An estimated 100 000 km of steel cord conveyor belts are in use around the world. They have been in service for over 80 years as the most economical and environmentally friendly solution for carrying bulk materials over long distances. Steel cord conveyor belts for inclined shafts, which transport raw coal directly and continuously from underground to the surface, are particularly important in coal mining (see Figure 1).
As conveyor belts can only be spliced or made endless on site for reasons of weight and size, they have to be transported from the factory to the place of use in smaller individual lengths. The average length of such a single belt is around 300 m. With very high-strength belts, this still results in individual weights of 50 t, and diameters of 4 m. Round reels are common, but racetrack reels are also used.
100 000 km of conveyor belts in use divided by 300 m results in a total of over 300 000 splices in use.
Splice geometry
Before the belts can be connected, the appropriate splice layout must be determined. This results from the diameter of the ropes and the rope pitch, i.e. the distance between the ropes. There must be enough rubber between the steel cords in the splice to be able to transfer the tensile forces. This results in one to six-step splices. The lower the belt breaking strength, the smaller the number of stages can be, and the shorter the splice. As the cable diameter increases, more space must be created between them, which can only be achieved by graduation. An ST1000 belt (the number after the ST indicates the minimum belt breaking strength in Newtons per millimeter of belt width) can be connected in one-step design, and is 0.6 m long, while an ST10000 splice has six stages and is more than 10 m long.
It is important to note that no rope is directly connected to another rope. All forces are transmitted via the surrounding rubber. This results in shear forces which, depending on the splice geometry, lead to very different loads during operation.
The general aim is to achieve, at least, the nominal breaking strength of the belt statically, as well as the highest possible fatigue strength, i.e. dynamic strength or Wöhler test, in accordance with DIN 22110.
This is a preview of an article that was originally published in Issue 1 of World Coal 2025.
Subscribe to World Coal for free to read this article in full and many more here.