1. Influence of Wire Rope Lubrication
Wire ropes are helical wire bundles formed by twisting steel wires that meet mechanical property requirements according to specific rules. Their main components include rope steel wires, lubricating grease, and rope cores (fiber cores or steel cores). Rope steel wires include high-quality carbon steel wires, stainless steel wires, and galvanized steel wires or galvanized aluminum alloy steel wires made by coating high-quality carbon steel wires. Most types of wire ropes are coated with lubricating grease, and the fiber cores are impregnated with hemp core grease. A few types of wire ropes are not coated with lubricating grease due to usage limitations. Decades ago, many wire rope manufacturers and users had already recognized that lubrication conditions are crucial to the service life of wire ropes. Reference [1] lists fatigue test data for a certain specification of wire rope in tabular form (Tables 1 and 2).
Table 1. Fatigue Test Data for a Certain Specification of Wire Rope
| Oiling Condition | Tension/N | Bending Fatigue Cycles at Initial Wire Breakage | Bending Fatigue Cycles at 10% Wire Breakage per Lay |
|---|---|---|---|
| Normal Oiling | 25.48×106 | 34,500 | 48,500 |
| No Oiling | 25.48×106 | 16,800 | 22,500 |
Table 2. Fatigue Test Data for Different Oiling Parts of Wire Rope
| Oiled Parts | Bending Fatigue Cycles | Core Oil Content Mass Fraction/% | |
|---|---|---|---|
| Before Test | After Test | ||
| All Steel Wires and Core Oiled | 4,907,375 | 25.52 | 16.42 |
| Core Oiled Only | 1,782,425 | 25.28 | 15.37 |
| No Oiling on Steel Wires or Core | 514,400 | — | — |
As can be seen from Tables 1 and 2, when neither the steel wires nor the core of the wire rope are oiled, the number of bending fatigue cycles decreases sharply, proving that lubrication conditions significantly affect the service life of wire ropes. Fatigue tests simulate the actual working conditions of wire ropes, and the actual service life of wire ropes is directly proportional to the fatigue test life.
Currently, wire rope manufacturers carry out lubrication treatment during the initial production stage, typically applying lubricating grease using strand coating or rope coating methods during manufacturing. The most reasonable grease application method is now considered to be spraying grease before the steel wires are closed during stranding, i.e., improving the grease application method to ensure the most uniform distribution of lubricating grease on the surface of the rope-making steel wires, thereby guaranteeing the lubrication effect between any two mutually contacting steel wires inside the wire rope.
2. Wear During Wire Rope Usage
In addition to lubrication treatment during production, wire ropes can also undergo lubrication maintenance during daily operation. Through research on failed wire ropes used in port bridge cranes and fracture surface inspection and metallographic analysis, the nature of wire breakage was found to be fatigue fracture, with the fracture source located at surface damage or wear points. The broken wire surfaces exhibit many defects such as wear and extrusion spalling, and the wear at the fracture site of some steel wires is very severe, with the fracture cross-sectional area being only 60% of the original wire. Through inspection and analysis of the internal wire fractures of scrapped wire ropes, it was confirmed that most wire fracture sites have obvious surface wear, and the fracture nature is fatigue fracture. This indicates that during actual use, the surface of the steel wires first undergoes significant wear, and then fatigue fracture occurs under alternating stress, leading to the final failure of the wire rope.
2.1 Impact of Lubrication on Wire Rope Life
Wear of wire ropes includes external surface wear and internal steel wire surface wear. External surface wear of wire ropes is caused by relative sliding with contacting objects, such as friction between the wire rope and the groove of a diverting sheave, which occurs on the visible part of the outer wires of the outer strands. Internal steel wire surface wear occurs when adjacent steel wires in the wire rope slide relative to each other during loading and unloading. The wear on the surface of internal steel wires in failed wire ropes confirms the existence of such relative sliding, as relative sliding is a prerequisite for wear.
The asynchronous elastic elongation deformation between internal steel wires of wire ropes causes relative sliding, leading to wear on the steel wire surfaces under the action of friction. Generally, the relative sliding amount is on the micrometer scale, and the resulting wear is called fretting wear. The main related factors of fretting fatigue include contact pressure, slip amplitude, fretting frequency, friction force, fretting environment, and the combined effect of fretting and alternating stress, which accelerate fatigue crack initiation and propagation, thereby significantly reducing the service life of wire ropes. Fretting wear between steel wires and the resulting fatigue fracture are one of the main causes of wire rope failure.
For wire ropes used in corrosive environments, such as galvanized wire ropes for marine fishery, chloride ions in seawater cause severe corrosion on the surface of steel wires. The combined effect of corrosion and wear usually accelerates each other, and corrosion fatigue is the main cause of failure for wire ropes used in corrosive environments.
The lubricating grease inside wire ropes serves two purposes: one is to improve lubrication between adjacent internal steel wires, and the other is to effectively isolate the steel wires from air, providing anti-corrosion protection while lubricating. When lubrication fails, the oxidation rate of steel wires in the atmosphere increases rapidly, especially in coastal high-temperature and humid atmospheres with high salt content. Good lubrication can effectively reduce the friction coefficient between internal steel wires of wire ropes, thereby slowing down the rate of fretting wear and delaying the occurrence of fretting fatigue. This is how lubrication affects the service life of wire ropes. Factors influencing lubrication effectiveness include lubricating grease application amount, distribution uniformity, and chemical-physical properties.
2.2 Impact of Lubrication on Wire Rope Life
Based on the relevant data collected above, it is evident that during the twisting process, steel wires undergo combined tension, torsion, and bending loads, resulting in wedge-shaped, stepped, and mixed fracture surfaces. Microscopic analysis and metallographic testing of the fracture surfaces show that transverse cracks form on the steel wire surface due to the combined effect of torsion and bending loads, and crack propagation eventually leads to wire fracture. The occurrence of wear before wire fracture confirms the existence of relative sliding between steel wires. Good internal lubrication of wire ropes can reduce the friction coefficient, slow down wear, and thus extend service life. Previous measures to improve wire rope service life mainly focused on structural improvements and enhancing the mechanical properties of rope-making steel wires. By analyzing the impact mechanism of lubrication on wire rope service life, technical measures targeting internal steel wire surface wear, such as reducing the friction coefficient through lubrication or improving the wear resistance of steel wire surfaces through surface treatment technology, provide a new approach to extending wire rope service life. With continuous innovative technologies in the wire rope field, the service life of wire ropes will be significantly extended.
The VMAXX®️ Wire Rope Cleaning and Lubrication System, combined with specialized wire rope lubricating grease, effectively improves wire rope lubrication and extends service life. The wire rope first passes through a specially designed cleaner to remove失效的油脂 (failed grease) and dust-hardened particles from the surface and grooves. The cleaned wire rope then enters the lubricator, which consists of a wire rope clamp, wire rope seal, and quick oil inlet connector. The VMAXX®️ pneumatic pump stores pressurized lubricating grease in a sealed chamber to pressurize and lubricate the wire rope, promoting effective penetration of the grease into the internal structure for comprehensive lubrication.
The VMAXX®️ system injects specialized grease under high pressure to the surface of wire ropes requiring maintenance and penetrates into the core, providing reasonable and thorough lubrication protection to maximize service life. It is widely used in scenarios with high stress, severe wear, high cost, and difficult replacement, while also saving costs and improving work efficiency.