18.5.1 Materials
Careful consideration must be given to the materials from which the sling is made. The material characteristics and mechanical properties of the various natural and man-made fibre ropes vary greatly. It is not possible, nor indeed desirable, in a document of this nature to detail all of the characteristics or properties and it is strongly recommended that the supplier’s advice is always sought and followed when selecting fibre rope slings for specific applications. The following therefore only outlines the more important of these for comparison purposes.
18.5.1.1 Construction of man-made fibres
Man-made fibres are produced by extrusion and the size and shape of the fibres can be controlled to obtain the most desirable characteristics for a particular duty. Four types of fibre construction are generally used in the manufacture of ropes used for the production of fibre rope slings:
Multifilaments are continuous fibres of very fine circular section less than 50 microns in diameter.
Monofilaments are continuous fibres of larger diameter than multifilaments.
Staple spun fibres are discontinuous lengths of fibres produced by selectively cutting either multifilament or monofilament fibres. This construction of man-made fibre is the one that nearest resembles natural fibre.
Fibrillated film is extruded as a tape which, after hot stretching, is caused to split longitudinally during the spinning to allow the yarn so formed to assume a roughly circular cross section.
Continuous fibres, as their name suggests, are produced in long lengths without a break. It may be assumed that they run from one end to the other of a length of cordage without discontinuity.
18.5.1.2 Chemical resistance
The various materials have very differing resistances to chemicals. Natural fibre rope slings offer little or no resistance to chemicals, their fumes or to certain gases. Man-made fibre rope slings however offer selective resistance to chemicals as follows:
Polyamide [nylon] is virtually immune to the effects of alkalis. It is attacked by moderate strength acids. It also suffers loss of strength on wetting which can be as much as 15%.
Polyester is resistant to moderate strength acids but is damaged by alkalis.
Polypropylene is little affected by acids and alkalis but is damaged by solvents, tars and paints.
18.5.1.3 Comparative strength
The strength of a fibre rope is somewhat less than the aggregate strength of the yarns used in its construction and as the size of rope is increased so the proportional strength of the rope is less. A size for size comparison shows that:
Polyamide [nylon] is the strongest fibre rope.
Polyester is weaker than polyamide [nylon] but stronger than polypropylene fibre rope.
Polypropylene is the weakest man-made fibre rope but is stronger than the natural fibre ropes.
Manila is the strongest natural fibre rope but is weaker than those of man-made fibre.
Hemp and Sisal are approximately the same and are slightly weaker than manila fibre rope.
18.5.1.4 Unit weight
The cross section of the various fibres differs, allowing them to pack differently. Their densities also vary. These factors affect the weight of the finished ropes and thus, size for size, man-made fibre ropes tend to be lighter than those of natural fibre.
18.5.1.5 Rope surface
The nature of the fibres affects the surface finish of the rope. This in turn affects the operative when handling the rope and the ropes suitability for handling certain loads. Manila and sisal tend to be coarse fibres giving a rough, abrasive surface to the rope, whilst man-made fibres tend to be smoother. Frictional resistance of the rope surface is greater in natural fibre rope than in man-made fibre ropes and as a result the splicing requirements differ. Splices with fewer tucks are often possible in natural fibre ropes.
18.5.1.6 Elongation
All ropes elongate under load. In the case of natural fibre ropes, this is mainly due to bedding down of the yarns in the strands and the strands in the rope. Although this also occurs in man-made fibre ropes, the fibres themselves elongate resulting in greater rope elongation than with natural fibre ropes. As a general rule, once an initial amount of bedding down has taken place the rope will return to its original length when the load is removed.
18.5.1.7 Deterioration
Natural fibre ropes deteriorate with age and as natural lubricants dry out, the fibres become brittle. Mould and fungi, e.g. mildew, will readily grow on damp natural fibre ropes living on the cellulose and greatly weakening the rope. Man-made fibre ropes do not suffer in the same way, mould only being able to live on surface contamination, but they can be affected by ultra-violet light and therefore may suffer solar degradation if exposed to strong sunlight or other sources of ultra-violet radiation.