Choosing the correct wire rope is not just about the specification in the crane manual and price. The would-be purchaser is typically faced with a bewildering range of products from various manufacturers around the world, all with their own naming conventions and brands.
There are simple non-negotiables to ensure a correct and safe selection of a wire rope for your crane. These include factors such as breaking load, diameter, length and turn characteristics. The discerning buyer should also consider the commercial issues.
Assuming you have identified the minimum specification of wire rope that will work safely on your crane - what other factors should be considered before ordering?
The old cliché, “If you can't measure it, you can't manage it,”is very apt here. Often an operator will view performance in terms of time lapsed and complain that a particular rope has not lasted as long as previous ropes. It is common to hear something like, “My last set lasted two years but these have only lasted 18 months.”However, after careful questioning, it is revealed that the crane is doing a lot more work.
It is, therefore, important to record rope performance and measure it in a meaningful way. If coal is moved by the ton then measure the rope in thousands of tons. If you have ship to shore cranes and lift containers then measure in TEUs (20-foot equivalent units).
Choosing quality rope from a reputable wire rope manufacturer will ensure that you get maximum rope life and a consistent rope performance
Only very rarely will an elapsed time be a reliable indicator, which is when the crane is lifting the same load on a repetitive cycle so that the rope is doing the same job every lift.
When you are sure of your current rope performance then you can look at rope types that will extend this performance. These include compacted ropes, which have a significantly longer life than conventional ropes where fatigue and wear are the prime factors.
Making an economic decision on whether to buy a high performance rope is the same thought process and math as buying a premium car tire versus a budget one. If you buy cheap, then generally you will need to replace your tires earlier than if you had chosen a more expensive one. Quite simply it is about value for money over the life time of the wire rope not the initial purchase cost.
The calculation you then need to make is “does the extra life that I get outweigh the additional cost I need to pay for a higher performance product?”
One of the factors that will help you make this calculation is to look at the cost of installation, which varies enormously depending on your crane. If you have a small overhead crane or a truck mounted mobile crane with one rope then this is very different from the cost of changing a set of ropes for a ship to shore container crane or for a steelworks crane.
Generally, a mobile crane rental company will have sufficient labor to make changing a rope a quick and easy job. Changing a set of ropes on a 200 ton capacity crawler crane, however, could require a team of up to 12 people and take many hours. To ensure safe access, the crane will also require scaffolding before the job even begins. In cases like this, the cost of a set of ropes is insignificant compared to the cost of the installation. Thus if you purchase a solution that reduces the number times you have to do this job then you can save significantly more on installation costs than the cost of additional ropes.
It is also important to establish whether your ropes are reaching the end of their natural life and wearing out or if they are they being replaced due to fatigue or damage. A good wire rope supplier will offer a service to help you determine why the rope has reached the end of its life. Many publications discuss how to make this determination and wire rope examiner courses are available, which can help determine what has happened.
If a rope is being damaged then, in the majority of cases, it is time to look at operating procedures and driver training. A rope will last much longer with a good operator than with a reckless one. You may be able to fix the cause of the damage but, if not, the choice is simple. Fit a rope that meets the minimum criteria but is the cheapest one you can find. There is no point in fitting an expensive one as it too will get damaged - so it is a waste of money.
Consequences of downtime
Choosing quality rope from a reputable wire rope manufacturer will ensure that you get maximum rope life and a consistent rope performance. The better standard of ropes will perform consistently so that you can safely use a planned maintenance program and change the ropes when you want to rather than when you have to, which is usually when there is a container vessel waiting to be unloaded or when your boss is demanding maximum output from your production crane.
The consequences of having a vital crane out of action on an unplanned basis can be astronomical in terms of lost production or, in the case of port operators, demurrage costs.
If ropes are changed less often, will this necessarily lead to increased production? To answer this, we can look at a real life example of an unloader crane used to unload ore and coal for a leading steel plant. First, the installation costs - labor and scaffolding - were $60,000. The cost of the original ropes was $20,000. The original rope life was seven months, making the annualized cost at more than $136,000. The new high performance ropes cost $32,000 but gave a life of 19 months, so the annualized cost was reduced to $58,000 - almost a 60% reduction in costs associated with the wire rope and installation.
The big benefit, though, was increased availability of the asset, which enabled at least one additional vessel to be unloaded - the real value of this was $ 1 million.
When faced with sums like these, the additional cost of using high performance ropes becomes insignificant.
If your ropes are wearing out or being replaced due to fatigue then it may be more suitable to choose a wire rope that has an enhanced life. If your ropes are being replaced due to damage, and you cannot effectively stop this happening, then your best option is to fit the cheapest rope that is fit for the purpose.
As we have seen, selecting a high performance wire rope in the right circumstances will reduce your overall wire rope costs and, on critical applications, installation savings may outweigh the costs saved on wire rope. If you can increase the availability of your crane to produce more product or to do extra work, then choosing a high performance crane rope might have been one of smartest decisions you took to increase your profitability. act