Basic Differences Between Lubricating Oils and Greases
| Comparison Item | Lubricating Oil | Grease |
|---|---|---|
| Form | Liquid, good fluidity | Semi-fluid or semi-solid paste, with adhesiveness |
| Composition | Base oil (approx. 90%) + additives (approx. 10%) | Base oil (70%–90%) + thickener (8%–30%) + additives ( |
| Heat dissipation capability | Good; heat can be carried away through circulation | Poor; not conducive to rapid heat conduction |
| Sealing and dust protection | Fair | Good; effectively isolates moisture, dust, and impurities |
| Application scenarios | High-speed or circulating lubrication points such as engines, gearboxes, and hydraulic systems | Low-speed heavy-duty bearings, locations requiring long-term lubrication and difficult to replace |
Grease can be understood as lubricating oil with a thickener added (similar to a sponge structure), which adsorbs and locks the base oil in a three-dimensional network, serving to hold and slowly release the lubricating oil; its lubricating performance mainly depends on the quality of the base oil.
The eight major functions of lubricants
Cooling and heat dissipation—preventing friction-generated heat from raising component temperatures, avoiding aggravated adhesive wear and corrosive wear; sealing protection—isolating moisture, oxygen, and harmful media in humid air, preventing leakage of internal media; cleaning dirt—carrying away abrasive particles generated by friction and external impurities, reducing wear; reducing friction and wear—forming a lubricating film to lower the friction factor between two moving parts; rust prevention—protecting metal surfaces from corrosion; vibration damping—providing damping and vibration absorption; reducing power consumption—improving equipment operating characteristics and extending equipment life; power transmission—transmitting power in equipment such as hydraulic systems.
Composition of lubricating oil
Lubricating oil consists of base oil and additives. Base oil is the main component and determines the basic properties of the lubricating oil; there are four main categories: paraffinic, naphthenic, aromatic, and synthetic. In engine oil, base oil typically accounts for about 90%, and its quality is crucial to performance, providing the most fundamental lubrication and cooling functions. Additives are used to compensate for and improve the deficiencies of base oil and impart new properties; common ones include anti-wear agents, antioxidants, rust inhibitors, detergent dispersants, viscosity index improvers, and more.
Factors to consider when selecting lubricating oil
Movement speed—at high speeds, low-viscosity oil is suitable; at low speeds, high-viscosity oil can be used; operating temperature—at high temperatures, oil with higher viscosity or good oxidation resistance should be selected; load conditions—when bearing heavy loads or impact loads, oil with high viscosity or good extreme-pressure performance should be selected; fit clearance—when the clearance of the moving pair is large, high-viscosity oil should be selected; when the clearance is small, low-viscosity oil should be selected; surface roughness—when roughness is high, high-viscosity oil should be selected; otherwise, low-viscosity oil should be selected; working environment—in humid conditions or with chemical media, oil with good demulsibility or corrosion resistance should be selected.
Suitable applications for common greases
| Grease types | Main Features | Operating Temperature Range | Applications |
|---|---|---|---|
| Calcium-based Grease | Good water resistance, not resistant to high temperatures | -10℃ ~ 60℃ | Humid environments or parts in contact with water, such as agricultural machinery and water pump bearings |
| Sodium-based Grease | High temperature resistance, not water resistant (emulsifies on contact with water) | -10℃ ~ 110℃ | Dry high-temperature environments, such as motor bearings and fan bearings |
| Lithium-based grease | Combines heat resistance and water resistance, with good versatility | -20℃ ~ 120℃ | General-purpose applications such as most industrial bearings, motors, and automotive wheel hubs |
| Extreme-pressure lithium grease | Contains extreme-pressure additives, resistant to heavy loads | Depends on base oil | Heavy-duty gears, rolling mill bearings, mining equipment |
| Molybdenum disulfide lithium grease | Contains molybdenum disulfide, extreme pressure and vibration resistance | Depends on base oil | Impact load, vibration, or low-speed heavy-load locations |
| Complex calcium-based grease | High temperature resistance, high load | -20℃ ~ 150℃ and above | High-temperature and high-load conditions, such as continuous casting machine and sintering machine bearings |
| Urea-based grease (polyurea grease) | Long life at high temperature, good oxidation stability | -20℃ ~ 180℃ | High-temperature, long-life maintenance-free applications, such as sealed bearings |
Key Points for Selecting Lubricating Grease Based on Operating Conditions
In addition to referring to the equipment manual, actual operating conditions must also be comprehensively considered:
Sodium-based grease should not be used in humid environments (it easily emulsifies and washes away when exposed to water); calcium-based or lithium-based grease should be selected instead;
For parts in contact with acidic gases, grease with good acid resistance should be selected;
In high-temperature environments, grease with a dropping point at least 20–30°C higher than the operating temperature should be selected;
For heavy loads and low speeds, select a high-viscosity base oil with extreme pressure additives and slightly higher consistency (NLGI No. 2 or No. 3);
For high speeds and light loads, select a low-viscosity base oil and lower consistency (NLGI No. 1 or No. 2) to avoid oil churning and heat generation;
Centralized lubrication systems require NLGI No. 00 or No. 1 grease with good pumpability.
Selection of Grease Grades (NLGI Numbers)
| Lubrication Method or Usage Requirements | Recommended Grades (Penetration Grades) |
|---|---|
| Centralized Lubrication System (Pump Required) | Grade 00 or Grade 1 |
| Manual application with grease gun or grease cup | Grade 1, Grade 2, or Grade 3 |
| Sealed areas with long-term use without grease replacement | Grade 2 or Grade 3 (thicker, less prone to loss) |
| General Rolling Bearings | Grade 2 or Grade 3 |
| Open Gears or Gearboxes | No. 0 or No. 1 (softer, easier to penetrate tooth surfaces) |
Why equipment temperature rises or wear persists after adding grease
Insufficient grease leads to poor lubrication, increased friction and heat; excessive grease increases churning resistance and impairs heat dissipation; mismatched grease type or poor quality prevents an effective oil film from forming; newly added grease is not distributed quickly and evenly, causing excessive local friction; mixing different types of grease destroys structure or triggers additive reactions, degrading performance; lack of a running-in period—the correct practice is to run at 30%–60% of standard speed for a period after greasing so the grease spreads evenly before running at full speed.
Can grease be diluted with lubricating oil?
No. Grease is a colloidal structure composed of a thickener and base oil. Adding lubricating oil after the grease has formed cannot be evenly dispersed even with stirring, and it easily separates and leaks out during use, causing the grease to lose its adhesion and sealing properties. If a softer grease is needed in winter, choose a lower-numbered No. 1 or No. 2 product directly rather than adding lubricating oil yourself.
Can different types of grease be mixed?
In principle, no. Mixing different thickeners (such as lithium-based and calcium-based) may destroy the colloidal structure, causing the grease to soften or harden; different additives (especially extreme-pressure agents) may react chemically and produce corrosive substances. When changing product types, the old grease should be thoroughly removed, preferably flushed out with the new grease before refilling.
Is more grease always better for wheel hub bearings?
No. If the hub cavity is overfilled with grease, most of it will be thrown onto the cavity wall and cannot participate in bearing lubrication, while increasing churning resistance and raising temperature. Grease melted at high temperature may seep into the brake drum and cause brake failure. The correct practice is "dry hub lubrication"—apply an appropriate amount of grease only to the bearing rolling elements and raceway surfaces, and do not fill the cavity.
Why does grease harden?
High temperatures cause the base oil to evaporate or bleed excessively from the thickener, hardening the grease; centrifugal force from high-speed rotation throws out the base oil, thickening the residual grease; overly long service cycles (over six months to a year) lead to base oil oxidation and volatilization; oil separation occurs under sustained heavy pressure; and prolonged storage or excessively high storage temperatures also accelerate aging. Hardened grease cannot provide adequate lubrication and should be replaced promptly.
Is it normal for lubricating oil to turn black?
Oil turning black usually has three causes: the lubricating oil itself oxidizes and deteriorates; metal powder from equipment wear mixes in; and external contaminants (dust, damaged seals) enter. A slight darkening after a period of use is normal, but it should not become very black (engine oil is an exception—diesel engine oil turns black due to the action of detergent-dispersant additives, which is normal). If the oil turns black rapidly without external contaminants, it may be a quality issue, and you should consider changing the brand or grade.
What causes oil to turn white?
It is generally emulsification after water ingress, turning the oil milky white or cloudy. Water entering the oil destroys the oil film, accelerates oxidation, and causes additives to fail. If whitening is found, immediately check whether the oil seal is damaged, trace the water source, thoroughly clean the system before changing the oil, and store oil drums in a rain-sheltered place.
Can old and new oil be mixed when changing oil?
Not recommended. Old oil contains harmful substances such as wear particles, oxides, and moisture; mixing it with new oil lowers the new oil's grade and shortens its service life. When changing oil, drain the old oil as completely as possible, and preferably flush the system once with new oil before refilling.
Can used lubricating oil be left to settle and then reused?
No. After use, all performance indicators of the oil have declined, additives are depleted, and oxidation products and fine particles cannot be removed by simple settling. Reusing it is equivalent to using a non-compliant product and will aggravate equipment wear.
The "Five Fixeds" of equipment lubrication management
Fixed points — clearly define the location and number of each lubrication point on the equipment; Fixed time — add oil, inspect, and replace oil at specified intervals; Fixed quality — select lubricants of the specified grade and quality according to equipment requirements; Fixed quantity — control the amount added each time to avoid over- or under-lubrication; Fixed personnel — assign dedicated personnel and establish a responsibility system.
