Lubrication of Rolling Bearings

When a rolling bearing is operating, the rolling elements and raceways are not completely free from friction. Especially under high-speed and heavy-load conditions, the pressure in the contact area can be very high. Without proper lubricant protection, metal surfaces may come into direct contact, causing wear and damage.

After the lubricant enters the bearing contact area, it forms a very thin lubricating film. Although this film is very thin, sometimes even less than 1 micron, it plays an extremely important role.

1. Why Do Rolling Bearings Need Lubrication?

The main purpose of lubrication is to reduce direct contact between metal surfaces and help the bearing run smoothly and reliably. Proper lubrication can improve bearing performance, reduce wear, and extend service life.

Main Functions of Bearing Lubrication

• Reduce friction and wear: The lubricating film separates two metal surfaces and helps prevent direct friction.
• Lower bearing temperature: Bearings generate heat during operation. Lubrication can reduce frictional heat, and some lubrication methods can also help carry heat away.
• Improve load-carrying capacity: A good lubricating film can increase the effective contact area and reduce local contact pressure.
• Extend bearing service life: With proper lubrication, the bearing can withstand repeated loads more steadily, which helps improve fatigue life.
• Prevent rust, dust, and corrosion: Lubricants can form a protective layer on metal surfaces, reducing the effects of moisture, dust, and corrosive media.

In simple terms, without good lubrication, a bearing is difficult to operate reliably for a long time. With proper lubrication, bearing life can be significantly improved.

2. Common Lubrication Methods for Rolling Bearings

Rolling bearing lubrication methods can generally be divided into three main types:

• Oil lubrication
• Grease lubrication
• Solid lubrication

Different lubrication methods should be selected according to different working conditions.

3. Oil Lubrication

Oil lubrication uses lubricating oil to lubricate the bearing. It is suitable for applications involving high speed, high temperature, heavy load, or conditions where heat dissipation is required.

Common Oil Lubrication Methods

• Drip oil lubrication
• Oil bath lubrication
• Splash lubrication
• Oil circulation lubrication
• Jet oil lubrication
• Oil mist lubrication
• Oil-air lubrication

Advantages of Oil Lubrication

Oil lubrication has a relatively wide operating temperature range, good heat dissipation performance, and makes it easier to filter and replace the lubricant. In some high-speed equipment, oil lubrication is more suitable than grease lubrication.

In addition, if the equipment also contains other friction components such as gears or chains, the lubricating oil system can lubricate these parts at the same time.

Where Is Oil Lubrication Usually Used?

Oil lubrication is commonly used for:

• High-speed bearings
• High-temperature bearings
• Heavy-load bearings
• Equipment requiring continuous heat dissipation
• Mechanical systems requiring centralized lubrication

However, oil lubrication systems are relatively complex and have higher requirements for sealing, oil supply, oil return, and maintenance.

4. Grease Lubrication

Grease lubrication is one of the most common lubrication methods for rolling bearings. Grease is a semi-solid lubricant composed of base oil, thickener, and additives.

In general, grease contains:

• About 70% to 95% base oil
• About 5% to 30% thickener
• A small amount of additives, which still play an important role

Advantages of Grease Lubrication

The biggest advantages of grease lubrication are its simple structure, convenient sealing, and low maintenance cost.

Grease is not easy to flow away, and it does not need to be replenished or circulated as frequently as lubricating oil. Therefore, grease lubrication is very common in ordinary motors, mechanical equipment, conveyor systems, agricultural machinery, construction machinery, and mining machinery.

Where Is Grease Lubrication Usually Used?

Grease lubrication is commonly used for:

• Low-speed or medium-speed bearings
• Medium-temperature operating conditions
• Equipment that is difficult to lubricate frequently
• Dusty working environments
• Bearings requiring better sealing
• Applications where oil leakage or product contamination is not allowed

For example, many bearings used in food machinery, conveyor equipment, electric motors, and agricultural equipment use grease lubrication.

5. Solid Lubrication

When oil lubrication and grease lubrication cannot meet the operating requirements, solid lubrication can be used.

Solid lubrication is usually used in special working conditions, such as:

• High-temperature environments
• Vacuum environments
• Strong radiation environments
• Conditions where lubricating oil or grease may fail easily
• Applications where liquid lubricant contamination is not allowed

Solid lubrication can use solid lubricant powders, coatings, or low-friction materials to reduce direct contact between metal surfaces.

Common solid lubricating materials include molybdenum disulfide, graphite, PTFE, and other materials.

6. How to Choose the Right Lubricant

Lubricant selection is very important. Many bearing failures are not caused by poor bearing quality, but by improper lubricant selection or incorrect lubricant quantity.

When selecting a lubricant, the following factors should mainly be considered.

1. Operating Temperature

Temperature directly affects the viscosity of lubricating oil or grease.

If the operating temperature is low, a lower-viscosity lubricant should be selected to avoid difficult bearing start-up.

If the operating temperature is high, a higher-viscosity lubricant with good high-temperature resistance should be selected.

When the temperature is above 200°C, ordinary lubricating oil and grease may no longer be suitable. In this case, high-temperature grease, special synthetic oil, or even solid lubricant should usually be considered.

2. Bearing Speed

The higher the speed, the less viscous the lubricant should generally be.

If a high-speed bearing uses a lubricant with too high a viscosity, running resistance will increase and serious heat generation may occur.

Therefore:

• High-speed bearings: suitable for low-viscosity lubricating oil or high-speed grease.
• Low-speed bearings: suitable for higher-viscosity lubricating oil or thicker grease.

A simple way to understand this is:

High speed needs “light” lubrication, while low-speed heavy-load operation needs “thicker” lubrication.

3. Bearing Load

The greater the load, the higher the pressure in the bearing contact area, and the easier it is for the lubricating film to be squeezed out.

Therefore, heavy-load bearings usually require lubricants with higher viscosity and stronger oil film strength. If necessary, lubricants with extreme-pressure additives or anti-wear additives should also be selected.

If the lubricant does not have enough load-carrying capacity, dry friction, wear, burns, or even bearing fracture may occur inside the bearing.

4. Working Environment

The working environment of the bearing also affects lubricant selection.

If the bearing operates in a humid environment, in water exposure, in corrosive gases, in dusty conditions, or at low temperatures, a lubricant with suitable performance should be selected.

Working Environment	Recommended Lubricant Features
Humid environment	Good water resistance
Corrosive gases	Good rust prevention and corrosion resistance
Dusty environment	Good sealing performance; grease lubrication is preferred
Low-temperature environment	Good low-temperature fluidity
High-temperature environment	High-temperature resistance and good oxidation resistance
Shock and vibration	Strong oil film strength and good anti-wear performance

5. Bearing Precision and Surface Roughness

If the bearing has high machining precision and the friction surfaces are relatively smooth, a lower-viscosity lubricant can be used to reduce energy consumption and temperature rise.

If the bearing surface is relatively rough or the contact condition is poor, a higher-viscosity lubricant should be selected to improve oil film protection.

7. How to Choose Lubricating Oil Viscosity

If the lubricating oil viscosity is too low, the oil film will not have enough load-carrying capacity and may break easily. As a result, metal surfaces inside the bearing may come into direct contact, causing increased wear.

If the lubricating oil viscosity is too high, it will increase churning resistance, causing higher bearing temperature and higher energy consumption.

Therefore, oil viscosity should not be selected simply by thinking “the higher the better” or “the lower the better.” It should be selected according to the actual working conditions.

In general:

• Higher speed → choose lower viscosity
• Lower speed → choose higher viscosity
• Higher load → choose higher viscosity
• Higher temperature → choose lubricating oil with good high-temperature resistance and viscosity-temperature performance
• Precision bearings → choose lubricating oil with moderate viscosity and good stability

8. How to Choose Bearing Grease

When selecting grease, the following key factors should be considered:

• Base oil type
• Thickener type
• Dropping point
• Penetration
• Water resistance
• Oxidation resistance
• Extreme-pressure and anti-wear performance
• Operating temperature range

1. The Dropping Point Should Be Higher Than the Working Temperature

The dropping point of the grease must be higher than the actual operating temperature of the bearing. In general, it is recommended that the dropping point should be 20°C to 30°C higher than the working temperature.

If the dropping point is too low, the grease may soften, leak, and lose its lubricating function at high temperatures.

2. High-Speed Bearings Should Not Use Grease That Is Too Thick

Grease has poorer fluidity than lubricating oil. If a high-speed bearing uses grease that is too thick, churning resistance will increase and the bearing temperature will rise.

3. Heavy-Load Bearings Should Use Grease with Good Anti-Wear and Extreme-Pressure Performance

If the bearing load is high, grease with anti-wear and extreme-pressure additives should be selected to improve oil film strength and reduce wear.

9. Why Should the Lubricant Quantity Not Be Too High?

Many people think that more lubricant is always better. This is incorrect.

If there is too little lubricant, the bearing may suffer from insufficient lubrication, wear, and overheating.

However, too much lubricant can also cause problems:

• Increased churning resistance
• Higher bearing temperature
• Faster lubricant aging
• Higher energy consumption
• Grease leakage
• Abnormal heating in high-speed bearings

Therefore, the lubricant quantity should be appropriate.

In simple terms:

Lubrication is not about using more lubricant. It is about using the right amount.

10. Basic Principles for Lubricating Oil Quantity

1. Oil Bath Lubrication

For oil bath lubrication, the oil level should generally reach the middle of the lowest rolling element of the bearing, which is about half the height of the rolling element.

If the bearing speed is relatively low, the oil level can be slightly higher.

2. Splash Lubrication

Splash lubrication usually uses rotating parts such as gears to bring up the lubricating oil and splash it onto the bearing area.

This method is commonly used in gearboxes and similar equipment.

3. Oil Circulation Lubrication

Oil circulation lubrication is suitable for high-speed, high-temperature, or heavy-load equipment. The lubricating oil continuously flows into the bearing and carries away heat and contaminants.

This method provides good lubrication performance, but the system structure is more complex. It usually requires an oil pump, oil tank, filter, and cooling system.

11. How to Determine the Grease Filling Amount

The grease filling amount should be determined according to bearing speed.

The general principles are as follows:

Bearing Speed Condition	Grease Filling Amount
High speed or close to the limiting speed	About 1/3 of the bearing internal free space, or less
Medium-speed operation	About 1/3 to 2/3 of the bearing internal free space
Low-speed operation	More than 2/3 may be acceptable
Low speed with high sealing requirements	The entire space may sometimes be filled

For most ordinary bearings, grease should usually not be overfilled.

Especially for high-speed bearings, too much grease can easily cause heat buildup and may even lead to bearing damage.

12. Why Do Bearings Need Grease Relubrication?

During long-term operation, grease gradually ages. The base oil may evaporate or be lost, and part of the grease may also be pushed out through the sealing gap.

Therefore, many pieces of equipment need regular grease replenishment.

Adding fresh grease has two main functions:

• Restoring lubrication performance
• Pushing out old grease and contaminants to reduce the entry of external impurities into the bearing

In general, the grease relubrication interval should be determined according to bearing type, speed, temperature, load, sealing condition, and working environment.

13. Key Principles of Rolling Bearing Lubrication

Rolling bearing lubrication can be summarized as follows:

• High-speed bearings require heat control and low resistance. At high speeds, the lubricant should not be too viscous, and the bearing should not be overfilled.
• Heavy-load bearings require strong oil film strength. When the load is high, a lubricant with higher viscosity and good anti-wear performance should be selected.
• High-temperature bearings require high-temperature-resistant lubricants. Ordinary oils and greases can fail easily at high temperatures, so special lubricants should be used for high-temperature applications.
• Wet or corrosive environments require rust and water protection. Stainless steel bearings also need proper lubrication, and the protective effect of the lubricant should not be ignored.
• More lubricant is not always better. Excessive lubricant can cause temperature rise, leakage, and higher energy consumption.
• Regular relubrication and maintenance are important. Lubricants age over time and should be inspected and replenished according to actual working conditions.

14. Conclusion

The main purpose of rolling bearing lubrication is to form a stable lubricating film between the rolling elements and raceways. This reduces direct metal-to-metal contact, lowers friction, reduces wear, controls temperature rise, and extends bearing service life.

In actual applications, it is not enough to simply say that “lubrication is present.” The lubrication method should be selected according to bearing speed, load, temperature, working environment, and structural features.

For general working conditions, grease lubrication is convenient, economical, and the most commonly used option. For high-speed, high-temperature, or heavy-load conditions, oil lubrication is usually more suitable. For extreme environments, solid lubrication or special lubrication solutions may be required.

In one sentence:

The right lubrication method, the right lubricant, and the right lubricant quantity are the key to stable rolling bearing operation and long service life.