I . Lubricant Classification
Lubricants can be divided into four categories according to their physical states: liquid lubricants, semisolid lubricants, solid lubricants and gaseous lubricants.
1. Liquid Lubricants
Liquid lubricants are the most widely used and varied category of lubrication materials, including mineral lubricating oil, synthetic lubricating oil, animal and vegetable oil, and water-based liquids.
The characteristic of liquid lubricants is that they have a wide viscosity range, offering a wide selection for mechanical components operating under various loads, speeds and temperatures.
(1) Mineral Lubricating Oil: It is currently the most used type of liquid lubricant, representing around 90% of the total volume of lubricating oil. It is usually formed by adding additives to mineral base oil.
(2) Synthetic Lubricating Oil: Refers to lubricating oil produced by chemical synthesis.
(3) Animal and Vegetable Oil: Refers to lubricants extracted from animals and plants.
(4) Water-based liquids: These are lubricants that contain water, including types of solutions and types of emulsions.
2. Semisolid lubricants (grease)
Also known as grease, semi-solid lubricants present a semi-fluid state at normal temperature and pressure and have a colloidal structure.
3. Solid Lubricants
Solid lubricants work mainly in three ways: The first type forms a solid lubricating film on the friction surface, operating in a similar way to boundary lubrication.
The second type, soft metal solid lubricants, utilizes the low shear strength of soft metals to provide lubrication. The third type involves substances such as graphite with a layered structure that takes advantage of its structural characteristic for lubrication.
The most commonly used solid lubricants for equipment lubrication are molybdenum disulfide, graphite, and polytetrafluoroethylene.
4. Gas lubricants
Gases, as a type of fluid, obey the physical laws of fluid lubrication. Therefore, under certain conditions, gases can serve as lubricants just like liquids.
The advantages of gaseous lubricants include a small coefficient of friction, less frictional heat generation at high speeds, low temperature rise, flexible operation and a wide working temperature range.
Disadvantages are their low density and load capacity, making them only suitable for pneumatic devices in the range of 30-70 kPa and hydrostatic devices not exceeding 100 kPa.
II. Lubricant Composition
1. Base Oil
Base oil is the main component of lubricants, representing 80% to 95% of the total volume, and serves as a carrier for additives. Base oils are broadly categorized into mineral oils and synthetic oils.
(1) Mineral Oil.
In our country, mineral oils are normally classified into paraffinic base, intermediate base and naphthenic base.
(2) Synthetic Oil.
Synthetic base oils are produced through chemical reactions of various compounds. They have greater chemical purity, superior physical and chemical properties compared to mineral oils, therefore they have a wider range of applications and a longer useful life. They represent the future direction of lubricant development.
Currently, synthetic oils are widely used in aerospace machines and their application in industrial machines is rapidly expanding. Synthetic oils are generally divided into synthetic hydrocarbon oil, ester oil, polyisobutylene oil, polyethers, silicone oil, etc.
2. Additives
Additives are secondary constituents added to lubricants that significantly improve certain characteristics or introduce new properties. Its functions are as follows:
(1) Detergents.
Used primarily in internal combustion engine oil to remove lacquer and carbon buildup on cylinder walls and piston rings. They also evenly disperse gum and soot particles in the oil, preventing the formation of larger particles.
(2) Antioxidants.
They slow down the oxidation reaction of the lubricating oil, thus extending its useful life.
(3) Antiwear agents.
These improve the oil's resistance to wear and abrasion, reduce equipment wear and prevent seizing or sintering.
(4) Oiling agents.
They reduce the coefficient of friction and improve lubrication performance.
(5) Metal deactivators.
They form a passive film on the metal surface to minimize the corrosive impact of the oil on the metal and the catalytic oxidation of the oil by the metal.
(6) Viscosity index improvers.
They increase the viscosity index of the oil, improving its visco-thermal performance.
(7) Rust inhibitors.
They act on the metal surface to prevent rust or corrosion when in contact with water.
(8) Pour point depressants.
They lower the pour point of the oil, delaying the formation of paraffin crystals at low temperatures, thus improving the fluidity of the oil at low temperatures.
(9) Antifoams.
They alter the oil's tendency to foam and cause surface bubbles to burst quickly.
(10) Emulsifiers and anti-emulsifiers.
Emulsifiers are used in oil emulsification to form a uniform and stable emulsion with water. Anti-emulsifiers are used in general lubricants to quickly separate water from oil.
3. Thickeners
Thickeners are a critical component of lubricating grease and distinguish it from lubricating oil. Lubricating grease is composed of thickeners, base oil and additives, forming a solid or semi-solid substance when the thickener is dispersed in the base oil.
Thickeners generally affect the consistency, dropping point and water resistance of the grease, and sometimes also its load-bearing capacity.
III. Lubricating oil selection
1. Factors in choosing lubricating oil
Lubricating oil selection is mainly based on three factors: the actual working conditions of the equipment, the equipment manufacturer's specifications or recommendations, and the oil manufacturer's regulations or suggestions.
In practice, the choice of lubricating oil is mainly based on the equipment manufacturer's recommendations. However, the actual load, speed and temperature conditions of the equipment must also be considered.
When selecting a lubricating oil, the following performance indicators are crucial:
1. Viscosity:
Viscosity is a criterion for classifying and grading different types of lubricating oils and plays a decisive role in identifying and determining quality. The viscosity of lubricating oil for equipment is determined according to design or calculated data, referring to relevant tables.
2. Pour point:
The pour point indirectly reflects the low-temperature fluidity of lubricating oil during storage, transportation and use. It has been proven that the usage temperature should be 5-10°C higher than the pour point.
3. Flash point:
This is a key safety indicator for storage, transportation and use of lubricating oil. The principle for defining the flash point of lubricating oil is to leave a safety margin of half, that is, it must be half higher than the actual temperature of use.
For example, if the maximum oil temperature in the lower casing of an internal combustion engine does not exceed 120°C, the minimum flash point of the engine oil should be set at 180°C.
As there are many performance indicators of lubricating oil and the differences between different types are significant, the final decision must be made rationally, considering the working conditions of the equipment, the manufacturer's requirements and the specifications and introductions of the petroleum product.
2. Replacing lubricating oil
Each lubricant has its own performance characteristics, so it is essential to make the correct and reasonable selection, avoiding replacement whenever possible. If replacement is really necessary, the following principles must be respected:
(1) Replace with oil of the same type or with similar performance characteristics.
(2) The viscosity must be comparable, the viscosity of the replacement oil must not exceed ±15% of the viscosity of the original oil. Preference should be given to those with slightly higher viscosity.
(3) Replace with a higher quality oil whenever possible.
(4) The equipment environment and operating temperature must also be considered.
3. Mixing lubricating oil
Mixing different types, brands, manufacturers and conditions (new or old) of oil should be avoided as much as possible. The following types of oil are strictly prohibited from being mixed:
(1) Special and specific oils cannot be mixed with other types of oil.
(2) Oils that require emulsion resistance should not be mixed with oils that do not meet this requirement.
(3) Ammonia-resistant turbine oil should not be mixed with other types of turbine oil.
(4) Anti-wear hydraulic oil containing zinc cannot be mixed with anti-silver hydraulic oil.
(5) Gear oil should not be mixed with helical gear oil.
However, the following oils can be mixed:
(1) Products from the same manufacturer with similar quality.
(2) Products from different brands from the same manufacturer.
(3) Different types of oil, as long as they are mixed in a composition that does not contain additives.
(4) Different types of oil that do not show abnormal effects in mixing tests.
(5) Internal combustion engine oils with a variety of additives and large quantities have different performance characteristics. When oil properties are not understood, care is needed to avoid adverse effects or even lubrication accidents.
4. Selection of lubricating grease
When selecting a lubricating grease, the main consideration should be its function, i.e. its role in lubrication, friction reduction, protection and sealing.
For friction-reducing greases, the main factors include resistance to high and low temperatures, load and rotational speed.
For protective greases, the focus is on the media and materials in contact, particularly the protective and stability properties for metals and non-metals. For sealing greases, considerations must include the materials and media in contact and the compatibility of the grease with the material (especially rubber) to select the appropriate lubricating grease.
The choice of lubricating grease must consider the operating temperature, rotation speed, load size, working environment and grease supply method of the machinery. General considerations include the following factors:
(1) Temperature.
The impact of temperature on lubricating grease is significant.
It is generally believed that when the operating temperature of the lubrication point exceeds the upper limit of the grease temperature, evaporative loss, oxidative degradation and colloidal contraction of the grease base oil accelerate.
For every temperature increase of 10°C to 15°C, the grease oxidation rate increases by 1.5 to 2 times and the useful life of the grease decreases by half. The operating temperature of the lubrication point also changes with the ambient temperature.
In addition, factors such as load, speed, continuous operation and overfilling of grease can also affect the operating temperature of the lubrication point.
For environments with high temperatures and machines operating at high temperatures, high temperature resistant grease must be used. The general grease temperature should be 20°C to 30°C below its dropping point (temperature).
(2) Rotational speed.
The higher the operating speed of the lubricated components, the greater the shear stress suffered by the lubricating grease and the more significant the damage to the fibrous structure formed by the thickener, thus shortening the useful life of the grease.
If the operating speed of the equipment doubles, the useful life of the lubricating grease is reduced to one-tenth of its original life.
Components operating at high speeds generate more heat and at a faster rate, potentially diluting the lubricating grease and causing it to leak. Therefore, a thicker lubricating grease should be used in such scenarios.
(3) Load.
Choosing the right lubricating grease according to the load is a fundamental aspect to guarantee effective lubrication.
For high-load lubrication points, lubricating grease with high viscosity base oil, high thickener content and superior extreme pressure and anti-wear properties should be selected. The penetration of the lubricating grease cone is directly related to the load it can support during use.
For high load conditions, lubricating grease with lower cone penetration (higher viscosity) should be selected.
If the application involves heavy, impact loads, lubricating grease with extreme pressure additives, such as those containing molybdenum disulfide, should be used.
(4) Environmental Conditions.
Environmental conditions refer to the working environment and the environment surrounding the lubrication point, such as air humidity, dust and the presence of corrosive substances.
In humid environments or situations involving contact with water, water-resistant lubricating grease should be selected, such as calcium-based, lithium-based, calcium complex or calcium complex sulfonate greases. Under severe conditions, anti-rust lubricating grease should be used instead of sodium-based grease with low water resistance.
In environments with strong chemical media, synthetic greases resistant to chemical media, such as fluorocarbon greases, should be used.
(5) Other factors.
In addition to the points mentioned above, the cost-benefit of the lubricating grease must also be considered when selecting it.
This involves a comprehensive analysis of whether the use of grease prolongs the lubrication cycle, the number of grease additions, grease consumption, bearing failure rate and maintenance costs, among other factors.
(6) The relationship between grease viscosity and application.
Table: Applicability range in relation to grease viscosity.
| NLGI Degree | Scope of application |
| Grade 000, Grade 00 | Mainly used to lubricate gears and open gearboxes. |
| 0 Degree | Mainly used to lubricate open gears, gearboxes or centralized lubrication systems. |
| 1st grade | Mainly used to lubricate needle bearings or roller bearings operating at higher speeds. |
| 2nd series | Most widely used to lubricate anti-wear bearings operating under medium load and medium speed. |
| 3rd series | Mainly used to lubricate anti-wear bearings operating under medium load and medium speed, as well as automotive wheel bearings. |
| 4th series | Mainly used to lubricate bearings and shaft collars in water pumps and other high load, low speed applications. |
| 5th grade, 6th grade | Mainly used for lubrication in special conditions, such as ball mill neck lubrication. |
Reference indicators for grease failure
| Project | Reference indicators for lubricating grease failure |
| Drip Point | Lubricating grease should be discarded when the dropping point is within the following ranges: 1. The dropping point of lithium-based lubricating grease (temperature) falls below 140°C. 2. The dropping point of lithium-based composite lubricating grease (temperature) falls below 200°C. 3. The dropping point of calcium-based lubricating grease (temperature) falls below 50°C. 4. The dropping point of calcium-based composite lubricating grease (temperature) falls below 180°C. 5. The dropping point of sodium-based lubricating grease (temperature) falls below 120°C. |
| Viscosity | When the cone penetration of the lubricating grease changes by more than +20%, the grease must be discarded. |
| Oil content | If the percentage of oil content in used lubricating grease to oil content in new grease falls below 70%, the grease must be discarded. |
| Ash content | When the ash content change rate of the tested sample exceeds 50%, the grease should be discarded. |
| Corrosion | If the lubricating grease fails the copper strip corrosion test, it must be discarded. |
| Oxidation | When the lubricating grease generates a strong rancid odor or the acid value of lithium-based grease exceeds 0.3 mg/g (KOH), it should be replaced with new grease. |
| Mechanical Impurities | If particles larger than 125 μm are mixed into the lubricating grease during use, it must be replaced with new grease. |
