Definition of bearing life

Service life of rolling bearing: under rotating conditions, the number of turns or working hours of rolling bearing from the beginning of use to fatigue peeling.

Contact fatigue has a very random nature. When the bearing steel is determined, the contact life (the number of stress cycles, which can also be simply understood as the speed of the bearing) conforms to the Weibull distribution law. In this distribution law, we can determine a certain range of bearing damage rate distribution (for example, 10% of bearing damage) as the calculation standard of the number of stress cycles of rolling bearings. If special industries have higher requirements on the reliability of bearings, then 1% bearing damage rate can also be taken as the calculation standard of bearing fatigue life.

If it is not clear enough, let's briefly and vividly understand the definition of life: suppose 100 sets of the same centrifugal pumps are used and equipped with 100 sets of the same bearings, all of which operate under similar conditions. When operating for 10,000h, one set of bearings is observed to be invalid (99% of the bearings are not invalid), that is, L01=10,000h. If 10 sets of bearings are observed to be invalid (90% of the bearings are not invalid) after 30,000h, L10=30,000h.

Bearing manufacturers and associations have investigated a large number of bearing failures (several million sets of bearings) for many years. Based on such a large amount of data, it is found that bearing failures are not common and have no direct relationship with typical fatigue peeling. Therefore, 90% reliability criterion can usually avoid typical fatigue peeling failure and provide a solid and reliable bearing application scheme. This is the origin of L10 life, which we often say.

Bearing reliability factors

Bearing life is a probabilistic life based on specific assumptions. If we want to use bearings well and improve the reliability of bearings, we must understand these assumptions and try to meet them.

There is no doubt that the reliability of the bearing comes from three parts. The first part is guaranteed by the bearing manufacturer to ensure that the bearing is carefully designed (stress, speed, structure, cage, lubrication, sealing, etc.), made of advanced materials (steel with high purity), carefully manufactured (strength, hardness, accuracy, etc.), correctly transported and stored, and the second part is guaranteed by the OEM manufacturer to ensure that the bearing is correctly selected and fully lubricated. The third part is guaranteed by the user to ensure that the bearings are correctly installed, lubricated and maintained.

The first part and the second part belong to the design and manufacturing link, and will not be discussed here. The reliability of the maintenance phase is mainly related to the application environment and working cycle of the bearing. According to the bearing manufacturer SKF report, only a small number of bearings have reached the fatigue limit (catalogue life). According to the statistics of bearing replacement, generally only 10% of rolling bearings reach L10 life (according to the definition, 90% can reach this life).

Bearings will not only fail normally (expected fatigue life), they will also be damaged early (early failure replacement). In order to make the bearing have a normal life expectancy, we need to ensure that the following fault causes will not occur at any time after installation and commissioning.

1 Mechanical causes

Exceeding the rated dynamic load capacity of the bearing will reduce the bearing fatigue life disproportionately. For most bearings, doubling the bearing load will reduce about one eighth of the normal bearing life. Please refer to the life index p of the bearing life calculation formula (about 3 for ball bearings and 3.3 for roller bearings). The mechanical impact of misalignment (Fig. 1) and unbalanced installation on the bearing will also have a similar effect of increasing load.

Fig. 1 Service life of misaligned bearing

2 Incorrect lubrication

There are many important lubricant properties that will dramatically reduce the life and reliability of bearings when changed or damaged. These indicators include additives (AW, EP, etc.), acid value, viscosity, pressure viscosity coefficient and viscosity index. The use of the wrong lubricant, degraded lubricant, mixed incompatible lubricant and/or contaminated lubricant will result in the loss of lubricant performance.

3 Lubrication pollution

The "top killers" of lubrication pollution include particles and water (Figure 2: Water pollution and bearing life curve, Figure 3: Particle pollution and bearing life curve). Of course, there are many others, which are not listed here.

Figure 2 Moisture Pollution and Bearing Life

Figure 3 Particle pollution and bearing life

4 High temperature operation

Heat is also a pollutant. It is the cause and result of most lubrication and mechanical problems. For example, excessive lubrication (excessive grease) is a common cause of grease lubricated bearings heating.

5 Insufficient lubrication

After the bearing runs for a long time, the lubricating oil or grease will dry up, and proper re lubrication must be carried out. Incorrect relubrication intervals are usually the primary cause of insufficient lubrication, but cold start, dry start and lubrication leakage problems can also lead to insufficient lubrication. Other factors causing insufficient lubrication are also related to the failure of other lubricating parts, such as oil cup, oil slinger, oil slinger, single point device, centralized system, etc.

More about SUNSPEED Rolling Bearings:

Rolling bearings support and guide rotating or oscillating machine elements such as shafts, axles or wheels and transfer loads between machine components. They provide high precision and low friction and therefore enable high rotational speeds while reducing noise, heat, energy consumption and wear. Rolling bearings are cost-effective and exchangeable machine elements that typically follow national or international dimension standards.

Roller bearings also known as rolling-element bearings are similar to ball bearings in that they are designed to carry a load while minimizing friction. However, roller bearings transmit loads using cylinder rolling elements, rather than balls, to maintain the separation between moving parts of the bearing. These versatile bearings can contain single or multiple rows of rolling elements; multiple rows can significantly improve radial load capacity. Also, the use of different roller shapes can further reduce friction and support both radial and axial loads.

SUNSPEED formerly known as Seven Color Bearing co., Ltd. set up in 1996, is a large bearing enterprise in China. The company focuses on 10 categories of bearing products with a total of more than 5000 types. The products are as follows: deep groove ball bearing, tapered roller bearing, self-aligning ball bearing, spherical roller bearing, cylindrical roller bearing, angular contact bearing, thrust ball bearing, thrust roller bearing, needle roller bearing, joint bearing, etc. Bearings are widely used in motor, machine tool, automobile, wind power, mining and other fields.

The company’s private brand “LSCB” is China’s most promising brand, and we always adhere to the principle of “quality first, service first”. The company has established a set of complete sales network and after-service support system, and products are sold both in China and abroad. In order to realize the long-term sustainable development of the company, all SUNSPEED people will keep adhering to the people-oriented concept as well as the principle of “quality first, service first”, sparing no effort in achieving the long-term development of the company.

The SUNSPEED Company intend to stay at the front of this market. If you have any questions please do not hesitate to call or write us!