Unit 3: Bow, skew & weave analysis
Fabric Quality Analysis notes · PTU syllabus (BSTD 401-20)
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Unit summary
Fabrics must be straight and their structure correctly identified. This unit covers bow and skew in woven and knitted fabrics, and weave analysis with a pick glass to derive the weave, drafting and lifting plans and colour patterns.
After this unit you can
- Define and measure bow and skew
- State acceptable limits and causes
- Analyse a fabric's weave
- Derive draft, lifting plan and colour pattern
PTU syllabus topics
- Definition and calculation of bow and skew for woven and knitted fabrics
- weave analysis using a pick glass to derive weave
- drafting and lifting plans
- warp/weft colour patterns
Bow %
(Depth of bow / fabric width) × 100
Skew %
(Distance of skew / fabric width) × 100
Cause
Uneven tension in finishing (tentering)
Why it matters
Checks and stripes look crooked in garments
Topic 1
Bow and skew
Definition
Weft (or courses) curved across the width
Weft (or courses) straight but slanted across the width
Measurement
Maximum distance between the curved weft and a straight line ÷ width × 100
Distance one end of the weft is displaced ÷ width × 100
Causes
Uneven tension or speed across the stenter
Uneven feed at the stenter or tension differences
Correction
Weft straighteners (mahlo) on stenters
Same
Example
Fabric width 150 cm; weft bows 2.25 cm at the centre → bow = 1.5%. Many buyers accept up to 2% for woven fabrics (stricter for checks and stripes).
- Knitted fabrics: spirality (courses slanting) — caused by twist liveliness of yarns; measured as angle or percentage; controlled by yarn twist, compacting and finishing.
Topic 2
Weave analysis using a pick glass
- 1
Identify face and back, warp and weft (selvedge parallel to warp; warp usually straighter and finer)
- 2
Count EPI and PPI
- 3
Under the pick glass, lift threads with a needle to see interlacements
- 4
Record each warp end's interlacing with successive picks on graph paper (filled square = warp over weft)
- 5
Find the repeat (smallest unit)
- 6
Name the weave — plain, twill (direction and float), satin (move number), etc.
Topic 3
Drafting and lifting plans
- Design (weave plan)
- Interlacing pattern on graph paper
- Draft (drawing-in plan)
- Shows which heald shaft each end is drawn through — straight, pointed, skip, broken
- Lifting plan (peg plan)
- Shows which shafts are lifted on each pick
- Denting plan
- Number of ends per dent
- Rule
- Design = draft combined with lifting plan
Example
A 2/2 twill with straight draft on 4 shafts: lifting plan pick 1 — shafts 1, 2; pick 2 — 2, 3; pick 3 — 3, 4; pick 4 — 4, 1.
Topic 4
Warp and weft colour patterns
- Record the sequence of coloured warp ends (warp pattern) and weft picks (weft pattern), e.g., 8 white, 4 blue repeated. Combined with the weave, colour patterns produce colour-and-weave effects — houndstooth (2/2 twill with 4 and 4 colour order), log cabin, pinstripes, checks.
Key terms
- Bow
- Curved weft across fabric width
- Skew
- Slanted weft across fabric width
- Spirality
- Slanting of courses in knitted fabric
- Draft
- Order of drawing ends through heald shafts
- Lifting plan
- Order of shafts raised on each pick
Quick revision
- Bow and skew definitions, measurement, causes, limits, correction; spirality.
- Weave analysis steps; repeat; naming the weave.
- Design, draft, lifting and denting plans.
- Warp and weft colour patterns; colour-and-weave effects.
Important exam questions
Practice questions written to the PTU exam pattern for this unit's syllabus: short answers (Section A style) and long answers (Sections B and C style).
Short-answer questions
- Q1.Distinguish bow and skew.
- Q2.Calculate bow for a 1.8 cm displacement on 120 cm width.
- Q3.How do you identify warp and weft in a fabric sample?
- Q4.What is a draft?
- Q5.What is a lifting plan?
- Q6.What is houndstooth?
Long-answer questions
- Q1.Explain bow and skew in woven and knitted fabrics.
- Q2.Explain the method of weave analysis.
- Q3.Explain drafting and lifting plans with an example.
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