At the Mennens location in Dongen, we pre-stretch steel cables on our 150-metre-long pre-stretching track. This is a very important task, because otherwise the steel cable can stretch by up to several metres when you put it into operation. This is disastrous, as steel cables usually need to be of an exact length, for instance for bridges, locks and attractions. A few millimetres can make all the difference. Hans Borst, Project Manager, will gladly tell you more about how we pre-stretch cables.
Strands and wires
“How can a steel cable stretch, since it is made of steel?”. It is indeed quite astonishing. Yet it is nevertheless true. A steel cable consists of different strands, which in turn are made up of different wires. These wires and strands are woven (twisted) together in a very particular way. In between there is some air. If you use a steel cable, it will generate tensile force. The air between the wires and strands is, as it were, expressed. This brings them closer together, reducing the diameter of the cable and stretching the cable.
Five metre stretch
How far a steel cable stretches depends on the length of the cable. The longer the cable, the more it can stretch. One example of a very long cable is at a theme park attraction, which belongs to one of our customers. Boats are transported through the water via a steel cable of up to 1 kilometre in length. If we do not pre-stretch the cable, it will stretch by 0.5% (on average) when put into operation. This means that the steel cable stretches five meters. The system will then soon malfunction. You will have to constantly make adjustments and, in the worst case, even shorten the cable. By pre-stretching you save costs and prevent downtime. And it contributes to safety.
Minimum breaking load
How many kilograms should you stretch the steel cable? That depends on the minimum breaking load of the cable. In our pre-stretching track, we pre-stretch the steel cables to a third of that minimum breaking load. 48 hours, according to the NEN standard 6786. The pre-stretching track can pull up to 100,000 kilos. And if we double it, via an orbital sheave, up to 200 tons. The bank is self-tensioning. This means that it maintains the tension and automatically adjusts if there is stretch in the cables. In this way we maintain the required tensile force for 48 hours.
We stretch in a special way that is not used anywhere else in Europe. With certain techniques we can produce the steel cable with a precision of a few millimetres. This is great for projects such as bridges, locks and cable-stays. For instance, suppose the steel cables of a bridge that opens and closes dozens of times a day stretch to a certain length. This means that the entire set-up of the bridge is no longer correct and it will become faulty.
Over time, a steel cable might start to retract again. That is why at Mennens we aim to assemble the pre-stretched cables as soon as possible after pre-stretching, preferably within three weeks. This minimises the retraction of the stretch, and the production length of the cables remains accurate to a few millimetres.
Customer-specific pre-stretching procedures
In addition to pre-stretching done in this way, at Mennens we can also pre-stretch using a method that we refer to here as 'yo-yo pre-stretching'. You put a predetermined tension on the cable and hold it for about five minutes. Then you check whether permanent stretching occurs. Then you release the tension and apply tension again. You continue this way until there is no longer any permanent stretching. This will take you about two hours. So much simpler and cheaper. For each project, we determine the most ideal way. We advise on this and leave the choice to the customer.
Are you interested in finding out whether pre-stretching makes sense for your project? Please contact us via one of our branches.
Steel cable, make the right choice
Safe and efficient movement of loads, both vertically and horizontally. Get the maximum return from your installation, with the certainty of business continuity. This requires the correct (application of) steel cable. You can read all about this in the Steel Cable File.