For the best fabrics, we have to choose the best yarns.
But how can we know the yarn quality? You must understand key quality parameters.
In this article, we’ll introduce one of the most important yarn quality parameters – yarn evenness.
1. Yarn Evenness
Yarn evenness is a measure of the level of variation in yarn linear density or mass per unit length of yarn.
In other words, it refers to the variation in yarn count along its length.
A yarn with poor evenness will have thick and thin places along the length, while an even yarn will have little variation in mass or thickness.
2. Importance of Yarn Evenness
While yarn may vary in many properties, evenness is the most important quality factor,
because variations in other yarn properties are often caused by yarn count irregularity.
For example:
- Twist tends to concentrate in thinner places.
- This leads to diameter differences and affects fabric appearance.
- Irregular yarns also vary in strength along their length.
Yarn properties affected by evenness include:
→ Weight per unit length
→ Twists per inch
→ Diameter
→ Strength
3. Yarn Faults’ Classification
Terms like evenness, unevenness, regularity, and irregularity are used to describe yarn uniformity.
In textiles, uniformity is expressed by evenness or regularity — or unevenness or irregularity.
A yarn that is not uniform is considered irregular or contains faults.
These vary in cross-sectional size and length.
Yarn faults are classified into three types:
a. Unevenness / Irregularity
This refers to the yarn evenness we typically measure.
b. Imperfections:
- Thick places: Local linear mass is >35% above average
- Thin places: Local linear mass is <30% below average
- Neps: Local mass increases ≥100% (tiny entanglements)
c. Objectionable Yarn Faults
Not acceptable — these defects cause major fabric quality issues.
4. Expression of Yarn Evenness: U% & CV%
Yarn unevenness is defined as the variation in fineness or mass per unit length.
It is commonly expressed using U% or CV%.
1). U% – Percentage of Mean Deviation (PMD):
- The average deviation from the mean mass, expressed as a percentage.
- Also known as Unevenness.
2). CV% – Coefficient of Variation:
- The ratio of standard deviation to average mass variation.
- A higher CV% means more irregular yarn.
5. Relationship Between CV% and U%
Modern testers like the Uster Evenness Tester can measure both U% and CV%.
Because CV% is calculated using squared deviations, larger faults impact CV% more strongly than U%.
Therefore, CV% is preferred in modern yarn quality assessments.
6. Classification of Yarn Unevenness (Irregularity)
There are two types of variation typically seen in yarns:
1). Random Variation
- Occurs without a fixed pattern
- Caused mainly by natural fiber property differences
2). Periodic Variation
- Occurs at fixed intervals or lengths
- Often due to mechanical faults in machinery
7. Conclusion
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