Abstract:
A drawing mechanism for the doubling and drafting of fibre slivers, has a drawing mechanism frame for accommodating the drawing mechanism, which has at least two pairs of rollers each comprising an upper roller, and a lower roller, and has means for adjusting the spacing of at least one of the lower rollers in relation to another lower roller, in each case having a mounting device for accommodating the lower roller, wherein lower rollers are arranged to be driven by a drive device comprising at least one drive element endlessly revolving around pulley wheels. 
     In order, by simple means in terms of construction, to make possible a considerable reduction in the work and time required for adjustment of the lower roller(s) and, accordingly, of the extent(s) of the drawing zone(s), the mounting device(s) are made adjustable by the drive device.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
   This application claims priority from German Patent Application Nos. 102 42 391.1 and 103 29 837.1, which are incorporated herein by reference. 
   BACKGROUND OF THE INVENTION 
   The invention relates to an apparatus at a draw frame or other textile machine having a drawing mechanism for the doubling and drafting of fibre slivers. 
   Certain known forms of draw frame have a drawing mechanism frame for accommodating the drawing mechanism, which has at least two pairs of rollers each comprising an upper roller and a lower roller, and means for adjusting the spacing of at least one of the lower rollers in relation to another lower roller, in each case having a mounting device for accommodating the lower roller, and lower rollers are arranged to be driven by at least one drive element endlessly revolving around pulley wheels. 
   In a known apparatus (DE-OS 20 44 996), the mountings of the intake and middle lower rollers are displaceable on the frame of the machine so that the extent of the drawing zone can be matched to the particular fibre staple. A tensioning pulley wheel, which is displaceable in a guideway in the frame of the machine, allows the length of the toothed belt to be modified in accordance with the changed spacing between the axes of the middle roller and a guide pulley wheel, brought about by displacement of the intake roller. The middle roller is driven by a further toothed belt. The latter toothed belt is tensioned by a tensioning pulley wheel which is fastened to the machine frame and which can pivot about one axis; as a result, it can also be matched to changed spacings between the axes of the intake roller and middle roller. It is disadvantageous that displacing devices for displacement of the intake roller and the middle roller and additional tensioning devices for re-tensioning of the toothed belts after the displacement operations are necessary, requiring a considerable outlay in terms of construction. In addition, it is disadvantageous that a number of work steps are required for the displacement operations and the subsequent re-tensioning operations. The belt tension is destroyed by the displacement process. Where the displacement is carried out manually, spacers are inserted between the mountings, the mountings being pushed against the spacers so that, in this case too, the amount of set-up work is considerable. Finally, the displacement and re-tensioning operations result in a doubling of potential error sources when setting the spacings and belt tensions. 
   It is an aim of the invention to provide an apparatus of the kind described at the beginning that avoids or mitigates the disadvantages mentioned and that especially is of simple construction and allows a considerable reduction in the work and time required for adjustment of the lower roller(s) and, accordingly, of the extent(s) of the drawing zone(s). 
   SUMMARY OF THE INVENTION 
   The invention provides a drawing mechanism having a drawing mechanism frame, at least two pairs of rollers each comprising an upper roller and a lower roller and having a mounting device for accommodating the lower roller, means for adjusting the spacing of at least one of the lower rollers in relation to another lower roller, and at least one drive device comprising a drive element endlessly revolving around pulley wheels, wherein the drive device can be used for adjusting the position of said at least one lower roller. 
   The measures according to the invention make it possible, by simple means, for the mountings and, as a result, the extents of the drawing zones (nip line spacings) to be adjusted in a short time. For the purpose of adjusting the extents of the drawing zones, elegant use is made of existing structural elements necessarily present in the draw frame, for example, a pulley wheel and the drive belt. Separate apparatuses for adjustment are not required. As a result of the fact that the drive belt can be in tension before, during and after adjustment, further apparatuses for re-tensioning the drive belt after the adjustment are not required, which allows the extents of the drawing zones of the drawing mechanism to be changed in a short time by means that are especially simple in terms of construction. 
   Advantageously, a said mounting device of a said lower roller is adjustable by means of a moving force applied to a pulley wheel of said drive device, which moving force is converted into an adjusting movement for the mounting device. As well or instead, a said mounting device of a said lower roller is advantageously adjustable by means of a moving force applied to a drive element of said drive device, which moving force is converted into an adjusting movement for the mounting device. Advantageously, the drive element is stationary and the pulley wheel is rotated. Advantageously, the pulley wheel is stationary and the drive element is moved. Advantageously, the rotation of the pulley wheel or the movement of the drive element is converted into the adjusting movement of the slider. Advantageously, at least one guide pulley wheel is attached to each slider (mounting); and the roller-driving pulley wheel or guide pulley wheel(s) act, in each case one after the other, on both sides of the tensioned drive element. Advantageously, the rotation of the pulley wheel or the movement of the drive element is accomplished manually. Advantageously, the slider is linearly displaceable. 
   Advantageously, the drive element is a toothed belt. Advantageously, an endless flexible toothed belt is present. Advantageously, the pulley wheels comprise toothed belt wheels. Advantageously, the pulley wheels comprise guide pulley wheels. Advantageously, at least one driving pulley wheel is provided. Advantageously, driven pulley wheels are present. Advantageously, the drive element loops around the pulley wheels. Advantageously, the drive element and the pulley wheel are in engagement with one another. Advantageously, the pulley wheel for adjustment of a slider is the drive pulley wheel of a lower roller (roller-driving pulley wheel). Advantageously, the slider is displaceable during adjustment. Advantageously, the slider is arranged to be stopped. Advantageously, the stopping arrangement is releasable. Advantageously, a display device for the position of the slider is present. 
   Advantageously, a drive motor is used for rotation of the pulley wheel. Advantageously, a drive motor is used for movement of the drive element. Advantageously, the drive motor is used for the lower rollers. Advantageously, a separate drive motor is used. Advantageously, belt shortening or belt lengthening is arranged to be automatically evened out during adjustment. Advantageously, the evening-out of belt length is carried out at a slider by two guide pulley wheels. 
   Advantageously, the lower rollers are arranged to be adjusted singly and independently of one another. Preferably, a roller-driving pulley wheel and a guide pulley wheel are attached to the slider of the intake roller and a roller-driving pulley wheel and a guide pulley wheel are attached to the slider of the middle roller. Advantageously, the drive element runs around the pulley wheels at the slider of the intake roller and around the pulley wheels at the slider of the middle roller in a mirror-reflected arrangement. Advantageously, the drive element is in tension before, during and after the displacement. Advantageously, the drive motor is in communication with an electronic control and regulation device. Advantageously, a measuring element is connected to the control and regulation device. Advantageously, the measuring element is capable of registering fibre-related and/or machinery-related measurement variables. Advantageously, adjustment of the slider is carried out when the drawing mechanism is in operation. Advantageously, adjustment of the slider is carried out when the drawing mechanism is not in operation. Advantageously, adjustment of the slider is carried out during can-changing. Advantageously, the draw frame is self-adjusting. Advantageously, adjustment of the slider is carried out by inputting adjustment variables. Advantageously, the adjustment variables can be input manually. Advantageously, a memory for adjustment variables is connected to the control and regulation device. Advantageously, the slider for the intake roller and the slider for the middle roller are arranged to be connected by a rigid connecting element. Advantageously, the connecting element is releasably connected. The spacing of the pairs of rollers in relation to one another may be adjustable without fibre material. The spacing of the pairs of rollers in relation to one another may be adjustable with fibre material. Advantageously, the extent of the preliminary draft zone can be adjusted. Advantageously, the extent of the main draft zone can be adjusted. Advantageously, the extent of the preliminary draft zone and the extent of the main draft zone can be adjusted. Advantageously, each lower roller has its own associated drive motor. Advantageously, the intake and middle lower rollers are arranged to be driven by one drive motor. Advantageously, a brake, stopping arrangement or the like is associated with the stationary pulley wheel. The brake, stopping arrangement or the like may be mechanical, electrical or electromagnetic. Advantageously, the drive motor is a self-braking motor. Advantageously, the drive motor drives a further drive train, which has a free-wheel arrangement or the like. 
   Advantageously, the mounting device consists of the mounting and the slider. The mounting and the slider may be fastened to one another, for example by bolts. The mounting and the slider may be of integral construction. 
   The invention further provides an apparatus at a draw frame having a drawing mechanism for the doubling and drafting of fibre slivers, having a drawing mechanism frame for accommodating the drawing mechanism, which has at least two pairs of rollers each comprising and upper and a lower roller, having means for adjusting the spacing of at least one of the lower rollers in relation to another lower roller, in each case having a mounting device for accommodating the lower roller, wherein lower rollers are arranged to be driven by at least one drive element endlessly revolving around pulley wheels, characterised in that at least one pulley wheel and the tensioned drive element are used for adjusting the mounting device, wherein a moving force applied to the pulley wheel or to the drive element can be converted into the adjusting movement for the mounting device. 
   The invention further provides a draw frame comprising a drawing mechanism according to the invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a diagrammatic side view of an autoleveller draw frame for use with an apparatus according to the invention together with a general circuit diagram; 
       FIG. 2  is a perspective view of a side of the draw frame showing the displaceable mounting of the intake and middle lower rollers; 
       FIGS. 3   a  and  3   b  show the drive for the intake and middle lower rollers for the draw frame according to  FIG. 1 , in a side view ( FIG. 3   a ) and plan view ( FIG. 3   b ); 
       FIG. 3   c  is a partial side view of a drive belt; 
       FIGS. 4   a  to  4   d  show, in diagrammatic form, the sequential procedure for shortening of the preliminary and main draft zones; 
       FIGS. 5   a  and  5   b  show the intake and middle lower rollers before displacement ( FIG. 5   a ) and after displacement ( FIG. 5   b ); 
       FIGS. 6   a  and  6   b  show, in diagrammatic form, an electromagnetic braking apparatus for a toothed belt wheel; 
       FIG. 7  shows a locking device for a slider; 
       FIG. 8  shows a connection element (bridge) for connecting two sliders; 
       FIG. 9  is a partial side view of an embodiment comprising a drawing mechanism having three roller combinations, each having its own drive motor; 
       FIG. 10  is a side view of a drawing mechanism with input devices for manual and/or memory-assisted input of adjustment values for changing the nip line spacings in the drawing mechanism; and 
       FIG. 11  is a front view of a roller pair with an upper roller lifted off from a lower roller. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   In accordance with  FIG. 1 , a draw frame  1 , for example a draw frame known as an HSR draw frame (trade mark) made by Trützschler GmbH &amp; Co. KG, has a drawing mechanism  2 , upstream of which is an intake  3  of the drawing mechanism and downstream of which is an exit  4  from the drawing mechanism. The fibre slivers  5 , coming from cans (not shown), enter the sliver guide  6  and, drawn by the draw-off rollers  7 ,  8 , are transported past the measuring element  9 . The drawing mechanism  2  is designed as a 4-over-3 drawing mechanism, that is to say it consists of three lower rollers I, II, III (I delivery lower roller, II middle lower roller, III intake lower roller) and four upper rollers  11 ,  12 ,  13 ,  14 . Drafting of the fibre sliver combination  5 ′ from a plurality of fibre slivers  5  is carried out in the drawing mechanism  2 . Drafting is composed of preliminary drafting and main drafting. The roller pairs  14 /III and  13 /II form the preliminary draft zone and the roller pairs  13 /II and  11 ,  12 /I form the main draft zone. 
   The attenuated fibre slivers  5  reach a web guide  10  in the exit  4  from the drawing mechanism and, by means of the draw-off rollers  15 ,  16 , are drawn through a sliver funnel  17 , in which they are combined to form one fibre sliver  18 , which is then deposited in cans. Reference letter A denotes the work direction. 
   The draw-off rollers  7 ,  8 , the intake lower roller III and the middle lower roller II, which are connected to one another mechanically, for example by toothed belts, are driven by the control motor  19 , it being possible, in the process, for a desired value to be specified. (The associated upper rollers  14  and  13 , respectively, revolve by virtue of the motion of the lower rollers.) The delivery lower roller I and the draw-off rollers  15 ,  16  are driven by the main motor  20 . The control motor  19  and the main motor  20  each have their own controller  21  and  22 , respectively. Control (speed-of-rotation control) is carried out in each case by means of a closed control loop, a tachogenerator  23  being associated with the control motor  19  and a tachogenerator  24  being associated with the main motor  20 . At the intake  3  of the drawing mechanism, a variable proportional to the weight of the fibre slivers  5  fed in, for example their cross-section, is measured by an intake measuring element  9  known, for example, from DE-A-44 04 326. At the exit  4  from the drawing mechanism, the cross-section of the delivered fibre sliver  18  is ascertained by an exit measuring element  25  associated with the sliver funnel  17  and known, for example, from DE-A-195 37 983. A central computer unit  26  (control and regulation device), for example a microcomputer with a microprocessor, sends a setting for the desired value for the control motor  19  to the controller  21 . The measurement values of the two measuring elements  9  and  25  are sent to the central computer unit  26  during the drawing process. The desired value for the control motor  19  is determined in the central computer unit  26  from the measurement values of the intake measuring element  9  and from the desired value for the cross-section of the delivered fibre sliver  18 . The measurement values of the exit measuring element  25  are used for monitoring of the delivered fibre sliver  18  (delivered sliver monitoring). By means of this control system, it is possible for variations in the cross-section of the fibre slivers  5  fed in to be compensated, and for the fibre sliver to be made more uniform, by appropriately regulating the drafting process. Reference numeral  27  denotes a display monitor,  28  an interface,  29  an input device,  30  a pressure rod and  31  a memory. 
   With reference to  FIG. 2 , each of lower rollers II, III has an associated mounting device comprising a respective mounting  33   a ,  34   a . The trunnions Ia, IIa, IIIa (see  FIG. 3   b ) of the lower rollers I, II and III are mounted so as to be capable of rotation in mountings  32   a ,  33   a ,  34   a  ( 32   b ,  33   b ,  34   b  are located on the other side of the drawing mechanism and are not shown). The mountings  33   a  and  34   a  are bolted onto sliders  35   a  and  36   a , respectively, which are displaceable in the direction of the arrows C, D and E, F, respectively, along a bar  37   a . The two ends of the bar  37   a  are fixedly mounted in mounting blocks  38 ′ ( 38 ″ not shown), which are attached to the frame  39  of the machine. 
   Displacement of the sliders  35   a ,  35   b ;  36   a ,  36   b  at the same time causes the mountings  33   a ,  33   b ;  34   a ,  34   b  and, as a result, the lower rollers II and III, respectively, to be displaced and moved in directions C, D and E, F, respectively. The associated upper rollers  13  and  14  are correspondingly moved (in a manner not shown) in directions C, D and E, F, respectively. By that means, the nip line spacings between the roller combinations are modified and set. 
   Locking of the sliders  35   a ,  35   b ;  36   a ,  36   b  is accomplished by means of a catch device, stopping device or the like, one suitable form of stopping device being shown in  FIG. 7 . 
   Referring to  FIG. 3   a , the lower rollers II and III are driven from the right-hand side of the draw frame, seen in the direction of material flow A, by means of a common loop mechanism in the form of toothed belt wheels  40 ,  41  and a toothed belt  47 . The different speeds of rotation of the lower rollers II and III are achieved by means of change-gearwheels at the drive trunnions Iia, IIIa provided with different numbers of teeth. The toothed belt  47  runs in direction B (that is to say contrary to the work direction) onto the control drive, which is in the form of a servo motor  19 . The lower roller I is driven from the left-hand side of the machine by means of a loop mechanism in the form of toothed belt wheels and a toothed belt  47 ′. For that purpose, the toothed belt  47 ′ runs on the left-hand side from the toothed belt disc  40 ′ at the lower roller I in direction G onto the servo motor  20 . 
   In operation, that is to say when the fibre slivers are running in direction A, the toothed belt  47  moves in direction G. Starting from the toothed belt wheel  47  arranged on the drive motor  19 , the toothed belt  47  runs successively over a toothed belt wheel  45 , a smooth guide pulley wheel  46 , the toothed belt wheel  40  (roller-driving pulley wheel for the lower roller III), the toothed belt wheel  41  (roller-driving pulley wheel for the lower roller II), a smooth guide pulley wheel  42  and a toothed belt wheel  43 . As shown in  FIG. 3   c , the belt  47  has a toothed side  47   a  and a smooth side  47   b . By means of its teeth, the toothed belt  47 , by means of teeth  47   a  ( FIG. 3   c ), is in positive engagement with the toothed belt wheels  40 ,  41 ,  43 ,  44 , and  45 . The smooth side  47   b  (reverse) ( FIG. 3   c ) of the toothed belt  47 , opposite the toothed side, is in contact and in engagement with the smooth guide pulley wheels  46  and  42 . The toothed belt  47  loops around all the pulley wheels  40  to  46 . In operation (when the fibre slivers are running in direction A during drafting), the toothed belt wheels  40 ,  41 ,  43 ,  44 , and  45  rotate clockwise and the guide pulley wheels  42  and  46  rotate anti-clockwise. 
   The toothed belt wheels  40 ,  41  are associated with the mountings  34   a  and  33   a , respectively, whereas the guide pulley wheels  42 ,  46  are attached to the sliders  35   a  and  36   a , respectively, in a manner allowing rotation. Because of the rigid attachment between the mounting  34   a  and the slider  36   a  and between the mounting  33   a  and the slider  35   a  (for example, by means of bolts), there are associated with the lower rollers II and III, in each case, one toothed belt wheel  40  and  41 , respectively, and one guide pulley wheel  46  and  42 , respectively. The toothed belt  47  runs around the pulley wheels  40 ,  46 , on the one hand, and around the pulley wheels  41 ,  42 , on the other hand, in a mirror-reflection arrangement (see  FIG. 3   b ). 
   The zone between the pairs of rollers  13 /II and  14 /III is designated VV (preliminary drafting) and the zone between the pairs of rollers  12 /I and  13 /II is designated HV (main drafting) (see  FIG. 4   a ). When, in accordance with  FIG. 3   a , the nip line spacing between the roller pairs  14 /III and  13 /II is to be increased, at least one pair of rollers must be moved away from the respective other pair of rollers. For that purpose the slider  35   a  may be displaced towards the right, which may be accomplished in two ways:
         a) The slider  35   a  is unlocked. A pulley wheel, for example the toothed belt wheel  44 , is stopped so that there is no possibility of rotation. Stopping may be accomplished, for example, by mechanical or electromagnetic means. As a result the toothed belt  47  is stationary and cannot be moved. The toothed belt wheel  41  is then rotated anti-clockwise, for example manually using a crank or the like, whereupon the guide pulley wheel  42  likewise rotates, clockwise, as a matter of necessity. In the process, the rotary movement of the toothed belt wheel  41  is converted into a longitudinal movement of the slider  35   a  in direction C, the toothed belt wheel  41  and the guide pulley wheel  42  winding along opposite sides of the stationary toothed belt  47 , thereby “shortening”, as it were, the toothed belt  47  at one pulley wheel and “lengthening” it at the other pulley wheel. The length of belt required during that “winding along” at the toothed belt wheel  41  is made available at the guide pulley wheel  42 . The lower roller II is thereby displaced in direction C by means of the slider  35   a  and the mounting  33   a.      b) The slider  35   a  is unlocked. The toothed belt wheel  41  is stopped so that there is no possibility of rotation. As a result the guide pulley wheel  42  is also stopped of necessity. Then, clockwise rotation is brought about by means of the drive motor  19 . The toothed belt  47  moves in direction G, likewise “shortening” the belt  47  at one pulley wheel and “lengthening” it at the other pulley wheel. The length of belt actually required between the toothed belt wheels  40  and  41  is made available between the toothed belt wheels  43  and pulley wheel  42 . The rotary movement of the toothed belt wheel  44  and the movement of the toothed belt  47  is thereby converted into a longitudinal movement of the slider  35   a  in direction C. The lower roller II, mounted in the mounting  33   a  (which is rigidly connected to the slider  35   a ), is likewise moved in direction C as a result.       

   In practice, it is often the case that, in accordance with  FIGS. 4   a  to  4   d , first the preliminary draft zone VV is modified and then the main draft zone HV. In the case of shortening of the draft zones VV and HV, the slider  36   a  is displaced in the direction of the arrow E from the position according to  FIG. 4   a  into the position according to  FIG. 4   b . As a result, the nip line spacing in the preliminary draft zone VV is reduced from “a” to “a′”. Then, in accordance with  FIG. 4   c , the sliders  36   a  and  35   a  are rigidly connected to one another by means of a bridge  50 . Finally, the rigidly coupled sliders  36   a  and  35   a  are moved, in accordance with  FIG. 4   d , in the direction of the arrows E and C, from the position shown in  FIG. 4   c  into the position shown in  FIG. 4   d . As a result, the nip line spacing in the main draft zone HV is shortened from “b” to “b′”.—A corresponding procedure is used in the case of lengthening the preliminary and main draft zones, that is to say the coupled sliders  35   a  and  36   a  are displaced in the direction of the arrows F and D (see  FIG. 2 ), as a result of which the main draft zone HV is lengthened. Then, the sliders  35   a  and  36   a  are uncoupled from the bridge  50 . Finally, the slider  36   a  is moved in the direction of the arrow F (see  FIG. 2 ), as a result of which the preliminary draft zone VV is lengthened. 
   With regard to the fibre slivers  5  in the drawing mechanism  2 , it should be noted that, in the case of shortening of the draft zones VV and HV, a small amount of stretching, in direction B, of the fibre slivers  5   IV  upstream of the pair of rollers  14 /III can occur on displacement in accordance with  FIGS. 4   a ,  4   b , but because of the length (about 1.5 m) of the spacing between the transport rollers  7 ,  8  and the pair of rollers  14 /III this is without significance. In the case of shortening, a sagging loop does not form in the preliminary draft zone VV because in the case of displacement referring to the pairs of rollers  14 /III and  13 /II either one or both pairs of rollers are rotatable because the drives to both pairs of rollers are coupled by way of the toothed belt  47 . In contrast, in the case of shortening of the main draft zone HV, a sagging loop is formed in fibre slivers  5 ″, which is drawn out or drawn straight by rotation of the pair of rollers  12 /I in the work direction A by means of the main motor  20 .—In the case of lengthening of the draft zones VV and HV, the pair of rollers  12 /I is, in a first step, rotated backwards in direction B, whereupon a sagging loop is intentionally formed in the fibre slivers  5 ″. When the main draft zone HV is subsequently lengthened by displacement of the coupled sliders  35   a  and  36   a  in direction D and F, the artificially formed loop is, in the process, once again drawn out or drawn straight. Finally, after uncoupling of the bridge  50 , the slider  36   a  is displaced in direction F. As a result of the above-mentioned coupling of the drives to the intake and middle lower roller pairs by means of the toothed belt  47 , the length of the fibre slivers  5 ′ in the preliminary draft zone VV remains unaffected. Possible slight longitudinal compression of the fibre slivers  5   IV  upstream of the pair of rollers  14 /III is, in respect of the drafting and the constitution of the fibre slivers  5   IV  without significance. 
     FIGS. 5   a ,  5   b  show a suitable construction for bringing about the displacement of the sliders  36   a  and  35   a . The nip line spacing in the preliminary draft zone VV is lengthened from “a” ( FIG. 5   a ) to “a″” ( FIG. 5   b ). The sliders  36   a  and  35   a  are displaced one after the other according to the arrows E and C, respectively. Displacement is accomplished by stopping the toothed belt wheel  40  or fixing it with a holding brake or the like and then actuating the drive motor  19 , whereupon the toothed belt  47  moves. In continuation thereof, the sliders  36   a  and  35   a  are displaced in accordance with  FIGS. 4   a ,  4   b  and, subsequently,  FIGS. 4   c ,  4   d.    
   In  FIG. 6   a  there is shown an electromagnetic holding brake for braking the toothed belt wheel  44 . The brake has a rod-shaped iron core  53  surrounded by a plunger coil  54 . Mounted on one end face of the iron core  53  is a brake shoe  55 , for example made of plastics material or the like. The iron core  53  is displaceable in the direction of the arrows M, N. When current flows through the plunger coil  54 , the iron core  53  is moved in direction M, in accordance with  FIG. 6   b , so that the brake shoe  55  is pressed against the smooth cylindrical surface of the shaft  44   a  of the toothed belt wheel  44 . As a result, the toothed belt wheel  44  is fixed (stopped) so that it cannot rotate, for as long as voltage is applied to the plunger coil  54 . 
   In  FIG. 7  there is shown a stopping device for slider  36   a  and corresponding lower roller III. A pneumatic cylinder  60  having a piston rod  61  is attached to the slider  36   a . When subjected to pressure from the pneumatic cylinder  60 , the piston rod  61  is moved out in the direction of the arrow P and comes to rest, with a high degree of contact pressure, against the machine frame  61 . The slider  36   a  is fixed (stopped) so that it cannot be displaced with respect to the bar  37   a , for as long as compressed air is applied to the pneumatic cylinder  60 . Lower roller II may be provided with an analogous arrangement. 
   In accordance with  FIG. 8 , there is provided, as the bridge  50  between the sliders  35   a  and  36   a , a flat piece of metal (plate), which is fastened in the region of one of its ends  50   a  to the slider  36   a , for example using bolts. In its region  50   b  facing the slider  35   a , the flat piece of metal has an elongate hole  50   c , through which a bolt  62  can engage in a threaded hole (not shown) in the slider  35   a . By means of this bridge  50 , the sliders  35   a  and  36   a  can be rigidly connected to one another, releasably, at different spacings with respect to one another. 
   In accordance with  FIG. 9 , in contrast to  FIG. 1 , each lower roller I, II and III is driven by its own drive motor  20 ,  52  and  19 , respectively, as shown, for example, in DE-OS 38 01 880. The motor  20  drives the toothed belt wheel  55  of the lower roller I by way of the toothed belt  56 ; the motor  52  drives the toothed belt wheel  41  of the lower roller II by way of the toothed belt  57 ; and the motor  19  drives the toothed belt wheel  40  of the lower roller III by way of the toothed belt  47 . Attached to the slider  36   a , in addition to the smooth guide pulley wheel  46 , is a further smooth guide pulley wheel  51 . The endless toothed belt  47  loops around, in succession, the pulley wheels  44 ,  46 ,  40 ,  51  and  43 . The toothed belt wheels  44 ,  40  and  43  are in engagement with the teeth of the toothed belt  47 , whereas the smooth guide pulley wheels  46  and  51  are in engagement with the smooth reverse side of the toothed belt  47 . The sliders  35   a  and  36   a  are rigidly connected to one another, releasably, by means of the bridge  50 . When they are not connected by the bridge  50 , the sliders  35   a  and  36   a  are individually displaceable and when they are connected by the bridge  50  they are jointly displaceable. 
   In accordance with  FIG. 10 , the drive motor  19  for lower rollers II and III is in communication with the electronic control and regulation device  26 . Adjustment values for modification of the draft zones VV and HV (that is to say the extents of the drawing zones) either can be entered manually by way of the input device  29  or can be called up from a memory  31  for particular categories of fibre material. 
   Adjustment of the nip line spacing in the preliminary draft zone VV and/or the main draft zone HV can be carried out with the fibre slivers  5  inserted. 
   Displacement can be carried out with the upper rollers  11  to  14  in the loaded state.  FIGS. 1 and 10  show inserted fibre slivers  5  and loaded upper rollers  11  to  14 . With the fibre slivers inserted and the upper rollers  11  to  14  loaded, the sliders  35   a ,  36   a  or mountings of at least one lower roller II, III are unlocked, the sliders or mountings are set to the desired nip line spacing a, a′; b, b′ by means of a displacement device, for example in accordance with  FIGS. 3   a ,  3   b ;  5   a ,  5   b  and then the sliders  35   a ,  36   a  or mountings are locked again (for example in accordance with  FIG. 7 ). 
   Displacement can also be carried out with the upper rollers  11  to  14  lifted off. The upper rollers  11  to  14  may be lifted off completely from the lower rollers I to III in the manner shown in DE-OS 197 04 815, the upper roller  14  being swung out on a portal  58  about a pivot mounting  59 . However, it may also be sufficient for the upper rollers  11  to  14  to be unloaded and to be lifted off from the lower rollers I to III only to a slight degree such that the fibre slivers  5  are not caught by the pairs of rollers during displacement of the draft zones VV and HV but can slide through the roller nip without being adversely affected. 
   The invention has been illustrated using the example of the adjustment of the nip line spacings of a drawing mechanism of a draw frame. It likewise encompasses the adjustment of drawing mechanisms of other machines, for example carding machines, combing machines, fly frames and ring spinning frames.