Stress test in the torture chamber

Test centres with chain conveyor test and torsion test systems for cables and wires

View of the Test Centre

"They're only cables" is unfortunately what many developers are still thinking. Even worse is when they don't even consider the electrical lines which will be installed when designing their machines. But the fact is that such a defective component can put a machine out of operation. After all, the requirements placed on these C-parts are continuously increasing, so they should never be underestimated. In the HELUKABEL test centres, electrical lines are really put through the paces. But that doesn't happen overnight.

Since 1988, HELUKABEL has been constructing cables and wires at the Windsbach site, in the region where "cableing" is virtually at home. The success proved Managing Director Helmut Luksch right, and not only in regard to his site selection. The high degree of quality assurance is another attribute which helped make sure that the production area of the former chocolate marshmallow factory had to be doubled after each of the first and second decades. And just four years after the second doubling, the factory, with its 9000 m² of covered production area, is once more bursting at the seams. So participation in the Kabelwerk Meißen cable factory in 2012 came just in time.

They deserve more attention

Many machine constructors have ignored the necessity of testing the electrical wires in demanding applications in the past and have had to pay the price for the ignorance. After all, in fact they're not merely cables and certainly not in highly dynamic applications such as can be found in the manufacturing of machine tools. The machines are becoming smaller, the masses to be moved slighter and their axes faster. The upshot is that when the installation space becomes tighter, the bending radii of the wires also have to become smaller. On top of that comes the continuously increasing dynamic in the machinery. Whereas formerly, the axes were accelerated with maximal 0.5 m/s², nowadays they are accelerated in high-end machines with up to 50 m/s². All that leads to higher mechanical stress for the wires. So if applicable in advance - at best already during the development phase - it should be tested to see if the wires for installation can withstand the ambient conditions.

"If a machine from the export-champion Germany breaks down in India, the damage repair is much more costly than if that happens right here in our neighborhood", explains Thomas Pikkemaat, Commercial Plant Manager of the Windsbach factory and Product Manager for Drive Technology at HELUKABEL. It is exactly in the high-end machines where in his opinion the wires in their function as C-parts are paid much too little attention. But when such a 500,000 euro machine in automobile production comes to a standstill due to a simple cable lug, the downtime costs quickly add up to vast sums.

The causes of failure are manifold

The wires routed in chain conveyors are exposed to especially high stress factors. Due to the constant bending and extending during the movement of the chain conveyor, they must be able to meet very high mechanical requirements.

"The stress on the wires in a chain conveyor depends on the traveling distance, the bend radius, the speed and the acceleration. When customers make inquiries we always request these four pieces of technical data", the Product Manager explains. "The better the customer can specify these variables, the more detailed we can design his wires for him. Unfortunately, that can't be calculated; it can only be determined empirically. Naturally we have an extensive database with comprehensive, empirically established figures which we initially involve. But if we don't find what we need there, tests have to be run" states Pikkemaat. Since there is a difference between a plant in 3-shift operation that runs continuously and a plant running in single shift operation that also has regular standstill times, the guarantee is given on the number of cycles. For chain-pulled conveyor lines that is five million cycles. Naturally, such a test also needs its time: For example in the 5 m plant that is continuously run with very high accelerations, it takes about 3/4 of a year.

If a wire in the chain conveyor fails, there are three possible error sources: Cable fault, abrasion or the corkscrew effect. If during the design a strand is used, for instance, that is not designed for the application, the result is a cable fault. It makes itself known in advance through an increase of the wire resistance. If a wire routed in the chain-pulled conveyor has too much or too little play, the relatively sharp-edged plastic parts of the chain links rub against the flexibly routed chain wires. The sheathing abrades a long time before a maximum number of 5 million cycles is reached. Added to that are other design errors of the cables and wires. If for instance the design was made for the false bend radius, they will probably break before the service life comes to an end. That is really treacherous because the wire works at the beginning and it take some time before the weak points start to reveal themselves.

To be able to prevent such damages during use, in its production facilities HELUKABEL operates seven chain-pulled conveyors and two torsion test rigs

Chain conveyor systems from 1 to 40 m

Hearing protection is what is needed in the Windsbach Test Centre when the electric cables and wires in their chain conveyors are practically shot back and forth with up to five-times the acceleration of gravity. They are all tortured together for months and no mercy is granted. "Only if the permanently-tested cables and wires in the various test plants withstand the specifications of our customer in the test plant do we assume the guarantee for their intended application case" explains Pikkemaat. Windsbach has a new acquisition with the 5 m chain-pulled conveyor plant (speed 10 m/s; acceleration 50 m/s²). "We can test applications on it that many suppliers are not in a position to test. We purchased the plant based on our customers' requirements. In this plant we still have upside potential; there are barely any machines with a five meter axis that have to be accelerated to five (g)", Pikkemaat comments.

In total, tests are performed on seven plants with 1, 3, 5, 18 and 40 m traversing distances (speeds up to 7 m/s; accelerations up to 50 m/s²). Chains with conventional bending radii can be used on all the plants and of course HELUKABEL keeps them available for replacement.

Torsion test for large sizes too

HELUKABEL also has quite a bit to offer to test the torsion characteristics. The new entry here in Windsbach is a torsion test device for robot applications. The increased requirements, especially in machine tool manufacturing and in robotics are prompting more and more - but unfortunately still too few - users to test their electrical wires for their torsional stability. "Here also, I can only advise the constructors to get an idea of their application onsite" Pikkemaat warns. The new plant provides a rotation angle of ±720 degrees at a length of restraint of 2 m and a rotational speed of 360°/s. Impressive and a rarity is the test tower for torsion testing for applications in wind power plants. The cables are stressed here with the greatest possible torsion of ±150°/m. That means the test conditions are many times more extreme than in reality. In the 8-meter high tall girder mast, the loop is reproduced 1:1 to the wind power plant. Up to 20 cables can be tested simultaneously here.