The service life of a bearing refers to the number of revolutions or the number of hours that the bearing has experienced before pitting occurs under a certain load. A bearing within this service life should not suffer from initial fatigue damage on any of its bearing rings or rolling elements.

What is the bearing life?

The life that 90% of the bearings can achieve is defined as the bearing life, and it can also be considered that the reliability of the bearing is 90%. Therefore, the higher the reliability of the bearing, the shorter its life. The bearing life is commonly expressed in terms of the operating time or the number of rotations. For a batch of bearings, the bearing life is discrete, but the life distribution follows the parametric Weibull distribution.

In our daily practical use, it can be clearly observed that bearings with the same appearance under the same working conditions have significantly different actual lives. There are many factors that affect the service life of bearings. Today, the editor will briefly introduce what impact the maintenance and rust prevention of bearings have on their service life.

Bearing maintenance cycle

How often should bearings be maintained?

Theoretically, bearings can be used for 20,000 to 80,000 hours. However, the specific service life depends on factors such as wear during use, working intensity, and subsequent maintenance.

How to maintain bearings

In order to enable the bearing to fully exert and maintain its due performance for a long time, it is necessary to earnestly carry out regular maintenance (regular inspection).  

Through proper regular inspections, it is possible to detect faults at an early stage and prevent accidents before they happen, which is of great importance for enhancing productivity and economic efficiency.Regarding storage, all bearings are coated with an appropriate amount of anti-rust oil and wrapped with anti-rust paper when they leave the factory. As long as this packaging is not damaged, the quality of the bearings will be ensured.

However, for long-term storage, it is advisable to store the bearings on a shelf that is 30 cm above the ground under the conditions of humidity lower than 65% and a temperature of around 20°C. In addition, the storage location should avoid direct sunlight and contact with cold walls. **Cleaning**: When removing the bearings for inspection, first make a visual record of them by means such as photography. Besides, confirm the amount of remaining lubricant and take a sample of the lubricant before cleaning the bearings.

Steps of Bearing Maintenance

1. The bearings should be strictly replaced regularly, and the replacement cycle should be reasonably set according to the operating conditions of the bearings.

2. New bearings must be inspected before use. The inspection items include whether the packaging (preferably with an instruction manual and a certificate of conformity attached) is intact; whether the markings (manufacturer's name, model) are clear; and whether the appearance (rust, damage) is in good condition.

3. For new bearings that pass the inspection, under general operating conditions (for motors with more than 2 poles), they do not need to be cleaned; new sealed bearings do not need to be cleaned.

4. Before changing the oil of the bearings, the bearing covers and the bearings themselves must be cleaned. The cleaning is divided into rough cleaning and fine cleaning. The oil for rough cleaning is clean diesel oil or kerosene, and the oil for fine cleaning is clean gasoline.

6. After the bearings are cleaned, they should rotate flexibly by hand. Radially and axially shaking the bearings by hand can preliminarily determine whether there is looseness or excessive clearance. Check the clearance if necessary. Replace the bearings if serious wear, rust, or metal spalling of the balls or roller cages is found.

7. After the cleaning and inspection of the bearings are completed, dry the cleaning agent with a white cloth (or let it dry naturally), and then add qualified lubricating grease. It is not allowed to add different types of lubricating grease into the same bearing.

8. When adding oil, there should be no dust in the surrounding environment. Add oil with clean hands. Slowly rotate the entire bearing with one hand, and use the middle finger and index finger of the other hand to press the oil into the bearing cavity. After adding oil to one side, then proceed to the other side. Remove the excess grease according to the number of poles of the motor.

9. The amount of oil added to the bearings and bearing covers: The amount of oil in the bearing cover should be 1/2 to 2/3 of the capacity of the bearing cover (take the upper limit for motors with a higher number of poles); the amount of oil in the bearings should be 1/2 to 2/3 of the cavity between the inner and outer rings of the bearings (take the upper limit for motors with a higher number of poles).

10. The end covers of motors with oil replenishment holes and oil drainage holes must also be cleaned when changing the oil to keep the channels unobstructed. The oil injection holes must be filled with oil when adding oil.

11. Motors with oil replenishment holes must have their oil replenished regularly. The oil replenishment cycle should be determined according to the operating requirements and conditions of the motor (generally, for a 2-pole motor operating 24 hours a day, it is 500 hours).

12. When replenishing oil, the oil replenishment port must be clean. The amount of oil replenishment should be limited to a maximum increase of 2°C in the bearing temperature (for a 2-pole motor, use an oil gun to inject oil quickly twice, observe for 10 minutes, and decide whether to continue adding oil according to the situation).

13. When installing and removing the bearings, it is necessary to ensure that the force application points are correct (the inner ring on the shaft bears the force, and the inner and outer rings of the end cover bear the force), and the force is evenly distributed. It is best to use the press-fitting method (for small motors) and the hot shrinking method (for motors with large interference and large motors).

14. When installing the bearing, apply a small amount of grease evenly on the contact surface. After the bearing is installed, it is necessary to check the clearance between the inner ring of the bearing and the shaft shoulder (it is better to have no clearance).

15. When using the hot shrinking method for the bearing, the heating temperature should be controlled between 80°C and 100°C, and the time at 80°C to 100°C should be controlled within 10 minutes. When heating with oil, ensure that a mineral oil with good thermal stability and no corrosiveness is used (it is best to use transformer oil), and both the oil and the container should be clean. Set up a metal mesh at a height of 50 to 70 mm from the bottom of the oil tank, and place the bearing on the mesh. For large bearings, use hooks to hang them up.

16. Regularly conduct inspections of the motor and record the operating status of the motor (motor vibration, temperature of the motor and the bearing, operating current of the motor). Generally, for 2-pole motors above 75KW, it should be inspected once a day. When abnormal operating conditions occur, strengthen the inspections and inform the relevant parties.

17. All maintenance work on the bearings must be recorded, which serves as the basis for setting the regular replacement cycle of the bearings and determining the quality of the bearings.  

The cleanliness of bearings 

The cleanliness of bearings has a considerable impact on the service life of bearings. The higher the cleanliness of the bearings, the longer their service life. Lubricating oils with different cleanliness levels have a significant influence on the service life of ball bearings. Therefore, improving the cleanliness of the lubricating oil can extend the service life of the bearings. In addition, if the particle size of the contaminants in the lubricating oil is controlled below 10um, the service life of the bearings will increase several times.

(1) Impact on vibration: Cleanliness seriously affects the vibration level of bearings, especially the vibration in the high-frequency band is more significant. Bearings with high cleanliness have a low vibration velocity value, especially in the high-frequency band.

(2) Impact on noise: Experiments have been conducted on the influence of dust in the bearing lubricating grease on noise, which proves that the more dust there is, the louder the noise.

(3) Impact on lubrication performance: The decrease in the cleanliness of bearings not only affects the formation of the lubricating oil film but also causes the deterioration and accelerates the aging of the lubricating grease, thus leading to a decline in the lubrication performance of the lubricating grease.

Methods of Bearing Rust Prevention1. Surface cleaning: The appropriate method must be selected according to the nature of the surface of the rust-proof object and the current conditions. Commonly used methods generally include solvent cleaning, chemical treatment cleaning, and mechanical cleaning.

2. Surface drying: After cleaning, it can be dried with filtered dry compressed air, or dried with a dryer at 120 - 170 °C, or dried with a clean gauze.

3. Immersion method: For some small items, the method of immersing them in rust-proof grease is adopted to make their surfaces adhere to a layer of rust-proof grease. The thickness of the oil film can be achieved by controlling the temperature or viscosity of the rust-proof grease.

4. Brushing method: It is used for outdoor construction equipment or products with special shapes that are not suitable for immersion or spraying. When brushing, attention should be paid not only to avoiding accumulation but also to preventing missing painting.

5. Spraying method: For some large rust-proof objects that cannot be oiled by the immersion method, turntable bearings are generally sprayed with filtered compressed air at a pressure of approximately 0.7Mpa in a clean place. The spraying method is suitable for solvent-diluted rust-proof oil or thin-film rust-proof oil, but perfect fire prevention and labor protection measures must be taken.

It should be noted that the following types of acid solutions cannot be used for bearing rust removal: sulfuric acid, hydrochloric acid, dilute sulfuric acid, and dilute hydrochloric acid. Because these acid solutions will damage the good metal parts, these liquids must never be used! In daily life, there are also several liquids that can remove rust without harming the good metal parts, but the effects vary. First, dilute oxalic acid, with a ratio of 3:1 to water, that is, 3 parts of dilute oxalic acid and 1 part of water. This method is slow, but the effect is very good, and it is available everywhere. The second is gun oil, also known as mechanical rust removal oil, which is not easy to buy. This kind of oil can remove rust quickly and has a very good effect.

The service life of bearings cannot accurately assess the service time of bearings in all application scenarios. Therefore, in recent years, some new life assessment methods have been proposed for some operating conditions, such as noise life, lubrication life, and precision life, etc.

01 Fatigue Life

During the operation of rolling bearings, they bear a certain load. Even under a static load, for the steel balls and raceways, the stress is cyclic. The material will undergo rolling fatigue, thus reaching the service life of the bearing. The phenomenon is that a part of the material on the rolling surface suddenly peels off like fish scales, and this phenomenon is called fatigue spalling (Flaking).

02 Noise Life

For bearings used in household appliances, the load borne by the bearings is very small, and the service life of the bearings is very long. However, after the bearings have been in operation for a period of time, wear, increased vibration noise, deterioration of the lubricating grease, etc. will also occur. Due to the strict requirements for the noise of bearings in household appliances, when the noise of the bearings exceeds the allowable range, it is considered that the bearings can no longer be used. The total time during which the bearings operate with low noise is called the noise life of the bearings.

03 Lubrication Life

Bearings for household appliances usually use maintenance-free grease-lubricated sealed bearings. Although only a small amount of lubricant is required for bearing lubrication, after a long period of operation, due to the deterioration and leakage of the lubricating grease, the bearing lubrication is poor, resulting in the failure of the bearings. The total time during which the bearings operate in a good lubrication state is called the lubrication life of the bearings.