Method and apparatus for limiting stresses in weft yarn advancing towards a weft insertion mechanism
BACKGROUND OF THE INVENTION 
     The present invention is concerned with a method of influencing the motion of a weft yarn which has to be drawn off a stock bobbin and runs towards a weft insertion mechanism of a loom, and in particular a projectile loom, At any given time in a weaving cycle the weft yarn is transferred for a weft insertion to a weft insertion member which passes through the shed, where the weft yarn is additionally accelerated at any given time by a compressed-air nozzle arranged between the stock bobbin and the weft insertion mechanism, and after the weft insertion is drawn back by a predetermined length by a deflector element movable transversely to the direction of weft insertion and deflected locally from a yarn path running essentially stretched into a yarn path cranked like a loop, and upon a following weft insertion is returned from the latter at any given time towards the yarn path running stretched. 
     The invention is further concerned with a loom for the performance of the above-described method. 
     EP-A-0 155 432 discloses a loom which contains between the stock bobbin and the deflector element a nozzle arrangement with a compressed-air nozzle for accelerating and a compressed-air nozzle for braking the weft yarn which is being fed to the weft insertion mechanism, as well as a compressed-air nozzle arranged between the deflector element and the weft insertion mechanism for transferring the weft yarn to the insertion member. The nozzle intended for accelerating the weft yarn in the known arrangement is acted upon by compressed air during the greater part of the weft insertion process in order to avoid the weft yarn being drawn off by the insertion member alone. Towards the end of the weft insertion process when the insertion member arrives at the catching side of the loom, the feed of compressed air to this accelerator nozzle is shut off and the braking nozzle acting in the opposite direction is acted upon by compressed air. The yarn transfer nozzle is activated each time only before the weft insertion in order to introduce the end of the weft yarn which has to be gripped into the insertion member. An accelerator nozzle which is active during essentially the whole weft insertion period demands a relatively elaborate arrangement and control of the compressed-air supply and consumes a relatively large amount of compressed air. In the case of looms of high weft insertion power, when a loop-shaped length of weft yarn is being returned towards the stretch position, in particular upon stretching out the portion of weft yarn being steadily accelerated by the accelerator nozzle and the weft insertion member, a &#34;snatch&#34; may additionally occur which, when handling sensitive yarn material, e.g., wool, may lead to weft yarn breakage. 
     SUMMARY OF THE INVENTION 
     The problem underlying the invention is to achieve a controlled feed of the weft yarn to the weft insertion mechanism and a correspondingly simplified loom which, at relatively low costs, allows gentle guidance of the weft yarn to be fed to the weft insertion mechanism, and by which a snatch loading of the weft yarn, in particular upon returning the deflected length of weft yarn towards the stretched yarn path, is avoided. 
     Through the method in accordance with the invention the weft yarn is additionally accelerated only during the relatively short initial phase which is critical for snatch loading, by a correspondingly brief feed of compressed air. It has been found that in this way at minimum consumption of compressed air and with a simple nozzle arrangement, in certain circumstances with one single compressed-air nozzle, a reliable return of the weft yarn into the stretched position which is an optimum for the weft insertion is guaranteed, the tension in the weft yarn which is to be inserted being influenced only during an exactly definable time interval which is variable according to various parameters. The occurrence of a &#34;snatch&#34; upon stretching the deflected length of weft yarn may accordingly be prevented and an inadmissible loading of the weft yarn be thereby avoided. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The single Figure schematically shows the weft yarn path of a projectile loom constructed in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     On the illustrated loom a weft yarn 1 is being drawn off a stationary weft yarn stock bobbin 2 which is arranged outside the shed 4 of the loom, formed by the warp threads 3. The weft yarn 1 leaving the stock bobbin 2 is wound onto a drum store 5, drawn off it overhead in the axial direction and led through a yarn brake 7, a compressed-air nozzle 6 and a deflector device 10, into a weft insertion mechanism 11. The deflector device 10 contains two stationary guide eyes 8, 8&#39; and a deflector element 16 arranged to be movable between them and adjustable transversely to the direction of weft insertion for taking up slack in the yarn during certain portions of the weaving cycle as is further described below. In the weft insertion mechanism 11 the end of the weft yarn 1 is transferred by a transfer nozzle or, as shown, a yarn clamp 12 to a weft insertion member or element such as a projectile 14 provided with a weft yarn clamp 13. The projectile 14 may be shot in known manner by a striker lever 15 into the shed 4 to insert the weft yarn 1. On the catching or receiving side of the loom (not shown) the projectile 14 is braked and pushed back by a predetermined amount into a yarn release position. 
     Slack in the weft yarn resulting from this reverse motion of the projectile 14 is taken up by moving deflector element 16 out of an essentially stretched yarn path 1a (represented in dash-dot line) into an angled yarn path 1b cranked like a loop (also shown in dash-dot line) so that the weft yarn 1 in shed 4 is kept taut. The weft yarn 1 is then beaten up in known manner by a reed (not shown) into the tip of the shed 4 against the cloth 18 being formed there and is cut off by a shear 17 on the weft side. The resulting end of the weft yarn 1 remaining outside the shed 4 is carried back by the yarn clamp 12 from the region of the shear 17 to the position shown for transfer to a further projectile 14 during the next weaving cycle. Additional slack generated during this return motion of the yarn clamp 12 is taken up by moving deflector element 16 from the yarn path 1b into angled yarn path 1c (shown in solid line). 
     Each weaving cycle involves one revolution of a control shaft 20 of the loom, from which the drives and control functions of all of the units of the loom are derived. A trigger-activated control valve 21 connects a compressed-air nozzle 6 with a compressed-air supply line 22 which is provided with a pressure gauge 23 and connected to a source of compressed air (not shown). The control valve 21 may be adjusted in any way, in accordance with the illustration via an electromagnetic positioning mechanism, in dependence upon the angular position of the control shaft 21, between a closed position as shown, blocking the feed of compressed air to the compressed-air nozzle 6, and an open position permitting compressed air to flow. During the greater part of the weaving cycle the control valve 21 is held in the closed position and it can be opened with switching element only during a predefined fraction of the weaving cycle, which corresponds to a partial revolution of the control shaft 20 through an angle of rotation α. 
     As illustrated the switching segment 24 extends over an angle of rotation α=about 90° which corresponds to the starting phase of the weft insertion process when the projectile 14 is being shot into the shed 4. This starting phase also includes at least a portion of the period when the deflector element 16 is being returned from the deflected yarn path 1c towards the stretched yarn path la. Accordingly, compressed air at relatively high pressure, e.g., about 2 to 3 bar, is fed to nozzle 6 only during this starting phase so that portion 1&#39; of the weft yarn 1 located upstream of the guide eye 8 is additionally accelerated by a powerful thrust of compressed air in the direction of the weft (arrow 9) while deflector 16 returns to its position 16&#39; shown in phantom lines. This prevents an excessive stressing of weft yarn 1 as it becomes straightened out from yarn path 1c. When, after the partial revolution of the control shaft 20 through the angle α, the deflector element 16 has reached its position 16&#39; in the region of the yarn path 1a, the control valve 21 is closed, thereby cutting off the flow of compressed air during the remainder of the weaving cycle when the weft yarn 1 is drawn off the drum store 5 exclusively by projectile 14, inserted into the shed 4 and, in a final phase of the weft insertion process, braked by actuation of the yarn brake 7. 
     The start and duration of the actuation of the compressed-air nozzle 6 during a weaving cycle are variable within a range of adjustments determined by the type of loom and its output data to take account of the yarn material which is being woven at the time. The angle of rotation α of the control shaft 20 which determines the actuation of the compressed-air nozzle 6 may therefore be freely chosen within certain limits, e g , within a range of α=60° to 120° and be exactly set to correspond to a given range of motion of the deflector element between the positions 16 and 16&#39;. 
     The compressed-air feed to the compressed-air nozzle 6 may according to one embodiment be so controlled that the maximum air pressure for the additional acceleration of the weft yarn 1 becomes fully effective in the second half of the period intended for the return of the deflector element 16 or of the weft yarn 1 towards the yarn path la. In such an event the portion 1&#39; of the weft yarn 1 upstream of the guide eye 8 becomes preaccelerated to the desired speed just before weft yarn 1 reaches the stretched position from the rest position. In this manner a snatch loading of the weft yarn 1 is avoided. 
     The pressure of the compressed air for the additional acceleration of the weft yarn 1 may usefully be built up during the first half of the period intended for the return of the weft yarn towards the straightened-out yarn path 1a. During this partial phase portion 1&#39; of the weft yarn 1 upstream of the guide eye 8 may be preaccelerated from the rest position to the desired speed independently of the portion 1&#34; which is downstream of the guide eye 8&#39; and already in motion through the firing of the projectile 14. 
     For influencing the build-up of pressure in the compressed-air nozzle 6 according to one embodiment of the invention, the start of the compressed-air feed may be varied to commence before the return of the weft yarn 1 towards the yarn path 1a begins as determined by the firing of the projectile 14. Such variations are made within a time interval which corresponds with a predetermined fraction, e.g., 10% of the aforesaid period or respectively of an angle of control shaft 20 rotation β of about 0% to 10%. 
     According to one embodiment of the invention, on a loom having a weft insertion speed of about 40 m/sec, an equalized loading of the weft yarn 1 during the weft insertion process may be achieved by controlling the compressed-air feed to the compressed-air nozzle 6 so that the portion 1&#39; of the weft yarn 1 upstream of the guide eye 8 is preaccelerated so that it has a speed which is a predetermined fraction, e.g., half, the speed of the portion 1&#34; of the weft yarn 1 that is being accelerated by the projectile 14 and is located downstream of the guide eye 8&#39;. 
     A gentle loading of the weft yarn 1 may further be achieved by controlling the compressed-air feed to the compressed-air nozzle 6 so that the anticipated speed of the compressed air for the preaccelerating portion 1&#39; of the weft yarn 1 is reached essentially within the first third of the period provided for the return of the weft yarn 1 towards the yarn path 1a, in the case of the example shown: after a partial revolution of the control shaft 20 through an angle of rotation gamma=about 25% after the firing of the projectile 14. 
     According to a modified embodiment, in addition to the compressed-air nozzle 6 blowing in the direction of weft insertion (arrow 9), a second compressed-air nozzle (not shown) blowing in the opposite direction may also be provided which, in a known manner, discharges compressed air for braking the weft yarn 1 during an end phase of the weft insertion process, say, via a control valve which may be energized accordingly. In that case the yarn brake 7 may if necessary be omitted or, e.g., for the processing of fine weft yarns, be preserved merely as a member for securing and releasing the weft yarn 1 and energized accordingly in certain angular portions of the weaving cycle. The compressed-air nozzle 6 and the braking nozzle may also be arranged in a combined common unit by energizing them separately from one another. 
     An arrangement is also possible in which the yarn brake 7 is between the compressed-air nozzle 6 and the deflector device 10. The invention may also be employed for looms having some other weft insertion member, e.g., a gripper belt.