Patent Publication Number: US-2006016504-A1

Title: Warp let-off device for loom

Description:
BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to a warp let-off device for a loom, and more specifically relates to a brake unit for a warp beam for feeding warp.  
      2. Description of the Related Art  
      In a typical warp let-off device for a loom, a predetermined tension is applied to a warp sheet, and a warp beam for feeding warp is rotated in an unwinding direction as the tension increases, so that the warp is fed. For example, Japanese Unexamined Patent Application Publication No. 2003-193355 discloses a warp let-off device which provides a braking force against the warp tension by a non-return function of a worm gear mechanism while rotating a worm included in the worm gear mechanism with a dedicated motor to drive a worm wheel. Accordingly, a warp beam rotates together with the worm wheel, and warp is fed at a predetermined tension.  
      In a weaving process of the loom, the warp tension greatly varies because of shedding motion and beating motion, and vibration occurs in every pick (every cycle). The tension variation is transmitted to the warp beam around which the warp is wound, and serves as a vibrating force which causes a so-called rotational vibration of the warp beam. In the rotational vibration, slight rotation of the warp beam in the unwinding direction and that in the winding direction (reverse rotation) are repeated because of torsional deformation of a shaft or backlash of a gear.  
      The warp length between a cloth fell and the warp beam varies due to the rotational vibration, and accordingly weaving bars (uneven density) occur. As the radius of the warp beam increases, the radius of the vibrating force applied to the warp beam increases, and therefore the rotational vibration of the warp beam easily occurs. In addition, as the weight of the warp beam increases, the energy of the rotational vibration increases, and therefore large weaving bars easily occur. In particular, in a weaving process of high-density fabric, there is a problem that large tension vibration occurs due to looseness in the beating motion.  
     SUMMARY OF THE INVENTION  
      Accordingly, an object of the present invention is to reduce variation in warp tension in a warp let-off device for a loom, and to thereby prevent weaving bars. Another object of the present invention is to reduce the required capacity of a drive source and a transmission for driving a rotating shaft of a warp beam, and to increase range of use conditions, such as allowable warp tension and pick density, so that versatility of the loom is increased.  
      In order to achieve the above-described objects, according to the present invention, a warp let-off device for a loom includes a drive unit which transmits rotation of a drive source to a warp beam with a transmission and rotates the warp beam in an unwinding direction while applying a braking force against a warp tension to the warp beam, and a brake unit which applies a braking force to the warp beam in addition to the braking force applied by the drive unit.  
      According to the present invention, in the warp let-off device for the loom, the brake unit applies the braking force to the warp beam separately from the braking force applied by the drive unit. Therefore, during the weaving process, the rotational vibration of the warp beam can be reduced by applying the braking force to the warp beam. Accordingly, weaving bars are prevented from being caused due to the rotational vibration of the warp beam.  
      According to the present invention, the brake unit may be provided with a braking-force reducing means for reducing the braking force applied by the brake unit when the warp beam rotates in the winding direction.  
      As described below in “Description of Let-Off Drive Using Worm Gear Mechanism”, torque required of the drive unit during the rotation in the winding direction is higher than that required during the rotation in the unwinding direction. Accordingly, since the braking force of the brake unit is reduced by the braking-force reducing means during the rotation of the warp beam in the winding direction, load of the drive unit during the rotation of the warp beam in the winding direction is reduced. Therefore, even when the capacity of the drive unit is constant, the range of use conditions, such as allowable warp tension and pick density, is increased, and accordingly the versatility of the loom is also increased. In addition, instead of increasing the range of use conditions, the capacity of the drive unit may be reduced. In such a case, the cost of the device can be reduced.  
      In order to set the braking force of the brake unit within the range of 0 to a predetermined value during the rotation of the warp beam in the winding direction, the braking-force reducing means may include a one-way rotation transmission interposed between the brake unit and a rotating shaft of the warp beam. In this case, a commercial one-way clutch or a coil spring can be used, and the above-described advantages can be obtained with a simple structure.  
      The braking-force reducing means may also include an actuator which reduces a contact pressure between a circular brake plate and brake discs, the circular brake plate being integrated with a brake shaft. In this case, the contact pressure can be easily changed, and adequate contact pressure adjustment can be performed.  
      The warp let-off device may be an electrical let-off device in which the drive unit includes a dedicated motor separated from a loom motor. Alternatively, the warp let-off device may be a mechanical let-off device in which the drive unit includes a transmission for transmitting the rotation of a loom motor after changing the rotational speed thereof. The structure for applying the braking force against the warp tension in the drive unit may include a dedicated motor, a drive shaft, and a non-return function (self-lock worm gear) of a worm gear which meshes with a warp beam gear (worm wheel). In this case, the braking force against the warp tension is applied by the worm gear. Alternatively, the structure for applying the braking force against the warp tension may include a dedicated motor having a braking function, a driving shaft, a drive gear which meshes with the warp beam. In this case, a brake signal is output to the dedicated motor from a control unit and the braking force against the warp tension is applied by the dedicated motor. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a side view of a warp let-off device for a loom;  
       FIG. 2  is a sectional view of the main part of a warp let-off device for a loom according to a first embodiment;  
       FIG. 3  is a sectional view of the main part of a warp let-off device for a loom according to a second embodiment;  
       FIG. 4  is a schematic diagram showing the main part of a warp let-off device for a loom according to a third embodiment;  
       FIG. 5  is a schematic diagram showing the main part of a warp let-off device for a loom according to a fourth embodiment;  
       FIG. 6  is a schematic diagram showing the main part of a warp let-off device for a loom according to a fifth embodiment;  
       FIG. 7  is a diagram showing the main part of the warp let-off device shown in  FIG. 6  viewed from the arrow A; and  
       FIG. 8  is a schematic diagram showing the main part (circular brake plate) of a warp let-off device for a loom according to a sixth embodiment. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       FIG. 1  shows the basic structure of a warp let-off device  1  for a loom according to embodiments of the present invention. With reference to  FIG. 1 , the warp let-off device  1  includes a drive unit  3  for driving a warp beam  2 . The drive unit  3  includes a dedicated motor  32  which functions as a drive source  4  and a transmission  5  which transmits the rotation of the dedicated motor  32  to the warp beam  2 . In this example, the transmission  5  includes a worm  34  which is fixed to a drive shaft  33  integrated with an output shaft of the dedicated motor  32 , a gear  50  which meshes with the worm  34 , a gear  51  which is integrated with the gear  50  and which is rotatably supported by a rotating shaft  52  together with the gear  50 , and a warp beam gear  35  which meshes with the gear  51 . The warp beam gear  35  is attached to a rotating shaft  21  which supports the warp beam  2 , and rotates together with the warp beam  2 .  
      In this example, the gear  50  is a worm wheel, and the worm  34  and the gear  50  form a worm gear mechanism with a large reduction ratio which has a non-return function (self-lock function). Accordingly, although the rotation of the dedicated motor  32  is transmitted to the warp beam gear  35  after the rotational speed is reduced, reverse operation cannot be performed. More specifically, rotational force of the warp beam gear  35  cannot be transmitted to the worm  34 .  
      The drive unit  3  rotates the warp beam  2  while applying a braking force to the warp beam  2  against a warp tension T applied to warp  9  which is fed toward a cloth fell  8  from the warp beam  2 . The braking force against the warp tension T is obtained by the non-return function (self-lock function) of the worm gear mechanism (the worm  34  and the gear (worm wheel)  50  meshing therewith). The warp  9  is fed from the warp beam  2  in the form of a sheet, is guided by a guide roller  6  and a tension roller  7 , and is conveyed toward the cloth fell  8 . In general, the tension roller  7  is moveably supported by a tension supplying means (not shown) to apply a predetermined warp tension T to the warp  9 .  
      The warp let-off device  1  for the loom according to the present invention is characterized by including a brake unit  10  for applying a braking force to the warp beam  2  separately from the braking force applied by the drive unit  3 . Examples of the brake unit  10  will be explained in the embodiments below. A one-way rotation transmission  13  is used in embodiments other than a second embodiment, and an actuator  31  for reducing a contact pressure against a brake disc  25  is used in the second embodiment.  
     First Embodiment (FIG.  2 )  
      In the first embodiment, as shown in  FIG. 2 , a brake unit  10  includes a one-way rotation transmission  13  disposed between a brake gear  11  and a brake shaft  14  as a braking-force reducing means  12 . With reference to  FIG. 2 , the brake unit  10  includes the brake gear  11  which meshes with the warp beam gear  35 , a circular brake plate  23  which functions as a braking means for applying a braking force to the brake gear  11 , a pair of brake discs  25 , and the one-way rotation transmission  13  which applies a braking force to the rotation of the warp beam  2  in an unwinding direction (hereafter called forward rotation) but applies no braking force to the rotation of in the winding direction (hereafter called reverse rotation). The one-way rotation transmission  13  is, for example, a one-way clutch, and the warp beam gear  35  is concentrically attached to the rotating shaft  21  which supports the warp beam  2 .  
      The brake gear  11  and the pair of brake discs  25  are all supported by the brake shaft  14 . The brake shaft  14  is supported by a ball bearing  15  and a bearing housing  16  in such a manner that the brake shaft  14  can rotate with respect to a frame  17 . The brake gear  11  is attached to an end portion of the brake shaft  14  with a needle bearing  18 , the one-way rotation transmission (one-way clutch)  13 , a sleeve  19 , and a key  22  of the sleeve  19  interposed therebetween at a position such that the brake gear  11  meshes with the warp beam gear  35 .  
      The brake unit  10  includes a plurality of columns  20  attached to the bearing housing  16 , a circular brake plate  23  which is non-rotatably supported by the columns  20 , the pair of brake discs  25  which clamp the circular brake plate  23 , and a brake spring  27 . Rotation of the pair of brake discs  25  with respect to the brake shaft  14  is locked by a spline or the like, and the brake discs  25  are supported such that they can move along the shaft. Brake linings  24  provided at the periphery of the brake discs  25  come into frictional contact with the surfaces of the circular brake plate  23 , and accordingly the braking force is applied to the brake shaft  14 .  
      The brake spring  27  is shaped like a coil provided around the brake shaft  14 . One end of the brake spring  27  is in contact with one of the brake discs  25  which is adjacent to the brake spring  27 , and the other end is in contact with a spring receiver  28  fitted around the brake shaft  14 . The spring receiver  28  is in contact with an adjustment nut  26  engaged with an external screw  29  on the brake shaft  14 , and compresses the brake spring  27  to generate a frictional force corresponding to the required braking force between the circular brake plate  23  and the pair of brake linings  24 . The brake disc  25  adjacent to the one-way rotation transmission (one-way clutch)  13  is pushed toward the ball bearing  15  with a receiving ring  30  disposed therebetween. Accordingly, the brake unit  10  generates the required braking force from the frictional force between the circular brake plate  23  and the pair of brake linings  24 .  
      In the above-described structure, the needle bearing  18  allows the rotation of the brake gear  11  with respect to the brake shaft  14 . However, because of the one-way rotation transmission function of the one-way rotation transmission (one-way clutch)  13 , although the brake gear  11  allows the rotation of the warp beam gear  35  in the winding direction (reverse rotation), the brake gear  11  rotates together with the brake shaft  14  wile the warp beam gear  35  rotates in the unwinding direction (forward rotation), and thereby applies the braking force of the brake unit  10  against the warp tension T. Thus, the one-way rotation transmission  13  is interposed between the rotating shaft  21  of the warp beam  2  and the braking-force reducing means  12  for applying the braking force of the brake unit  10  to the rotation of the warp beam  2  in the unwinding direction, but applies no braking force of the brake unit  10  to the rotation in the winding direction, that is, in the direction against the warp tension T.  
      As a result, when the warp tension T greatly varies because of shedding motion and beating motion during a weaving process and the tension variation serves as a large vibrating force applied to the warp beam  2 , a braking force is applied to the rotation of the warp beam  2  in the unwinding direction. The braking force applied at this time is the sum of the braking force obtained by the non-return function (self-lock function) of the worm gear mechanism (the worm  34  and the warp beam gear (worm wheel)  35  meshing therewith) and the braking force of the brake unit  10 . Accordingly, the rotational vibration of the warp beam  2  can be prevented and the weaving bars do not occur.  
      In addition, when the warp beam  2  rotates in the reverse direction (winding direction), the one-way rotation transmission (one-way clutch)  13 , which functions as the braking-force reducing means  12 , allows the reverse rotation of the warp beam  2  (rotation in the winding direction). Accordingly, the braking force of the brake unit  10  is not applied to the reverse rotation of the warp beam  2  (rotation in the winding direction), and is reduced to substantially 0. Therefore, although the brake unit  10  is provided, the load of the drive unit  3  does not increase in the reverse rotation of the warp beam  2 , and is lower than the load in the rotation in the unwinding direction.  
      As a result, as described below in “Description of Let-Off Drive Using Worm Gear Mechanism”, even when the capacity of the drive unit  3  (braking force applied by the drive unit  3 ) is constant, the torque required of the drive unit  3  can be reduced by installing the one-way rotation transmission (one-way clutch)  13  in the brake unit  10  as the braking-force reducing means  12 . Thus, the range of use conditions, such as allowable warp tension and pick density, is increased, and accordingly the versatility of the loom is also increased. In addition, instead of increasing the range of use conditions, the capacity of the drive unit  3  (driving force) may be reduced. In such a case, the cost of the device can be reduced.  
      In the example shown in the figure, the one-way rotation transmission (one-way clutch)  13 , which functions as the braking-force reducing means  12 , is interposed between the brake unit  10  and the rotating shaft  21 . However, the one-way rotation transmission (one-way clutch)  13  may also be installed between the brake discs  25  and the brake shaft  14 . In addition, the one-way rotation transmission  13  may also be an electromagnetic clutch instead of one-way clutch. In such a case, the electromagnetic clutch may be excited manually or in association with the control operation of the loom when the braking force is required, and the excitation may be cancelled to set a free state when the braking force is not necessary, such as when the reverse rotation is performed.  
     Second Embodiment (FIG.  3 )  
      As shown in  FIG. 3 , in the second embodiment, a braking-force reducing means  12  of a brake unit  10  includes an actuator  31 , for example, an air cylinder, for reducing a contact pressure between a circular brake plate  23  integrated with a brake shaft  14  and a pair of brake discs  25 . The pair of brake discs  25  are brought into contact with or moved away from the circular brake plate  23  depending on whether the warp beam  2  is rotated in the forward or reverse rotation. A brake gear  11  is directly fixed to the brake shaft  14  with a key  22 .  
      With reference to  FIG. 3 , the actuator  31  is attached to an attachment plate  36  fixed to columns  20  at one end thereof. The amount of projection (projecting force) of an actuator rod  37  of the actuator  31  is controlled with a pressure source  38 , a valve  39 , and a control unit  40 . The actuator rod  37  extends through the attachment plate  36 , and applies a predetermined projecting force to an end portion of a brake spring  27  via a spring receiver  28  attached at the end of the actuator rod  37 . Accordingly, when the brake shaft  14  rotates, a predetermined frictional force (braking force) is applied between the circular brake plate  23  and the pair of brake discs  25 .  
      When the warp beam  2  rotates in the unwinding direction (forward rotation), the control unit  40  controls the valve  39  to move the actuator rod  37  in the projecting direction, so that the pair of brake discs  25  receive a predetermined contact pressure and the braking force is applied to the warp beam  2 . When the warp beam  2  rotates in the winding direction (reverse rotation), the control unit  40  controls the valve  39  to retract the actuator rod  37  so that the pair of brake discs  25  move away from the circular brake plate  23  by a distance small enough not to cause separation of the brake discs  25  from the circular brake plate  23 . Accordingly, the braking force between the circular brake plate  23  and the pair of brake discs  25  is reduced or substantially eliminated.  
      The actuator  31  may be a hydraulic air cylinder or an electromagnetic solenoid instead of an air cylinder. Alternatively, the actuator  31  may be a unit obtained by combining an electric or hydraulic motor with a mechanism, such as a screw mechanism, for converting rotation to linear motion.  
     Third Embodiment (FIG.  4 )  
      As shown in  FIG. 4 , in the third embodiment, the brake unit  10  and a braking-force reducing means  12  are installed at a position where a rotating shaft  21  for supporting a warp beam  2  is disposed. In  FIG. 4 , a one-way rotation transmission (one-way clutch)  13 , which functions as the braking-force reducing means  12 , is fitted around the rotating shaft  21 . The one-way rotation transmission  13  (one-way clutch) slips while the rotating shaft  21  rotates in the reverse direction. In addition, a sliding bearing  41 , which functions as brake unit  10 , is disposed between the one-way rotation transmission (one-way clutch)  13  and a loom frame  42 . A braking force is generated by friction (sliding resistance) between an inner ring  43  and an outer ring  44  of the sliding bearing  41 . In this case, the frictional force increases, that is, the braking force increases as the diameter of the rotating shaft  21  increases.  
      When the warp beam  2  rotates in the unwinding direction (forward rotation), the rotating shaft  21  and the inner ring  43  of the sliding bearing  41  rotate together, and the braking force is applied to the warp beam  2  by the sliding bearing  41 . When the warp beam  2  rotates in the winding direction (reverse rotation), only the rotating shaft  21  rotates and no braking force is applied to the warp beam  2 .  
     Fourth Embodiment (FIG.  5 )  
      The structure of the fourth embodiment is similar to that of the first embodiment ( FIG. 2 ) except a torsional coil spring  45  is provided as the one-way rotation transmission  13  instead of a one-way clutch. As shown in  FIG. 5 , the torsional coil spring  45  is disposed around a brake shaft  14  between a pair of brake discs  25  and a brake gear  11 . The torsional coil spring  45  is wound around the peripheral surface of the brake shaft  14 , and is fixed to the brake shaft  14  at one end and to a boss portion of the brake gear  11  at the other end.  
      When the warp beam  2  rotates in the unwinding direction, the brake gear  11  rotates in a direction to twist the torsional coil spring  45 . Accordingly, after the torsional coil spring  45  is twisted by a predetermined amount or to the limit, the rotation of the brake gear  11  is transmitted to the brake shaft  14 . As a result, when the warp beam  2  rotates in the unwinding direction, the braking force of the brake unit  10  is applied to the warp beam  2 .  
      When the warp beam  2  rotates in the winding direction, the brake gear  11  rotates in a direction to untwist (release) the torsional coil spring  45 , and therefore the torsional coil spring  45  rotates while applying an urging force to the brake gear  11 . At this time, the rotation of the brake gear  11  is not transmitted to the brake discs  25 , so that the braking force of the brake unit  10  is not applied to the warp beam  2 . The amount of reverse rotation is limited to an amount such that the braking force is not exceeded (such that the brake discs  25  do not rotate during the reverse rotation). Alternatively, the spring constant of the torsional coil spring  45  is set such that the braking force is not exceeded (such that the amount of twist is sufficient relative to the amount of reverse rotation).  
     Fifth Embodiment (FIGS.  6  and  7 )  
      The structure of the fifth embodiment is similar to that of the fourth embodiment ( FIG. 5 ) except a helical compression spring  46 , a brake connecting member  47 , and a gear connecting member  48  are used instead of the torsional coil spring  45 , as shown in  FIGS. 6 and 7 . The brake connecting member  47  is fixed to a brake shaft  14 , and the gear connecting member  48  is fixed to a brake gear  11 . The brake connecting member  47  and the gear connecting member  48  are connected to each other with the helical compression spring  46 .  
      When the warp beam  2  rotates forward, the brake gear  11  rotates in a direction to compress the helical compression spring  46 . Accordingly, after the helical compression spring  46  is compressed to the limit or a compressing force of the helical compression spring  46  reaches a value corresponding to a braking force of a brake unit  10 , the rotation of the brake gear  11  is transmitted to the brake discs  25 . Accordingly, the braking force of the brake unit  10  is applied to the warp beam  2 .  
      When the warp beam  2  rotates in the reverse direction, the brake gear  11  rotates in a direction to cancel the compressed state of the helical compression spring  46 . Accordingly, the brake gear  11  rotates while receiving an urging force from the helical compression spring  46 . At this time, the rotation of the brake gear  11  is not transmitted to the brake discs  25 , so that the braking force of the brake unit  10  is not applied to the brake gear  11 . The amount of reverse rotation is limited to an amount such that the braking force is not exceeded (such that the brake discs  25  do not rotate during the reverse rotation). Alternatively, the spring constant of the helical compression spring  46  is set such that the braking force is not exceeded (such that the amount of twist is sufficient relative to the amount of reverse rotation).  
     Sixth Embodiment (FIG.  8 )  
      The structure of the sixth embodiment is similar to that of the first embodiment ( FIG. 2 ) except a circular brake plate  23  disposed between the pair of brake discs  25  is set to be rotatable within a predetermined range, as shown in  FIG. 8 , instead of using the one-way clutch as the one-way rotation transmission  13 . With reference to  FIG. 8 , the circular brake plate  23  is fitted to columns  20  with long attachment holes  49  extending along the circumferential direction, and can rotate along with the pair of brake discs  25  within the range corresponding to the attachment holes  49 .  
      When the warp beam  2  rotates forward, the pair of brake discs  25  and the circular brake plate  23  disposed between the brake discs  25  rotate together as the brake shaft  14  rotates. However, when the inner walls of the attachment holes (long holes)  49  formed in the circular brake plate  23  come into contact with the columns  20  at one end thereof, the circular brake plate  23  stops. Therefore, the pair of brake discs  25  rotate while exerting a frictional force, and the braking force of the brake unit  10  is applied to the warp beam  2 .  
      When the warp beam  2  rotates in the reverse direction, the pair of brake discs  25  and the circular brake plate  23  disposed between the brake discs  25  rotate together as the brake shaft  14  rotates. Then, when the inner walls of the attachment holes (long holes)  49  formed in the circular brake plate  23  come into contact with the columns  20  at the other end thereof, the circular brake plate  23  stops. Thus, the braking force of the brake unit  10  is not applied until the inner walls of the attachment holes (long holes)  49  in the circular brake plate  23  come into contact with the columns  20 . The amount of reverse rotation of the warp beam  2  is limited such that the inner walls of the attachment holes  49  in the circular brake plate  23  do not come into contact with the columns  20  (such that the braking force is not applied in the reverse rotation). The sixth embodiment is applied to a mechanical drive unit, for example, a drive unit including a transmission which changes the reduction ratio depending on the warp tension T. When the drive unit  3  is electrical, reverse rotation is repeatedly performed by the dedicated motor  32 . Therefore, the amount of reverse rotation is limited by the drive unit  3 , or the size(range) of the attachment holes  49  is increased.  
      Description of Let-Off Drive Using Worm Gear Mechanism  
      As described above, rotational driving of the warp beam  2  in the unwinding direction (forward rotation) is regarded as braking/loosening driving for applying a braking force against the warp tension T, and the direction of load is the same as the rotating direction. Accordingly, worm gear mechanisms are often used in the let-off device. The worm gear mechanism (the worm  34  and the warp beam gear (worm wheel)  35 ) constantly applies a braking force to the warp beam  2  in order to prevent the warp beam  2  from slipping while the warp  9  is fed in the let-off motion. A torque of the drive unit  3  required during the forward rotation of the warp beam  2  is determined depending on the braking force of the worm gear mechanism (the worm  34  and the warp beam gear (worm wheel)  35 ), that is, depending on the warp tension T.  
      In the case in which the worm gear mechanism is used for the let-off motion, the relationship between the motor torque TF against the warp tension T required of the dedicated motor  32  connected to the worm gear mechanism in the forward rotation and the motor torque TR required in the reverse rotation is expressed as TF&lt;TR. In addition, the ratio between the required motor torques TR/TF is about 3 or 4 (when the angle of lead is 10°). This means that the motor torque TR required in the reverse rotation is three or more times the motor torque TF required in the forward rotation even when the brake unit  10  is not provided. Therefore, there is a demand to reduce the motor torque TR required in the reverse rotation.  
      In the known structure, the reduction ratio of the dedicated motor  32  is increased to obtain a large motor torque TR in the reverse rotation. However, when the reduction ratio is increased, the maximum rotational speed of the warp beam  2  is reduced and low-density weaving cannot be performed. If settings are made such that low-density weaving can be performed, allowable warp tension T is reduced. In addition, when the outer diameter of the warp beam  2  is large or the warp tension T is high, the torque required of the drive unit  3  of the warp beam  2  is increased. In the case in which the above-described brake unit  10  is installed in the warp let-off device  1  for the loom using the worm gear mechanism, when the warp beam  2  rotates forward, the required warm-gear braking force is reduced by the amount corresponding to the braking force of the brake unit  10  (hereafter called a warp-beam braking force). Therefore, the torque required of the drive unit  3  in the forward rotation of the warp beam  2  is calculated as (warp tension T)−(warp-beam braking force B).  
      However, when the warp beam  2  rotates in the reverse direction, opposite to the forward rotation, the warp beam  2  must be rotated with a torque high enough to overpower the sum of the warp tension T and the warp-beam braking force B. Therefore, the torque required in the reverse rotation of the warp beam  2  is calculated as (warp tension T)+(warp-beam braking force B). Accordingly, the capacity of the drive unit  3  is determined on the basis of the torque required in the reverse rotation of the warp beam  2 . Therefore, a motor with high capacity relative to the torque required in the forward rotation of the warp beam  2  must be used as the dedicated motor  32 . In addition, the generated torque cannot be sufficiently exploited, and the range of use conditions, such as the allowable warp tension and the range of pick density, that is, the number of picks per inch (the rotational speed of the warp beam must be increased as the pick density is reduced), etc., is limited.  
      As described above, the drive unit  3  according to the present invention is not limited to electrical drive units, and mechanical drive units may also be used. The braking force of the brake unit  10  may be applied over the entire period of the let-off motion. Alternatively, the braking force of the brake unit  10  may be applied only under weaving conditions which tend to cause weaving bars or vibration of the warp beam  2  in a loom which changes the weaving conditions in the weaving process. In addition, the transmission  5  may be controlled by the brake unit  10 . In detail, the braking force of the brake unit  10  connected to the one-way clutch may be applied to the drive shaft  33 . Such a structure is suitable for the case in which a dedicated motor having a braking function is used instead of the worm gear mechanism.