Keighley Dobby (negative- double lift)

DOBBY MECHANISMS -KEIGHLEY DOBBY (NEGATIVE- DOUBLE LIFT)

Fall Term Project

Submitted by:Aybala ÖZCAN

Submitted to: Prof.Dr.Mehmet TOPALBEKİROĞLU

10.01.2013

DOBBY MECHANİSMS

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 30heald

frames. Picks per repeat are virtually unlimitedin dobby shedding. Due to their complexity, dobby mechanisms are moreliable to

produce fabric faults than cam systems.

DOBBY MECHANİSMS

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

and lowers the heald shafts. Today, the trend is away from negative dobby to

electronically controlled positive dobby mechanisms, whichcan operate at very high speeds.

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

Mechanical Dobby Electronic Dobby

The mechanical dobby chain and pegs are used

Computer controlled shaft selection

The construction of shaft sequences is done by building a mechanical dobby chain.

Construction is done on computer screen with weave software’s.

A little bit tougher way to design fabric.

More intuitive way to design fabric.

Unable to visualize the repeated designs in fabric in mind.

Can visualize the repeated weave design on the screen.

Loading and swtching of weave drafts takes longer time.

Loading and switching of weave patterns can be done in seconds without getting up from the loom.

Tedious work in designing and producinh fabric.

Emove tedious work in designinh and producing the fabric.

DOBBY MECHANİSMS

Depending on their performance they are divided into single-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 original positions 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 MECHANİSMS

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 are eliminated.

• Suitable for high speed operations.

DOBBY MECHANİSMS The dobby consists of three principal mechanisms:The drive mechanism:An auxiliary shaft is permanently

drivenfrom the weaving machine.-It operates two steel bars (knives), having a regular

reciprocating motionor-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 thelifting mechanism.

The lifting mechanism operates the heald shaft motion.

DOBBY MECHANİSMS

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 and lowers the heald shafts.

Today, the trend is away from negative dobby to electronically controlled positive dobby mechanisms, which can operate at very high speeds.

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

Selection electro magnetic unit

ratchet

jack

drive

Lifting:controllable crank disk (cam unit) and jack

STÄUBLI ROTARY DOBBY OF THE SERIES 2600

Jack

Cam Unit

Connecting Rod

DriverRatchet

Control Lever

Selector

Magnet Bar

Selection:1-magnetbar; 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

In the diagram, the heald shaft has been raised by moving thetop end of the baulk B away from its stop bar S

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

The knife K was able to engage the hook H, because a peg in the lag forming part of the pattern chain had raised the right-hand end of thefeeler F, which thus allowed the rod R to lower the hook H, onto theknife K . In the diagram, there is no peg to support the right-hand end of the feeler F, which has therefore fallen, this fall allowing its upturned left-hand end to raise the hook Hclear of the knife K 

KEIGHLEY DOBBY (NEGATIVE- DOUBLE LIFT)

As the action continuous, the top end of the baulk will be returned toits 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 pickNecessarily takes place while the hook is in contact with its stop bar because only then can the hook engage or disengage its knife

THE MECHANİSM OF PEGGING THE LAGS FOR A KEIGHLEY DOBBY

THE MECHANİSM OF PEGGING THE LAGS FOR A KEIGHLEY DOBBY

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 of the 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 the weave in order to have a pattern chain sufficiently long to encircle therotating barrel that presents the lags to the feelers.

HEALD SHAFT CONNECTİONS FOR NEGATİVE DOBBİES There are many different methods

of connecting the centers of the baulksto their heald shafts.

Negative dobbies are always designed to raise the shafts, and theconnections are therefore made between the baulks and the tops of theshafts.

In diagram A, the main jack, J, has a fixed fulcrum at F. It is connectedby a link L to a secondary jack, J, which is fulcrummed at F.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 bead justed.

HEALD SHAFT CONNECTİONS FOR NEGATİVE DOBBİES

In diagram B, an elbow lever, L,which is fulcrummed at F, carries atoothed quadrant , Q, which engages a similar toothed quadrant, Q. This iscarried on an extension of lever L, which is fulcrummed at F. The upperend of the lever L, 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:FOURPOSSIBLE ARRANGEMENTS

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

HEALD SHAFT CONNECTIONS FOR NEGATIVE DOBBIES

Type of connections which necessitate thepositioning of bearings, and of numerous metal partsin rubbing contact with each other,above the warpmay causesome problems.

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

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

SPRİNG UNDERMOTİONS FOR NEGATİVE DOBBİES

Similar to the cam shedding, spring reversing motion is an inefficientarrangement 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 weaving lightweight 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 spring tension when the shafts are raised, is the Kenyon undermotion.

DİSADVANTAGES OF NEGATİVE DOBBİES

Negative dobbies consume much energy to overcome the warp tension, the heald shaft weight and the reversing motion pull during heald shaft lifting.

This results in increased irregularity in the operation of the weaving 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 which operate with a constrained heald shaft motion in bothdirections, i.e. with positive heald shaft lifting and lowering.

REFERENCES

1-http://tr.scribd.com/doc/54758469/8-Weaving-C4c-Dobby-09

2-http://tr.scribd.com/doc/104507645/Shedding-Dobbies

3-http://tr.scribd.com/doc/62223628/E-DOBBY