Post 317 – by Gautam Shah
A Rope is composition of fibres, filaments, yarns, thin stripes or wires, twisted or braided or kept together by ties, over winding, binders like an adhesive or fused partially. The geometric composition (it is not a composite, as there is no matrix agent) that remains flexible yet becomes lengthier and stronger in form. The composition retains its ‘winding’ or compactness through its manipulations such as bending, pulling, twisting, etc. Rope of very a thin diameter, is called string twine or a strand. Wire ropes are also called cables.
Rope is formed by several methods. The basic technique involves twisting fibres (short-staples or long-filaments) to form yarn. Twisted ropes are made by twisting the yarn into strands, then three or more strands further twisted into rope. For Braided ropes the yarn is braided. Double-braided rope has a core of braided yarn and covered with braids. In the basic twisted rope structure, alternate stages are twisted in opposite directions to give torsional stability. In the right-laid or Z-twist, (compared with left-laid or S-twist), the rope twist is seen as a spiral in upward direction.
The braided or plaited rope structure provides torsional balance by crossing and re crossing rope components in maypole fashion. Smaller or thin diameter ropes are called cords. Cordage was made by braiding two or three strands of yarns of same thickness, and then combining several of them by twisting in the opposite direction. The ends of twisted rope were tied up to keep them from unravelling. The finished rope was beaten with a wooden implement or brushed to even out the stresses.
The texture and the nature of a rope are determined by the fineness, stiffness, strength, and stretchability of the fibres or filaments used in its construction. Strength of a rope is chiefly governed by the degree of twist in the rope and strands. A greater twist, normally lowers the strength. Rope strength is not affected by repeated pulls or tensions, on the contrary it induces inner spatial adjustment of the yarn. Ropes of natural fibres deteriorate mainly due to fibre degradation caused by mould growth, whereas synthetic ropes deteriorate due to exposure to sunlight, elevated temperatures, and chemicals. Ropes when mishandled develop strand kinks (cockles). These are also caused by irregular twisting of rope.
Ropes are made from natural fibres like, Cotton, Manila (hemp), coir, jute or sisal. Cotton ropes are, weaker and stretch, but being soft is used for drawing water from wells and similar handling purposes. Manila ropes are stronger and have good salt water resistance. Short-staple fibres of cotton, wool, Rayon and long-filaments of synthetics like Nylon, polypropylene, polyester and acrylic are spun into ropes. A heavier fiber or wire creates a stiffer rope than raw fibres that are finer. Wire ropes are stronger, stiffer, heavier, and less extensible when compared to fiber ropes. Early wire ropes were strands of wires strapped together by clips, and to prevent individual wires from flaying on breaking, a slight twist was given during the clipping. Electric sky grid wires are conductive ropes.
Ropes and cables are used for their tensile capacity, such as in cranes, elevators, draggers, barges, tents, animal harness, curtains, for sails for the boats, suspension bridges, and pre-stressed concretes.