Lubricant Classification and Selection: A Comprehensive Guide | MachineMFG (2024)

I.Classification of Lubricants

Lubricants can be divided into four categories according to their physical states: liquid lubricants, semi-solid lubricants, solid lubricants, and gas 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 broad selection for mechanical components operating under various loads, speeds, and temperatures.

(1) Mineral Lubricating Oil: This is currently the most extensively used type of liquid lubricant, accounting for about 90% of the total volume of lubricating oil. It is generally formed by adding additives into mineral base oil.

(2) Synthetic Lubricating Oil: This refers to lubricating oil produced through chemical synthesis.

(3) Animal and Vegetable Oil: This refers to lubricants extracted from animals and plants.

(4) Water-based Liquids: These are water-containing lubricants, including solution types and emulsion types.

2. Semi-solid Lubricants (Grease)

Also known as grease, semi-solid lubricants exhibit a semi-fluid state at normal temperature and pressure, and they possess a colloidal structure.

3. Solid Lubricants

Solid lubricants function primarily in three ways: The first type forms a solid lubricating film on the friction surface, operating similarly to boundary lubrication.

The second type, soft metal solid lubricants, utilize the low shear strength of soft metals to provide lubrication. The third type involves substances such as graphite with a layered structure which leverages its structural feature 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, comply with the physical laws of fluid lubrication. Therefore, under certain conditions, gases can serve as lubricants just like liquids.

The advantages of gas lubricants include a small friction coefficient, less frictional heat generation at high speeds, low temperature increase, flexible operation, and a wide working temperature range.

The disadvantages are their low density and load-bearing capacity, making them suitable only for pneumatic devices within the 30-70 kPa range and hydrostatic devices not exceeding 100 kPa.

II. Composition of Lubricants

1. Base Oil

Base oil is the primary component of lubricants, accounting for 80% to 95% of the total volume, and serves as the carrier for additives. Base oils are broadly categorized into mineral oils and synthetic oils.

(1) Mineral Oil.

In our country, mineral oils are typically classified into paraffinic base, intermediate base, and naphthenic base types.

(2) Synthetic Oil.

Synthetic base oils are produced through chemical reactions of various compounds. They possess higher chemical purity, superior physical and chemical properties compared to mineral oils, thus have a broader range of applications and longer service life. They represent the future development direction of lubricants.

Currently, synthetic oils are widely used in aerospace machinery, and their application in industrial machinery is rapidly expanding. Synthetic oils are generally divided into synthetic hydrocarbon oil, ester oil, polyisobutylene oil, polyethers, silicone oil, etc.

2. Additives

Additives are minor constituents added to lubricants that significantly enhance certain characteristics or introduce new properties. Their functions are as follows:

(1) Detergents.

Primarily used 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 within the oil, preventing the formation of larger particles.

(2) Antioxidants.

They slow down the oxidation reaction of the lubricating oil, thereby extending its service life.

(3) Anti-wear agents.

These improve the oil’s resistance to wear and scuffing, reduce equipment wear, and prevent seizing or sintering.

(4) Oiliness agents.

They reduce the friction coefficient and enhance 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 oil’s viscosity index, enhancing its visco-thermal performance.

(7) Rust inhibitors.

These act upon the metal surface to prevent rust or corrosion when in contact with water.

(8) Pour point depressants.

They lower the oil’s pour point by slowing the formation of paraffin crystals at low temperatures, thus improving the oil’s low-temperature fluidity.

(9) Defoamants.

They alter the oil’s tendency to foam and cause surface bubbles to burst quickly.

(10) Emulsifiers and anti-emulsifiers.

Emulsifiers are used in emulsifying oil to form a uniform and stable emulsion with water. Anti-emulsifiers are used in general lubricants to separate water quickly from the 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 grease’s consistency, drop point, and water resistance, and sometimes its load capacity as well.

III. Selection of Lubricating Oil

1. Factors in Choosing Lubricating Oil

The selection of lubricating oil is primarily based on three factors: the actual working conditions of the equipment, the specifications or recommendations from the equipment manufacturer, and the regulations or suggestions from the oil manufacturer.

In practice, the choice of lubricating oil is mainly based on the recommendations of the equipment manufacturer. However, the equipment’s actual load, speed, and temperature conditions 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 quality identification and determination. The viscosity of lubricating oil for equipment is determined according to design or calculated data, referring to relevant charts.

2. Pour Point:

The pour point indirectly reflects the low-temperature fluidity of lubricating oil during storage, transportation, and usage. It has been proven that the usage temperature must be 5-10°C higher than the pour point.

3. Flash Point:

This is a key safety indicator for storing, transporting, and using lubricating oil. The principle for setting the flash point of lubricating oil is to leave a safety margin of half, i.e., it should be half higher than the actual usage temperature.

For example, if the maximum temperature of the oil in the bottom shell of an internal combustion engine does not exceed 120°C, the minimum flash point for engine oil should be set at 180°C.

Since there are many performance indicators for lubricating oil, and the differences between different types are significant, the final decision should be made rationally, considering the equipment’s working conditions, the manufacturer’s requirements, and the oil product’s specifications and introductions.

2. Substitution of Lubricating Oil

Each lubricant has its own performance characteristics, so it’s crucial to make the correct and reasonable selection, avoiding substitution whenever possible. If substitution is indeed necessary, the following principles should be adhered to:

(1) Substitute with the same type of oil or one with similar performance characteristics.

(2) The viscosity should be comparable, the substitute oil’s viscosity should not exceed ±15% of the original oil’s viscosity. Preference should be given to those with a slightly higher viscosity.

(3) Substitute with a higher quality oil when possible.

(4) Consideration should also be given to the equipment’s environment and operating temperature.

3. Mixing of 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 must not be mixed with oils that do not have this requirement.

(3) Ammonia-resistant turbine oil must not be mixed with other types of turbine oil.

(4) Zinc-containing anti-wear hydraulic oil cannot be mixed with anti-silver hydraulic oil.

(5) Gear oil must not be mixed with worm gear oil.

However, the following oils can be mixed:

(1) Products from the same manufacturer with similar quality.

(2) Products of different brands from the same manufacturer.

(3) Different types of oil, if mixed in a composition that does not contain additives.

(4) Different types of oil that show no abnormal effects in mixing tests.

(5) Internal combustion engine oils with a variety of additives and large quantities have different performance characteristics. When the oil properties are not understood, caution is required to avoid adverse effects or even lubrication accidents.

IV. Selection of Lubricating Grease

When selecting a lubricating grease, the primary consideration should be its function, namely, its role in lubrication, friction reduction, protection, and sealing.

For friction-reducing greases, 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 properties and stability for metals and non-metals. For sealing greases, considerations should 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 should consider the operating temperature, rotational 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, evaporation loss, oxidative degradation, and colloidal shrinkage of the base oil of the grease accelerate.

For every increase in temperature by 10℃ to 15℃, the oxidation speed of the grease increases by 1.5 to 2 times, and the lifespan 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 ambient temperatures and machines operating at high temperatures, high-temperature-resistant grease should be used. The temperature of the general grease should be 20℃ to 30℃ below its dropping point (temperature).

(2) Rotational Speed.

The higher the operating speed of the lubricated components, the greater the shear stress experienced by the lubricating grease, and the more significant the damage to the fibrous structure formed by the thickener, thereby shortening the lifespan of the grease.

If the equipment’s operating speed doubles, the lifespan of the lubricating grease reduces to one-tenth of its original duration.

Components operating at high speeds generate more heat and at a faster rate, potentially thinning the lubricating grease and causing it to leak out. Therefore, a thicker lubricating grease should be used in such scenarios.

(3) Load.

Choosing the right lubricating grease according to the load is a key aspect in ensuring effective lubrication.

For high-load lubrication points, lubricating grease with a high viscosity base oil, high thickener content, and superior extreme pressure and anti-wear properties should be selected. The cone penetration of the lubricating grease is directly related to the load it can handle during use.

For high-load conditions, lubricating grease with a smaller cone penetration (higher viscosity) should be selected.

If the application involves both heavy and 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 surrounding media of the lubrication point, such as air humidity, dust, and the presence of corrosive substances.

In damp environments or situations involving contact with water, water-resistant lubricating grease should be selected, such as calcium-based, lithium-based, complex calcium, or complex sulfonate calcium greases. Under severe conditions, rust-preventive lubricating grease should be used instead of sodium-based grease with poor 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-effectiveness of the lubricating grease should also be considered when selecting it.

This involves a comprehensive analysis of whether using the grease extends 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 with respect to grease viscosity.

Reference Indicators for Grease Failure