7.5.6 Rigging Arrangement

The rigging arrangement must be given full consideration including the need for diverter pulleys, the use of pulley blocks and suitable anchorages. The diameter of sheaves should ideally be 18 times the diameter of the wire rope, but never less than 12 times, to avoid the rope crushing under load and bending fatigue taking place.

Consideration must also be given to the use of accessories and structures. Temporary arrangements may call for the use of tripods (shearlegs) or davits for winch and/or pulley block suspension.

It should be noted that when pulley blocks are used, the load imposed on the supporting structure is increased by the value of the hoisting effort and the effects of friction. This additional load is also imposed on any equipment used to connect the top hook or eye of the pulley block to the structure. Care should therefore be taken to ensure that the structure, together with all above the hook equipment, e.g. beam clamps, shackles, etc., is of adequate capacity. Table 1 shows the increase in load for various rigging arrangements assuming 8% per sheave for friction.

Where the rigging arrangement calls for a single fall of rope, the rope termination must be considered, e.g. hook or eye and shackle, etc., and steps must be taken to prevent the rope rotating thus unlaying itself, e.g. by the use of a swivel. In some cases, the rope termination may be of insufficient weight to keep the rope under tension. This can result in the rope jumping out of the grooves of pulleys or becoming twisted. To prevent this, a small purpose made weight, sufficient only to keep the rope in tension, may be fitted to the rope immediately above the terminal fittings.