Monthly Archives: Dec 2018

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White Paper 2 : Bearings Fatigue Life & Reliability

Fatigue Life & Reliability

Where any part failure may result in damage to the entire system and repair of damage is not possible, in such application greatly increased reliability is demanded of each component. Aircraft, satellites, or missiles are good example. This concept is being applied generally to durable consumer goods and may also be utilized to achieve effective preventive maintenance of machines and equipment.

The rating fatigue life of a rolling bearing is the gross number of revolutions or the gross rotating period when the rotating speed is constant for which 90% of a group of similar bearings running individually under similar conditions can survive without suffering material damage due to rolling fatigue. In other words, fatigue life is normally defined at 90% reliability. There are other ways to describe the life. For example, the average value is employed frequently to describe the life span of human beings.

However, if the average value were used for bearings, then too many bearings would fail before reaching the average life value. On the other hand, if a low or minimum value is used as a criterion, then too many bearings would have a life much longer than the set value. In this view, the value 90% was chosen for common practice. The value 95% could have been taken as the statistical reliability, but nevertheless, the slightly lower reliability of 90% was taken for bearings empirically from the practical and economical viewpoint. However, 90% reliability is not acceptable for parts of aircraft or electronic computers or communication systems these days and a 99% or even 99.9% reliability is demanded in some of these cases.

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White Paper – Bearing Fatigue Life and Reliability

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White Paper 1 : Concept of Bearing Life and Bearing Load Carrying Capacity

Concept of Bearing Life and Bearing Load Carrying Capacity

 

  • Concept of fatigue theory

The concept of the fatigue theory depends on the calculation s in the ISO 281 with the accordance of the Lundeberg and Palmgren’s theory for the calculation of the bearing rating life.

The actual bearing operational life is however more than the calculated life. Thus the life adjustment factor comes in to the role for the calculation of the bearing life.

The life adjustment factor is influenced by the following factors,

  • The bearing load
  • The fatigue limit of the material
  • The extent to which the surfaces are separated by the lubricated
  • The cleanliness in the lubrication gap
  • Additives in the lubricants
  • The internal load distribution
  • Frictional condition in the bearing

 

  • Dynamic load carrying capacity of bearing and its dependency on bearing life

The basic dynamic load rating of rolling bearings is defined as the constant load applied on bearings with stationary outer rings that the inner rings can endure for a rating life (90% life) of one million revolutions. Radial load on the center of the bearing defines the basic load rating of radial bearings and an axial load of constant direction and magnitude defines the basic load rating of the thrust bearings.

The dynamic load carrying capacity is described in terms of the basic dynamic load rating and the basic rating of bearing life.

The fatigue life of the bearing depends on the following factors,

  • Load acting on bearing
  • Operating speed of bearing
  • Statistical probability of the first appearance of the bearing failure

The basic dynamic load rating is defined by C. Constant radial load for radial bearing is Cr and for axial load is Co.

 

  • Bearing life calculation

Bearing application decides the various function included in the rolling bearings. These functions should work defect free for the maximum number of period. Bearings will eventually fail to perform satisfactorily due to an increase in noise and vibration, loss of running accuracy, deterioration of lubricant, or fatigue flaking of the rolling surfaces even if bearings are properly mounted and correctly operated.

Bearing life is defined as, satisfactory performance of bearing function even after the continuously operated. Bearing life depends on the factor of evaluation of noise life, grease life abrasion life or rolling fatigue life.

Factors which cause bearing failure other than the above mentioned factors are seizure due to heat, fracture, scoring of the rings, damage of seals or cage, or any other damage occurs.

As a result of errors in bearing selection, improper design or manufacture of the bearing surroundings, incorrect mounting, or insufficient maintenance; if such condition occurred then failure should not be interpreted as normal bearing failure.

 

To know more click on the below link …

White Paper – Concept of Bearing Life and Bearing Load Carrying Capacity

 

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