Wire Rope Slings

ABSTRACT The magnitude of structures being lifted offshore with wire rope slings is rapidly becoming heavier and heavier. Lifts of 75 to 250 tons are not uncommon. Faced with such load magnitudes, the rigger is many times faced with a delicate problem. He has a multiple choice. He can go by the book and specify a sling based on a Design Factor (DF) of 5, even though such a sling may be inordinately large, heavy, cumbersome, expensive, and, at times, not readily available, if at all. Or, he can lower his sights and specify a sling based on a lower design factor, to down as low as 3. To adopt this latter choice; however, the rigger must be keenly aware of all the factors that affect the strength of the wire rope sling and then be able to apply them if the lift is to be made with any degree of confidence and acceptable safety. It is to this end - the defining, both quantitatively and qualitatively, of the sling strength factors - that this paper is directed. INTRODUCTION 1.The Rated Capacity (RC) of a wire rope sling is the maximum load (W) that can be lifted by a particular sling in a particular application. It is based on:1.1 The Design Factor (DF).1.2 The Nominal Strength (P) of the wire rope used to fabricate the sling.1.3 The Terminal Efficiency (TE) of the fitting or splice used to terminate the sling extremities.1.4 The Number of Legs (N) in a bridle sling or the number of component ropes (n) in a Multipart or Hand-Tucked CableLaid Grommet Slings.1.5 The Nominal Fabrication Factor (NFF) as applicable to Multipart and Grommet Slings.1.6 The type of Hitch, i.e. Vertical (V) Choker Hitch (CH), Basket Hitch (BH), or Bridle (B).1.7 The Angle of Loading (A).1.8 The Ratio of Diameter of Curvature (D) around which the sling is bent to the Nominal Diameter (d) of the wire rope.1.9 The diameter of Pin (L) used in the Loop Eye of slings. DESIGN FACTOR (DF) By definition, the Design Factor is the ratio of the Effective Design Sling Strength (S) to the Calculated Total Static Load (W). It is expressed by the equation - (Mathematical equation available in full paper) Ordinarily, wire rope sling rated capacities as published in Manufacturing catalogs and in OSHA Regulations are based on a Design Factor of 5. However, as stated above, it can be expedient to use Design Factors as low as 3 for extremely heavy loads, provided that the effective design sling strength has been accurately determined. At this point, it might be helpful to realize that in equation [1] "W" and "RC" (Rated Capacity of the sling) are equivalent terms. Therefore, by transposition, the Rated Capacity of the sling is expressed by the equation: (Mathematical equation available in full paper)