Every crane user will encounter loose strands of wire rope and the formation of "Birdcage". This deformation of the wire rope is usually interpreted as a result of the wire rope being subjected to impact loads (even in the old version of ISO 4309). However, in most cases, the defects of loose strand and "bird cage" of steel wire rope are caused by other mechanical reasons, and the most common reason is the torsion of steel wire rope around its own shaft.
Taking a small section of steel wire rope in Figures 1 and 2 as an example, if both ends of the wire rope are fixed and then twisted around the central line in the middle part, one end of the wire rope is elongated and the other end is shortened. The result is that the outer strand on one side is too long, while the inner strand on the other side is too long, as shown in figures 3 and 4.
But how can the wire rope on the crane twist around its own shaft? This happens to each pulley and drum. When the wire rope enters the pulley at a deflection angle, it first connects with the side edge of the rope groove, then rolls downward and slides to the bottom of the rope groove, as shown in Fig. 5. In this process, the wire rope will twist around its own shaft, and the greater the deflection angle, the greater the torsion angle.
Figures 6 and 7 show the cross section of the wire rope entering the pulley, where the opening of the rope groove is 35 ° according to the American Standard, when the wire rope is at 1 ° angle When the deflection angle of 5 ° enters into the same pulley, it will contact the side of the rope groove at a relatively high position, and the torsion angle of the rope groove will reach 50 ° after rolling downward.
Of course, different from the movement of the gear on the rack, since the movement of the wire rope from the side of the groove to the bottom of the groove is a combination of rolling and sliding, so the torsion angle of the wire rope largely depends on the friction coefficient between the wire rope and the rotary table.
Solution 1:
Lubricating wire rope
The simplest way is to lubricate the wire rope so as to reduce the friction coefficient between the wire rope and the rotary table. When the well lubricated wire rope enters the rotary table, the torsion produced is much smaller than that of the dry and corroded wire rope.
Solution 2:
Replacing plastic pulley with steel pulley
The friction coefficient between steel and plastic is larger than that between steel and steel. Therefore, under the same geometric conditions, the torsion of steel wire rope entering steel pulley is much smaller than that of plastic pulley. Therefore, plastic pulley is only suitable for small deflection angle.
Solution 3:
Use wire rope with internal plastic cushion
The steel wire rope with internal plastic lining has better resistance to single strand loose strand and "bird cage" defects than other forms of wire rope.
Solution 4:
Use pulley with large groove opening
When the opening of the rope groove is large, the wire rope has entered the deep part of the rope groove before contacting with the side of the rope groove. Figure 8 is a cross-sectional diagram of the process of a wire rope entering a rope groove with an opening of 60 ° at a deflection angle of 5 °. The contact position between the wire rope and the groove side is much lower than that in Figure 7, and the torsion angle of the wire rope is only 25 °. It can be seen that by enlarging the opening of rope groove from 35 ° to 60 ° the torsion angle of wire rope is reduced by 50%. Therefore, the German standard DIN 15061 stipulates that the minimum opening of the rotary table rope groove is 45 ° and the opening degree of 52 ° is better.