by Thomas Pikkemaat, HELUKABEL Commercial Plant Manager, Windsbach and Product Manager, Drive Technology
Test centers with chain conveyor and torsion test systems for cables and wires are critical in preventing cable failure and downtime.
“They’re only cables” is unfortunately what many developers still think. 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. The true test of a cable’s quality is in the rigorous testing that happens before cables are placed into mass production. But that doesn’t happen overnight.
Changing conditions require tougher designs
Cables deserve more attention. In the past, many machine constructors have ignored the necessity of testing electrical wires when they are placed in demanding applications and have paid the price for their ignorance. After all, they’re not “merely cables,” but a critical component when they are a part of highly dynamic applications such as manufacturing, for example, machine tools, handling equipment, robots, and so on. As technology improves, machines are becoming smaller, the masses being moved are lighter and their movements along multiple axes are faster. These technological enhancements have residual effects on the individual components including the cables. When the installation space becomes tighter, the bending radii of the wires must also become smaller. On top of that, the continuously increasing dynamics of the machine must also be taken into account. Previously, axial movements accelerated no faster than 0.5 m/sec². Nowadays, acceleration rates in high-end machines reach up to 50 m/sec². All these factors lead to higher mechanical stress on the wires. As early as possible—the best time being the development phase—cables should be tested to see if they can withstand the everyday conditions of the application in which they will be installed.
The reason for such rigorous testing at the beginning stages of cable development is quite simple. If a machine from the exporting country, such as the U.S. or Germany, breaks down in India, the repair costs are much higher than if that happens domestically. It is in these high-end machines where cables functioning as C-parts aren’t paid the amount of attention that they deserve. But when a $500,000 machine located in an automobile manufacturing facility comes to a standstill due to an improper cable being installed, the downtime costs quickly add up.
Causes of failure are many
The cables and wires installed in cable tracks are exposed to especially high stress factors. Due to the constant bending and extending during the movement of the track, they must be able to meet demanding mechanical requirements.
The stress on track cable depends on the traveling distance, bend radius, speed and acceleration. When customers make inquiries, these four pieces of technical data are critical. The better the customer can specify these variables, the more detailed a cable can be designed for his/her needs, or suggesting a cable from stock can be more narrowly defined. Unfortunately, when these critical pieces of data aren’t known, such as in the case of a newly designed piece of equipment, the values of traveling distance, bend radius, speed and acceleration have to be determined empirically.
Naturally, manufacturers that specialize in continuous-flex cables should have extensive databases with comprehensive, empirical and established figures to which they initially refer. However, if they don’t find what they need there, tests must be run. Because there is a difference between a plant in a three-shift operation that runs continuously and a plant running a single-shift operation with regular downtimes, the guarantee is given on the number of cycles. For track cables, that is five million cycles. Furthermore, such a test also needs time—for example in a 16-ft (5-m) test track that is continuously run with high accelerations, it takes about 3⁄4 of a year.
If a cable in the track fails, there are three possible sources of the failure—cable fault, abrasion or the corkscrew effect. If during the design a strand is used, for instance, that is not intended for the application, the result is a cable fault. It makes itself known in advance through an increase in the wire resistance. If a cable routed in the track has too much or too little play, the relatively sharp-edged plastic or metallic parts of the track links rub against the flexibly routed track cables. The jacket abrades over time before a maximum number of five million cycles is reached. This action exposes the conductors to the elements and can lead to improper machine function or failure. Finally, the corkscrew effect is caused by the inner conductors blocking each other during flexing or bending cycles. This effect occurs for many different reasons, including improper installation in the drag chain or poor cable construction. The worst possible case is production mistakes during the construction of the correct cable.
Additionally, other design errors are associated with cables and wires. If, for instance, the design was made with an incorrect bend radius, the cable will probably break before its service life comes to an end. This is extremely frustrating to engineers and maintenance technicians because the cable works properly at the beginning and takes time before the weak points start to reveal themselves, which leads to unexpected downtime and unforeseen costs to replace the faulty cable.
The right testing equipment
To prevent such damage during use, many cable manufacturers operate numerous cable track testing machines at their manufacturing or R&D facilities, as well as torsion test rigs among the many other types of testing equipment to ensure cable quality. The following is a summary of the most common equipment used to test cable and wire.
Cable track testing—Rigorously tests track cables that move short, medium and long distances at slow and fast speeds, as well as various acceleration rates and different bending radii.
Torsion apparatus—Tests a cable’s ability to be continually twisted and un-twisted in such applications as robots. Torsion levels up to ± 720° are applied to the cables being tested.
Bending apparatus—Tests the cable’s ability to resist being bent to its intended bending radius in flexible applications before damage to the cable occurs both in the conductor materials and insulation/jacket compounds.
Oven and cooling equipment—Ovens are mainly used to simulate artificial aging. Cables are subjected to temperatures between -70 and 180° C in substances such air, oil and other fluids to speed up aging to determine if a jacket compound can withstand being exposed to these substances over extended periods of time. This equipment also simulates how jacket compounds will react in environments where extreme cold or hot temperatures are present.
Tensile strength and elongation—Tests how far compounds can be stretched before failing, as well as their ability to return to form after being stretched. Materials are tested directly after production with the tests being repeated after the material is put into the oven for artificial aging. The delta of the results before and after the aging process must stay within a certain range.
Scrape-off apparatus—Tests the outer jacket durability and resistance to abrasion.
Fire test apparatus—Flame resistance and flame retardant tests are in accordance with different standards, such as DIN EN 60332-1/2/3, UL 2556 and CSA FT4. These tests simulate how cable materials react when fire is present, such outgassing, extinguishing flame, and so on.
Voltage testing—The defined maximum voltage is put on the cable to test its maximum voltage resistance.
Torsion test for large sizes too
Some manufacturers also test cable-torsion characteristics. The latest piece of testing equipment is a torsion test device for robot applications. Increased requirements, especially in machine tool manufacturing and in robotics, are prompting more demand for greater torsion resistance, but unfortunately still too few users test their electrical wires for their torsional stability. The new testing apparatus provides a rotation angle of ± 720° at a length of restraint of 7 ft (2 m) and a rotational speed of 360°/sec.
More unique is a test tower for torsion testing cables and wires used in wind turbines. Machines stress cables and wires with torsion to ± 150°/m. This test condition pushes cables to greater extremes than they would ever experience in real applications. In the 27-ft (8-m) tall girder mast, a wind turbine’s cable loop is reproduced 1:1 and can test up to 20 cables simultaneously.
Case in point: Cable track test systems from 3 to 130 ft (1 to 40 m)
Consider how at Helukabel’s Windsbach Test Center in Germany, electric cables and wires in chain conveyors are shot back and forth at up to 5 g. Operators need hearing protection, as equipment rigorously tests all cable for months. Only when the cables and wires withstand customer specifications in the test equipment does the manufacturer approve the cables for their intended application use. The Windsbach test center recently acquired a new 16-ft (5-m) chain-pulled conveyor testing machine that runs to 32 ft/sec (10 m/sec) and accelerates to 164 ft/sec² (50 m/sec²).
At the center, operators with seven machines having 3, 10, 16, 60 and 130 ft (1, 3, 5, 18 and 40 m) traversing distances are used. Cables are routinely tested with speeds up to 7 m/sec. Chains with conventional bending radii work on all the machines.
HELUKABEL
www.helukabel.com
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