YARN TWIST TEST:-

YARN TWIST TEST:-

The test procedure is to untwist the yarn  untill all its twist has been removed and then to continue Twisting the yarn in same direction until it's original position length. The basis of the method is assumption that the amount of twist put in is equal to the twist that has been removed.

image of a twist tester. Ref by textile Apex.

Direct counting method:-

The essence of the method is to unwind the twist in the yarn untill  the fibers are parallel to the yarn axis and to count how many turns are required to do this. A suitable instrument has two jaws at a set distance apart of the jaw is fixed  and is capable of being rotated . The rotating jaw has a counter attach to it  to number the whole turns and fraction of a turn. Before starting any test the sample should have been conditioned in the standard atmosphere. 

image of yarn twist measurement technique ref by India mart.

Testing is started at least one meter from the open end of yarn, as the open end of yarn is free to untwist, so that the level of twist may be lower in that region. As the yarn is being clamped in the instrument, it must be kept under a standard tension (0.5 CN/tex) as the length of yarn will altered by too high or too low a tension. The twis is removed by turning the rotatable clamp until it is possible to insert a needle between the individual fibers at the non-portable clamp end, and to traverse the needle across to the rotatable clamp. The use of a magnifying glass lense may be required in order to test the fine yarn. The twist direction and the mean turns per centimeter or per meter are reported.

Continuous Twist test:-

This apparatus is designed to reduce the amount of handling needed on concecutive twist and to speed up the testing process. Yarn passed through jaw end and is wounded up on a rotating drum as it is removed. Twist is assigned by the same principle as on the other tester but after removing the twist ,it is put back  in to the yan by ratating the counter back to  zero. The rotating clamp, as opened and its jaws moved forward to meet the fixed clamp; the jaws are then clamped on the yarn. The fixed clamp is opened and moving jaws are returned to starting position taking a new length of yarn with of yarn with them, the drum taken up the slack in the yarn.

image of continuous twist tester. Ref  Textile study center.

Untwist-twist method:-

This method is based on the fact that yarn contract in length as the level of twist is increases. There of if the twist is subsequently removed, the yarn will increase in length reaching a maximum when all twist is removed. The method uses a piece of equipment such as that shown in the figure, in which the end of the yarn distance from the counter is attached to a pointer which is capable of magnifing it's changes in length.

image of twist-untwist method twist tester.ref by Textile learner.

At the Starting of the test, the yarn is placed under a suitable tension, either by a clip, on weight or by weighted arm as shown . The last  procedure is to untwist the yarn until twist has been removed and then to continue Twisting the yarn in the reverse direction until it returns to its original length. The basis of this method is assumption that the amount of twist put in is equal to the twist that has been removed. However, this is not necessarily the case for woollen yarn, the method may results up to 20% below the true value , whereas for worsted yarn , the result may be 15% higher owing to fiber slippage. One sources of error in the method is that at the point of total twist removed to the fibers in the yarn are unsupported. So that any tension in the yarn may cause the fiber slip , paste on another, so increasing the length of the yarn. The difference in the length of unnoticed will causes an error in measurement of turns per length unit. Another source of error is the fact that with some yarn, when the twist is removed, the amount of twist to bring it back to the same length is not equal to twist taken out. 

Because of these problems the method is not recommended for determining the actual twist of the yarn , but only for used as a production control method . There is a US standard for this method, but it earns that the measured values are listing a guage length 250 to 125 mm .

However the method is easy to used and has less operator variability than the standard method so that it is often used for measuring the twist in single yarn 

Ref:-https://textilrleapexblogspot.com