Bearing life l10, Bearing life calculation, formula

Bearing life

Even in bearings operating under normal conditions, the surfaces of the raceway and rolling elements are constantly being subjected to repeated compressive stresses which causes flaking of these surfaces to occur. This flaking is due to material fatigue and will eventually cause the bearings to fail. The effective life of a bearing is usually defined in terms of the total number of revolutions a bearing can undergo before flaking of either the raceway surface or the rolling element surfaces occurs.

Other causes of bearing failure are often attributed to problems such as seizing, abrasions, cracking, chipping, scuffing, rust, etc. However, these so called "causes" of bearing failure are usually themselves caused by improper installation, insufficient or improper lubrication, faulty sealing or inaccurate bearing selection. Since the above mentioned "causes" of bearing failure can be avoided by taking the proper precautions, and are not simply caused by material fatigue, they are considered separately from the flaking aspect.

A bearing can be viewed as a system of components: raceways, rolling elements, cage, seals (if present) and lubricant. The performance of each component contributes to or determines the performance and life of the bearing. Consider these aspects:

  • Rolling contact fatigue on the rolling elements and raceways - this is the primary aspect that dictates bearing life in most applications
  • Permanent deformation of rolling elements and raceways because of heavy loads acting on the bearing, while it is stationary or oscillating slowly, or high peak loads acting on the bearing while it is rotating
  • Cage type or cage material - these may limit the operating speed or the permissible acceleration or temperature
  • Speed limit of contacting seal lips - this can determine the maximum allowable speed, which affects operating temperature, thereby affecting life
  • Lubricant life - when the lubricant deteriorates, the resulting poor relubrication condition quickly reduces bearing life

The operating conditions of the application determine which of these factors most influence the performance and life of the bearing.

The effect of Rolling contact fatigue or permanent deformation on rolling elements and raceways is directly related to bearing size. Effects of cage type and material are not related to bearing size. In capped bearings, the effects of the lubricant and integral seal are only indirectly related to bearing size.

Size selection based on bearing rating life

Bearing life L10

A group of seemingly identical bearings when subjected to identical load and operating conditions will exhibit a wide diversity in their durability.

When you want to avoid fatigue failures of the bearing before your application reaches its desired lifetime, you can use a statistical approach to determine the bearing size. The rating life L10 is the fatigue life that 90% of a sufficiently large group of identical bearings operating under identical conditions can be expected to attain or exceed.

The rating life L10 is a proven and effective tool which can be used to determine a bearing size that is adequate to avoid fatigue failures. Compare the calculated rating life to the service life expectations of the bearing application.

Basic rating life

If you consider only the load and speed, you can use the basic rating life, L10. The formula is as follow: 

If the speed is constant, it is often preferable to calculate the life expressed in operating hours using, the formula is as follow:

Where,

For modern high-quality bearings, the calculated basic rating life can deviate significantly from the actual service life in a given application. Service life in a particular application depends not only on load and bearing size, but also on a variety of influencing factors including lubrication, degree of contamination, proper mounting and other environmental conditions.