History repeats itself; and this is no different when applied to crane maintenance. If you use the same approach towards a typical maintenance procedure you will ultimately achieve the same result. Generally, this is considered a good thing when applied to maintenance because predictability allows for more consistency, thus eliminating unscheduled downtime.
On the other hand, simply performing a routine inspection, time after time without any documentation, prevents the user from obtaining important information that is critical to the overall operating cost of the equipment employed.
When you look at the applicable overhead crane code from country to country you will easily find the criteria for retiring a crane wire rope from service. They clearly outline criteria when to replace wire rope.
However, they generally do not provide the rope inspector with any clues or guidelines as to how or why it happened.
The clues are not in the retirement criteria but they can be found during the rope inspection itself. It is these clues that can help the crane user to take a course of action to improve wire rope life and achieve the cost benefits that go along with this improvement.
The best recommendations from wire rope engineers are those where they have had the opportunity to examine the rope in service at or near the end of the rope’s life. They are able to look at the affected area of the wire rope and examine the equipment that is part of the rope path. From this examination, they are usually able to determine the primary cause of rope retirement. They usually fall into one of three categories: rope related, equipment related and/or operator related.
Of course, it is not always possible to have a wire rope engineer examine a rope in every instance. That is the job of the rope inspector. Since it is this person that has the opportunity to examine the rope in all conditions, it would be appropriate for these inspectors to learn what the rope inspection is actually telling them. The best way to accomplish this is to start documenting the inspections in a wire rope inspection log.
The wire rope inspection log becomes a valuable tool because you can see the rope’s history at a glance. By saving these logs for individual ropes, an inspector can look for trends that might require a course of corrective action. As an example, if you keep seeing ropes coming out of service due to corrosion, then you would review the type of lubricant being applied and its frequency of application.
An overhead hoist is basically a wire rope fatigue machine. The rope never touches itself. It simply cycles back and forth through sheaves while it is coming on and off the drum. Hence, ropes are usually retired because of broken wires. This may be simply due to fatigue, however, the inspector should check the type and nature of the broken wires and see if there is a pattern to their location. Unusual wire breaks or patterns should be further investigated as they may be equipment or operator related.
If the problem is equipment related and not picked up at the individual inspection, a review of previous forms may show a downward trend in rope life that might be related to wear on the sheaves and/or hoist drum. This is the type of problem that will have premature damage in the same area rope after rope. It is a good idea to keep previous rope inspection histories so that you can make this comparison. Often, the report is thrown out once the rope is retired from service and valuable information is lost.
If the cause is operator related (a popped rope core due to shock loading, for example), then you have documentation that will enable you to help the operator recognise the problem if it occurs more than once. One of the more common operator problems is from side loading the wire rope on an overhead crane. This is hard on the rope but can also cause the rope to jump a drum groove that could sever or severely damage the rope. This is an operational problem and should be pointed out to employees as being a dangerous practice.
One of the solutions in improving rope life after reviewing the inspection logs may be simply changing the rope construction. With this inspection information in hand you can now consult your wire rope supplier to see if there are any rope constructions that will provide longer rope life. Wire rope catalogues are helpful in making your decision, but not all rope constructions shown in catalogues are necessarily available from stock. This is why it is recommended that you contact your supplier for availability of different rope constructions.
With little or no change in cost, you might be able to select one of many general purpose rope constructions available to increase your operating rope life. Most often, overhead hoists are already fitted with 6×37 classification wire ropes that are best suited for fatigue applications. If your crane is fitted with a 6×19 classification rope, then switching to a 6×37 classification wire rope will increase fatigue life. If your hoist is already fitted with a 6×37 classification rope and you are looking for increased performance (lower operating costs), then your next option would be to consider a high performance wire rope construction.
Examples of these high performance ropes are Dyform-6 and Dyform-8 (shown right). During the outer strand manufacturing process, the wires are drawn through a special Dyforming die that compacts the finished strand and results in a smooth flat surface around its periphery. This compacting process provides the rope with both a greater surface area and increased metallic area. This results in greater wire rope fatigue life due to the lower bearing pressures and stress levels. An added benefit is reduced wear on the crane’s sheaves and drums because of the rope’s increased surface area. It is more evident in the Dyform-8 construction because of the two extra strands in its construction. Dyform-8 is also available with a plastic covered core which enhances its fatigue life and should be considered for the most severe crane applications.
An American steel company located in the Midwest found itself experiencing short rope life on its anti-sway block P&H crane. The rope originally specified for the crane was a 6×37 IWRC wire rope. This general purpose wire rope lasted only four to five weeks under normal usage. The rope was removed from service after meeting the broken wire retirement criteria as outlined in the ASME B30.2 code. Because it kept good wire rope inspection records, the company realised that it needed to start looking for a better performance wire rope construction. After experimenting with (and documenting) different rope constructions, the company finally settled on Dyform-8. This construction achieved 13 months of continuous service, a more than elevenfold increase in performance over the original rope construction.
Dyform-6 and Dyform-8 are just two of many high performance rope constructions available on the market today. Though costing more than general purpose ropes, their increased operating life usually exceeds their premium. Again, consultation with the rope supplier is essential in making the correct selection to suit your particular application and design factors.
It must be pointed out that the condition of a crane’s sheaves and/or drum may be causing less than desired wire rope life. There will be no improvement in rope life by putting a high performance wire rope in a worn, undersize sheave. Check the sheave and drum groove profiles before any new wire rope is installed.
Also, if you feel that you are not receiving adequate performance from your current wire ropes, there is an important point to check before opting for high performance wire ropes. Are you receiving the ropes that you think you are getting? To make sure that you are obtaining the wire rope you ordered, request a copy of the actual wire rope manufacturer’s test certificate. Performance can vary from manufacturer to manufacturer which further illustrates the importance of keeping good records that also give you the opportunity to compare brands.
New cranes
Wire rope is a large part of the crane’s operating cost over its lifetime and this is reflected in the Crane Manufacturers Association of America (CMAA) specification 70 – ‘Specifications for top running bridge & gantry type multiple girder electric overhead travelling cranes’ – where higher duty cranes use larger drum and sheave diameters which provide for better rope life. The chart (next page) lists these classes along with their guide for the type of rope construction and minimum bending ratio requirements.
When considering the purchase of a new crane, the user can review these classes and select the most economical crane based on their requirements. Obviously, the smaller the sheave and drum diameters, the less expensive the crane. However, wire rope life (measured in cycles) will be adversely affected. Additionally, other crane components may not be suitable for high duty-cycle operation. Therefore, it is extremely important to work with the crane manufacturer to select the right class for the right application.
As mentioned earlier, there are many wire rope constructions on the market. As a sensible approach to designing or looking at new unproven overhead hoisting systems, it is best to select ropes conservatively like the general purpose ropes as outlined in the CMAA table. This will allow the user to employ high performance wire ropes like Dyform-6 and Dyform-8 if the application needs it. If a crane designer selects the ultimate rope for a new and untried system, if rope life proves to be less than desirable the crane user may not have the option to change to a high performance rope alternative. This could lead to higher than expected maintenance costs. If the crane is fitted with high performance ropes from the factory, it is advisable to verify the crane’s performance with the crane manufacturer.
The message for crane manufacturers is: it is fine to use these high performance ropes on new systems, but they should be tested for wire rope reliability before being released to consumers.
History
The first step to evaluating rope performance is history. Wire rope always tells its story through proper examination and the rope inspector will soon find that keeping good rope records will allow one to see what type of problems are being experienced; or what gains can be realised by changing rope constructions or manufacturers. These well documented inspections will tell inspectors what they need to know. By examining the past, you can improve your future. John Manka is a wire rope consultant with 24 years experience in the industry
