ast year in Cranes Today (CT Aug 03 p.34) I explained some of the key changes being made to the revision of ISO 4309 Cranes – Wire ropes – Code of practice for examination and discard. Mention was made of the new vocabulary, terminology and definitions relating to the three basic types of stranded rope that are used on cranes: single layer, rotation resistant and parallel closed ropes. Mention was also made of the new rope designation system recently adopted by EN 12385-2 Steel wire ropes – Safety – Part 2: Definitions, designation and classification and the since published ISO 17893:2004 Steel wire ropes – Vocabulary, designation and classification that will begin to impact on how ropes are to be described in certificates, catalogues, handbooks and other such documents in the future.
Without such a well defined system, it is not surprising that the same rope supplied by different rope manufacturers has been described in many different ways. Not too helpful for the purchaser or end user who might wish to compare one rope with another; or the rope examiner who needs to satisfy himself that the crane rope under examination matches that described on the certificate. As both of the above mentioned standards have adopted the same designation system, the stage is now set for global harmonisation of the way in which ropes are described, putting pressure on rope and crane manufacturers and others in the supply chain to promote its use for the benefit of others downstream.
Not surprisingly, the new designation system for stranded ropes comprises the same principal elements as before. These are:
• nominal diameter
• construction
• core
• rope grade
• finish of wires
• type of lay and direction
Apart from rope strength, which in the future will be characterised only as minimum breaking force for all crane ropes, these elements, when combined, are sufficient to fully describe the rope.
Just think about the implications of simply ordering a 6 x 19 rope without any other reference to the strand detail. What would you expect to receive? A stiff rope containing large outer wires (see Table 1 i.) offering a higher degree of abrasion and corrosion resistance; or perhaps a more flexible rope offering improved bending fatigue life (see Table 1 ii.); or maybe something else (see Table 1 iii.) which is occasionally supplied as a small hoist/winch rope or for use in the manufacture of a sling. As can be seen, there are three completely different rope constructions that would satisfy this number of wires.
In each of the above constructions it will be noted that a symbol succeeds the number (19) of wires in each strand. This is the rope designation system at work, distinguishing one 6 x 19 rope construction from another. But there is another 6 x 19 rope construction that is missing from the Table 1. Confusing matters even more, the popular 6 x 25F (Filler) construction was often referred to in some quarters as 6 x 19F to distinguish it from another well-known construction having six triangular shaped strands of 25 wires, designated 6 x 25TS. But today this latter rope is more associated with deep shaft mine hoisting and some drilling applications rather than boom hoisting, which is more adequately served by single layer compacted strand ropes, or even some specialist compacted ropes.
For all rope constructions, even those ropes incorporating compacted strands and/or cushion cores, which are often marketed as ‘higher performance’ ropes, the new designation system can be readily applied. Tables 2, 3 and 4 give examples of how the system is to be used for a few of the more common single layer, rotation resistant and parallel closed rope constructions respectively. For good measure, rope makers are also encouraged to enter the brand name of the rope on the certificate to help end users and examiners more readily identify any specific handling and maintenance information, including discard criteria, that might be provided elsewhere.
The examples in Table 2 are only a fraction of the growing number of constructions of single layer ropes that are supplied into the marketplace. Indeed, single layer ropes having as few as three outer strands and as many as 10 can be seen in operation on various types of cranes throughout the world. When this is considered along with the number of wires in each strand, the different grades of rope, the direction and type of lay, and the finish of the wires, there are countless options available. No wonder confusion abounds, but at least the new rope designation system is geared to accommodate them all.
Representing just a few of the more common 6- and 8-strand rope constructions, the examples in Table 2 cover quite a wide rope diameter range; but what about the relative merits of 6- and 8-strand ropes? There is virtually no sense in extolling the virtues of one versus the other without also giving consideration to the strand construction and the diameter of the outer wire in relation to the sheave/drum diameter. Putting aside the effects of tensile strength of wires and type of rope lay and direction, it is this relationship that will more often than not have greatest influence on bending fatigue life. However, when taking the effects of multi-layer spooling and fleet angle into account, a rope having improved bending fatigue resistance may not always have sufficiently large outer wire or strands to resist potential mechanical damage occurring at cross-over points at the drum.
The next category of rope to consider is the more sophisticated rotation resistant rope. Generally, rotation-resistant ropes incorporate inner rope centres that are laid in the opposite direction to the outer layer of strands. (See Table 3 for typical examples.). But there are notable exceptions to this arrangement, such as some three- and four-strand ropes that can also be designed to have similar rotational properties.
As rope development has advanced, particularly in terms of increases in strength coupled with improvements in rotation resistance, the need for a well-structured designation system has become even more necessary. Take the 34 x 7 rope construction: for many years it was produced in three closing operations (17 strands laid over 11 strands laid over 6 strands laid over a fibre or wire strand centre – see Table 3, above). But this is quite different from the 34 x 7 of today that is produced in only two operations, (see vii. above). To distinguish one from the other, a symbol ‘(M)’ or ‘(W)’ is inserted directly after ‘34’ to denote that the inner rope element is a multi-layer cross lay or Warrington rope respectively. For the same rope grade, the strength of the 34(W) x 7 – WSC can be more than 20% higher than the 34(M) x 7 – WSC rope and the amount of rotation when one end of the rope is free to rotate is significantly reduced.
The third type of stranded rope is the parallel closed rope. (See Table 4 for typical examples of construction and core designation.) In appearance, such a rope could easily be confused with a single layer rope that has the same number of outer strands, but the big difference is that the whole rope is manufactured in one closing operation, as distinct from the single-layer rope in which the core is manufactured first in a separate operation. Such parallel closed ropes, usually having either eight or nine outer strands laid parallel to the inner underlying strands forming a parallel-closed wire rope core, have the potential of offering increased breaking strength and fatigue performance, but have limited resistance to rotation, thus requiring both ends of the rope to be prevented from rotating when in service.
Without giving consideration to any of the other elements of rope designation, which are equally important, it should now be obvious as to why it is necessary for such a system to be applied. When adopted by manufacturers and suppliers of ropes, those involved at the sharp end of the business will be better able to assess the finer differences between one rope and another.
