The calculation of cable pulling forces is one of important section of the design documentation. It assumes at least the following calculations:
✅ cable pulling forces (should be no more than 30 N/mm2 for a cable with an aluminium core and no more than 50 N/mm2 for a cable with a copper core);
✅ pressure on the side wall of the cable at the points of the route turning (should be no more than 1.5 kN/m).

Let us focus on the pressure on the side wall. The upper photos show cases:
1️⃣ The cable was bent using a large number of rollers;
2️⃣ The cable was bent using a single roller.

In the second case, as can be seen, under the influence of the cable pulling force (F), the cable was strongly pressed to the roller, and this led to dangerous deformation of the cable. Let us try to calculate the pressure on the side wall. Such a formula is given, for example, in the catalogues of cable manufacturers, but it is not always clear which of radii should be used in this formula. Consider the two options shown on the left and right diagrams:
👉 cable bending radius (R);
👉 roller radius (r).

At first glance, it seems that it is necessary to use the permissible bending radius of the cable (R), because it is prohibided to bend a cable with a radius less than this value. It is usually about 20 cable diameters (d), as shown on the left diagram but the exact value of (R) depends on the cable design, insulation material, cable laying temperature, etc. If we perform calculations using (R), it turns out that the pressure on the side wall is quite acceptable (0.9 kN/m).

However, it is clear that with the help of a single roller it is impossible to ensure that the cable bends along a curve having a radius (R). Highly likely, the cable will bend as shown in the right diagram. Then it is necessary to substitute into the formula not the permissible bending radius (R), but the radius of a single roller (r). Of course, in this case, the side wall pressure will be more than acceptable level. Thus, for the right diagram, where a single roller is used, there are two problems at once:
❌ the pressure on the side wall is too high;
❌ the cable is bent with a radius that is noticeably less than the minimum permissible (R).

So, the meaning of the post is simple:
➡️ turns should be arranged by several rollers, and the exact number should be calculated (in the right diagram, with a single roller, the pressure was 9 kN/m, and to reduce it to acceptable 1.5 kN/m, it is necessary to have at least 6 rollers for every 1 m of turn);
➡️ if someone’s calculations show that a single roller is enough, then it’s probably because of the wrong radius that was used in the formula.

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