Twist Measurement in Yarn

1. Introduction

The most obvious way to measure the number of twists per unit length of yarn is simply to untwist a known length of the yarn and check visually to see whether the twist has been completely removed. 

2. General specification required for twist measurement

Twist testing methods depends on the specification, accuracy, demanded, from of the sample etc. In general the following points may be followed for getting reliable result. They are-

• Usually twist is not distributed uniformly along a yarn. Speaking technically, the amount    of twist in a materials is inversely proportional to its thickness. This from is 'twist x   weight=constant''. And for to avoiding having bias in twist measurement, twist should be determined at fixed intervals along the yarn that is 1 yard of distance which is suitable.
• Withdrawal of yarn over the end of the package adds twist to a yarn, whereas withdrawal from the side of the package does not. The yarn should be withdrawn from the package in the manner in which it would be normally used in the next stage of processing. The package should be mounted vertically on a peg and free to rotate about the peg as the yarn is withdrawn gently from it.
• The actual test specimen should be handled manually such that the fingers do not touch the length over which the twist is actually measured.
• In the case of plied and cabled spun yarn and continuous filament yarn, at least 20 specimens are to be tested if the length is 500 mm according to BIS (British Standards Institution). In the case of single spun yarn, BSI recommends 50 specimens of 25 mm each.

3. Techniques of twist Measurements

The most common techniques that have been used for measuring yam twist are listed below.

• Straightened fiber technique or the direct count method.
•  Twist contraction technique or untwist and re-twist method.
•  Twist to break technique.
•  Microscopic

The straightened fiber technique

Principle:

Used only for testing single spun yarns, this technique involves the untwisting of the test specimen until all of the twist is removed. The completeness of the untwisting is verified by visual examination of the straightening of all of the fibers in the strand – this is why the name 'straightened fiber technique'. The number of turns required for the untwisting is counted and the number is divided by the length of the test specimen to arrive at the twist of the test yarn turns per unit length

This technique is also known as the 'direct count method', as the twist in the yarn is directly counted.

Construction:

The below figure depicts the salient features of a single yarn twist tester incorporating the straightened fiber technique. The instrument consists of two pillars, A and B, mounted on a rigid base C. On pillar A are mounted a fixed jaw D, a guide pulley E, a tensioning arrangement F, a magnifying lens G and a blackboard H. Pillar B carries a jaw I that can be rotated manually either way by means of handle K and the number of revolutions is recorded in the revolution counter J. A length of 25 mm (or one inch), the test length normally used, separates the faces of the jaws D and I. A test specimen L can be seen mounted between the two Jaws.

Test Procedure: 

The first step is to set the revolution counter to zero.

Next clamp the test yam in the rotatable jaw. Open the static jaw and lead the yarn through it and then over the guide pulley; attach a small weight to the yarn to give it the required tension and then close the jaw to clamp the yarn.

Check, by means of lens G if necessary, the twist direction of the test specimen. Then using the handle, rotate jaw I in a clockwise or anti-clockwise direction so as to untwist the yarn.

Note the twist in the yarn being removed. When most of the twist has been removed, push a sharp needle through the middle of the partially untwisted strand so it nearly touches the static jaw; then, looking through lens G, gently move the needle towards the other jaw. Give the handle a final rotation either way until the needle is in the closest possible position to the rotatable jaw.

Finally, the number of turns of the rotatable jaw required to untwist the test specimen is read off the revolution counter and recorded.

At least 50 tests are conducted in this manner and the mean TPI and CV% are calculated.