Example 3: tested, tested, tested! “Millions of double strokes” in an energy chain
Profibus cables in permanentlymoving industrial use
For users, it is hard to get an overview of the cable market. Competition between cable suppliers is intensifying and manufacturers are outshining one another in their promises to “guarantee service life for cables used in energy chains”. Catalogues claim ten million - or even as many as 50 million - double strokes when it comes to the service life of cables used in applications involving movement.On taking a closer look at figures claimed, one must ask how testing was done, or how realistic tests carried out actually were (for example length of travel, test radii, etc.) in order to be able to provide such a guarantee. Even information stating that cables are tested in accordance with VDE (Association of German electrical engineers) 0472, Part 603, test method H, is not helpful when it comes to determining the service life of a cable in energy chains, since the roller testing stand cannot provide any conclusive results and there is no VDE test for special cables in energy chains.
Differences in service life
At the beginning of 2002, a test to determine the service life of profibus cables in a real application was commissioned in igus’ test laboratory. The aim was to examine any differences in the service life of igus’ CFBUS.001 chainflex® cable and another market leading profibus cable. The parameters required for the test were selected on the basis of data contained in the competitor’s catalogue:
| Catalogue details | Test item “A” Twin-core profibus cable |
Test item “B” igus® Chainflex® CFBUS.001 |
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|---|---|---|---|---|---|---|---|
| Cross section | (2 x AWG24)C | (2 x 0.25 mm2)C | |||||
| Guaranteed lifetime | Min. 4.0 Mio. Cycles | To be determined in a test |
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| Bending radius | > = 60 mm | 85 mm | |||||
| Diameter | 8,0 mm | 8,5 mm | |||||
| Catalogue details | as in 2002 | as in 2002 | |||||
Test parameters according to catalogue data of the competition
A gliding application was chosen as a suitable test structure since profibus cable systems are usually used here because of their data integrity, particularly over long lengths of travel and long transmission distances.
In order to be able to carry out non-destructive testing and hence achieve a large number of bending cycles in a short period of time, a genuine profibus transmission path was erected. In a PC at the fixed end of the test chain there was a profibus master insert card. A connection to a profibus slave was located on the moving end. This enabled the transmission rate to be determined with the help of a diagnosis program. Any data packets which might have been transmitted incorrectly could be indicated. The highest-possible transmission rate of 12 megabits/s was set.
The fundamental test, which commenced at the beginning of 2002 and is still in progress today, showed that only a relatively low number of cycles (420,000) resulted in the total failure of test item “A“, which, according to the competitor’s catalogue, should have functioned safely for at least 4.0 million cycles. The actual service life achieved deviates from that indicated in the catalogue by a factor of ten.
On the other hand test item “B“, the CFBUS.001, is still undergoing testing without any faulty data transmissions. So far, it has accomplished more than 14.0 million cycles.
Structure and materials
The main reason for the major differences in service life is the differing structural parameters of test item “A“ and test item “B“ (CFBUS.001), as well as the different materials used for producing the cables. The conductor insulation of the bus comprised of a foam material for all the test items. The electrical assets of this material ensured better transmission properties were achieved. A disadvantage of this material, however, was its weakness under reverse stresses. The forces which affect the bus pair should be absorbed by the element sheathing in order to alleviate the mechanical stress of the conductor insulation.Highly-elastic element sheathing
For this reason, test item “B“ (igus®) was provided with a mechanically superior, extruded TPE inner, or element, gapfilling sheathing, in order to protect the bus pair against mechanical influences during the bending procedure. The element sheathing must be highly elastic. A mechanically inferior element sheathing made of inexpensive filling material only serves to make the bus pair round, just like frequently used fillers or banding. It is not able to protect the buses from the high degree of mechanical stress present in the chain. Tensile and compression forces which occur mainly influence those parts of the cable core in which there is a break in the element sheathing.
| Test parameters | |||||||
|---|---|---|---|---|---|---|---|
| Distance of travel: | S = 5,0 m | ||||||
| Speed, approx: | V = 3,5 m/s | ||||||
| Acceleration, approx.: | a = 7,5 m/s2 | ||||||
| Radius, approx.: | 55 mm | ||||||
The sheathing of test item ”B“ (CFBUS.001) is on the one hand characterized by a mechanically superior, gusset-filled TPE element jacket, which mechanically relieves the bus pair, fixes the cores in a defined position and bends. The sheathing of test item ”B“ (CFBUS.001) is on the one hand characterized by a mechanically superior, gusset-filled TPE element jacket, which mechanically relieves the bus pair, fixes the cores in a defined position and bends. The extremely short pitch of the core strands and special cable also ensure that no great tensile or compression force has an effect on a long length of core.
UL and CSA approval
chainflex® CFBUS cables are now also available for all standard field bus systems, complete with UL and CSA approval and DESINA compliance. The highly abrasion resistant, flameretardant TPE outer jacket is extruded onto the fully braided shield with an adjusted twisted angle in order to provide the cable with additional stability.
The bus elements braided with a particularly short strand pitch are protected by means of a gap-filling, extruded TPE inner jacket.
As with all chainflex® cables, the new standard field bus cables of the CFBUS series are now available ex stock, without any cutting costs or extra charges for small quantities.
