Dobby Shedding Mechanisms

Dobby Shedding Mechanisms

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, Dolby mechanism are more reliable to produce fabric faults than cam systems.
  • Dobby mechanisms are classified as negative, positive and rotary dobbies, they can be mechanical or electronic.
  • In negative dobby shedding, the heald frames are lifted by the lobby and lowered by a spring reversing motion.
  • In positive lobby shedding, the lobby both raises and lowers the heald shafts.
  • Today, the trend is away from negative lobby to electronically controlled positive lobby mechanisms, which can operate at very high speeds.
  • Rotary dobbies have been largely replaced the Hattersley dobbies in the last decade.

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 weave 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 (centre_closed shed).
       - Advantageous in the wool weaving in the past.
       - The speed of single-lift dobbies is limited to 160 to 180 rpm.

  • 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, but the shed opening is achieved     every pick.
       - Most of the motions in lobby occur at half time loom speed.
       - Open shed is produced; unnecessary, wasted movements are eliminated.
       - Suitable for high speed operations.

The lobby 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 monition. It operates coupling rings on a rotary lobby.

The selection mechanism: It is operated by a lobby 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: It operates the heald shaft motion.

Keighley Dobby (negative - double lift)

  • For each heald shaft
  • a baulk B
  • two hooks H1 and H2
  • two feelers, F1 and F2.

Keighley dobby,dobby shedding mechanism
  • The stop bars S1 and S2 extend the full depth of the dobby.
  • The knives K1 and K2 reciprocate in horizontal slots. They complete one reciprocation every two picks, and in this case they are driven from a crank on the bottom shaft.
  • The heald shaft is connected indirectly to the centre of the baulk B.   
In the diagram, the heald shaft has been raised by moving the top end of the baulk B away from its stop bar S. This has happened because the top knife k has previously engaged the hook H, and has drawn the top end of the baulk B away from its stop bar S, This action causing the baulk ton pivot about the point of contact between 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 the feeler F, which thus allowed the rod R to lower the Hook H, onto the knife 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 H clear of the knife K. 

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 the right without disturbing the bottom end of the baulk.The shaft will therefore be lowered and will remain down for the next pick. 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 shaft raised.

Some form of spring under motion, acting through the shaft and its connections to the baulk, keeps one end of the baulk in contact withers stop bar while the other end is being displaced. Alternatively, it keeps both ends of the baulk in contact   with the stop bars when the shaft is not being raised.


THE FULL CYCLE OF OPERATIONS of dobby shedding mechanism, dobby shedding mechanism

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 contact withers stop bar because only then can the hook engage or disengage its knife.


    dobby shedding mechanism types of dobby shedding
  • The method of pegging the lags for a Keighley lobby.
  • 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 or similar 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 the rotating barrel that presents the lags to the feelers.

                                DISADVANTAGES OF NEGATIVE DOBBIES 

  • 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 weaving machine.
  • Another disadvantage is the somewhat uncertain bottom position 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 both directions, i.e. with positive heald shaft lifting and lowering. 

dobby shedding mechanism,negative dobby shedding mechanism

positive dobby shedding mechanism, Hattersley Positive Dobby

Dobby Shedding Mechanisms Dobby Shedding Mechanisms Reviewed by Suraj Gupta on May 03, 2020 Rating: 5

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