw in movement the N and S polarities of the bar  45  with the slider  41 . 
   The windings  72  may each be wound around only one stem of the Ts, or differentiating the windings according to different lengths around one or more stems of the Ts  71 , as shown in FIG.  2 D. With this expedient it is possible to use power supplies with various phases with a more regular trend of the moving magnetic field produced. 
   The magnetic field generated in the bar  65  of the stator  60  is hence extremely regular and slides easily in both directions. 
   The sliding speed of the magnetic field is regulated by varying the frequency with which the direction of the energizing current in the windings changes. Generally, the length of the stator corresponds at least to the length of the slider with an increase in its travel, according to the height of bobbin required. 
   Generally, energizing of the bar  65  may also be produced with direct current and periodically switching the connection of the N/S polarity of the generator. 
   Operation of the thread-guide is obtained by energizing the windings of the series with an inverse frequency of the energizing current which causes the N and S polarity frequency of the bar  65  to slide forward and backward so as to draw the slider with it. 
   The operating layout of the linear motor device according to the present invention is described with reference to the schematic representation in FIG.  2 E. 
   The magnetic core which forms the bar  45  of the slider is positioned above the energized electromagnetic bar  65  of the stator with interposition of the gap t. The different alternate N/S polarities of the permanent magnets  52  of the magnetic bar  45  positioned side by side are shown. The lower part represents the bar of the stator, in which modulations of the currents supplied to the windings produce a magnetic field wit axial movement, which precedes the field of the slider angularly offset (generally by 90° e) attracting it with its movement. In this way the S-N-S-N . . . respectively of the stator  65 , which however continue to invert their positions, while the slider translates by one step, to create the translation force of the motor. 
   Modulation of the currents in the windings moves the slider, determining acceleration, braking and inversion of motion, and also allows control of the mutual position between stator and slider. 
   By regulating times, intensity and direction of the energizing currents in the windings  72 , the control unit G obtains the desired backward and forward motion as regards amplitude and travel inversion points, speed and accelerations of the thread-guide rod  22 . To simplify the drawing, the electrical connections of the windings with the control unit G are indicated with lines  68 . 
   For management of the alternate motion control of the thread-guide rod  22 , by the control unit G of the spinning frame, reading of the relative position between slider  41  and stator  60  in the axial direction of motion is delegated to a conventional positioning sensor per se known, equipped with the suitable precision and reading promptitude. This reading is transmitted to the control unit G which processes it and produces the consequent energizing commands. For example, in the embodiment illustrated in  FIG. 2B  the stator  60  is equipped on an extension  69  of its left side with an optical sensor  70 , currently indicated as “encoder”, which reads the axial coordinate of an optical scale of reference  50  positioned instead on the side of the left wing  46  of the slider  41 . Control of energizing of the windings  72  is managed by the control unit G on the basis of moment by moment reading of the position of the optical scale of reference  50  in relation to the encoder  70 . 
   For this purpose, the control unit G of the spinning frame is provided with relative means for said control of energizing on the basis of reading by the encoder. 
   The length of the energized electromagnetic body  64  available for axial excursion of the slider  41  is of a size that allows parallel excursion of the rod  22  to take place without drawbacks for all the foreseeable strokes L for the range of heights of bobbin designed and required for the pick-up section  3  of the spinning stations. 
   Alternate linear motion drive of the thread-guiding rod  22  is due to the parallel and corresponding alternate motion of the slider  41  which is controlled in alternate excursions by the control unit G of the spinning frame. It therefore contains the instructions and software required to command and control alternate axial movement as regards instantaneous speed, amplitude of axial excursion, coordinates of the ends of its travel, and so on. 
   According to a preferred embodiment of the invention the linear electric motor, constituted by the slider  41  and the stator  60 , is obtained with a core of magnetic material  45  based on a mixture of rare earths which, with the same magnetic attraction/repulsion force, allow a considerable reduction in the weights of the moving equipment and their inertias, compared with traditional ferrous magnets. 
   The alternative embodiment in  FIGS. 3A and 3B  shows the same assembly of thread-guiding rod  22  with thread-guides  21  and guides  40  already described with reference to  FIGS. 2A , B. 
   According to the embodiment shown in  FIG. 3 , it is the upper part of the projections or tracks  61  of the stator  60  to be equipped for its entire length with a series of rollers  75  with horizontal axis, or equivalent rolling bodies, carried by a suitable retaining element which allows the lower part to project to allow the slider  41  to rest and translate horizontally on the stator  60  with rolling friction, guaranteeing a strictly controlled distance between the two bodies at all times. 
   Similarly, fitted on the two opposite sides  48  of the stem of the T, for the entire length of the two vertical inner walls of the projections  61  is a frame to support a series of directional rollers  76  with vertical axis, again to maintain the body of the slider  41  and the thread-guiding rod  22  correctly aligned with its guides  40  in its backward and forward motion, sliding with rolling friction in relation to the stator  60 . 
   Compared with the previous embodiment, the alternative embodiment shown in  FIGS. 3A , B allows a significant reduction in the masses moved by alternate motion, thus decreasing the inertia forces deriving from sudden braking and acceleration in the points where motion is inverted. 
   The alternative embodiment shown in  FIG. 4  concerns the layout of the linear drive motor, while the mechanical layout to guide and support the slider and stator to guide the thread-guiding rod  22  assembly does not differ from the one already described with reference to the previous  FIGS. 2A , B, C, D and  3 A, B. 
   According to the embodiment shown in  FIG. 4 , the slider  80  is connected integrally with the thread-guide bar  22  and is preferably produced with a magnetic bar constituted by a composite sandwich of permanent magnets  81  with light connection materials, such as glass fibres, stainless steel sheets and so on, positioned similarly to those of the bar  45  according to the FIG.  2 C. The slider  80  is positioned with its N-S-N-S . . . polarities directly facing two opposite stators  85  with structure and windings entirely analogous to those illustrated with reference to FIG.  2 D. 
   The two opposite stators may be energized with parallel connections to produce on their surfaces facing the two opposite faces of the slider  80  a series of N and S polarities which move correspondingly and axially to attract with it the opposite N and S polarities of the magnetic bar of the slider  80 . 
   In correspondence with the two side faces of the slider  80  both the position sensor and the elements to support and guide the slider are positioned, entirely analogous to those illustrated with reference to  FIGS. 2A , B and  3 A, B; these are not shown in  FIG. 4  to simplify the drawing. This preferred embodiment of the invention makes it possible to fully satisfy the requirements of dynamic performance of the thread-guide greatly reducing residual magnetic attraction, with the elimination of ferrous materials throughout the body of the slider. This embodiment also allows balance of the forces of attraction between the slider and the stators positioned opposite, which greatly reduces the load on the bearings to guide and support the slider. These forces of attraction may exceed the useful forces of axial attraction even by one order. 
   The thread-guiding device according to the present invention has substantial improvements compared with prior art devices. Of these, at least the following improvements deserve a mention. The drive control with linear motor controlled by the control unit of the open-end spinning frame allows modulation of the bend ratio in relation to the pick-up speed so as to prevent negative phenomena of ribboning and uneven density and winding of the body of the bobbin. It also allows precision winding to be obtained. 
   It is important that a mechanical system to convert rotatory motion of the conventional electric motor in alternate rectilinear motion is not required, as is the case in traditional systems. This allows elimination of their most critical components, such as the motor bearings and the mechanical connection between rotary motor and thread-guide in alternate rectilinear movement. In the device according to the invention the masses moved by alternate motion, with the sudden accelerations and decelerations required, are considerably reduced with improvements in the performances obtained. 
   The linear electric motor is subjected to less stress as the forces exerted between its static part and the part in translating alternate motion are forces caused by magnetic fields, with fewer eddy currents. 
   In general, on open-end spinning frames bobbins are normally produced by winding with considerably different degrees of feed and dimensions. Electrical control of the linear motor allows a large variation of modulations in the motion of the thread-guide rod  22  and avoids ribboning phenomena even when working with bobbins of various degrees of feed, or started at different times. Linear motor drive makes it possible both to vary the length of travel and, with a constant length of the thread-guide travel, to modulate the motion inversion points. This makes it possible to reduce the M profile of the bobbin, axially distributing on a discrete stretch the peak of increase in density and compactness which it produces at the two bases of the bobbin  25 . 
   The effect of the M-shaped terminal bulges of the side profile of the bobbin may be further flattened, by getting the linear motor to make centred axial excursions of reduced length at discrete intervals of time; this makes it possible to fill the distance between the terminal bulges with centred and shorter windings; these bulges are inevitably caused by the increased deposit of yarn in points in which the instantaneous speed of the thread-guide is lower. 
   A further possibility to improve the quality of the winding may be obtained by modulating the wind ratio in the middle stretch of the travel, decreasing the linear speed of the thread-guide in the central stretch of its travel, controlling energizings of the linear motor or modulating the magnetization intensity of the magnetic body fitted in the slider  41 . 
   The electronic control system of the thread-guide according to the invention is capable of optimally performing all the functions required of the mechanical system generally employed on open-end spinning frames of the most recent conception, but with noteworthy advantages compared to this, the most important points of which are set forth below. 
   With modulation of the electric energizings imparted to the linear motor constituted by the slider  41  and by the stator  60  in the thread-guiding device according to the present invention the following are obtained:
         backward and forward movement of the thread-guiding rod common to the entire machine front with extremely low weight equipment. In the conventional mechanical system the same movement is obtained with a heavy and complicated assembly, based on a rotary cylindrical cam with a groove to which a sliding guide moved by horizontal motion is connected;   a range of possible modulations of the alternate motion to avoid ribboning. In the conventional mechanical system modulation requires overlapping a sinusoidal motion over -a basic uniform motion, obtained with an epicyclical mechanism, or with the disturbance of said basic motion with coupling/uncoupling devices, with effects not strictly controlled for the various feed conditions of the bobbins;   modulation both of the travel and of the axial motion inversion points of the thread-guide to prevent or reduce the formation of denser bulges at the ends of the bobbin. The conventional mechanical system can only reduce the phenomenon with modest axial translations of the thread-guide control.       

   In addition to these functions of the conventional thread-guide, the electronic control system of the thread-guide according to the invention is capable of producing further performances and of solving recurrent problems in the open-end spinning frame with mechanically controlled thread-guide, especially when forming bobbins at different degrees of feed on the machine. 
   Modulated control of the linear motor energizings allows the thread-guiding device according to the invention to produce precision bobbins in layers, also working on staggered bobbins, that is without the need for all the bobbins on the machine to be of the same diameter and length, or with the same degree of feed with all the bobbins having been started and terminated at the same time. This possibility is not offered by conventional mechanical systems. The procedure consists in winding on the bobbin layers of the same length, produced with a constant pitch and with a wind ratio that decreases as the diameter increases. At the end of each layer the initial wind ratio is returned to and another layer is deposited and so on. In this way very dense and uniform bobbins are obtained, similar to those of the precision bobbins, even when working with bobbins with staggered feed. The only difference between the bobbins is due to the fact that, at the start of a new bobbin, the first layer is in a random condition between the start and end of the single layer but, by operating with a large number of layers to constitute the bobbin, this circumstance has no influence whatsoever on the quality or appearance of the finished bobbin.