Patent Publication Number: US-6666012-B2

Title: Process and device for pneumatic retention of a yarn

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a process for pneumatic holding of yarn spun from fibers under different work conditions as well to a device to carry out this process. 
     In textile machines, in particular in spinning machines such as open-end spinning machines, it is customary to hold a yarn by means of suction air (DE 23 50 843 A1) to carry out a given work phase. For example, the yarn is cut, merely stored, or transferred from one position to another during this work phase. In this process, the problem occurs, however, that the end of the pneumatically held yarn is affected by the air stream and therefore less than perfect quality is available for the subsequent work steps, so that errors in work or even interruption of the work process may result. 
     OBJECTS AND SUMMARY OF THE INVENTION 
     It is the principal object of the present invention to create a process and a device by means of which excessive stress on and thereby damage to the yarn end, and thus a disturbance of the work process resulting from such damage, may be avoided. Additional objects and advantages of the invention will be set forth in part in the following description, or may be obvious from the description, or may be learned through practice of the invention. 
     This principal object is attained by the invention through adapting the intensity of the air stream holding the yarn to the prevailing work conditions. Due to the fact that the intensity of the airflow brought to bear upon the yarn is adapted to the current operating conditions, the lowest possible intensity of the air stream can be selected for this purpose. The yarn is treated with care in the time interval during which it is exposed to the air stream, while the intensity of the air stream remains strong enough to carry out its task reliably. 
     The term “working conditions” is not only to be understood in the sense of the present invention to relate to the work phases following the pneumatic holding of the yarn, but also relates to the different yarn characteristics with regard to yarn thickness, fiber material spun into the yarn and yarn structure as well as yarn twist, fiber incorporation, etc. 
     The working conditions may be different depending on the textile conditions and/or the operating position. Therefore, it is possible according to an advantageous embodiment of the process to change the intensity of the air stream brought to bear on the yarn end in work phases following each other in time and in adaptation to the task to be accomplished in that case. This change of intensity allows secure handling of the yarn with the least possible flow intensity in an optimal manner. 
     Complementing or as an alternative to, an adaptation of the flow intensity to different work phases, it is an advantageous to adapt the air stream acting upon the yarn end to the character of the yarn. The “character of the yarn” in this case should be understood to be the yarn thickness, the fiber material spun into the yarn, the twist of the yarn or its structure, etc., which may show considerable differences, depending on the spinning process used. 
     According to the invention, the air stream for the reception of a yarn is set at a high intensity, since the task to be accomplished as a rule consists of releasing the yarn from the influence zone of another element in which it is held by that element. In addition, the air stream does not necessarily act upon the end of such a yarn to be accepted, but in some cases upon a middle area of the yarn, so that the latter must be taken up in form of a loop. For this process, an intensive air stream is useful. 
     In a further development according to the invention of the described process, the yarn to be cut is held under tension by an air stream of high intensity, so that the cutting point is accordingly determined very precisely as related to the yarn to be cut. 
     If the task consists merely in holding the yarn after cutting, the intensity is maintained advantageously only until the severed yarn end has been removed. The intensity is then reduced to a value sufficient for the yarn end formed by the cutting process to be held securely on the one hand, while, on the other hand, it is exposed to the lowest possible air stream so that the yarn end may not be frayed and may essentially preserve its character. In an advantageous further development of the process according to the invention, such a reduction of the air stream intensity is effected before the yarn is taken out of the range of influence of the air stream. This reduction occurs whether or not the yarn was to be cut in the pipe in which it had been held previously. 
     In order to reduce the air stream acting upon the yarn, a secondary air stream can be fed to this air stream outside the area in which the yarn is located. 
     In another advantageous variation of the described process, the cross-section of a line conveying the yarn and the air stream can be changed to control the intensity of the air stream acting upon the yarn. 
     To carry out the process, a device for pneumatic holding of a yarn by means of a line connected to an air source is used. Thanks to the air control device according to the invention it is possible to control the air stream which can be brought to bear upon the yarn in such manner that it is as weak as possible on the one hand, while on the other hand it is strong enough that it securely holds the yarn as required by the current operating conditions in an optimal manner. 
     The air control device can be designed in different ways. Thus, it is possible to design the air control device of the object of the invention in an advantageous manner using control openings or chokes. Beyond this, it is possible with the design of the air control device of the invention to actuate one or several intermediate positions in addition to the two end positions that can also be adjustable. In that case, it is advantageous to provide a monitoring device to adapt the intensity of the airflow to different yarn thicknesses. 
     In addition, a yarn severing device can be assigned to the air control device and, if applicable, can be an integral part of the air control device. 
     The process and the device according to the invention make it possible in a simple and optimal manner to hold the yarn carefully. The process and the device are based on the realization that, in adapting the air stream to different operating conditions, it is possible to hold the yarn securely, depending on the prevailing operating conditions, when the intensity of the airflow is as strong as possible as well as when it is reduced. This not only results in a careful treatment of the yarn but, depending on the design of the air control device, also in savings in energy, since the air source need provide less air for certain operating conditions than for other operating conditions. 
    
    
     Examples of embodiments of the invention are explained below through drawings. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows an open-end spinning station as well as a service unit with an air control device according to the invention, shown in a schematic side view; 
     FIG. 2 shows a cross-section of an air control device according to the invention; 
     FIG. 3 shows a cross-section of a variant of an air control device according to the invention; and 
     FIG. 4 shows a diagram of the intensity of the air stream in function of different operating conditions. 
    
    
     DETAILED DESCRIPTION 
     Reference will now be made in detail to the presently preferred embodiments of the invention, one or more examples of which are shown in the figures. Each example is provided to explain the invention, and not as a limitation of the invention. In fact, features illustrated or described as part of one embodiment can be used with another embodiment to yield still a further embodiment. It is intended that the present invention cover such modifications and variations. 
     The device according to the invention is explained below through the example of an open-end spinning machine. On this machine, FIG. 1 shows a workstation  1  on the left by means of a line of alternating dashes and dots, while a service unit  2  is indicated on the right side of the figure by means of a dash-dot-dot line. Normally, an open-end spinning machine has a plurality of identical workstations  1  adjoining each other along which the service unit can be moved. 
     The workstation  1  of an open-end spinning machine selected as an example shows only a schematically drawn open-end spinning device  10  which is fed a fiber sliver B during spinning operation. The fiber sliver B is opened in a known manner into individual fibers in the open-end spinning device  10 , and these are then incorporated continuously into the end of a yarn (not shown). 
     The open-end spinning device  10  is provided with a yarn draw-off pipe  100  through which the spun yarn is drawn off by means of a draw-off device  11  from the open-end spinning device  10 . In the direction of yam draw-off towards the draw-off device  11 , a winding device  12  is provided with a winding roller  120  by means of which a package, or a bobbin,  121  can be driven for the winding of the yarn spun in the open-end spinning device  10 . For this purpose, the bobbin  121  is mounted so as to be capable of rotation between two bobbin holders  122 . By swinging the bobbin holders  122  the bobbin  121  can be lifted off from the winding roller. 
     The service unit  2  is provided with a yarn take-up device  25  with a suction pipe  250  which is connected to an air source  6  via a suction line  251  and a connection line  252 . Between the suction line  251  and the connection line  252  is a valve  60  by means of which the negative pressure which can act in the suction pipe  250  can be controlled. The valve  60  is connected for control to a control device  5  by means of a line  55 . 
     In the embodiment shown, the suction pipe  250  has an outlet  253  which extends parallel to the surface line of the bobbin  121  over the entire length of the latter. 
     The suction pipe  250  is mounted on a swivel axis  255  so that it can be swiveled by means of a swivel arm  254 . The swivel arm is associated with a swivel drive  256  which is connected for control to the control device  5  by means of a control line  51 . 
     In the embodiment shown, the suction pipe  250  is furthermore assigned to a yarn-severing device  4  with, e.g., two blades  40  and  41  interacting in the manner of scissors. The yarn-severing device  4  is provided with a drive (not shown) which is connected for control via a control line  50  to the control device  5 . 
     A yarn transfer device  20  with a swivel arm drive  200  of a swivel arm  202  with a suction pipe  203  at its end and mounted on a swivel axis  201  is connected for control via an additional control line  52  to the control device  5 . This suction pipe  203  is connected via a negative-pressure line  204  to a valve  61  which is connected to the connection line  252  by means of a connection line  205 . The valve  61  is connected via a control line  56  to the control device  5 . 
     By swiveling it, the suction pipe  203  can be presented to a yarn end preparation device  22  fixedly installed in the service unit  2 . The yarn end preparation device  22  is connected via an overpressure line  220 , a valve  62 , and a connection line  221  to the overpressure side of the air source  6 . The valve  62  is connected via a control line  57  to the control device  5 . 
     Another yarn transfer device  23  with a swivel drive  230  and a swivel arm  232  mounted on a swivel axle  231  supports a suction pipe  233  at its free end. This suction pipe is connected via a negative-pressure line  234  to a valve  63  which in turn is connected via a connection line  235  to the connection line  252 . The valve  63  is connected via a control line  58  to the control device  5  with which the swivel drive  230  is also connected for control via a control line  580 . 
     The valves  60 ,  61 ,  62  and  63  thus assume in turn one after the other the function of an air control device  9  as explained below. 
     An auxiliary drive  21  of the bobbin  121  is furthermore connected via a first control line  53  with its swivel drive  210  to the control device  5 . The swivel drive  210  is assigned to a swivel arm  212  mounted on a swivel axle  211 . The swivel arm  212  supports on its free end an auxiliary drive roller  213  driven in a conventional manner. This auxiliary drive roller  213  can be presented to the bobbin  121  when the latter is lifted off from the winding roller  120  in order to drive the bobbin  121  in a known manner during a yarn search or for renewed winding up of a pieced yarn. To control this drive of the auxiliary drive roller  213 , the auxiliary drive  21  is connected via a second control line  59  to the control device  5 . 
     The structure of the service unit  2  has been described above only to the extent absolutely necessary to understand the object of the invention. It goes without saying that the service unit  2  contains in addition, in a known manner, a plurality of other devices, aggregates lines, etc., not shown which are of no interest here. 
     The control device  5  is connected via a control line  540  to a higher-order control device  54  which controls and monitors the aggregate and devices of the open-end spinning machine and in particular of their workstations  1  (see control line  541 ). The control device  5  and/or its higher-order control device  54  are programmed in such manner (by adjustment or by means of a suitably designed software) that it brings the air control device  9  (valves  60 ,  61 ,  63  and  63 ) in timely sequence into the necessary position to control the intensity of the air stream to be brought to bear on the yarn F. 
     The operation of the device, the structure of which has been described above, shall be explained below with the help of FIGS. 1 and 4. On its ordinate, FIG. 4 shows the intensity of the air stream L. On its abscissa (time axis t), FIG. 4 shows different work phases I, II, III, IV, V and VI, during which the yarn F is located within the yarn take-up device  25  which is chosen here to stand for other yarn take-up devices (e.g.,  24 ) or yarn transfer devices (e.g.,  20 ,  23 ), where the control of the air intensity is effected in similar manner. 
     For the description below of the device whose structure is described above, it shall be assumed that breakage of the drawn-off yarn has occurred at the shown work station  1  and that the spinning process has been interrupted as a result. At the same time, the feeding of the fiber sliver B to the open-end spinning device  10  is interrupted. Furthermore, the bobbin  121  is lifted from the winding roller  120  by swiveling the bobbin holders  122  and is thereby stopped. 
     When the service unit  2  reaches the affected workstation  1  and stops there on its patrol along the open-end spinning machine or as a result of a call-up signal normally triggered in case of a yarn breakage, the piecing process is initiated. For this process, the auxiliary drive roller  213 , controlled by the control device.  5  (first control line  53 ), is brought to bear upon the bobbin  121  which has been lifted away from the winding roller  120 . The auxiliary drive roller  213  is now driven in such manner (second control line  59 ) that the bobbin  121  rotates in counterclockwise direction as shown in the drawing of FIG.  1 . 
     Simultaneously with the presentation of the auxiliary drive roller  213  to the bobbin  121 , the swivel arm  254  with the suction pipe  250  under control by the control device  5  (control line  51 ) is swiveled towards the bobbin  121  that has been lifted away from the winding roller  120  (see the positions  254 ′,  250 ′ of the swivel arm  254  and of the suction pipe  250  whose outlet  253  now assumes the yarn receiving position  253 ′, these positions being indicated by broken lines). A negative pressure is produced in the suction pipe  250  through actuation of the valve  60  (control line  55 ). 
     When the bobbin  121  is now driven in a counterclockwise direction by the auxiliary drive roller  213 , the yarn end E (not shown in FIG. 1) located on the bobbin surface reaches the area of the outlet  253  of the suction pipe  250  which is presented to the bobbin  121  (see yarn receiving position  253 ′). The yarn end E is then sucked into outlet  253  due to the negative pressure prevailing in this suction pipe  250 . 
     Since the yarn end E has been “rolled into” the windings of the yarn wound on the bobbin  121  to a certain degree by the rotation of the bobbin  121  between the moment at which the yarn breakage occurs and the lifting of the bobbin  121  from the winding roller  120  and its reaching a stopped position, the yarn end E must first be disengaged from these windings on the bobbin  121 . For this reason, the valve  60  between the suction line  251  and the connection line  252  is adjusted before the take-up of the yarn F (control line  55 ) so that a suction air stream of higher intensity flows in the suction pipe  250  and thereby also at its outlet  253  (see flow intensity L a  in FIG. 4; phase I: yarn take-up). This strong suction airflow seizes the yarn end E and pulls it further into the suction pipe  250 . At the same time, due to the rotation of the bobbin  121  imparted to it by the auxiliary drive roller  213 , an ever greater length of the yarn F comes within the zone of influence of this air stream L sucked into the suction pipe  250  and the yarn F is carried off by it. 
     When the suction pipe  250  has taken up securely and in sufficient length the yarn F unwound from the bobbin  121 , the swivel arm  254  (control line  51 ) is swiveled so that the suction pipe  250  moves with its outlet  253  away from the bobbin  121  (Phase II: swiveling of the yarn take-up device  25 ). When this swiveling of the yarn take-up device  25  has caused the yarn F to be deflected more at the outlet  253  of the suction pipe  250 , the friction or retention force acting upon the yarn F is so great that the intensity of the air stream L can be lowered if the circumferential speed of the bobbin  121  substantially matches the swiveling speed of the yarn take-up device  25  (see flow intensity L b  in phase III: continued swiveling of the yarn take-up device  25 ). Since the yarn F has not yet been shortened at this point in time by the yarn-severing device  4 , the reduction of the flow intensity can be dispensed with during this phase III (see flow intensity L a1  in phase III, indicated by broken lines). 
     Finally, the yarn take-up device  25  assumes the yarn transfer position, indicated in FIG. 1 by a solid line, in which the yarn F is released at a given time for further handling and is later transferred to the yarn transfer device  20 . When this yarn transfer position has been reached, the drive of the auxiliary drive roller  213  and thereby also the bobbin  121  is stopped (control line  59 ). 
     At a point in time coordinated with the piecing process, the yarn-severing device  4  (control line  50 ) is actuated by the control device  5  so that the blades  40  and  41  execute a movement relative to each other and sever the yarn F located between them (phase IV: yarn severing process and removal of the severed yarn end). For this severing of the yarn F, it should assume as stretched a position as possible within the suction pipe  250  so that the length of the yarn F extending from the bobbin  121  to the yarn-severing device  4  may be defined exactly. For this reason, the negative pressure in the area of the suction pipe  250  has as before a high airflow intensity L a . The reduced airflow intensity L b  of the air stream L holding the yarn F during the phase III is increased by adjusting the valve  60  (control line  55 ). In the case of this high intensity being maintained during phase III (airflow intensity L a1 ), the air flow intensity continues to be held at this high suction level. 
     Following the severing and removal of the yarn end E, the intensity of the air stream L is reduced. This reduction is effected through appropriate actuation of the valve  60  (control line  55 ) (Phase V: holding of the shortened yarn end). In this manner, the yarn F held in the suction pipe  250  is no longer exposed to an aggressive air stream. The intensity of the airflow is lowered as far as possible (flow intensity L b ) without affecting the holding of the yarn F and can, among other things, also assume a different value than during phase III. The yarn F is held securely in this manner, as before, by the prevailing negative pressure and the suction air stream flowing through the suction pipe  250 . Thus the yarn F is held in a careful manner so that the free yarn end E is not untwisted thanks to the lowered flow intensity L b  of the air stream L. This lowered flow intensity L b  allows the former structure and twist of the yarn end E to be maintained. 
     At the latest, when the yarn F has been shortened to a predetermined length by severing its yarn end E, the swivel drive  200  of the yarn transfer device  20  is triggered by the control device  5  via the control line  52  in such manner that it reaches the yarn take-up position (indicated in FIG. 1 by a full line). By partial or complete closing of the valve  60  (control line  55 ), the intensity of the air stream L is further reduced, if necessary down to a value of zero (flow intensity L 0 ), and the yarn F which had been held until then by the suction pipe  250  is released (phase VI: release of the yarn). Simultaneously, with the closing of valve  60 , the valve  61  (control line  56 ) is opened so that a suction air stream of high intensity flows in the suction pipe  203 , causing the yarn F released from the suction pipe  250  to be sucked into the suction pipe  203 . The work phases and flow intensities relating to the yarn transfer device  23  are not shown in FIG. 4, as already indicated. 
     The suction pipe  203  is brought into its yarn transfer position through renewed actuation of the swivel drive  200  (control line  52 ) (see positions  202 ′ and  203 ′ of the swivel arm  202  and of the suction pipe  203  indicated by broken lines). In this position, the outlet of the suction pipe  203  is located across from the outlet of the yarn end preparation device  22 . 
     The yarn end preparation device  22  is connected to an overpressure line  220  which is configured with the yarn end preparation device  22  in the form of an injector (not shown) in such manner that a sucking action is produced at the outlet of the yarn end preparation device  22  towards the suction pipe  203 . When the suction air stream in the suction pipe  203  is taken out of action by closing the valve  61  (control line  56 ) and an air stream flowing into the yarn end preparation device  22  is produced at the same time through actuation of valve  62 , the yarn F released from the suction pipe  203  will follow this air stream and will enter the yarn end preparation device  22 . The compressed-air stream, which is introduced with great intensity and possibly with tangential orientation components in the form of one or several injector openings (not shown) into the yarn end preparation device  22 , acts in a known manner on the yarn end E to thin it out in the desired manner for subsequent piecing. When this preparatory action has been accomplished—as can be determined by time control or monitored optically—the intensity of the air stream acting upon the yarn end E is reduced for the remainder of the time during which the yarn end E is still held by the yarn end preparation device  22  through actuation of the valve  62  (control line  57 ). 
     Finally, the suction pipe  233  of the yarn transfer device  23  is presented to the outlet of the yarn end preparation device  22  by the control device  5  through actuation of the swivel drive  230  (control line  580 ). By opening valve  63  (control line  58 ), a negative pressure is produced in the suction pipe  233 . At the same time, intensity of the air stream from the yarn end preparation device  22  acting upon the yarn end F is lowered considerably or eradicated completely through extensive or complete closing of the valve  62  (control line  57 ), so that the yarn F is released and thus enters into the suction pipe  233 . 
     The suction pipe  233  is now brought into its yarn transfer position indicated by broken lines by actuating the swivel drive  230  (control line  580 ) (see positions  232 ′ and  233 ′ of the swivel arm  232  and of the suction pipe  233 ). A negative pressure required for the transportation of the fibers, which have been opened in a known manner from the forward end of the fiber sliver B and presented to the spinning device  10 , is produced in the spinning device  10 . This negative pressure takes effect as far as the outermost outlet of the yarn draw-off pipe  100  so that the free yarn end E of the yarn F held by the suction pipe  233  comes under the influence of this suction air stream flowing into the yarn draw-off pipe  100 . The valve  63  is then closed (control line  58 ) so that the suction pipe  233  releases the yarn F. At the same time, a predetermined yarn length is fed back in one or several steps in a usual and therefore not shown manner in the direction of the yarn draw-off pipe  100 . The predetermined yarn length is then sucked through the yarn draw-off pipe  100  into the spinning device  10  where the actual piecing takes place in the known manner. 
     The yarn F that is fed back into the spinning device  10  can be delivered by the bobbin  121  which is again driven in a back-feeding direction by the auxiliary drive roller  213 . The yarn length required for piecing, however, can be provided in the form of a yarn reserve already delivered at an earlier point in time than the yarn length delivered by the bobbin  121  into the suction pipe  250 . This yarn reserve is released at the proper time for the piecing, back-feeding though the yarn draw-off pipe  100  into the spinning device  10 . Such a yarn reserve can also be formed during the swiveling motion of the suction pipe  203  or  233  from their position indicated in FIG. 1 by a full line into their position  203 ′ or  233 ′ indicated by a broken line. 
     Following piecing, when all the elements of the service unit  2  required for piecing have again assumed their starting position, the service unit  2  leaves the serviced workstation  1  and is again available for service at other workstations  1 . 
     The process and the device can have many variants within the scope of the present invention, in particular by replacing individual characteristics with their equivalents or by other combinations of these characteristics or their equivalents. Thus, the work station  1  where the described process as well as the described device find their application need not be a part of an open-end spinning device  10 . This process and this device can rather be used on any textile machine or device in which a yarn is to be handled, whether it is for the transportation of the yarn from one position into another (see the suction pipes  250 ,  203  and  233  in the described example) or for improved treatment in combination with a transportation of the yarn F from one position into another (see yarn take-up position and yarn transfer position of the suction pipe  250 ) or without such a yarn transportation (see yarn end preparation device  22 ). 
     The pipe receiving the yarn F also need not have an enlarged or widened outlet  253  if this pipe does not have the task of taking up the yarn F from the bobbin  121  like the suction pipe  250 . This becomes clear through the above example of the suction pipes  203  and  233  where the yarn F is conveyed to the respective outlets of the suction pipes  203  or  233 . 
     The pipe receiving the yarn F may also be a pipe through which a yarn F provided for piecing, which is not unwound from the bobbin  121 , is transported from its end away from the outlet in the direction of the outlet, which in this case also need not have a greater diameter, where the yarn F is in turn grasped by a grasper or similar device and is conveyed to the yarn draw-off pipe  100 . 
     The control of the intensity or speed of the airflow is effected according to the above description by corresponding adjustment of the valve  60 ,  61 ,  62  or  63 . It is, however, also possible to adjust the intensity of the airflow by means of a central control of the air source  6 , since, as the above description shows, the yarn F reaches the valves  60 ,  61 ,  62 ,  63 , one after the other in time, whereby these valves  60 ,  61 ,  62 ,  63  merely have the task of switching the air stream on or off in the required zone. Depending on whether the valves  60 ,  61 ,  62 ,  63  or the air source  6  control the intensity of the airflow, these valves  60 ,  61 ,  62 ,  63  or this air source  6  constitute the air control device  9  for the air stream L acting upon the yarn. 
     Only one air source  6  was mentioned above, without indicating whether it supplies overpressure or negative pressure, it is because this air source  6  serves as negative pressure source or as overpressure source, depending on the connection of the connecting line  252 ,  205 ,  221 ,  235 , etc. It goes without saying that when either negative pressure or overpressure is needed, a negative-pressure source as well as an overpressure source can be provided as autonomous air sources, i.e., independent of each other, instead of the shown combined compressed-air source  6 . 
     By means of the yarn end preparation device  22 , yarn end E is imparted an especially suitable form for piecing by untwisting or fraying. It is possible to assign the yarn end preparation device  22  to the suction pipe  250  having it located between the outlet  253  and the yarn-severing device  4 . 
     Depending on the design and task of the pipe holding the yarn F, a yarn severing device  4  can be assigned to it functionally and thereby also to the air control device  9  (valve  60 ,  61 ,  62 ,  63  or air source  6 ). This is not obligatory, however, as appears from the above description. If it is provided, the yarn-severing device  4  can be designed in different ways, e.g., as shown, in the form of scissors, a knife, a grinding roller, etc. 
     In a device of this type, e.g., in the spinning device  10  shown in FIG. 1, not every yarn take-up device  24 ,  25  or yarn transfer device  20 ,  23  need necessarily serve to receive the yarn F with the assistance of a negative-pressure or overpressure air stream. Therefore, these devices do not absolutely require a suction pipe  240 ,  250 ,  203  or  233 . It may also be advantageous if, e.g., a suction pipe  203  in which the intensity of the air stream L is controlled in adaptation to the different work phases I to VI interacts with a yarn transfer device  23  which is provided with a controllable holding device, e.g., a pair of rollers (not shown), instead of a suction pipe  233 . If the two rollers of such a pair of rollers can be driven as needed in one or the other direction, then this pair of rollers can take over or assist the back-feeding of the yarn length required for piecing into the spinning device  10 . Also, such rollers can take over or assist the start of draw-off of the yarn F following successful piecing from the spinning device  10 . The yarn length required for the back-feeding to the spinning device  10  can be released by suitable reverse rotation of the bobbin  121  by means of the auxiliary drive roller  213 . 
     According to FIG. 1, a valve  60 ,  61 ,  62  or  63  located outside of the range of the length of yarn F is provided for the control of the air stream L acting upon the yarn F or the yarn end E. FIG. 2 shows in detail the example of an embodiment of a valve  7  in the form of a choke for the control of the intensity of the air stream acting upon the yarn end E. With the help of this valve, the cross-section of the line receiving the yarn F can be changed, as will be explained in further detail below. The yarn take-up device  24  shown can be in this case the yarn transfer device  20  or  23 , the yarn take-up device  25 , or also the yarn end preparation device  22 , or can be designed as a part thereof. 
     The yarn take-up device  24  shown contains the previously mentioned valve  7  between a suction line  240  and a connection line  242  connected to the negative-pressure side of the air source  6 . The suction line  240  and the connection line  242  are essentially aligned with each other and their two ends towards each other end in a housing  70  extending essentially perpendicularly to the longitudinal axes of the suction line  240  and the connection line  242 . The housing  70  receives a ram or piston  71  which is connected via a piston rod  72  (only indicated) to a drive  73  which in turn is connected by means of a control line  730  to the control device  5  (see FIG.  1 ). The housing  70  is closed by a cover  74  on its side across from the drive  73 . 
     When the full intensity of the airflow is to act upon the yarn end E, the piston  71  is with its face  710  in the position  710 a indicated by a broken line. The air stream can then flow unimpeded through the suction line  240 , the housing  70  and the connection line  242  and thus acts upon the yarn end E with the greatest possible intensity. 
     If the intensity of the air stream L brought to bear on the yarn end E is to be reduced for a given work phase I to VI, the piston  71  is pushed to the left as seen in the drawing of FIG.  2  through appropriate control by the control device  5  (control line  730 ), so that the cross-section in the area of housing  70  through which the air stream must flow is reduced. This causes the flow speed of air inside the housing- 70  to accelerate because of the reduction of the-cross-section, but the intensity is reduced due to the throttling achieved in the area of the suction line  240 . This throttling results in the airflow acting only with reduced intensity upon the yarn end E, which does not reach as far as into the housing  70  but merely extends into the suction line  240 . 
     If desired, the valve  7  can at the same time be designed as the yarn-severing device  4 , so that this yarn-severing device  4  is an integral part of the air control device  9  constituted by the valve  7 . In this case, the annular edge  711  between the circumferential surface  712  and the face  710  of the piston  71  is designed as a cutting edge. In addition, the longitudinal area  701  of the housing  70  is widened by a distance between the end  241  of the suction line  240  and the end  243  of the connecting line  242 , as compared with the area of the housing  70  for the movement of the piston  71 . This allows an anullar edge  700 , also in the form of a cutting edge, to be produced on the side of this longitudinal area  701  towards the cover  74 . If the drive  73  is actuated once the yarn F has been sucked into the suction pipe  240  by corresponding control while the piston  71  is completely pulled back creating an intensively acting airflow, then the piston  71  moves to the left, relative to FIG. 2, until the yarn F is finally severed by the interacting annular edges  700  and  711  (see the position  710   b  of the face  710  indicated by a dash-dot-dot line). The piston  71  is then withdrawn immediately so that the airflow can again act upon the yarn F. Depending on the intensity of the air stream L that is desired after the severing process, the piston  71  returns here only into the position of face  710  indicated by a continuous line or into the position  710   a  indicated by a broken line. 
     Depending on the desired intensity of the airflow, e.g., in adaptation to different yarn thicknesses, or depending on the type of fiber material being spun, it is also possible to provide another end or intermediate position of the piston  71  which is not shown. To prevent the yarn F from being temporarily subjected to no airflow at all during the severing process, it is possible, through appropriate design of the inner contour of the housing  70  and/or of the piston  71  to ensure that an airflow is always maintained and acting on the yarn F without taking it out of severing range, even when the yarn-severing device  4  constituted by the annular edges  700  and  711  becomes active. 
     As described below, it is possible to achieve a reduction of the airflow intensity also by changing it in a manner other than by reducing the line (suction line  240 , housing  70 , connection line  242 ) conveying the air stream L. Thus, it is possible to achieve a reduction of the intensity of the air stream L acting upon the yarn end E when the yarn end E extends not only into the suction line  240  but as far as into the longitudinal area  701  of the housing  70 , by enlarging the cross-section of the housing  70  in that longitudinal area  701 . In that case, the piston  71  is not only withdrawn to the position  710 a indicated by a broken line in which the face  710  is essentially in prolongation of the inner walls of the suction pipe  240  and of the connection pipe  242 , but beyond this in direction of drive  73  into position  710   c , so that a significant enlargement of the cross-section of the housing  70  is achieved. Large enough sizing of the longitudinal area  701  of the housing  70  in the longitudinal fiber direction indicated by axis A in FIG. 2 is naturally required for this. 
     Another alternative embodiment of a valve  8  for the control of the intensity of the air stream L acting upon the yarn end E is described below through FIG.  3 . Here too the suction line  240  of a yarn take-up device  24  as well as the connection lines  242  are shown, whose ends  241  and  243  face each other leaving a distance between them. These ends  241  and  243  are surrounded by a sleeve  80  which can be displaced in longitudinal direction along axis A and which, in its shown end position, closes the space  81  created by the distance between the ends  241  and  243  radially against the outside. On a portion of its circumference, the sleeve  80  is provided with an opening  82  which can be moved by axial displacement of the sleeve  8  from its shown end position to a position  82 a indicated by a broken line in which the space  81  is connected to the atmosphere or environmental air surrounding the sleeve  80 . 
     As shown in FIG. 3, the yarn F to be held extends with its yarn end E into the suction pipe  240  and does not reach as far as the space  81 . 
     If an air stream of high intensity is needed for the reception or handling of the yarn F, the space  81  is closed off from the outside by moving the sleeve  80  into the position shown in FIG. 3 by a continuous line, without changing the capacity of the air source. If on the contrary, a weaker negative pressure is needed to hold the yarn F in a given work phase, the sleeve  80  is moved by the control device  5  into a position in which the space  81  is opened to the desired extent to the air surrounding the valve  8 . In this manner, an air stream (secondary air stream) flows through the freed opening between the ends  241  and  243  of the suction line  240  and the connection line  242  into the space  81 , so that the intensity of the airflow is reduced correspondingly in the area of the suction line  240  in which the yarn end E is located. If a greater intensity of the air stream L is again required in the area of the yarn end E, the sleeve  80  is moved back into its starting position. 
     So far it has only been stated that the intensity of the negative pressure is modified in the area of yarn end E in adaptation to different work phases I to VI so that it is able to fully play its role, but still is kept as low as possible so that the yarn end E subjected to the air stream L caused by the negative pressure is held with the greatest care. In addition, it must also be considered that such an adaptation of the air stream intensity L can also be applied to other work conditions besides the work phase. For example, a thin yarn F naturally reacts with much greater sensitivity to such an air stream than a thick yarn F. For this reason, provisions are made to adapt the intensity of the air stream not only to the different work phases I to VI, but, if necessary, also as a function of the thickness of the held yarn F. This level of the intensity of the air stream L can first be determined empirically and can then be adjusted manually. It is, however, also possible to provide a sensor in the yarn holding device. Such a sensor (monitoring device  3 ) is shown in FIG. 1 in connection with the yarn take-up device  25 . This monitoring device  3  is connected for control via a control line  30  to the control device  5 . It has the task of determining the yarn thickness and transmitting it to the control device  5  which adjusts to the full or the reduced intensity of the air stream L as a function of this ascertained yarn thickness. For this purpose, the air control device  9  (valve  60 ,  61 ,  62 ,  63 ,  7  or  8 ) is brought into a corresponding intermediate position or is driven at the corresponding rotational speed (air source  6 ). 
     In FIG. 1 for example, with reference to the above-mentioned phases I to VI, L a , designates the maximum flow intensity with a thick yarn F and L a ″ designates the maximum flow intensity with a thin yarn F, while L b , designates the reduced flow intensity with a thick yarn F and L b , designates the reduced flow intensity with a thin yarn F. Of course, the values for and the relationships between the flow intensities L a , L a ′, L a  ″ on the one hand and the flow intensities L b , L b ′, L b ′ on the other hand can be selected as a function of the prevailing conditions. 
     Similarly, an adaptation to the character of the yarn with regard to the type of fiber material spun and/or to the yarn twist can be taken into account in determining the flow intensity of the air stream L. The structure of the yarn F that is different, e.g., in a wrap-around yarn or in an open-end yarn or in a yarn spun on a ring-spinning machine, is to be taken into account. 
     Depending on the type of textile machine, the workstation  1  can be designed in different ways. Thus, the work station  1  could be part of a spinning machine, a winding machine, or other similar device. Accordingly, the service unit  2  would then also be designed differently and could also be an integral component of the work station  1  if necessary. 
     It will be appreciated by those skilled in the art that various modifications and variations can be made in the present invention without departing from the scope of the invention. It is intended that the present invention include such modifications and variations as come within the scope of the appended claims and their equivalents.