Let-off mechanism
The function of the warp let-off mechanism is to deliver a quantitative amount of warp yarns with a certain tension in a timely manner during the weaving process according to the weft density requirements of the fabric structure, so as to maintain the continuous progress of weaving production. During the weaving process, the warp tension is uniform when the weft beam changes from large to small, which is the main indicator of the performance of the warp let-off mechanism. The size and stability of the warp tension depend to a large extent on the amount of warp let-off. Some fabric defects such as cloud weaving, long and short codes, etc. are caused by uneven warp let-off. Cloud weave is caused by uneven let-off of short segments, and long and short codes are caused by uneven let-off of long segments. The mechanical let-off mechanism has a complex structure and many connecting rods. The inertial force and friction of the machine parts will affect the sensitivity and accuracy of the let-off and tension control. The types of let-off mechanisms can be divided into three basic types based on the way the warp is sent out, namely: negative let-off mechanism, semi-positive and semi-negative let-off mechanism, and positive let-off mechanism. With the rapid development of electronic technology, the let-off mechanism has also been improved, and the electronic let-off mechanism has been produced. Electronic let-off has the characteristics of simple mechanism, sensitive response, precise operation, diverse functions, and simple operation, and is widely used in shuttleless looms. Due to the adoption of electronic let-off, the speed of air-jet looms and the appearance quality of fabrics have been greatly improved. At present, most of the warp let-off mechanisms of air-jet looms are electronic. 1. Warp let-off process requirements During the entire process of the weaving beam from full to empty, the warp let-off mechanism should be able to send out a certain amount of warp yarns with constant tension as needed to obtain a fabric with uniform density and flat cloth surface. 2. Classification of warp sending mechanism 1. According to the nature of its function, it can be divided into: passive type; semi-passive and semi-active type; active type. Negative let-off: During the weaving process, when the torque produced by the warp tension on the weaving beam is greater than the resistance torque produced by the braking force and other various resistances on the weaving beam, the weaving beam is dragged by the warp yarn and sends out the warp yarn, and the let-off action It is passive and relies on the tension of the warp yarn to pull the warp yarn out of the weaving beam. Using this kind of let-off mechanism, the woven fabric has a fuller appearance, but because it completely depends on the warp tension, the let-off length cannot be guaranteed to be uniform. Semi-positive and semi-negative let-off: It is equipped with a device to drive the rotation of the weaving shaft. The rotation amount of the weaving shaft is automatically controlled according to the warp tension. The amount of warp let-off is controlled by the warp tension, and the let-off action is active. The semi-passive and semi-positive let-off mechanism can be divided into three parts in terms of its structure: warp tension detection part; tension signal processing part and warp delivery part. This let-off method is widely used because the tension is basically constant and the let-off amount is accurate. Active let-off: Regardless of the warp tension during weaving, a constant amount of warp yarn is sent out according to the demand for warp yarn. The amount of warp let-off has nothing to do with the warp yarn tension. The let-off action is active. This method will cause disadvantages such as large tension fluctuations when the difference in yarn dryness is large. Mostly used for special fabrics. 2. According to the transmission properties, it can be divided into: mechanical let-off and electronic let-off. (1) Mechanical let-off mechanism (Figure 2-22): Transmission route: Pulley 1 driven by the main shaft drives continuously variable transmission 2. The output shaft O2 drives the weaving shaft 12 through gears 3, 4, 5, 6, clutch 7, worm 8, worm 9, gears 10 and 11 to send out the warp. The function of the brake disc 13 on the worm shaft is to prevent the idle inertia of the mechanism from producing excess let-off. The foot pedal 14 extends to the front of the machine. When the machine is stopped and the position of the weaving mouth is adjusted manually, the foot pedal can be stepped down to disengage the clutch 7, and then the handwheel is shaken to rotate the weaving shaft. Warp tension adjustment mechanism: During weaving, when the diameter of the weaving beam changes from large to small, the force of the warp tension on the back beam continuously collides with the tension frame and the connecting rod, and gradually increases, so that the output of the continuously variable transmission The speed increases until the warp tension drops back to the set value. In order to adapt to the need of uniform warp let-off, it plays the role of automatic adjustment. Start-up recoil device: The function of the recoil device is to remove the influence of warp elongation on warp tension caused by the loom being stopped, and to prevent the fabric mouth from shifting and causing a parking stall. In order to exert the recoil effect, the ZA loom is designed with a delay time of 0.215s in the starting circuit. Practice has proved that the backflush device has a certain effect on reducing fabric rungs. Buffer device: The buffer device of the let-off mechanism is a hydraulic buffer, which can reduce the vibration of the back beam due to shedding and beating up, and remove high-frequency fluctuations in warp tension (Figure 2-23). (2) Electronic let-off mechanism (Figure 2-24):
The electronic let-off mechanism adopts a tension sensor to detect changes in warp tension, and changes the speed of the AC servo motor through a computer to achieve the purpose of adjusting the warp tension. The warp mechanism can realize sensitive and careful control of the warp let-off amount, and can quickly adjust the warp tension changes. The difference in the total warp tension from the full shaft to the small shaft can be controlled to about 1kg. According to the set tension, the operation is automatically completed. The warp tension remains constant through self-adjusting work. The electronic let-off mechanism consists of: motor, deceleration device, warp tension detection device, microcomputer control part and active warp letting-off device. The warp tension acts on the tension spring 8 through the back beam 2 and the tension arm 7. The upper end of the spring core shaft is connected to the load cell 3, and the lower end is connected to the spring 8 with a screw.catch. The warp tension acts on the tension sensor through the tension arm, spring and core rod. The tension sensor sends out an electrical signal corresponding to the tension, and the microcomputer controls the rotation speed of the servo motor 1, and then drives the weaving shaft through the reducer 4, the small gear 6 and the large gear. Others are also equipped with an accessory device, a positive warp easing device. The eccentric wheel sleeve 9 of the warp easing device generates slack, and pushes the back beam 2 to swing once in each revolution of the main shaft, relaxing or tensioning the warp yarns in a timely manner, and balancing the tension fluctuations in the warp yarns caused by the shedding slack. The principle of the electronic let-off mechanism of the air-jet loom: The let-off mechanism of the air-jet loom can set the tension parameters on the keyboard according to the total warp tension required by the fabric. When the loom is running, the tension sensor detects the total tension of the warp yarns. The measured tension value is sent to the microcomputer control device, and the computer calculates the average tension of the warp yarn during one weft cycle of the loom, and then compares the calculated average tension with the set tension value. When the calculated value is greater than the set value , the servo motor increases speed and the warp tension decreases; the calculated value is less than the set value, the servo motor decelerates and the warp tension increases; if the two are equal, the original speed is maintained, and the let-off amount can ensure the constant total warp tension. At the same time, the diameter of the weaving shaft is calculated in real time according to the rotation speed of the weaving shaft. As the shaft diameter gradually becomes smaller, the rotation speed of the weaving shaft gradually increases. Features of the electronic let-off mechanism: ① The electric let-off mechanism can automatically remove the warp tension changes caused by inching reverse and forward rotation during warp and weft stops and shed operations before starting the machine, so as to avoid warp tension changes. The tension is unusual. ②The electronic let-off mechanism has the function of detecting and controlling the let-off amount. Therefore, the let-off amount can be accurately adjusted and compensated when the loom is restarted after stopping, eliminating the influence of elongation on tension. The warp elongation will be different depending on the length of downtime. Therefore, the warp let-off mechanism will automatically reverse before starting the machine. The amount of reversal will be divided into several levels according to the length of time, thus effectively overcoming the warp elongation caused by the warp elongation during shutdown. The tension change ensures that the warp yarn tension when starting the weft meets the weaving requirements and eliminates the possibility of rungs. ③The let-off servo motor can accelerate from 0 to 3000r/min within one hundredth of a second, indicating that the dynamic response of the let-off mechanism is particularly fast, thus ensuring that the warp tension can be corrected in time according to microcomputer instructions. At the same time, it can adapt to the high-speed operation of the loom. ④The warp tension can be set and displayed. The warp tension setting is represented by numbers, which also facilitates centralized management. To start the machine or change the weaving parameters, all operations are done on the keyboard, which is simple and easy. ⑤ During forward and reverse operation, the rotation of the weaving shaft can be controlled within 0.1 weft, which can meet the weft density requirements of 4 to 1083 threads/10cm (1 to 275 picks/inch). ⑥The function of the positive warp easing device: Cotton yarn is short fiber, and the fiber length is generally between 20 and 34mm. The strength of short fiber yarns is generally lower than that of long fibers due to the possibility of relative slippage between fibers. During the weaving process, the short fiber warp yarns are easily broken under certain tension, especially when the tension fluctuation is relatively large. When the shedding mechanism opens to a large opening, the warp tension reaches its peak, which can easily cause warp breakage. The warp easing device in the let-off mechanism of the air-jet loom moves the back beam of the let-off toward the front of the machine when the shedding amount is large, thereby reducing the peak warp tension, thus reducing warp breakage, improving the quality of the cloth, and at the same time improving the quality of the fabric. Improve the operating efficiency of the loom. To measure the performance of a warp let-off mechanism, it mainly depends on whether it can send out a certain amount of warp yarns with constant tension from large to small weaving beams during the weaving process. Because whether the warp tension is constant directly affects the quality of the fabric, uneven warp tension will cause defects on the fabric. The amount of warp tension depends largely on the amount of warp let-off. In addition, the total warp tension that the let-off mechanism can adapt to is an important indicator of the adaptability range of the let-off mechanism.
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