Maximum tensile strain of cables:

Everything you need to know

The maximum tensile strain of a cable is particularly important where cables are used in hanging applications. This is because the cable has to bear its own weight. This is the case, for example, with a storage and retrieval unit (RBG) during the lifting movement.

What does the maximum tensile strength of a cable mean and how do I determine it?

The maximum tensile strain indicates the force up to which a cable can be loaded without danger, as well as the maximum free-hanging length at which the cable can still be used safely. Depending on the structure of the cable, different maximum tensile strains may result. The maximum tensile strength for cables with copper cores is specified by the VDE 0298-4 standard. Here, the maximum permissible tensile strength for standard cables is defined as 15N per square millimetre cross section of the sum of the main cores.

A calculation example can illustrate the situation:

Assume a CF21.25.15.02.02.UL, i.e. a servo cable with a structure of (4G2.5+(2×1.5)C)C, is to be used hanging for 25m in an application. First, the weight of the cable must be calculated:

F = m x g x l = 0.271kg/m x 9.81m/s² x 25m = 66.5N

How high can the cable be loaded? The main cores have a cross section of 2.5mm², i.e:

4 cores with 2.5mm² cross section each -> 4 x 2.5mm² = 10mm²

10mm² x 15N/mm² = 150N max. permissible tensile strain

As the weight force of 66.5N is less than the maximum permissible tensile strain of 150N, the cable can be used in this application.

Which cable should be used for high tensile strains?

With small cross sections, such as those found in bus cables, the permissible load limit is quickly reached. For this reason, igus offers optimised cables especially for applications with particularly high tensile strains. A good example of such a cable is the CFSPECIAL182.045, a CAT5e Ethernet cable. Due to its special structure with an aramid braid in the two-layer outer jacket, this cable is particularly suitable for applications with very high tensile strain, e.g. in hanging installations.

The aramid braiding integrated in the outer jacket ensures that the tensile strain does not act on the inside of the cable, but is absorbed in the braiding. This ensures a longer service life and the good functionality of the cable. Enormously high tensile forces also occur in other applications, such as in forklifts. In such cases, detailed testing and analysis is required to find a suitable solution. The type of movement in this application is also different, as the cable is guided by rollers.

Conclusion:

Taking the maximum tensile strain into account is crucial when selecting the right cable. If this is not considered, the cable may elongate, which can lead to defects and, in the worst case, to machine failure.