A Brief Discussion on the Damage and Prevention of Steel Wire Ropes Used in Cranes

Steel wire ropes are commonly used flexible force transmission components in machinery. As for the steel wire ropes used in cranes, there are many specifications and varieties, and their application is rather complex. They have high requirements for strength, flexibility and wear resistance. With the long-term use of steel wire ropes, damage or breakage often occurs. In recent years, major and extremely serious lifting machinery accidents caused by this have occurred from time to time. Below, based on practical experience in recent years, the author briefly discusses several types of damage to steel wire ropes used in cranes and their prevention and control methods.

1 Wear and tear

Wear is the most common damage phenomenon of steel wire ropes, which is generally divided into three situations: external wear, deformation wear and internal wear.

1) External wear: During the use of the steel wire rope, its outer circumference comes into contact with the surfaces of objects such as pulley grooves, drum walls, and hooks, causing external wear. As a result, the rope diameter becomes thinner, the fine steel wires on the outer surface are worn flat, reducing the cross-sectional area of the steel wires bearing the load, and the breaking load of the steel wire rope is also correspondingly reduced. Single-cycle wear is more severe than full-cycle wear. Efforts should be made to change single-cycle wear to uniform full-cycle wear as much as possible, and uniform wear should be achieved throughout the entire length of the steel wire rope. If the steel wire rope is replaced in the middle of use, its service life can be extended by 30% to 40%.

2) Deformation wear: The surface damage of steel wire ropes caused by vibration and collision is a kind of local wear phenomenon. If the steel wire rope on the surface of the drum is impacted by other objects, the lifting steel wire ropes are entangled with each other, or the rope biting phenomenon occurs due to the deviation of the pulley from the center of the drum, all these will cause deformation and wear of the steel wire rope.

3) Internal wear: During use, due to the bending of the steel wire rope, the fine steel wires inside interact with each other, causing slippage. The contact stress between strands increases, and local indentations and deep depressions occur between the steel wires of adjacent strands. When repeatedly stretched and bent in a cycle, stress concentration occurs at the deep depressions, leading to breakage and thus internal wear. Generally, the pressure on the surface of fine steel wire is directly proportional to the pressure of the steel wire rope. Under the same tension, due to the different areas of compression, the pressure per unit area also varies. From the perspective of surface pressure wear, it is more advantageous for wire ropes to use line contact than point contact, and it is even more advantageous to use surface contact than line contact. Therefore, choosing wire ropes with either line contact or surface contact types is an effective way to reduce internal wear. In addition, the degree of bending and the movement speed of the steel wire rope both have an impact on its internal wear.

2 Fatigue

During the use of steel wire ropes, they mainly bear bending fatigue and fatigue caused by tension, twisting and vibration.

1) Bending fatigue: When a steel wire rope bends countless times and repeatedly passes over pulleys or drums, the steel wires become fatigued and their toughness decreases, eventually leading to wire breakage. Fatigue broken wires usually occur on the outer steel wire of the side with the most severe bending of the strands. Generally, the appearance of fatigue broken wires indicates that the steel wire rope is approaching the end of its service life.

2) Fatigue caused by stretching, twisting and vibration: The changing tensile stress of the crane binding steel wire rope at the beginning and end of starting and braking, and before and after bearing loads, can cause metal fatigue. In addition, the frequent twisting and vibration of the steel wire rope are also causes of fatigue. The principle of fatigue damage is that under the action of variable stress, the surface of fine steel wire forms initial cracks due to various slippages. The crack tips repeatedly undergo plastic deformation under the action of shear stress and expand until they break. The broken wire surfaces caused by fatigue are usually even and mostly occur on the surface of the steel wire, which is quite regular.

The main ways to prevent fatigue damage are as follows: First, under conditions permitting, increase the diameter of the drum and pulley as much as possible; Second, when arranging the layout of pulleys, try to avoid causing the steel wire rope to bend in the opposite direction. Tests show that the damage caused by reverse bending is approximately twice that of bending in the same direction. Thirdly, it is advisable to choose steel wire ropes with good structures as much as possible, such as WS and TX type wire ropes with good line contact.

3 Rust

Steel wire ropes are generally used outdoors and are subject to corrosion from sun exposure and rain. The damage is even more severe when used in environments with harmful gases and harsh conditions. Due to the difference in oxygen affinity on the surface of the damaged steel wire rope, a large number of small cells are formed, resulting in many circular corrosion pits that gradually deepen. These pits then become stress concentration points and sources of fatigue cracks. Meanwhile, corrosion reduces the cross-sectional area of the steel wire rope, lowering its elasticity and ability to withstand impact.

There are two methods to prevent rust and damage to steel wire ropes. One is to apply oil frequently; Secondly, for steel wire ropes that are used in harsh environments and have relatively less movement, special steel wire ropes such as galvanized and aluminum-coated ones can be selected. This is because a film of zinc hydroxide and aluminum hydroxide forms on their surfaces in the atmosphere, effectively preventing corrosion.

4 Deformation

Many rope-breaking accidents are caused by the fact that the steel wire rope has been deformed or damaged in advance but has not undergone sufficient recurrence, resulting in major disasters. The main reasons for deformation are as follows.

1) Trauma is caused by abnormal contact between the steel wire rope and other equipment during operation. The most obvious external injuries are when the steel wire rope slides into the groove in the pulley or jumps out of the baffle on the drum, resulting in the scrapping of tens or even hundreds of meters of steel wire rope due to local crushing damage. The key to preventing external injuries lies in improving the crane equipment. The pulleys should be equipped with reliable anti-slip groove retaining rings. The gap between the retaining ring and the outer ring of the pulley should not exceed 1/5 of the diameter of the steel wire rope. The steel wire rope on the drum should not be too loose to prevent the rope loop from jumping out of the retaining ring and being crushed when it is tightly wound.

2) When the steel wire rope is coiled up on the drum and then rolls against each other, it is prone to crushing. During operation, a creaking sound is produced, and in some areas, broken wires and flattened marks will quickly appear. Preventive measures include selecting the deflection Angle between the pulley and the drum in accordance with the design specifications. If necessary, a rope arranging device or rope pressing device should be installed in the hoisting mechanism.

3) The permanent deformation of a twisted steel wire rope after local twisting is called kinking. If the twisting direction is consistent with the rotation direction of the steel wire rope, it is called a positive kinking; otherwise, it is called a negative kinking. Ordinary steel wire ropes have self-rotation. If tension is applied to the ends of the strands without binding, the strands will rotate in the reverse twist direction, which is an internal factor causing kinking. To prevent kinking, the following measures can be taken: First, non-rotating steel wire ropes should be selected on important lifting equipment; The second is to install a rotor at the free end of the steel wire rope. Third, if any signs of twisting are detected, immediately stop the operation and release it to restore the original state.

5 Bite the rope

The phenomenon of wire rope biting usually occurs on multi-layer wound crane drums, and it is even more severe on double-layer wound cranes with grooves