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Similar to Abhi Rana)Dobby Shedding Mechanism
Abhi Rana)Dobby Shedding Mechanism
- 2. REQUIREMENT The cam sheddingsystem has limitation in terms of number of healds that can be effectively controlled during shedding. The problem arises when the number of picks in the repeat of the design is very high. Let us assume that the design is repeating on 10 picks (8/2 twill). The number of healds required in this case will be 10 and to control these healds 10 cams will be required. These cams will be mounted on the cam (or tappet) shaft which will rotate at one tenth r.p.m. as compared to that of crank shaft. Therefore, one rotation of the cam shaft will ensure insertion of 10 picks. As 360° rotation of the cam corresponds to 10 picks, one pick becomes equivalent to 36°. If dwell is one third of pick then the small dwell period (when heald is down) in this case will be = 1/3×36° + 36° = 48°. The duration of movement of heald (when the radius of the cam changes) will be 24° each for upward and downward direction. This will create the big dwell (when the heald is up) of 12° + 7×36° = 264°. Now, the above calculation reveals that the follower has to move from the lowest position to the highest position within 24° span which is available to it. The follower follows the contour of the cam profile which becomes steep when the span available for movement is low. Besides, the force acts on the follower in a direction which is perpendicular to the tangent drawn on the cam contour. However, the follower has to move vertically up or down. Thus an angle is created between the direction of applied force and the direction of movement of the follower. This angle becomes even more when the cam contour is steeper i.e. the span available for upward or downward movement is low which has resulted from higher number of picks in the design repeat. This leads to the fact only one component of the force applied becomes effective in creating the movement of follower. Thus very high force is actually required to create the desired movement of the follower which may lead to wear and tear as well as vibration in the system. One plausible solution to the aforesaid problem could be to increase the dimension of the cams (Figure 6.34). It can be observed that the cam contour, for a given span of follower movement, becomes less steep when the diameter of cam increases. However, this may create another problem in terms of power consumption and space availability in the system.
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- 4. INTRODUCTION Dobby is ashedding mechanism placed on the top of the loom in order to produce figured patterns by using large number of healds than the capacity of a tappet. Dobby is also known as a “witch or “wizard”. This is a compact, electronically guided shedding motion and capable of having up to 28 shafts. More complex and versatile shedding motion. A dobby loom, therefore, can have up to 28 shafts, and much greater weave repeat is possible. Design may be woven with two or more basic weaves and their variation. Such fabrics may be referred as dobby cloths or dobby weave, towels usually show geometric designs when pattern is provided by dobby.
- 5. SCOPE •It can producemore complex design. •Theoretically it can control maximum 48 heald frame but practically 36. •For cotton yarn it can control maximum 48 heald frame. •Production is less than tappet.
- 6. OBJECTIVES 1. To knowabout the different parts formed this shedding. 2. To learn the mechanism of dobby shedding.
- 7. MECHANISM Here a climaxdobby is described to explain the dobby shedding mechanism. Climax dobby is a double lift double jack-lever negative dobby. Here two jack levers are operated by a single bulk lever. They are joined with the bulk lever by a timber lever and a link. The double lever is fulcrum med at a point and is connected with S-lever. Dobby shedding mechanismThe S-lever controls two hooks, one upper hook and one lower hook. The hooks are controlled by feelers. One hook is controlled by hooked feeler and other by a connecting needle. The two hooks control two knives that are joined with a T-lever. The T-lever is fulcrum med and is controlled by an upright shaft and an L-lever. They get motion from motor and machine pulley. When the feeler comes in contact with a peg and when it is not in the contact it goes down.
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- 9. ADVANTAGES OF DOBBYSHEDDING ● In this loom many numbers of healds can be used for weave a figured fabrics. ● Particularly 12-24 healds can be used in a cotton industry to weave fabrics. ● In which fabrics are not possible to weave in tappet shedding loom and jacquard loom for increasing the production cost, to weave this kind of fabrics dobby are used extensively.
- 10. DISADVANTAGES OF DOBBYSHEDDING Comparatively cost is high than tappet loom. ● Less productivity than tappet loom. ● Less speed. ● The adjustment of dwell is complicated. ● Maintenance cost is so high.
- 11. CLASSIFICATION Dobby Shedding Working Principle Raising motion of healdframe According to card reading system
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- 13. CLASSIFICATION Raising Motion of HealdFrames Positive Drive Negative Drive
- 14. CLASSIFICATION Card Reading System EndlessPattern Card Magnetic Card
- 15. NEGATIVE DOBBY Principle ofNegative Dobby Shedding In this type of shed, lowering of the heald frame occurred mechanically.
- 16. NEGATIVE DOBBY Construction In thenegative shedding the heald frames are operated by the jack & lever. The levers are connected with the knife and the knifes are attached with the driving rod by means of connecting needle. A pattern is used here according to the weave plan. When the teeth of chain is come to the contact of chain drum then whole the arrangement moves together and lowering of the heald frame is occurred by the spring
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- 18. NEGATIVE DOBBY Working Principle Thelowering of the heald frame is happens here by spring or jack lever. When the pattern cylinder doesn’t find peg on the pattern drum then bauck lever and jack lever bring the heald frame in downward direction, therefore the the lowering of the heald frame is occurred by means of spring tension
- 19. NEGATIVE DOBBY Advantages Highly complexand critical fabrics can be weaved. At a time it can control many heald frame More faster than the positive dobby as well as over conventional loom Can produce close bottom shed Disadvantages In this shed only lowering of the shed is possible Due to more stress, yarn breakages more It is not good for heavier fabric It does not provide more high speed to the loom
- 20. POSITIVE DOBBY Principles ofpositive dobby In this type of shed lifting & lowering of the heald frame both is possible. lifting is occurred by means of jack & lever, and lowering is occurred by means of spring under tension.
- 21. POSITIVE DOBBY Construction andWorking Principle: Positive dobby shedding is the combination of three Cylinders, jack lever, spring and a shaft. Between the three cylinders a shaft is fulcrum in one side the upper cylinder moves as the anti clockwise and lower cylinder moves as the clock wise direction. The main cylinder when get motion from the shaft when it found peg or pattern plan then the cylinder attached with the upper half toothed disc. So that the heald frame is up,& when the pattern cylinder doesn’t found peg then the main cylinder attached with the lower half toothed disc and the spring retains the heald frame to the downward direction.
- 22. DOBBY LOOMS Positive Dobby Looms Keighley Dobby Double Lift DoubleJack Double Lift Single Jack Cam Dobby Paper Pattern Dobby
- 23. DOBBY LOOMS Positive DobbyLooms Knowle’ Positive Dobby Staubil Positive Dobby Rotary Dobby Cross Border Dobby Yamada Two Cylinder Cross Border Dobby Climax Cross Border Dobby Three Position Device
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