Method and apparatus for limiting stresses in weft yarn advancing towards a weft insertion mechanism

A weft yarn (1) leaving a stationary stock bobbin (2) is led through a compressed-air nozzle (6) and a deflector element (16) movable transversely to the direction of weft insertion (arrow 9), to the weft insertion mechanism (11) of a projectile loom. After each weft insertion the weft yarn (1) becomes deflected by the deflector element (16) from a yarn path (1a) running stretched into a cranked yarn path (1c) and returned towards the stretched yarn path (1a) during a following weft insertion. During a predetermined fraction of the weaving cycle which comprises the return of the weft yarn (1) into the stretched yarn path (1a), the compressed-air nozzle (6) is acted upon by compressed air so that the weft yarn (1) is additionally accelerated for a short time. After the return of the weft yarn (1) the compressed air feed to the compressed-air nozzle (1) becomes blocked during the remaining part of the weaving cycle. A gentle guidance of the weft yarn is thereby guaranteed with minimum consumption of compressed air and with a simple nozzle arrangement, and the occurrence of a "snatch" upon stretching the weft yarn is prevented. The method is particularly suitable for the processing of sensitive yarn material in looms of high weft insertion power.

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 "snatch" 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 
"snatch" upon stretching the deflected length of weft yarn may accordingly 
be prevented and an inadmissible loading of the weft yarn be thereby 
avoided.

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' 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 .alpha.. 
As illustrated the switching segment 24 extends over an angle of rotation 
.alpha.=about 90.degree. 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' 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' 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 .alpha., the deflector element 16 has reached 
its position 16' 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 
.alpha. 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 .alpha.=60.degree. to 120.degree. and be 
exactly set to correspond to a given range of motion of the deflector 
element between the positions 16 and 16'. 
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' 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' 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" which is downstream of the 
guide eye 8' 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 .beta. 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' 
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" of the weft yarn 1 that is being accelerated by the 
projectile 14 and is located downstream of the guide eye 8'. 
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' 
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.