Shedding Dobbies

1

SHEDDING

DOBBIES

Prof. Dr. Emel ÖnderAss.Prof.Dr.Ömer Berk Berkalp

TEK332E Weaving Technology II Emel Önder & Ömer B.Berkalp 2

Dobby mechanisms are more complicated than cam systems,

have higher initial and maintenance costs,

can produce more sophisticated weaves,

are normally built to control 12, 16, 20, 24, 28 up to 30 heald frames.

Picks per repeat are virtually unlimited in dobby shedding.

Due to their complexity, dobby mechanisms are more liable to produce fabric faults than cam systems.


Dobby mechanisms Dobby mechanisms are classified as negative, positive and

rotary dobbies, they can be mechanical or electronic.

In negative dobby shedding, the heald frames are liftedby the dobby and lowered by a spring reversing motion.

In positive dobby shedding, the dobby both raises andlowers the heald shafts.

Today, the trend is away from negative dobby toelectronically controlled positive dobby mechanisms, whichcan operate at very high speeds.

Rotary dobbies have been largely replaced the Hattersleydobbies in the last decade.


Dobby mechanisms

Depending on their performance they are divided intosingle-lift and double-lift dobbies.

Single-lift dobbies are the oldest. All system elements perform their function once every weaving

cycle to open a shed, and then they return to their originalpositions before a new cycle.

The shed is closed after every weft insertion and the pick is beatenup at the closed shed (center-closed shed)

Advantageous in the wool weaving in the past.

The speed of single-lift dobbies is limited to 160 to 180 rpm.


Dobby mechanisms

Double-lift dobbies:

All new type dobbies are double-lift in their action.

Its cycle occupies two picks.

System elements operate once every two weaving cycles, butthe shed opening is achieved every pick.

Most of the motions in dobby occur at half time loom speed.

Open shed is produced; unnecessary, wasted movements areeliminated.

Suitable for high speed operations.


Dobby mechanisms The dobby consists of three principal mechanisms: The drive mechanism: An auxiliary shaft is permanently driven

from the weaving machine. It operates two steel bars (knives), having a regular reciprocating motion

or

It operates coupling rings on a rotary dobby.

The selection mechanism: It is operated by a dobby card (or by some form of pattern chain or by a magnetic disc.)

It reads or checks the design information punched on a dobby card, and

transmits the necessary movement from the drive mechanism to the lifting mechanism.

The lifting mechanism operates the heald shaft motion.


Keighley Dobby (negative- double lift)Drive Mechanism:2 knives are permanently driven from a crank mechanism and reciprocate in slots within certain limits.

Lifting Mechanism

Selection Mechanism


Keighley Dobby (negative- double lift)

The stop bars S1 and S2 extend thefull depth of the dobby.

The knives K1 and K2 reciprocate inhorizontal slots. They complete onereciprocation every two picks, and inthis case they are driven from a crankon the bottom shaft.

The heald shaft is connectedindirectly to the center of the baulk B

Drive MechanismLifting Mechanism

Selection Mechanism

heald shaft connection

For each heald shaft: a baulk B two hooks H1 and H2

two feelers, F1 and F2.


Keighley Dobby (negative- double lift) In the diagram (Fig.4.24), the heald shaft has been raised by moving the

top end of the baulk B away from its stop bar S1.

This has happened because the top knife K1 has previously engaged thehook H1, and has drawn the top end of the baulk B away from its stop barS1, this action causing the baulk to pivot about the point of contactbetween its lower end and the stop bar S2.

The knife K1 was able to engage the hook H1, because a peg in the lagforming part of the pattern chain had raised the right-hand end of thefeeler F1, which thus allowed the rod R to lower the hook H1, onto theknife K1. In the diagram, there is no peg to support the right-hand end ofthe feeler F2, which has therefore fallen, this fall allowing its upturned left-hand end to raise the hook H2 clear of the knife K2.


Keighley Dobby (negative- double lift) As the action continuous, the top end of the baulk will be returned to

its stop bar, and at the same time the bottom knife will move to theright without disturbing the bottom end of the baulk.

The shaft will therefore be lowered and will remain down for the nextpick. In the absence of a peg, the shaft is lowered or remains down.

A peg will lift the shaft, and a succession of pegs will keep the shaftraised.

Some form of spring undermotion, acting through the shaft and itsconnections to the baulk, keeps one end of the baulk in contact withits stop bar while the other end is being displaced. Alternatively, itkeeps both ends of the baulk in contact with the stop bars when theshaft is not being raised.


The full cycle of operations by the simplified line diagrams.

In each diagram, the knives are shown at one extreme of their movement.Selection for the next pick necessarily takes place while the hook is in contactwith its stop bar because only then can the hook engage or disengage its knife.


Card Cylinder


The method of pegging the lags for a Keighley dobby

Pattern barrel


The method of pegging the lags for a Keighley dobby Each lag serves for two picks.

The holes in the lags are staggered to correspond with the positions ofthe feelers.

The pattern barrel is turned intermittently by a Geneva wheel orsimilar motion so as to present a new lag every second pick.

In the diagram of the lags, a filled circle represents a peg.

In practice, it would be necessary to peg two or more repeats of theweave in order to have a pattern chain sufficiently long to encircle therotating barrel that presents the lags to the feelers.


Heald shaft connections for negative dobbies There are many different methods of connecting the centers of the baulks

to their heald shafts. Negative dobbies are always designed to raise the shafts, and the

connections are therefore made between the baulks and the tops of theshafts.

In diagram A, the main jack, J1, has a fixed fulcrum at F1 . It is connectedby a link L to a secondary jack, J2, which is fulcrummed at F2. The healdshaft is suspended from cords attached to the outer ends of the two jacks.Alternative points of connection allow the lift of individual shafts to beadjusted.

In diagram B, an elbow lever, L1 ,which is fulcrummed at F1, carries atoothed quadrant , Q1, which engages a similar toothed quadrant, Q2. This iscarried on an extension of lever L2, which is fulcrummed at F2. The upperend of the lever L1, is connected by a strap S to the top of the jack J. Thispoint of attachment provides adjustment for the lift of the shaft.

A disadvantage of the systems of connection shown in C and D is that thetwo ends of the heald shaft do not necessarily receive the same lift.


Heald shaft connections for negative dobbies: four possible arrangements

In each diagram, the arrow represents the link from the center of the baulk to the heald frame .


Heald shaft connections for negative dobbies

Type of connections which necessitate the positioningof bearings, and of numerous metal parts in rubbingcontact with each other, above the warp may causesome problems.

It is essential to fix a tray underneath the moving partsto catch oil drips, which are always heavilycontaminated with dark-colored metallic impurities.

Stains on the warp produced by dirty oil are verydifficult to remove, and, if not completely removed,may cause tendering of the yarn during bleaching.


Placement of negative dobby


Spring undermotions for negative dobbies Similar to the cam shedding, spring reversing motion is an inefficient

arrangement because the tension in the springs is the least when theshafts are down, which is when the springs should be performing boththeir functions.

Nevertheless, this simple system has been widely used for weavinglightweight fabrics, which can be woven with a relatively low warptension.

As the warp tension increases, stronger springs have to be used toovercome the vertical component of the warp tension. This tends tobend the heald-shaft frames when the shafts are lifted and the springsare fully extended.

A widely used device, which is designed to avoid an increase in springtension when the shafts are raised, is the Kenyon undermotion.


Spring undermotions for negative dobbies

In negative dobby, spring undermotion keeps the ends of the baulks in contactwith their stop bars when they are not being displaced by the action of the dobby.

Two coil springs for each shaft are stretched between the shaft and a horizontalbar fixed to the loom frame near the floor (A).

One variation of a more modern application as used by Rüti (B), a similar unit ateach side of the heald shaft

Kenyon undermotion

x


Spring undermotions for negative dobbies

T is the tension in each of the connections to the heald shafts and S is the tension in the spring, then, by taking moments: TD= Sd,

DSdT

As the shaft rises, the spring will stretch and S will increase, but only slightly. Atthe same time, d will decrease substantially, and D will increase slightly. The slightincrease in D and the large decrease in d as the shaft rises both tend to reduce TThe tension T will therefore tend to decrease as the shaft rises, which is what werequire.


Ruti Dobby Knives K1 and K2 are actuated bycams C1 and C2 mounted on thecamshaft C.S.

The cams are negative in action, andthe cam followers are kept incontact with the cams by means ofsprings not shown in the diagram.

The desired dwell can be obtained.

The inclined knife track facilitatesthe hook engagement.

Together with the inclined knifetracks this drive produces a morecompact and efficient motion.

This feature is commonly found onmodern dobbies.


Selection Mechanism for a Paper Controlled Dobby In another development of the Keighley dobby, the chain

of lags is replaced by a loop of very tough paper or plasticsheet.

Pegs tend to wear, break, and fall out, which produceswrong lifts and faults in the fabric. Paper control eliminatesthese faults.

Holes punched in the paper correspond to pegs in the lags,a hole causes the corresponding shaft to be lifted.

Light feelers are used to detect the presence or absence of ahole, but the force required to move the hooks is notsupplied by feelers.


Selection Mechanism for a Paper Controlled Dobby


Selection Mechanism for a Paper Controlled Dobby Selection needles n1 and n2 are lowered onto the paper pattern by a cam

control. If there is a hole, the corresponding supplementary hook, h1 or h2, is

lowered into the path of one of the supplementary knives k1 or k2, whichthen moves the corresponding vertical rod, r1 or r2, out of the path of thevertically reciprocating lifting block, b1 or b2.

In this case, the main hook H1 is lowered onto its knife, and the healdshaft is lifted.

If there is no hole in the pattern, the corresponding lifting block, b1 or b2,lifts the main hook out of the path of its main knife, and the shaft is notlifted.

As a result of this indirect action, very little force is exerted by the selectionneedles on the pattern paper, which consequently has a long life.


Card cutting and copying machines

The paper pattern takes less time to prepare than the corresponding set of lags unless the repeat is small, in which case several repeats need cutting for the paper-pattern method.


Card cutting and copying machines


Disadvantages of Negative Dobbies

Negative dobbies consume much energy to overcome thewarp tension, the heald shaft weight and the reversingmotion pull during heald shaft lifting.

This results in increased irregularity in the operation of theweaving machine.

Another disadvantage is the somewhat uncertain bottomposition of the heald shaft and an irregular shed is formed.

To avoid this, positive dobbies have been developed whichoperate with a constrained heald shaft motion in bothdirections, i.e. with positive heald shaft lifting and lowering.


Knowles Positive Dobby: single- lift

Single lift dobby


Knowles Positive Dobby: single- lift


Positive Dobby (Hattersley dobby)

For weaving heavy fabrics, such as fancy woolens and worsteds,it is better to use a positive dobby, which eliminates the need fora spring undermotion.

It is necessary to provide means for returning the ends of thebaulks to their stop bars and for holding them there.

This can be achieved without radical alteration to the Keighleydobby.

The dobby just described happens to be paper-controlled, thecontrol mechanism.


Positive dobby (Hattersley dobby)

Drive

Selection

Lifting


Positive Dobby (Hattersley dobby) Push bars B1 and B2 are rigidly connected to the knives K1 and

K2. and reciprocate with them. As a knife returns after displacinga hook, its push bar pushes the corresponding end of the baulkagainst its stop bar.

The locking bar L1, then engages a notch in the hook becausethe hook has been pushed up by the selection mechanism. Thisprevents that end of the baulk from moving until the timearrives for the next selection.

In the diagram, the locking bar L1 will hold the bottom of thebaulk against its stop while the knife K1 and the push bar B1make one complete cycle.

Note that the locking bars can engage and disengage only at thetime of selection, when the corresponding end of the baulk isagainst its stop bar.


Staubli - 2212


Staubli - 2232


Staubli dobby-Type 300hook 5 is pulled to the right by the knife 9 or pushed to the left by the arm 8

two hooks 5 and 5a are mounted on common baulk 6.

The Type 300 dobbies were mounted on the majority of the gripper projectile and rapier weaving machines.


Staubli dobby-Type 300 The heald shaft lowering is about to be completed and the heald shaft

approaches the bottom position. Hook 5 still engages knife 9 and ispushed by arm 8 (not shown) to the left.

When the heald shaft has attained the bottom position, knife 9 is turnedanti-clock wise through angle . As a result of this hook 5 is lifted andengages with its upper claw the fixed bar 14.

If the heald shaft is to say at the bottom position during the nextrevolution of the weaving machine, the selection mechanism releases pin3; support level 12 is adjusted, due to the action of spring 13, under thehook and locks the latter in the disengaged position.

During the next machine revolution hook 5 stays locked between stop 7and bar 14. Knife 9 is rotated back and together with arm 8 is moved tothe right out of the hook.

If the heald shaft is to be lifted during the next machine revolution, theselection mechanism does not release pin 3 support lever 12 stays at rest.During the reverse rotation of the knife the hook is re-engaged and theheald shaft lifted


Hattersley dobby

Presently, only double-lift dobbies are manufactured.

In the Hattersley system long hooks of grey cast iron wereadjusted in horizontal position; they engage the knife by theeffect of dead weight.

The highest speed attained on the shuttleless weavingmachine is 500 rpm, which is equipped with a modifiedversion of Hattersley dobby where the hook mass has beenconsiderably reduced.

The biggest obstacle on the way to further speed increase isthe hook mass.


Rotary dobbies In order to achieve dobby shedding at faster

speeds, new generation rotary dobbies aredeveloped and introduced.

A rotary dobby can allow weaving machine speedsup to 1000-1500 rpm.

The term 'rotary' was chosen because the straight-line motion of the heald frames is derived fromrotating elements in the dobby.


Rotary dobbies ‘A controllable crank mechanism’, known as ‘cam

unit’, is built into dobby.

Each heald frame is controlled by a cam unit, a compactelement only 12 mm wide, which converts the irregularrotation of the cam shaft directly into the linear motionrequired for the heald frame drive.

The cam unit consists of a heart-shaped crank discwhich encloses a cam with ball bearings plus amovable key which is the only controlled part.


Heart-shaped crank disc

A positive dobby machine that operates on the rotary principle with pattern card control.

cam

ball bearings

coupling ring

key


Functional principle :The key connects the cam with thecoupling ring fixed on the mainshaft and makes the cam at theend of the crank disc, whichrotates in cycles of 1800, perform alifting motion. The indexing armcontrols the key according to thepattern. The reading device is aself-contained unit of very simpleconception. The pullers moveforwards and backwards andtransmit the information read bythe feeler needles via tractionelements and indexing arms to thekeys.

The reading device of the rotary dobby operates true to the picking sequence, regardless of the position from which the direction of rotation is reversed. With the combination of feeler needle, traction element, indexing arm, key and cam unit - a fundamental invention patented by Staubli -a control system with unsuperpassed safety factor has been created, unique in its simplicity and efficacy.


Mounting location of a rotary dobby on a weaving machine.


Rotary Dobby


Principal mechanisms on a rotary dobby Drive mechanism (Modulator):

Modulator (complementary cams), cam shaft and couplingring

The modulator transforms the regular rotary motion of theweaving machine into an irregular rotary motion.

By the use of complementary cams precise laws of motionresults, meeting the requirements of any type of weavingmachine.

Irregular rotary motion means that the cam shaft turns through1800 and then pauses momentarily for a new selection.

Coupling ring fixed on the cam shaft has the same irregularrotary movement.


Principal mechanisms on a rotary dobby

Selection mechanism: A ratchet placed on the outside of the cam,

enclosed by the crank, connects it with the driver. The ratchet is controlled according to the pattern by

the control unit which may be mechanical orelectronic.

A 180° rotation of the cam causes a lifting motion.


Principal mechanisms on a rotary dobby

Lifting mechanism (cam unit): The essential element is a crank mechanism

enclosing a cam with ball bearings. The cam unit is mounted on the cam shaft but not

fixed. Cam is eccentric and can freely rotate thanks to the

ball bearings. The ratchet placed on the outside of the cam

connects it with the driver, and by a 1800 rotation ofthe cam causes a lifting motion.

TEK332E Weaving Technology II Emel Önder & Ömer B.Berkalp 48FIMTEXTILE FIMTEXTILE -- RD 3000RD 3000

Drive

ratchet

Selection, electro magnetic control unit

Lifting: controllable crank disc (cam unit) and jack

jack


Stäubli Rotary Dobby of the Series 2600

JackMagnet Bar

Selector

ControlLever

RatchetDriver

ConnectingRod

Cam Unit


Stäubli Rotary Dobby of the Series 2600

Selection:1-magnet bar; 2-selector; 3-control lever

Harness frame motion: 4- ratchet ; 5-driver; 6- cam unit; 7-connecting rod; 8- jack

Drive: modulator


Stäubli Rotary Dobby of the Series 2600 The high performance rotary dobbies of Series 2600 with electronic

controls operate according to the rotary principle .

According to the pattern, the control elements transmit electrical signalsto magnet bar 1. This moves corresponding selector 2 which- dependingon its state on the magnet bar- presses on one of the two control levers 3.If ratchet 4 is underneath the control lever which is pressed down, theratchet is released (see Fig 4.35).

When the ratchet is engaged with the driver, it then couples cam unit 6.By this process the motion of the cam shaft is also transferred to the camunit. By way of connecting rod and jack the harness frame is then moved.

If the harness frame is to be moved to one of its end positions, thecontrol lever 3 which has not been pressed down, pushes ratchet 4 byspring force out of the driver 5, so that the harness frame remains eitherin upper or lower shed.


Rotary Dobby

TEK332E Weaving Technology II Emel Önder & Ömer B.Berkalp 54FIMTEXTILE FIMTEXTILE -- RD 840RD 840


M’ = middle shed position overdrawn upwards

M’’ = middle shed position overdrawn downwards

1.Jack in position A

2.Jack in position A= pile harness frame in upper shed H

1.Jack in position B

2.Jack in position B= pile harness frame in lower shed T

1.Jack in position A

2.Jack in position B= pile harness frame in middle shed M

1.Jack in position B

2.Jack in position A= pile harness frame in middle shed M

ROTARY DOBBY TYPE 2490 ROTARY DOBBY TYPE 2490 FUNCTIONAL PRINCIPLEFUNCTIONAL PRINCIPLE


The attendance to faults in a weaving machine with dobbyI- On a double-lift dobby the heald shafts are never

all aligned at a time so that the piecing of a brokenwarp end would be extremely difficult.

Some dobbies are, therefore, equipped with a healdshaft leveler which operates on the same principle asin the multi-shaft cam operated shedding motion.


The attendance to faults in a weaving machine with dobbyII- With a double-lift dobby it is extremely difficult to find the

shed containing the broken weft thread. During the reversing movement of a double-lift dobby the heald

shafts remain at rest for the two revolutions of the weavingmachine before the correct shedding order is started.

When the dobby is switched back to forward motion, the healdshafts stay again at rest for two revolutions of the weavingmachine.


The dobby placing on the weaving machine


The commercially available dobbies The heald shaft dwell: Dobbies are equipped with cams to control the

dwell position of heald shafts at different settingangles; 900 for the narrow weaving machines up to 1.05 m in

width, 1200 for the weaving machine widths up to 1.35 m, 1500 for the weaving machines 2.2 m wide and 1600 to 2000 for the weaving machines of very large

widths, especially the shuttleless weaving machines.


The commercially available dobbies The dobbies for different heald shaft pitches: The most common pitch of the heald shafts, and

consequently that of the dobby mechanism, is 12mm.

Only in special instances dobbies with otherpitches, i.e. 10, 14, 16 and 18 mm, aremanufactured.

Every unconventional pitch must be discussedwith the dobby manufacturer in advance and thepossibility of card punching for the unconventionalpitch must also be considered.


The commercially available dobbies The dobby lubrication: The dobby is divided by vertical planes whose spacing corresponds to the

heald shaft pitch, i.e. 12mm. These very narrow spaces mustaccommodate the individual dobby mechanism, which must operateseparately and independently of each other.

The oil-bath lubrication is impracticable, because a dobby oil box wouldhave to contain 150 to 200 liters of oil.

From this viewpoint, the only solution is the oil-spray lubrication becauseit needs only 3 to 5 liters of oil per dobby.

However, the dobby must be provided with a good dust-proof cover. Placed in the oil tank is a small gear pump which pumps the oil

continuously into the upper space above the dobby. The upper spacecontains distribution pipes with openings or a distribution pan providedwith openings in the bottom. The dripping oil lubricates perfectly andpermanently all parts of the dobby.

Shedding Dobbies

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