The oil state refers to the physical and chemical properties and performance of industrial oils such as lubricating oil, hydraulic oil, and transmission oil during use. It directly affects the reliability, lifespan, and economy of the equipment. The condition of the oil is closely related to multiple aspects, including basic properties, usage environment, working conditions, pollution control, and maintenance management, which need to be comprehensively analyzed

1、 Base oil and additive system

The core of oil is composed of base oil and additives, which together determine the initial performance and durability.

-Base oil types: divided into mineral oil, semi synthetic oil, and fully synthetic oil. Mineral oil has a lower cost, but its high-temperature oxidation stability is poor; Synthetic oils (such as PAO and ester oils) have more stable molecular structures and higher thermal decomposition temperatures, making them suitable for special temperature or high load scenarios. The viscosity grade of base oil (such as ISO VG 32/46/68) directly affects the thickness of the oil film and flow resistance. Low viscosity oil is energy-saving but has weak bearing capacity, while high viscosity oil is the opposite.

-Additive function: By compounding, specific properties can be improved, such as:

-Antioxidants (phenolic amines, sulfur phosphorus type): delay the oxidation of base oil, reduce the formation of acidic products and sludge;

-Wear resistant agents (ZDDP, organic molybdenum): form a protective film on the metal surface to reduce the friction coefficient;

-Clean dispersant (sulfonate, salicylate): inhibits carbon deposition, paint film deposition, and maintains system cleanliness;

-Viscosity index improver (poly (methyl methacrylate)): Expand the applicable temperature range to avoid low-temperature solidification or high-temperature thinning.

As the usage time increases, additives gradually become ineffective due to consumption, decomposition, or reaction with other substances, leading to a decline in oil performance.

2、 External interference in the usage environment

Environmental factors accelerate oil aging through physical or chemical processes, mainly including:

-Temperature fluctuations: High temperatures (>80 ℃) significantly accelerate the oxidation rate, with the oxidation rate doubling for every 10 ℃ increase; Localized overheating (such as near engine cylinder liners) may cause evaporation of light components and coking of heavy components. Under low temperature conditions, the precipitation of wax crystals can increase viscosity and even clog the filter, resulting in insufficient oil supply.

-Humidity and moisture intrusion: When water vapor in the air condenses into free water, it can cause three major hazards: ① disrupting the continuity of the oil film and exacerbating wear and tear; ② Promote the hydrolysis and failure of additives; ③ Inducing corrosion on the metal surface, the generated iron ions further catalyze oxidation. Some extreme pressure gear oils can also generate more corrosive acids when exposed to water.

-Oxygen and reactive gases: Oxygen in the atmosphere continues to dissolve in oil and undergoes free radical chain reactions with hydrocarbons, producing peroxides, alcohols, ketones, and even carboxylic acids, ultimately forming high molecular weight asphaltene and gum, manifested as darkening of color and increasing viscosity. If there are corrosive gases such as chlorine and hydrogen sulfide in the system, it will further exacerbate metal corrosion.

-Radiation and electromagnetic fields: Oil near nuclear industry or high-frequency motors may be affected by ionizing radiation, causing molecular chain breakage and generating free radicals, accelerating degradation.

3、 Power input for mechanical working conditions

The operating parameters of the equipment directly determine the stress level that the oil is subjected to:

-Load intensity: Under heavy load conditions (such as rolling mills, excavator slewing mechanisms), the compression stress between tooth surfaces can reach GPa, forcing the oil film to rupture instantly, exposing the metal micro protrusions to intense friction, generating a large amount of heat energy and triggering adhesive wear. At this point, the sulfur phosphorus based extreme pressure additive rapidly reacts to form a protective layer of iron sulfide/iron phosphate, but its consumption rate far exceeds normal operating conditions.

-Movement speed: High speed rotating components (such as turbine bearings) rely on dynamic pressure effects to maintain oil wedges. Excessive linear velocity will increase the proportion of agitation losses, which will be converted into heat; At low speeds and heavy loads, mixed lubrication is prone to occur, with boundary friction dominating and a significant increase in the number of wear particles.

-Impact load: Frequent start stop or sudden load changes (such as in a press machine) cause the oil film to repeatedly compress and release, resulting in a "pumping effect" that not only consumes energy but also reduces effective viscosity due to shear dilution. Long term exposure to this can cause fatigue and detachment of additives.

-Vibration and Turbulence: The turbulent state inside the pipeline enhances the convective mass transfer between the oil and the pipe wall, accelerating oxygen diffusion; The metal particles generated by micro vibration wear at the bearing clearance become heterogeneous catalysts, significantly increasing the oxidation rate.

4、 Types and sources of pollutants

Foreign pollutants are one of the main causes of oil failure, which can be divided into three categories according to their forms:

-Solid particle pollution: Originating from residual cleaning, peeling of abrasive particles from parts, and invasion of external dust. >Particles with a size of 5 μ m can scratch the mating surface, while hard particles with a size of<10 μ m (such as SiO ₂) can cyclically wash the surface and form cutting grooves when the filter cartridge fails. For every one level increase in NAS cleanliness level, the bearing life is reduced by about half.

-Liquid pollution: Fuel dilution (diesel engine fuel injector leakage) and coolant leakage (heat exchanger perforation) can significantly reduce viscosity and damage lubrication; Cross contamination of process media (such as cutting fluid mixed with guide oil) may lead to saponification reactions and the formation of flocculent substances.

-Gas pollution: In addition to the aforementioned water vapor and oxygen, the oil mist introduced into the compressed air system can also cause pollution. It is worth noting that modern devices commonly use labyrinth seals and breathing valve designs, which can block most particles but still cannot completely isolate small water molecules.