Ply & Lay

Amatsunawa deals with experts in the growing of jute plants, their processing and subsequent production into yarns. We also work with rope and rope machinery manufacturers. From all our interactions, we understand there is no such thing as a standard lay ratio in a jute rope. This is variable depending on a multitude of factors. Tight and loose are subjective by individual interpretation.

Subtle differences in jute filament colour (1. Grade 3 Tossa, 2. Grade 2 White, 3. Grade 1 Tossa, 4. Grade 2 Tossa, 5. Grade 2 Tossa, 6. Grade 4 Tossa, 7. Grade 3 Tossa)

It begins with the raw jute quality – the finesse and length of the filaments. A larger number of fine filaments can be twisted together tighter into a yarn of an equal count than can be made with coarse, low quality jute. This is due to the relationship between coherence and obliquity. Critically important batching oil is then required to assist filaments to coalesce. As filaments are rotated, strength increases up to the optimum twist. With further rotation, obliquity begins to decrease as fibres tend to lay increasingly more perpendicular than parallel to the length of the yarn, losing linear orientation. Lower grade jute will have a lower optimum twist. The direct effect is a lower overall longitudinal strength.

Yarn ply – left: single ply 14/1, middle: 2–ply 14/2, right: 2–ply 10/2

The yarn twist then sets the optimum counter–twist when wound together into strands. If the twist dynamics of the yarns and strands counter each other, the strands will hang straight and true. Strands must then be twisted together to make rope. The twist dynamics of the yarns in one direction, the strands in the opposite, and then the rope back in the original direction need to match to prevent the rope recoiling and corkscrewing on itself.

With three different dynamics at work, there is room to balance one against the other, and this is where the lay ratio of the rope is set. While not as aggressive as over–twisting the yarn, strands or rope tightened too excessively will pass an optimum point of strength, and be more prone to filament breakage.

Yarn ply in rope – (left to right): single ply 14/1, 2–ply 14/2, 2–ply 10/2, single–ply 48/1

Tightly laid rope becomes very hard and more difficult to work with. When still within a ratio for structural integrity, a loose laid rope will feel very soft, but is easier to damage.

Lay is commonly measured by a simple system with a retention one end and a wheel the other, over which the rope runs to be pulled by a weight, stretched with ø (mm) x ø (mm) ÷ 8 (kg) load so a clear 1 metre length can be measured, e.g. for ø6.0mm, 6 x 6 = 36 ÷ 8 = 4.5kg, etc. Manufacturers lay paper over the rope and rub e.g. a crayon along it to mark the indentations, measuring how many to calculate the twist of the rope. The ratio is the distance the strand has rotated divided by the rope diameter.

Lay ratio – left: looser, middle: medium, right: tighter

However, this method only returns the lay ratio of the rope (the winding repeats of the strands). Due to the aforementioned dynamics of the yarn and strands, a ratio of 4.5 can feel very loose to slightly tight, and 3 can feel extremely tight and hard to slightly loose.

The measurement of rope diameter in a pliant, compressible medium is complicated, especially when it is a spiral of 3 strands. Manufacturers measure the uncompressed diameter of a circumscribed circle that encloses all strands – extremely difficult to measure with flat surfaces of a typical two-dimensional gauge like a caliper.

Multiple–ply yarns complicated matters because a fourth twist dynamic is mixed into the equation. Relatively loose 2 and 3–ply yarns can feel very soft, and have an impact on the resulting twist dynamics of the strands and rope, and therefore the lay. Tightly wound multiple yarns can make the rope tight, hard and feel uncomfortably knobbly.

Coating – left: rope made from coated yarns, right: rope made from uncoated yarns.

Amatsunawa no longer specifies lay ratio, but through education in precisely how jute bondage rope will be used, we teach machine operators to sense the feel of the final rope, and pay specific attention to quality control.

©AMATSUNAWA 2019