Sunday, April 25, 2021

Rapier weft insertion system l Types of rapier looms l Weft yarn passage and working principle of rapier weft insertion system

 

Rapier loom weft insertion system:

This is the most versatile. It has a very wide weft count range. You can insert from very coarse to very fine yarn without making a big change. The journey of weft yarn gets constantly controlled throughout the picking motion. The fabric width can be adjusted within the machine's specification limit. The rapier looms are widely used in the textile industry. The number of weft colours ranges between 8 to 16. 

Classification of rapier looms:


A rapier loom can be categorized into the below classes according to the weft insertion mechanism used.

Single rapier loom:

That rapier loom in which only one rapier gets used to insert the the weft yarn in the shed is called the single rapier loom. The traveled distance of the rapier gets equal to the reed space used in the loom. This kind of loom works at a very low speed. The warp yarn exerts high pressure on the rapier. 

Double rapier loom:

That rapier loom in which the weft insertion in the shed is carried out with the help of two rapiers, This type of loom is known as the double rapier loom. Each rapier travels a half distance of the reed space used in the loom. Double rapier loom runs at very high speed. The pressure of the warp yarn on the rapiers gets reduced up to a maximum extent.

Flexible rapier looms:

In this kind of loom, the grippers are mounted on the flexible ribbon or tape made of Teflon and carbon fabric. Since the length of the ribbon is almost equal to the reed space used in the double rapier loom and almost half of each ribbon length enters into the shed during weft insertion so that the remaining half-length of the ribbon goes back inside the hollow rapier guide when the rapier reaches to the outer dead center position. This remaining length is that length that doesn't enter into the shed during weft insertion. This task gets possible due to the flexibility of the rapier ribbon. Due to the use of flexible rapier ribbons, this kind of loom is called a flexible rapier loom. This loom can run at higher rpm than rigid rapier looms. This kind of looms occupies less space too.

Rigid rapier looms:

If the grippers are mounted on the rigid rapier racks in any rapier loom, it is known as a rigid rapier loom. The rigid rapier runs at a slower speed than the flexible rapier loom. The rigid rapier loom occupies more space than the flexible rapier loom. 

Negative weft transfer rapier loom:

If the weft yarn gets entered between the gripping jaws of rapier grippers with the help of weft tension and an extra mechanism is not used to open and close the gripping jaws of each gripper during the weft transfer cycle, this kind of weft transfer is called negative weft transfer. If the loom has negative weft transfer during weft insertion, that loom is called a negative weft transfer rapier loom.

Positive weft transfer rapier loom:

If the gripping jaws of each rapier get opened and closed with the help of an extra mechanism throughout the complete cycle of weft transfer, this kind of weft insertion is known as positive weft transfer. The loom equipped with positive weft transfer is called a positive weft transfer rapier loom. 

Passage of weft yarn in the rapier loom:

The weft package is mounted on the weft creel. The number of weft packages is used according to the number of colours in weft yarn getting used. The yarn passes first from the thread guide and weft tensioner mounted on the weft creel. Next, the weft yarn passes through the weft accumulator. This weft accumulator ensures continuous yarn supply at a constant tension. When the weft package reaches to near the exhaustion position, the yarn tension gets increased to maximum. The weft accumulator neutralizes this effect and keeps the weft tension constant throughout the package. The weft tensioner mounted on the weft creel prevents the overlapping of adjacent yarn coils on the accumulator. The number of weft accumulators used gets equal to the number of colours getting used in the weft yarn. 

Now the yarn passes through the filling detector. The filling detector consists of either a single weft sensing channel or multi weft sensing channels. The filling detector monitors the complete journey of the weft from one side to another side. If weft insertion gets either failed or felt short in length, it stops the loom immediately. The yarn next passes through the thread guide which helps to keep the weft yarns separate. It also prevents entangling the different weft yarns together. The thread guide consists of many ceramic eyes in it. 
Finally, weft yarn passes through the eye of the weft selector needle. The weft selector needle gets selected according to the weft sequence feed in the control computer.   





Working principle of rapier weft insertion system:

The weft yarn gets held in the catch selvedge. Ten to twenty ends are used to make this catch selvedge. The ends are wrapped either on a separate reel or a catch selvedge creel is used in which required small yarn packages are mounted. A plain weave is always used in the catch selvedge. A catch plate mounted on the temple bracket very close to the end of the reed prevents the weft yarn from going toward the reed.

When the reed beats the last inserted pick, a fresh weft insertion cycle begins. First of all, the weft selector needle falls, and the weft yarn comes in front of the insert gripper. The weft yarn falls on the rapier guide and touches it.

The rapier starts traveling toward the sley center and the receiving rapier also starts to travel toward the sley center. The receiving rapier starts to move first. When the gripper passes under the weft yarn, the yarn gets entered into the slot of the insert gripper. The gripping pressure of the gripper is adjusted in such a way that it allows entering the weft yarn between the upper and lower gripping jaws of the insert gripper. An approproate weft yarn tension is also achieved by adjusting the yarn tensioner. The weft yarn should be at least reached the middle of the gripping jaws. There should be no yarn slippage between the gripping jaws of the insert rapier. When the insert gripper begins to enter the shed, the filling cutter cuts the weft yarn. A weft yarn support guide is mounted at the one end of the rapier guide near the entry point of the insert rapier. This yarn guide makes a required clearance between the reed and weft yarn. it also prevents reed damage.

Both the rapier travels toward the sley center. The receiving rapier reaches first at the sley center and rests there indwell period. The receiving rapier gets ready to receive the weft yarn from the insert gripper and it waits there. Now the insert rapier reaches the sley center and the yarn gripped in the insert gripper cross the tip of the hook receiving gripper. The three mm clearance between the tip of the hook of receiving gripper and weft yarn is ensured during setting. As the insert gripper comes in the dwell period, the receiving gripper starts to travel toward the fabric selvedge. The hook of receiving gripper pulls the weft yarn from the insert gripper. Since the receiving gripper has little more gripping pressure than the insert gripper so that yarn gets entered between the gripping jaws of receiving gripper. The insert gripper also starts to travel toward the fabric selvedge after passing its dwell period. The receiving gripper comes out of the shed first. A gripper opener fitted at the rapier guide opens the gripping jaws and releases the weft yarn outside of the catch selvedge. A suction nozzle sucks the tail of the weft yarn inside it and helps to keep it tight. The catch selvedge ends cross to each other at the same time and holds the weft yarn firmly. The insert rapier also comes out of the shed during this period. Now the weft yarn is beaten by reed and catch plate hold again the weft yarn. This cycle is repeated continuously. 


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