7.5.5 Selection of the Wire Rope

Required minimum breaking load. Wire rope used for lifting should, in most cases, have a working load limit of not more than one fifth of the wire rope’s minimum (or catalogue) breaking load. That is a factor of safety of 5:1, although this figure may be varied by a Competent Person for specific circumstances of use. Hence the factor of safety and load rating of the winch will determine the required wire rope’s breaking load, i.e. rope minimum breaking load = load rating multiplied by factor of safety.

Rope construction. The use of wire rope on a winch usually involves wrapping the rope around a drum of a relatively small diameter when compared to other wire rope applications. The choice of a wire rope with sufficient flexibility is therefore important. Ropes with strands comprising a single layer of wire, e.g. 6 x 7 (6/1) are generally unsuitable for winch applications due to their lack of flexibility. Winches also subject the rope to crushing forces, particularly when more than one layer of rope is accommodated on the winch drum. For this reason, ropes with wire cores are to be preferred. In some cases, e.g. when a winch is used as a lifting machine by passing the rope over a pulley such that the load is lifted by a single part of rope, a rotation resistant rope may be required due to the propensity of six stranded ropes to un-spin under such conditions. In these circumstances, the advice of the wire rope supplier should be sought.

Corrosion protection. During manufacture of wire rope, the rope maker applies a dressing which acts both as a lubricant and protection against corrosion. In all cases, winch ropes should be adequately protected from the effects of atmospheric corrosion when in use. In severe conditions, this may call for additional dressing to be applied and the manufacturer’s advice should be sought. Similarly, advice should be sought and followed with regard to the regular service dressing of the rope. In some cases, for example when it is known that the winch will be working in a wet environment, a galvanised rope should be selected. In special cases, e.g. severely corrosive environments, the use of a stainless steel rope may be appropriate and in such instances the advice of the rope supplier should be sought.

Effective and actual lengths of wire rope. The effective length of wire rope will depend on the manner in which the assembly is rigged. This will be the height of lift multiplied by the number of falls plus the length of the rope run from the drum to the top suspension sheave. The actual length of wire rope required will be longer than the effective length to allow for a number of dead turns to remain on the drum at all times. It is necessary to have a number of dead turns of rope remaining on the drum when the rope is in its fully extended position to provide a frictional grip between the rope and the drum thus preventing the load being imposed on the rope drum anchorage. This will require that at least 2 full turns of rope remain on the drum at all times. However, some manufacturers state that 3 dead turns must remain on their design of winch. The manufacturer’s recommendations should therefore be sought and followed, but under no circumstances should less than 2 full turns remain on the drum.