Grease Lubrication vs Oil Lubrication for Bearings: Which One Is Better?
Lubrication methods for bearings are generally divided into two categories: grease lubrication and oil lubrication. To fully exert bearing performance, it is vital to adopt a suitable lubrication method according to the operating conditions and application purposes of bearings.
Bearing Grease
Lubricating grease is a lubricant composed of base oil, thickener and additives. When selecting grease, users must choose products matching bearing operating conditions. Greases from different brands vary greatly in performance, which requires careful attention during selection.
Common greases for bearings include calcium-based grease, sodium-based grease, calcium-sodium-based grease, lithium-based grease, aluminum-based grease, molybdenum disulfide grease, etc.
The proper filling volume of grease inside a bearing accounts for 1/2 to 1/3 of the internal bearing space. The filling amount shall be reduced to 1/3 under high-speed operation. Excessive grease will easily lead to temperature rise of operating bearings.
Selection of Lubricating Grease
When selecting grease according to operating temperature, key indicators include dropping point, oxidation stability and low-temperature performance. The dropping point is generally used to evaluate high-temperature resistance. The actual operating temperature of bearings shall be 10–20°C lower than the grease dropping point. For synthetic grease, the operating temperature shall be 20–30°C below its dropping point.
For selection based on bearing load: greases with low penetration shall be adopted for heavy loads. Under high pressure, the grease shall feature not only low penetration but also high oil film strength and extreme pressure properties.
For selection based on environmental conditions: calcium-based grease is insoluble in water, suitable for dry environments with low moisture content.
Bearing Lubricating Oil
Oil lubrication is adopted when grease lubrication cannot meet the requirements under high-speed and high-temperature conditions. Circulating lubricating oil can take away a large amount of heat.
Viscosity is an important property of lubricating oil. Viscosity directly affects fluidity and the thickness of the oil film formed on friction surfaces. Generally, the viscosity of lubricating oil at bearing operating temperature is 12–15 cSt. Lower viscosity oil shall be selected for higher rotating speeds, while higher viscosity oil is required for heavier loads.
Common lubricating oils include machine oil, high-speed machine oil, turbine oil, compressor oil, transformer oil, cylinder oil and so on.
Oil Lubrication Methods
Oil Bath Lubrication
It is the most common lubrication method applicable to low and medium-speed bearings. Part of the bearing is immersed in the oil sump. Lubricating oil is carried up by rotating bearing components and flows back to the sump. The oil level shall be slightly lower than the center of the lowest rolling element.
Drip Lubrication
Suitable for bearing assemblies requiring quantitative oil supply. The recommended drip rate is one drop every 3–8 seconds. Excessive oil supply will cause bearing temperature rise.
Circulating Oil Lubrication
A gear pump delivers filtered oil to bearing assemblies. After passing through the bearing, the oil is filtered and cooled for reuse. Circulating oil removes heat to cool bearings, so this method is suitable for high-speed bearing assemblies.
Oil Mist Lubrication
Dry compressed air mixes with lubricating oil via an atomizer to form oil mist, which is injected into bearings. The air flow effectively cools bearings and prevents impurity ingress. This method applies to high-speed and high-temperature bearing assemblies.
Jet Lubrication
A pump delivers high-pressure oil through a nozzle into the bearing. The oil flows out from the other side into the oil sump. When bearings rotate at high speed, rolling elements and the cage generate strong airflow. Conventional lubrication methods struggle to deliver oil into the bearing, so high-pressure jet injection is required. The nozzle shall be installed between the inner ring and the center of the cage.
The selection of lubricants and lubrication modes for rolling bearings depends on bearing type, dimensions and operating conditions. Normally, rolling bearings adopt grease or oil lubrication; solid lubrication can be used under special conditions.
Among grease and oil lubrication, grease lubrication is the most widely used, accounting for approximately 80% of all rolling bearing applications.
1. Characteristics of Grease Lubrication
(1) Advantages of Grease Lubrication
A. Simple lubrication arrangement. Sealed bearings or non-sealed bearings without re-greasing demand require no additional lubrication equipment (in contrast, oil lubrication systems need oil pumps, pipelines, oil tanks, etc.).
B. Low risk of grease leakage, enabling simpler bearing sealing structure.
C. Convenient bearing maintenance.
D. Grease works as a sealing barrier to block external dust, moisture and other contaminants.
E. Helps maintain cleanliness of mechanical equipment.
(2) Disadvantages of Grease Lubrication
A. Higher bearing friction, poor heat dissipation, limited allowable rotating speed.
B. The dn value of grease-lubricated bearings generally cannot exceed 3×10⁵ mm·r/min.
C. At excessively high temperatures, base oil in grease accelerates evaporation and oxidative deterioration. The colloidal structure of grease changes and causes accelerated oil separation.
Every 10–15°C rise in bearing temperature halves grease service life. Except for special high-temperature greases, ordinary greases cannot operate under high-temperature conditions.
2. Characteristics of Oil Lubrication
A. Suitable for high-speed rotation.
B. Good high-temperature stability for high-temperature applications.
C. Removes friction heat generated inside bearings with excellent heat dissipation and cooling effect when the ambient temperature of bearings is high.
D. Easily flushes away wear particles and dust inside bearings.
E. Bearings and adjacent components such as gears can share the same lubricant for centralized lubrication.
Bearing Grease
Lubricating grease is a lubricant composed of base oil, thickener and additives. When selecting grease, users must choose products matching bearing operating conditions. Greases from different brands vary greatly in performance, which requires careful attention during selection.
Common greases for bearings include calcium-based grease, sodium-based grease, calcium-sodium-based grease, lithium-based grease, aluminum-based grease, molybdenum disulfide grease, etc.
The proper filling volume of grease inside a bearing accounts for 1/2 to 1/3 of the internal bearing space. The filling amount shall be reduced to 1/3 under high-speed operation. Excessive grease will easily lead to temperature rise of operating bearings.
Selection of Lubricating Grease
When selecting grease according to operating temperature, key indicators include dropping point, oxidation stability and low-temperature performance. The dropping point is generally used to evaluate high-temperature resistance. The actual operating temperature of bearings shall be 10–20°C lower than the grease dropping point. For synthetic grease, the operating temperature shall be 20–30°C below its dropping point.
For selection based on bearing load: greases with low penetration shall be adopted for heavy loads. Under high pressure, the grease shall feature not only low penetration but also high oil film strength and extreme pressure properties.
For selection based on environmental conditions: calcium-based grease is insoluble in water, suitable for dry environments with low moisture content.
Bearing Lubricating Oil
Oil lubrication is adopted when grease lubrication cannot meet the requirements under high-speed and high-temperature conditions. Circulating lubricating oil can take away a large amount of heat.
Viscosity is an important property of lubricating oil. Viscosity directly affects fluidity and the thickness of the oil film formed on friction surfaces. Generally, the viscosity of lubricating oil at bearing operating temperature is 12–15 cSt. Lower viscosity oil shall be selected for higher rotating speeds, while higher viscosity oil is required for heavier loads.
Common lubricating oils include machine oil, high-speed machine oil, turbine oil, compressor oil, transformer oil, cylinder oil and so on.
Oil Lubrication Methods
Oil Bath Lubrication
It is the most common lubrication method applicable to low and medium-speed bearings. Part of the bearing is immersed in the oil sump. Lubricating oil is carried up by rotating bearing components and flows back to the sump. The oil level shall be slightly lower than the center of the lowest rolling element.
Drip Lubrication
Suitable for bearing assemblies requiring quantitative oil supply. The recommended drip rate is one drop every 3–8 seconds. Excessive oil supply will cause bearing temperature rise.
Circulating Oil Lubrication
A gear pump delivers filtered oil to bearing assemblies. After passing through the bearing, the oil is filtered and cooled for reuse. Circulating oil removes heat to cool bearings, so this method is suitable for high-speed bearing assemblies.
Oil Mist Lubrication
Dry compressed air mixes with lubricating oil via an atomizer to form oil mist, which is injected into bearings. The air flow effectively cools bearings and prevents impurity ingress. This method applies to high-speed and high-temperature bearing assemblies.
Jet Lubrication
A pump delivers high-pressure oil through a nozzle into the bearing. The oil flows out from the other side into the oil sump. When bearings rotate at high speed, rolling elements and the cage generate strong airflow. Conventional lubrication methods struggle to deliver oil into the bearing, so high-pressure jet injection is required. The nozzle shall be installed between the inner ring and the center of the cage.
The selection of lubricants and lubrication modes for rolling bearings depends on bearing type, dimensions and operating conditions. Normally, rolling bearings adopt grease or oil lubrication; solid lubrication can be used under special conditions.
Among grease and oil lubrication, grease lubrication is the most widely used, accounting for approximately 80% of all rolling bearing applications.
1. Characteristics of Grease Lubrication
(1) Advantages of Grease Lubrication
A. Simple lubrication arrangement. Sealed bearings or non-sealed bearings without re-greasing demand require no additional lubrication equipment (in contrast, oil lubrication systems need oil pumps, pipelines, oil tanks, etc.).
B. Low risk of grease leakage, enabling simpler bearing sealing structure.
C. Convenient bearing maintenance.
D. Grease works as a sealing barrier to block external dust, moisture and other contaminants.
E. Helps maintain cleanliness of mechanical equipment.
(2) Disadvantages of Grease Lubrication
A. Higher bearing friction, poor heat dissipation, limited allowable rotating speed.
B. The dn value of grease-lubricated bearings generally cannot exceed 3×10⁵ mm·r/min.
C. At excessively high temperatures, base oil in grease accelerates evaporation and oxidative deterioration. The colloidal structure of grease changes and causes accelerated oil separation.
Every 10–15°C rise in bearing temperature halves grease service life. Except for special high-temperature greases, ordinary greases cannot operate under high-temperature conditions.
2. Characteristics of Oil Lubrication
A. Suitable for high-speed rotation.
B. Good high-temperature stability for high-temperature applications.
C. Removes friction heat generated inside bearings with excellent heat dissipation and cooling effect when the ambient temperature of bearings is high.
D. Easily flushes away wear particles and dust inside bearings.
E. Bearings and adjacent components such as gears can share the same lubricant for centralized lubrication.
