Positive Dobby
  • Positive Dobby



    positive dobby 

    When it comes to weaving heavy fabrics, such as fancy worsted and woolen fabrics, it is appropriate to use positive dobbies that do not require a spring return device. There is no recovery device in these systems. All movements of the frames are carried out by the existing dobby mechanism.

    Since there is no return device in positive dobby systems, there is a need for some mechanism to ensure that the swing arm, which has been pulled forward with the hook, is brought back up to the stopper traverse, or that one or both ends of the swing arm, which needs to be pulled forward, are kept against the stopper traverses. On the other hand, since the vertical component of the warp tension will cause the frames to stay in the closed shed position in the middle, uncontrolled movements of the swing arms should also be prevented.






    seen above When the mechanism is examined; Retraction traverses B1 and B2 are rigidly connected to the blades K1 and K2 and move with them in the horizontal direction. The blades only pull forward the hooks attached to them. The backward movements of the hooks are provided by the retraction traverses pushing the relevant end of the swing arm up to the stopper traverses (S1, S2). The hook of the frame that will not lift up is held up by means of the sinker and remains attached to the hook traverse located at its level; therefore, the blade cannot move with it as it goes forward. In this way, it is ensured that the end of the swing arm connected to the hook is held against the stopper traverse until the new pattern is read. In this kind of dobby, pattern reading is done by means of pattern cardboards.

    as below An example of a mechanically controlled positive dobby forming a closed mouth with a single stroke in the middle is seen. This type of dobby is used for thick woolen fabrics.






    Frames numbered 1 in the dobby are connected from the center with the vertical arm numbered 2 placed on the swivel joint. A and B have a number 3 hook with elbows and attached to the arm at point O. The rotational movement of the 9 main shaft is transmitted to the 7 and 6 blades, which move back and forth on the slides with its 4 cranks and 5 arms. When arm 3 reaches its upper position, hook A is pushed to the left with its blade 4. In the meantime, 3 limbs connected to the 2 arm from the O2 point take a rotational movement in the counterclockwise direction on the O1 rotation axis. As long as the selection mechanism 3 does not change the position of the arm, the voluntary movement produced by the dobby 2 carries the frame 1, which is connected to its member with a belt, between the middle position and the upper position. When arm 3 comes to its lower position, hook B is pulled to the right with its blade 5 and its 2 limbs rotate clockwise on the O1 rotation axis. As long as the movement program does not change, the second voluntary movement produced by the dobby mechanism moves the 1 frame between the middle position and the corresponding position. In order to move the position of the 3rd arm according to the desired movement program, the selection mechanism consists of a hexahedral prism number 10 and a chain number 11 with pattern links of two different diameters.

    as below Electronically controlled positive dobby forming a closed mouth with a single stroke in the middle is seen.






    as above The view of this dobby in three positions is given. In this dobby, which works according to the closed shed principle in the middle, all the frames come to the middle of the shed after each weft insertion and the selection process is made for the next weft.

    Then, according to the pattern, the frames move up or down to form the shed for the new weft. Therefore, the dobby shaft rotates at the same speed as the weaving machine. This dobby, which can drive up to 16 frames with a pitch of 30 mm, is produced with different pitches and can be attached to the weaving machine in different positions.

    As the working principle of the dobby is seen in the figure above, two blades (1 and 2 blades), driven by a cylindrical cam pair, oscillate in opposite directions in each weaving machine revolution from the middle position. The position of the 6 hooks, which are connected to the lower end of the 3 arm with a rotary joint, is controlled by the 4 and 5 pneumatic pistons. In the first part of the figure, with piston 4 moving to the forward position and piston 5 moving to the back position, hook 3 is attached to blade 1. The blade 1 is moved clockwise along the axis of rotation Oa by dragging the handle 4 of the blade.

    This movement corresponds to the upper muzzle position of the frame. In the second part of the figure, the blades are in the middle position. In this position, the selection process is made according to the movement plan of the frames. If the frame is to move to the lower position, the hook 5 is attached to the blade 4, with the piston 3 moving forward and the piston 2 moving back. By dragging the 2 blades and 4 arms, O2 moves counterclockwise on the rotation axis.

    This position corresponds to the bottom position of the frame. The movement of the 4 and 5 pneumatic pistons is electronically controlled by a pneumatic electromagnet valve. Since the movement of the frame in both directions is provided by the dobby knives, this dobby is in the positive dobby class.

    as below The kinematic diagram of the electronically controlled positive SKN dobby forming a closed mouth with a single stroke in the middle is shown. Two blades are used in the dobby to transmit motion to the frames. The frame comes to the top position with the 3 hooks of the number 2 blade to the left, and the bottom position takes place during the movement of the number 8 blade with the upper or lower shoulders of the 9 arm to the right. The stay of the frames in the upper position is achieved by simultaneously displacing all four of the blades with their hooks.





    The suspension of the frames in the lower position occurs when both hooks are separated from the blades 3. In the meantime, the hooks 2 are held on their immovable supports 6 with the help of the executing arms 4 and the movable supports 10 .

    A punched card (11) is used as the program carrier in the selection unit. In order that the 12 pins do not force the card, the unit is equipped with 1 blades and the number 13 horizontal pins that they manipulate. If the card is punched, pins 12 move down, putting pins 13 in contact with blades 1. The needles 13, displaced to the left, rotate the arms 6 along their own axis, causing the hooks 2 to come into contact with the blades 3.

    The dobby has four main action knives, two selection knives, and four rows of needles, preventing the dobby from working at high speeds. On the other hand, in order to read the program in the selection unit, the contact of the needles with the card is provided by a complex mechanical system.

    Kinematic diagram of Staubli 2232 type number dobby working with the same working principle It is seen as below. In the dobby, the action is given to the blades 4 and 4' from 1 and 1' cams. The 12 and 12' reinforcing blades assist in reducing the pressure between the needle and the card during program reading. The disadvantage of this dobby is the same as SKN dobbies.






    Maximum 300 rpm of this dobby. It works fast and its practical working speed is 250 rpm.

    Mechanically controlled forming single-stroke open nozzle as below

    • Positive Knowless dobby
    • English Dobcross dobby
    • American Crompton-Knowless dobby

    It is used in worsted and woolen weaving machines and its working principle differs from the systems mentioned in the previous sections.





    In this dobby, each frame is moved by a two-shouldered No. 1 lever. The upper shoulder of the lever is connected to the 2nd limb and the 4th gear that is movably placed on the 6th arm. The program carriers number 10, placed in the pattern chain number 16 of the selection unit number 9, come into contact with the lever indicated with the number 4, and the gear number 3 engages with the gears number 7 or 12 rotating in the opposite direction with constant speed.

    When the 6 gear comes into contact with the 7 gear, since it takes a rotational movement in the clockwise direction, the arm number 2 connected to it moves to the right and as a result brings the frame to the upper position. If the 6 gear comes into contact with the 12 gear, while the 6 gear moves in the opposite direction, the 2nd arm will move to the left and bring the frame to the lower position.

    In order to enable the gear, which rotates at constant speed, and the stationary gear to engage and to separate from contact after taking 180° rotation, one tooth of the 6 gear is taken in the engagement area and three teeth on the opposite forehead. A 7° surface of the 12 and 180 gears is covered with teeth. The realization of waiting in the upper and lower positions and the stable operation of the gears are provided with the help of 11 -24 cam-arm mechanism.




    Posted by %AM, 31% 354%2019 10%:%Jul in Weaving Read 1697 times

Positive Dobby