Example 3: tested, tested, tested! "Millions of cycles" in a cable carrier

Differences in service life

At the beginning of 2002, a test to determine the service life of profibus cables in a real-world application was commissioned by 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 catalog:

Test parameters according to catalog data of the competition

A gliding application was chosen as a suitable test structure since profibus cables 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 achieve a large number of bending cycles in a short period of time, a genuine profibus transmission path was constructed. In a PC at the fixed end of the test cable carrier 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 tthereby set.

The fundamental test, which began at the beginning of 2002 and is still in progress today, showed only a relatively low number of cycles (420,000) resulted in the total failure of test item “A“, which - according to the competitor’s catalog - should have functioned safely for at least 4 million cycles. The actual service life achieved deviates from that indicated in the catalog by a factor of ten.

On the other hand test item “B“, the CFBUS.001, is still undergoing testing without any problems in data transmissions. So far, it has accomplished more than 14 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 is comprised of a foam material for all the test items. The electrical assets of this material ensured better transmission properties. A disadvantage of this material 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 sheathing to fill any gaps and to protect the bus pair against mechanical influences during the bending process. The element sheathing needs to be highly elastic. A mechanically-inferior element sheathing made of inexpensive filling material only serves to make the bus pair round and is not able to protect the bus cable from the high degree of mechanical stress present inside a cable carrier. Tensile and compressive forces which occur mainly influence those parts of the cable core in which there is a break in the element sheathing.

The sheathing of test item ”B“ (CFBUS.001) is characterized by a mechanically superior, gusset-filled TPE jacket, which mechanically relieves the bus pair, fixes the cores in a defined position and is flexible. The extremely short pitch of the core strands and continuous-flex cable also ensure that no great tensile or compressive 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, flame-retardant, 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. The bus parameters required are fulfilled by means of a choice of coordinated insulating materials and production procedures.

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.