Projectile Loom | Projectile Weaving Machines | Working Procedure of Projectile Loom

Friday, May 18, 2012

Projectile Loom | Projectile Weaving Machines | Working Procedure of Projectile Loom

General operation
In this weaving machine the weft insertion is carried out by small clamp projectiles (fig. 45), which number depends on the weaving width and which with their grippers take out the weft yarn from big cross-wound bobbins and insert it into the shed always in the same direction. The projectiles work in sequence, that is they are launched in succession. They run therefore one after the other, describing in the space a continuous, endless route, as if they would be stuck on a conveyor belt.

The first projectile takes and holds in its back the weft in form of a tail; then, pushed by the release of the projectile thrower, it passes through the shed and deposits the weft inside the warp; subsequently the projectile falls and is collected by a device which, by passing under the array of the warp threads, takes it at reduced speed back to the starting point. Here the projectile goes up to take up a new weft; meanwhile the other projectiles have run after each other making the same operation.

Projectile guide
The limited weight and the reduced volume of the projectile make aprojectile guide necessary . The projectiles therefore do not come into contact with the threads, but run inside a sort of channel composed of the thin prongs of a rake, which form reminds a semiclosed hand. This rake goes up from under the threads at the moment of the projectile launch and has of course to fall back lowering itself at the slay stroke. To enable this movement, the rake is secured on the slay and is positioned very close to the reed; the rake’s laminas are not in contact with the warp, or touch it very lightly because the reed opens them the way. The latest models of the projectile machine have been equipped with new types of guide dents, which are divided and placed in alternate position, in order to reduce the stress on weft and warp threads. This permits to use in warp even very delicate yarns as for instance untwisted or entangled yarns and at the same time to cope with high quality requirements.

Projectile launching mechanism
The operational principle of the launching mechanism is the following a torsion bar 2 is anchored, at one side, to the fixed point 1, whereas the free end is connected by a toothed groove to the percussion shaft 3. The percussion lever 9, which is fixed to the percussion shaft 3, follows per force the movements of this last and consequently of the free end of the torsion bar 2. During its rotation, the cam 8 shifts the knee-joint lever 4+5, so that the torsion bar 2 is put under tension by the percussion shaft 3 and the percussion lever 9 is put in launching position (the scheme shows the launching mechanism with the torsion bar in the phase of maximum tension). The torsion bar 2 remains under tension until the roller 7 slides along the bend of lever 5. The particular shape of this lever makes so that the roller, when leaving it, presses its end, thus giving the starting point to the torsion bar for the articulation of the knee-joint lever 4+5. Subsequently the torsion bar 2 returns suddenly to its rest position imparting a strong acceleration to the projectile 11 through the percussion shaft 3 , the percussion lever 9 and the percussion element 10. The oil brake 6 serves to damp the stroke.
The projectile’s stroke time, that is the insertion time, is adjusted by modifying the torsion angle of
the bar through an angular shift of the anchorage point, which has proper adjustment windows.

Insertion cycle of the projectile machine

a) The projectile 1 is put in launching position; the weft is hold at its end by the weft carrier 2 and is controlled by the weft tensioner 3, by the weft brake 4 and by the eyelet 7 situated in proximity of the feeding bobbin 8;

b) The weft carrier 2 gets open after the projectile clamp has got hold of the end of the weft thread;

c) The projectile 1 is launched and crosses the shed dragging with itself the weft, while the weft tensioner 3 and the weft brake 4 operate in a way as to minimize the stress on the yarn (the critical phases are particularly the initial acceleration phase and the final stop phase in the collector box);

d) The projectile 1 on the one hand and the weft carrier 2 on the other take up the right position to build up the selvedge, while the tensioner arm opens to adjust the weft tension;

e) The weft carrier 2 closes while the selvedge clamps 5 get hold of the weft thread on both sides and the projectile clamp is opened to release the weft end;

f) The thread is cut by the scissors 6 on the launching side, while the projectile 1 is placed in the transport chain;

Loading of the torsion bar: a) torsion bar 2 in rest, knee-joint lever 4+5 in articulate position; b) loading phase; c) torsion bar in tension and kniejoint lever in stable position, before the launching control by roller 7.

g) The weft is beaten by the reed, while the weft carrier 2 moves back to its initial position and the weft tensioner 3 opens further to recover the thread piece and to keep it under tension. The projectile is brought back to the launching zone;

h) The selvedge needles 9 insert the weft ends into the subsequent shed (tuck-in selvedge), while a new projectile is placed in launching position.

Electronically controlled projectile brake
The present machines have the projectile brake adjusted by a microprocessor, and this permitted to increase the efficiency rate and to reduce the maintenance costs. The electronically controlled brake has the function of stopping the projectiles in the correct position, without any need of manual intervention (contrarily to previous mechanism). This result is obtained by means of a controlled double upper brake lining and of a lower fixed brake lining (Fig. 51 and 52). The mechanism works as follows: the sensor 1 and 3 detect the position of projectile 4 inside the collector mechanism and communicates it to a microprocessor which, on the basis of the received information, transmits a corresponding order to the stepping magnet 14. This last operates on a wedge-shaped guide element 13 which, by shifting the upper bracket lining 8, modifies the braking intensity. The sensor 2 controls instead the timely arrival of the projectiles in the collector mechanism.

Three cases are possible:
A) Position I (normal projectile position): the control co-ordinates S of sensors 1 and 3 are covered by the projectile;

B) Position II (projectile too far penetrated / insufficient braking): the control co-ordinate S of sensor III is not covered;

C) Position III (projectile insufficiently penetrated / excessive braking): the control coordinate S of sensor 3 is not covered.

In the first case the microprocessor does not answer; in the second and third case, it causes respectively the closing and opening of the brake, thus controlling the number of steps necessary to bring the projectile again to normal position.

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Friday, May 18, 2012