Fabric Club Of BD: Oct 28, 2010

Loom Break Mechanism

Labels: Weaving Manufacturing Technology | at 10/28/2010 11:57:00 PM

The brake stops the loom immediately whenever required. The weaver uses it to stop the loom to repair broken ends and picks.

Picture: Electrical Break Mechanism (Picanol Delta Air-Jet Loom)

Clutches:

A clutch is a form of connection between a driving & a driven member in the same axis. It is so designed that the two members in the same axis. It is so designed that the two members may be engaged or disengaged at will either by a hand operated device or automatically by the action of some power driven device. The common types of clutches which are used in weaving machines are,

i. Friction Clutch

ii. Electromagnetic Clutch

Friction Clutch:

A friction clutch is used in the transmission of power of shafts & machines which must be started & stopped frequently as in the case of weaving machines. The force of friction is used to start the driven shaft from the rest & gradually brings up to the proper speed without excessive slipping of the friction surfaces. In operating such a clutch the following care should be taken.

a) The friction surfaces should engage easily & gradually bring the driven shaft up to a proper speed.

b) The proper alignment of the bearings must be maintained & it should be located as close to the clutch as possible.

c) The heat generated due to friction should be rapidly dissipated & tendency to grab should be at a minimum.

d) Lateral displacement of the frictional clutch involves forces of high magnitude resulting in wearing of main shaft bearings. This can be avoided by using expanded clutches.

e) The surfaces should be backed by a material stiff enough to ensure a reasonably uniform distribution of pressure.

The friction clutches are of the two types;

i. Disc or Plate Clutches

ii. Cone Clutches

Disc or Plate Clutches:

In a disc or plate clutch as shown in fig.12.4 the driver A is rigidly keyed to the driving shaft B by means of a sunk key C & feather key E, so that it can move along the shaft. The driven member is faced with a friction lining F & is held against a driven member by means of axial pressure provided by a spring. The axial pressure exerted by the spring provides a frictional force in the circumferential direction when the relative motion between the driving & driven members tends to take place. If the torque due to this frictional force exceeds the torque to be transmitted, then due to this frictional exceeds the torque to be transmitted, then no slipping takes place & the power is transmitted from the driving shaft to driven shaft H.

Cone Clutch:

A cone clutch has a conical friction surface as shown in fig.12.5. The driver which is keyed to the driving shaft by a sunk key has an inside conical surface or face which exactly fits into the outer side of the conical surface of the driven member. Like the plate clutch, the driven pulley is mounted on the shaft with a feather key & the two conical surfaces can be engaged or disengaged by means of starting handle through levers. The contact surfaces of the clutch may be metal to metal, but more often the drive pulley is lined with felt or cork.

Electromagnetic Clutch:

With the conventional looms, the drive to clutch is usually controlled by means of a starting handle through a train of levers. The knocking of the starting handle should be done in such a manner that the loom is stopped with the shuttle in the starting handle side & the headls are at top centre in the event of a warp breakage or between bottom & back centre in the event of a weft breakage. With the high speed looms, it is very difficult to judge when to knock-off.

This difficulty can be overcome by using an electrically controlled clutch unit which is controlled by means of a push button.

Because of the following advantages, it has gained wide acceptance for high speed looms.

a) No physical strain is required to handle the weaving machine.

b) Electric power transmission enables the controls to be operated anywhere on the weaving machine. The control pluses given by the stop motions & safety devices of the weaving machines are easy to connect. Thus when the weft breaks or stop motion button is pressed, the loom is stopped at the back centre position, but when the warp breaks, it is stopped at the top centre so that drawing-in can take place without any further adjustment of the loom.

c) It ensures a quick & accurate start.

d) The loom can be run at a normal speed, or slow speed (inching) to a predetermined position. There is a provision to reverse the loom. The reversing motion takes the loom to the back centre for starting. Further, reversing can be done for a few picks for the purpose of pick finding. It may be mentioned that majority of the shuttles-less weaving machines cannot be operated in the reverse direction because they are equipped with unidirectional cams.

e) Variation in the loom speed during picking is less. The principle of working of electromagnetic clutch drive (2) is explained with reference to fig.12.7.

f) When the loom is to be started the pressing of starting button completes an electric circuit & energizes clutch solenoids so that the plate which is fixed on the main shaft spline is attracted to the driven fly wheel. This will result in rotation of a planet gear through the main shaft gear & thus the loom driving pinion will be rotated.

g) When the loom is to be stopped, the clutch solenoid is de-energized & the brake solenoid is energized & the break solenoid is energized so that the plate is taken away from the fly wheel on to the fixed motor casing. The timing is such that the loom is brought to rest exactly at the desired crank position depending upon the cause of stoppage.

h) When the loom is to be reserved, reversing solenoid is energized. Instead of the fly wheel gears driving a shaft gear with an additional tooth to given a forward drive, the shaft position is adjusted & the drive occurs on a gear with one tooth less so that the loom will run in reverse.

Tappets

Labels: Weaving Manufacturing Technology | at 10/28/2010 11:42:00 PM

Two tappets are connected to the bottom shaft at 180 degrees to each other because half a rotation of the bottom shaft is equal to one pick, and for each pick, one tappet will lower the heald shaft. The tappets have a portion corresponding to the dwell. This is used to arrest the movement of the heald shafts for a period of time. During this time, the shuttle is traversing from one box to the other. This period is usually 1/3 rd of a pick i.e. 120 degrees. See Figure 1.

Throw of a tappet:
Referring to the figure, it is clear that the difference between the heal s 1 and toe s 2 of a tappet is equal to its throw. If the difference is high then the throw of tappet is also high. Higher-throw tappets apply more force to the treadle lever. A higher-throw tappet is always connected to the back heald shaft. This is mainly to compensate for the difference in leverage in the treadle levers.

Figure 1: Throw of tappet

Lift between the Back and Front Heald Shafts:
This is due to the different connections of the heald shafts to the treadle levers. In Figure 2, ‘d’ represents the back shaft connection to one of the treadle levers and d 1 the front heald shaft connection to the other. Since d 1 is greater than d, the front heald shaft gets more lift than the back heald shaft.

Figure 2: Heald shafts connections

Depth of Shed:
Refer to Figure 3. By altering the positions of the lamb rod hooks on the treadle levers, the depth of shed is changed. By moving the lamb rod towards the fulcrum (distance d 1 ), the depth of shed is reduced and moving it away from the fulcrum (distance d), the depth of shed is increased. The depth of shed is altered when a shuttle of a different height is used.
Lamb rod hooks

Figure 3: Positions of lamb rod hooks

Advantages and Disadvantages of Tappet Shedding:

Advantages:
1. It is robust, simple and cheap.
2. It is capable of lifting a heavy weight with less wear and tear than other shedding mechanisms.
3. It can move heald shafts at great speeds.
4. It puts less strain upon the warp.
5. It consumes less power and gives greater output.
6. It requires less maintenance.

Disadvantages:
1. If the weave is changed, it will be necessary to change the tappet and the change gear wheel in the counter shaft arrangement. So work involved in changing the weave is more.
2. The capacity of a tappet to produce a pattern / weave is ver y much limited. A maximum of 8 or 10 tappets only can be used.
Faults that may Occur in Tappet Shedding Mechanism
1. If the tappet is faulty, it imparts a jerky movement to the heald shaft.
2. The tappet should always touch the bowls. Otherwise a severe blow is applied to the bowl and the vibration is transmitted to the heald shaft. End breakages may occur as a result of this.
3. Overshedding : If the depth of a shed is too much, strain on the warp will be more and end breakages may occur.
4. Undershedding : If the depth of shed is too low, the shuttle will not reach the other end and may be trapped in the shed or may fly out. Hence end breakages will occur.
5. Uneven shedding : Uneven shedding is caused by lifting one end of the heald shafts more than the other so the shuttle may move over some war p threads and fly out or get trapped in the shed.
6. If the shedding is mistimed, then other motions like picking and beat-up cannot be done smoothly and end breakages may occur.

Weft Feeler Mechanism

Labels: Weaving Manufacturing Technology | at 10/28/2010 11:03:00 AM

The function of the feeler mechanism is to sense the weft on the pirn and initiate the pirn changing mechanism to act when the weft has been almost exhausted on the pirn. If the weft is present on the pirn the feeler will allow the loom to continue working.

There are three main types of feelers;

1. Mechanical

2. Electrical and

3. Photo electrical

Mechanical Weft Feeler:

When the sley moves forward the feeler blade A passes through the slots in the front plate of the box and front wall of the shuttle and contacts the pirn. If there is sufficient weft yarn on the pirn, the blade A is pushed straight back into the feeler casing and no direction of a pirn transfer takes place. With the reserve bunch of weft, approximately a length of three picks of yarn is left on the pirn base. The feeler blade contacts the smooth polished surface of base pirn and slide side-ways, contacting the trip lever connecting rod B, which in turn raises the trip lever C through the bell crank lever D.

A tripper heel E attached to the weft fork hammer F, oscillating to and fro along with the weft fork hammer, comes in contact with the raised trip lever C and pushes it back in the direction of the arrow shown in figure. This will cause the change shaft G which runs across the width of the breast beam, to turn and effect a pirn change at the magazine and during the next forward movement of the sley with the shuttle on the magazine side box. The return spring in the feeler casing pulls the feeler blade to its normal position as soon as the contact of the blade with the pirn is over.

Electrical Two Pronged Feeler:

This type of feeler has been designed where the transfer of the fully wound pirn from the weft replenishing unit to the shuttle is initiated electrically. It can be used with advantage on looms weaving delicate weft. The feeler is mounted on a bracket fixed to the rear wall of the starting handle casing. The weft pirn is fitted with a metal sleeve on the barrel. A bunch of reserve of weft, sufficient for four picks across the loom must be wound at the base of the pirn. Under normal running conditions, as the sley moves forward and is almost at the front centre, the weft on the pirn contacts the feeler prongs which are pushed backwards into the feeler casing, against the pressure of return springs. However, when the pirn is empty, except for the reserve weft, the metal sleeve on the pirn barrel is exposed and comes in contact with the feeler prongs.

The feeler prongs are connected to an electrical circuit. The circuit is incomplete until contact is affected across the feeler prongs. As soon as the prongs come in contact with the metal sleeve on the pirn barrel the circuit is completed energizing the solenoid and the electrical magnet box so that the trip lever is lifted in line with the tripper heel which puts the pirn changing mechanism in action. The electrical circuit is broken when the sley moves backwards to break contact between the metal sleeve on the pirn and the feeler prongs.

Area of contact between the feeler and yarn is very small and does not damage the yarn. But it requires special type of pirns with a metal sleeve.

Optical Electronic Weft Feeler:

The weft pirn used for this type of feeler is covered with a reflective strip which has the property of reflecting a beam of light back to its source. The light source and the photocell are housed together in the feeler head and both the searching beam and the reflected ray pass through the same optical system.

Incident light ray is directed on the pirn constantly and as soon as the weft is exhausted the light ray is reflected back to the feeler head. On reaching the photocell, the reflected light is transformed into an electrical impulse and transmitted to the switch box, which contains the whole electrical supply for the feeler and feeds the appropriate selection mechanism in order to initiate the transfer of pirn. Advantage of this feeler is that there is no physical contact between the feeler and the weft yarn. Main disadvantage is that it is very expensive. Now-a-days for filament weaving, this feeler is extensively used.

Shuttle changing mechanism:

The shuttle changing automatic looms are suitable for weaving very delicate wefts like silk, rayon and fine counts of cotton yarns, because there is no hammer action on the weft package. As soon as the weft gets exhausted on the pirn the entire shuttle is replaced by a new shuttle with a fully wound pirn.

There are two main types of shuttle changing looms. They are;

1. One which does not stop for a change or in which changes is affected during the running of the loom.

2. One which stops for a few seconds for a change and restarts automatically.

Pirn changing and weft replenishing motion

Labels: Weaving Manufacturing Technology | at 10/28/2010 10:58:00 AM

The following parts or attachments are found essential for operating this mechanism:

· A large shuttle and pirn with a few changes in the design.

· Changes in the design of the shuttle boxes and sley.

· A rotary magazine to accommodate 24 to 30 fully wound pirns.

· A feeler mechanism on the starting handle side to detect the almost exhausted weft pirn.

· A mechanism to push the fully wound pirn from the magazine into the shuttle and at the same time eject the empty pirn.

· A self threading device in the shuttle.

· A device to cut the two ends of weft at the selvedge of the cloth at the pirn changing side. Among the two weft ends, one of which from the outgoing almost empty pirn and the other from a new fully wound in going pirn.

· A mechanism known as shuttle protector, to prevent changing mechanism from fraying to insert a new fully wound pirn in the shuttle, should the shuttle fail to be exactly in the correct position for receiving the bobbin.

Mechanism:

The change shaft which runs across the width of the breast beam is partially rotated so as to impart an upward movement to the shuttle protector lever A, which contacts the peg B and gives a forward throw to the shuttle protector C. The latch depressor D which moves along with the protector C releases its hold on the peg E with the result that the peg follows the depressor under pressure from the latch spring F and ultimately always the latch G to swing upwards into line with the bunter H which is fixed on the sley front. Under normal running conditions the spring loaded transfer latch G is held in the depressed position by means of a peg E which resists against the latch depressor D.

As the sley moves forward for beating up of weft and the shuttle having reached the battery end, the bunter H engages the notch on the latch G, forcing it backwards against the resistance of the hammer coil spring S thereby depressing the transfer hammer I fulcrummed on the stud J, together with the transfer depressor K. During the downward movement, the hammer and the depressor K imparts a sharp blow to the fully wound pirn that is immediately underneath, held by the battery. When a full pirn is forced into the shuttle, it expels the almost empty pirn out of the shuttle, making it pass through the slots provided in the bottom of the shuttle and the box and fall into a container. The new pirn which is forced into the shuttle is firmly held in the spring jaws of the shuttle.

Connected to the transfer hammer (I) is the feed pawl L, the catch of which rests in one of the teeth of the ratchet wheel M. As the hammer is depressed for the transfer of the new pirn into the shuttle, it lowers the feed pawl L so that the catch slips into the next tooth of the ratchet wheel. As soon as the transfer of the pirn has taken place, the receding sley breaks the contact between the bunter and the latch and enables the hammer to move up to its original position due to the pressure of the hammer coil spring S, and in doing it pushes the feed pawl L upwards aided by a spring underneath the pawl, and turns the ratchet wheel M one tooth bringing the next full pirn in the battery right below the hammer for the subsequent transfer.

Warp protector motion

Labels: Weaving Manufacturing Technology | at 10/28/2010 10:53:00 AM

The function of the warp protector motion is to stop when the shuttle fails to reach the shuttle box during picking. The shuttle failure or the shuttle trap inside the warp shed may cause many broken ends during the forward movement of the sley. In order to prevent this from occurring a device is necessary to stop the loom whenever the shuttle fails to reach the shuttle box. The causes for such a failure might be due to: slack ends, improper opening of the warp shed, wrong shed, wrong timing of picking, mechanical failure of parts such a broken picking straps, loose or badly worn picking tappets, picking shaft springs broken, faulty pickers, odd sized shuttles, shed too low and stop rod brackets loose.

There are two types of warp protector commonly available for shuttle looms.

o Loose reed warps protectors.

o Fast reed warps protectors.

On high speed shuttle looms, electromagnetic warp stop motions are used.

Loose reed warps protector:

The principle of the mechanism is that the reed is forced out of its support whenever the shuttle is trapped in the shed and this backward inside movement of the reed will cause a knock off device to act and stop the loom.

The reed A is held at the top of the slotted reed cap B. the bottom part of the reed is held firmly against the raceboard C by the reed case D which extends the whole width of the reed. This reed case is connected to a stop rod S by means of several brackets. The stop rod also extends the width of the sley and it is fixed to the sley below the raceboard. There are two, three or four frogs E, depending upon the width of the loom, mounted on the stop rod. In front of each frog there is a heater F fixed by means of a bracket to the breast beam.

During the normal working of the loom there are three devices to keep the reed firm:

o Frog E engaging the heater F.

Bowl G riding the bow spring H.
A light spiral spring I.

Frog and heater:

When the sley moves forward the frogs slide under the heaters thus locking the reed firmly for a good beat up of weft.

Bowl and bow spring:

During the backward movement of the sley the bowl G rides on the flat bow spring H and keeps the reed firm to enable the smooth flight of the shuttle during it traverse from one box to another.

Spiral spring:

The light spiral spring keeps the reed case tensioned all the time. A stop rod finger J is also mounted on the stop rod, and facing this finger is a serrated bracket K fixed to the starting handle L. When the shuttle is trapped in the warp shed it presses against the base of the reed during the forward movement of the sley, with the result the reed swung backwards turning the stop rod S through the reed case. When the stop rod is turned all the frogs and the stop rod finger are raised. During further forward movement of the sley the frogs ride over their respective heaters and the stop rod finger hits the serrated bracket and stop the loom. The frogs riding over the heaters will enable the reed case to move backwards easily.

The loose reed motion is only intended for light and medium weight fabrics. It is therefore necessary that the spiral spring I should only be strong enough to prevent the reed case from vibrating during running of the loom. If it is too strong the shuttle has to exert a greater force to push the reed back, which means more strain on the warp threads. Delicate warp used for light weight fabrics will not stand such strains with the result more warp breakage will occur.

Fast reed warps protector:

Fast reed warp protector is used for heavier fabrics because it works on the principle of fixed reed and the protector mechanism is operated by the shuttle box swell that reacts directly through the stop rod dagger to knock off the loom. Also, for heavier fabrics the beat up of weft by the sley should be very firm.

The stop rod A which runs beneath the sley has two fingers B fixed to it; one finger on each side of the shuttle box. These fingers with adjustable nuts are kept pressed against the swell C. To the same stop rod are fixed two daggers D, one each side of the shuttle box. The daggers face a sliding frog E mounted on the side frame. The sliding frog on the starting handle side carries the brake lever F at the rear and at the front it contacts the adjustable bolt that knocks of the starting handle. When the shuttle enters the box at either side, it passes the swell which makes the daggers rise above the frogs and the loom continues to run. If the shuttle fails to reach the box or if it rebounds owing to insufficient checking, then the swell will not be pushed back sufficiently to rise the daggers clear off the frogs with the result the daggers will dash against the frogs and push it backwards. Then the sliding frog will knock off the starting handle and the loom will stop. At the same time the brake lever F pulls the brake close on the brake drum to an almost instantaneous halt of the loom. The shock of the sudden stoppages taken by the two strong vertical springs S which are connected to the frog through a bolt G.

While setting the frogs with respect to the distance from the daggers, it is better to set so that the sley comes to a halt before the crank has passed the top centre. The sudden impact of the dagger on the frog is commonly known as bang-off. Sometimes frequent bang-off will cause the parts that are taking such force of the shock to fracture.

Electromagnetic warp protection:

The mechanism consists of a magnet in the end of the shuttle opposite to the shuttle eye. A coil B is mounted slightly off the centre position in the sley. As the shuttle passes over the coil, a pulse generated which is fed to an electrical control unit G. a second pulse is generated by a coil C and magnet D mounted on the disc E on the bottom shaft F and this occurs at a fixed time in each loom cycle. Under normal working these two pulses synchronize. A late passage or non-passage of the shuttle causes a break in the sequence of the two pulses. The solenoid then activate and then knock lever I will then be positioned in the knock off and catch and the loom will be brought to rest. The position of the knock-off catch depends upon the width of the loom, loom timings, speed of loom.

Advantages:

Banging-off shock is eliminated since there more time is available for stopping of the loom.
Unlike loose and fast reed methods of warp protection, there is no possibility of damage to the fell of the cloth since the loom is stopped before shuttle trapping can occur.

Mechanical Warp Stop Motion

Labels: Weaving Manufacturing Technology | at 10/28/2010 10:42:00 AM

There are two types of mechanical warp stop motion-

· Vibrator bar type

· Castellated bar type

Vibrator bar type:

The vibrator or oscillating bar type warp stop motions were previously being used on almost all types of looms. Now-a-days, these warp stop motions are being superseded by castellated bar type because of the following disadvantages:

· Open type drop wires cannot be used and hence pinning at the loom is consequently not possible.

· Maximum four banks can be used.

· Shaking movement due to the oscillating bar disturbs the lower edges of the drop wires thus causing them prone to jump and unnecessary friction is created between the yarn and the drop wires.

Working principle:

The eccentric A fitted on the bottom shaft B rocks the vibrating bar to and fro through the fork lever D, connecting rod E, oscillating shaft F. The fork lever D and the rod E are connected through a spring G so that the motion imparted to the vibrator bar is a negative one.

The two sides of the vibrating bar have five vertical serrations. On each side of the vibrating bar there is a fixed bar H having vertical serrations on the inner side. The drop wires I are supported by the warp threads. For each bank there is a bar J with smooth rounded edges which pass through the slots of the drop wires. These bars are stationary and serve the purpose of holding the drop wires in the event of an end breakage.

Under normal working conditions, the bottom ends of the drop wires are clear of the path of the oscillating bar which makes to and fro movement between the fixed bars during the one complete revolutions of bottom shaft. This movement gives complete up and down movement to the hitter through L-shaped link L, connecting rod M, cross lever N, and knock off rod O. K mounted on the knock off rod escapes the striker P on the sley every time the later comes forward. However, whenever a drop wire falls down on the drop wire bar due to a warp breakage, the path of the vibrating bar is obstructed; serrations of these drop wires prevent them to slip off and turn over. When its movement is obstructed the hitter K is in the middle of its path, the striker P on the sley hits the hitter as the sley moves forward. The hitter is pressed back resulting in the release of the starting handle from the knotch.

Castellated bar:

The mechanism consists of a slide A, slider bar B, the slide oscillating device and the knock off device. All these parts are illustrated in figure. The slide is placed into the groove of the slider bar which is secured firmly at both ends at the side brackets. The top of each bar is castellated. The number of bars used depends upon the density of warp (10-12 drop pins per cm. on each bar). Normally four bars are used.

The warp threads are drawn through the drop pins, heald eyes and reed dents at the same time in the preparatory department. When the warp beam is taken to the loom for gaiting of warp, the drop pins are threaded on to the slide bars. Then the slides are coupled to the mechanism by a pin passing through the holes C and the slots D. The slider bars are held firm in their end frames by bolts passed through the holes. The whole unit is placed behind the heald frames.

The warp tension has to be adjusted in order to keep the drop pin clear of the slides. When a warp thread breaks, the corresponding drop pin falls down in to the moving cut out of the slider. The free movement of slide A is arrested as the drop pin comes against the rigid cut put of the slide bar B and the knock off mechanism is actuated and the loom stops.

The slide A moves forward and backward by means of an eccentric in the driving box. The slide A is connected to the forked bracket which is attached to a tubular lever G fulcrummed at O. The lever G is oscillated by a cam coupling H. A small shaft S inside the drawing box consists of a chain driven wheel I, a double sided cam J and the cam coupling H. The motion to the shaft S is given from the crank shaft through a chain and chain wheel I. In the hollow part of the tubular lever G a spring loaded finger K passes through. The finger K during its oscillation above the double sided cam J clears the flat sides of the cam with each complete movement.

Picture: Castellated bar

If a slide is locked by a falling drop pin, the free motion of the lever G is arrested with a finger K positioning immediately above the cam J. The continuously rotating cam J with the projecting part lifts the finger K which in turn lifts the knock off lever L. The lever L is connected to a knock off finger M by means of a cable. When the lever is raised by the finger K, the cable pulls the knock off finger M in front of the striker N which finally knocks off the starting handle.

Weft-stop motion (Conventional Loom)

Labels: Weaving Manufacturing Technology | at 10/28/2010 02:01:00 AM

This motion enables to stop the loom immediately after a weft break or weft running out. In case the loom is allowed to run even after the weft breaks there will be no woven cloth except long threads of warp.

There are two types of weft stop motions on a conventional weaving machine:

a) Side weft fork motion.

b) Centre weft fork motion.

Side weft fork motion:

The basic principle of the side weft fork lies in the fork and grate. A metal grate is placed between the end of the reed and the shuttle box mouth on the starting handle side as shown in the figure. A weft fork made of light metal which has three prongs bent at right angles is situated in front of the grate. The complete weft fork motion is illustrated at figure.

A weft fork A with a single tail hooked at the end is held by a weft fork holder B at C. the other end of the holder is held by knock-over lever D, which is in contact with the starting handle when the loom is running.

The tail end of the fork is slightly heavier than the forked end. A hammer lever E fulcrummed at X is connected to a greyhound tail lever F, the bottom end of which is resting on a weft fork cam G which is fixed on the bottom shaft. During the rotation of the bottom shaft the cam raises the greyhound tail lever on every two picks and causes the hammer lever to rock towards the loom front.

A channel is cut in the wooden raceboard H opposite the weft fork so that when the sley comes forward to beat-up position the weft fork prongs will remain below the raceboard level until it is touched by a weft thread lying across the channel from the selvedge to the shuttle.

In this case the shuttle should be on the starting handle side. If the weft thread is not broken or missing, it will push the weft fork prongs, thus lifting the hooked tail clear of the hammer lever E. At the same time the rotation of the cam G makes the hammer lever move towards the front rest. In case the weft is absent either through breaking or from running out, the weft fork remains horizontal and the prongs pass freely through the bars of the grate. Then the hook tail of the fork is caught in the notch of the hammer lever E as shown in figure and when this lever moves towards the front rest it carries the fork along with its holder resulting in the weft fork lever D pressing against the starting handle S and knocking off the loom.

One fault in the mechanism described early is that the weft fork lever and the holder move in an arc of a circle because of the fixed fulcrum of the weft fork lever. This sometimes causes the prongs of the fork to hit against the side wall of the channel in the raceboard and cause damage. In the British made Northrop looms this arc of movement does not exist since the weft fork acts directly upon the starting handle with a straight backward push.

In the mechanism illustrated in figure the weft fork A is mounted on a sliding bracket B which slides forward and backward in a fixed bracket C. As usual the hook tail of the fork is caught in the notch of the hammer lever on weft failure and backward movement of this lever will push the knock-over lever D, thus the releasing the starting handle E. The spring S returns the sliding bracket B to its original position.

Important points to note while setting the weft fork mechanism:

1. The weft fork must be a possible source of weft cutting if it protrudes too far through the weft fork grate.

2. The grate must be smooth.

3. The weft fork prongs, during the forward movement of the reed, should not touch the grate wires or any part of the grate or raceboard groove.

4. The weft fork prongs protrude neither too less nor too far through the grate.

5. The clearance between the hook tail of the fork and the notch of the weft fork hammer is very important. If the clearance is too wide the weft thread may not keep the hook tail raised till the tail is clear off the weft fork hammer notch. This will result in unnecessary knock off of the loom even though the weft has not broken. On the other hand if the clearance is too close the hammer notch might prevent the hook tail from lifting when the weft thread applies pressure on the prongs.

6. The fork must be properly balanced so that the tail end is slightly heavier than the forked end.

7. An accumulation of fluff at the base of the grate will unnecessarily press the prongs of the fork thus raising the tail end when no weft is present. This will make the loom run without the presence of weft.

8. The side-play in the rocking rail and sley might cause the grate foul the fork. Sometimes, loose cranks might also cause this trouble.

9. Weft thread catching on the prongs because of inadequate tension will cause the loom to run on.

10. Bent prongs, binding of the fork through rust on the fulcrum pin, fork fulcrum worn out etc. might affect the good working of the mechanism.

11. Faulty timing of the hammer lever may cause the loom running even after the failure of weft.

12. Weak or late picking from the off side of the loom may cause the shuttle to strike the prongs and damage it.

13. Insufficient tension in the weft fail to lift the fork sufficiently causes the loom stoppage.

14. If the hammer lever begins to move too soon before the weft has had time to lift the fork tail clear, the loom will keep stopping.

Disadvantage:

Since this mechanism is situated only at the starting handle side of the loom, the stopping is affected only when the shuttle reaches the starting handle side. This will result in missing a maximum of two picks when the weft breaks or exhausts as soon as the shuttle leaves the starting handle side.

In case such a device is to be provided on both sides of the sley the cost factor and the complicated knocking off arrangement has to be thought of.

Centre weft fork motion:

This motion has been designed to feel the weft thread every pick and stop the loom in case the weft thread breaks or runs out, no matter which way the shuttle is running at the time. The shuttle can be housed in any one of the boxes.

It is for this reason the mechanism is situated in the centre of the raceboard. The loom is brought to a stop before the beat-up action takes place. It is not necessary that the shuttle should always be in the starting handle side box for effecting the loom stop. Therefore this device will not allow two missing picks before the loom stops. This device is useful for looms weaving pick and pick colored wefts. If there are two different colored picks weaving alternately and if one of the colored thread is broken, it is necessary to stop the loom immediately before another colored thread of the second pick is inserted in the shed. It also helps to weave faultless cloth free from pick finding marks or broken picks. With effective braking system, the loom can be stopped dead on the broken pick. In addition a device is incorporated to turn back the loom, opening the previous shed with a broken pick laid inside, so that the weaver can rethread without making any bad mark on the cloth. Centre weft fork motion is, therefore, suitable for weaving fabrics made of filament yarns, e.g. polyester, nylon and yarns made out of other delicate fibers. Though several types of centre fork motions are designed the basic principle remains the same. A channel is cut in the raceboard, at or near the centre depending upon the length of the weft from the shuttle eye to the fork and also on certain attachments like pirn changing and box changing. The weft fork with prongs is fulcrummed on a bracket fixed to the front of the sley. When the sley moves towards the back centre, the fork tilts upwards through the warp far enough to allow the shuttle to pass underneath and the weft is laid under the fork. During the forward movement of the sley the fork drops downwards upon the weft and is held from moving further down in the channel by the grid effect of the warp threads belonging to the bottom shed, supporting the weft thread against the light pressure of the fork. In this condition the weft fork holds the knock-off arm away from the knock-off lever. The fork is pulled out of the shed just before the reed reaches the fell of the cloth for the beat up of the weft. If, however there is no weft underneath the fork as the sley moves forward, the fork drops into the channel in the sley and the knock off arm D is moved into contact with the knock-off lever G thus stopping the loom. One important device which is necessary in all the centre weft fork motions is to enable the loom to restart after the weft replenishment without the presence of a weft thread across the shed. This means that the knock-off arm should be made ineffective for the first pick without the help of the weft thread.

A shield has been provided in all such motions to enable the sley to move forward, on the first pick, without stopping the loom. On successive picks the shield moves out to enable the weft thread to act as a preventive device to knock-off the loom.

The centre weft fork motion shown in figure has two important parts. The first part, the weft fork, is attached to the sley and moves with it. The second part consisting of, cam, knock-over lever, brake lever, the rod that connects the mechanism to the shipper lever, all attached under the breast beam, which is stationary.

The weft fork A is pivoted in a stud and is connected to a lever B pivoted in a bracket on the lower end of the stand by a connector rod C. An adjustable knock-off arm D which is connected to the lever B slides over the face of the cam E projecting from the breast beam assembly. The knock-off arm D is held against the cam face by a special spring S on the opposite end of the lever.

During the backward movement of the sley the fork is raised, and during the forward movement it drops down. The projecting stand F mounted under the breast beam has a knock-off lever G on one side and a first pick shield M on the other. The knock-off lever projects above the lug stop of the stand F. If the weft thread is not holding the fork from falling down in the sley channel, that is the absence of the weft, the knock-off arm D will follow the cam E during the forward movement of the sley and engage the knock-off lever G. When the knock-off lever is pushed back by the knock-off arm, a round bracket on the lower part of the lever will press a brake tube lever, turn the brake and stop the loom.

Immediately the loom is knocked-off, a flat spring J clamped to the shipper shaft pushed back the first pick shield through an intermediate lever N. Since the shield M is held by pins that follow the curved shape of the cam slots, a push at the back will enable it to rise above the top of the stand F and also above the top of the knock-off lever G. When the loom is started after the repair of the broken pick, the flat spring is moved away from the lever N but the shield F stays in position owing to the dwell in the cam slots. On the first pick the advancing knock-off arm, strikes the end of the cut out O in the shield pushing it forward into the normal position.

Improved centre weft fork motion:

One disadvantage of the motion described is that the whole mechanism is difficult to approach for any adjustments. The looms, namely, the German Zang, the Swiss Ruti and the American C & K for silk have designed to have all the working parts in an easily accessible position at the side of the loom. Therefore setting and adjustment of the parts is made easier.

Problems associated with centre weft fork motion:

1. Weft curls in the middle of the cloths.

Causes:

a) The prongs of the forks press the weft through the bottom shed.

b) Early or strong picking.

c) Irregular loom speed.

Remedies:

a) Correct tensioning of weft in the shuttle.

b) Shortening the prongs a little in case of rayon weft.

c) A longer setting for the prongs in case of nylon weft.

2. Loom stopping constantly although weft has not broken.

Causes: