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BEARING LOAD RATING AND LIFE

- Sep 07, 2018 -

BEARING LOAD RATING AND LIFE

Bearing life

Even  in bearings operating under normal conditions, the surfaces of the raceway and rolling elements are constantly, it is subject to repeat 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 factors which cause the bearing,s failure are often attributed to problems such as seizing, abrasions, cracking, chipping, gnawing and rusty, 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.

 

Basic rating life and basic dynamic load rating

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

 

This " life " disparity can be accounted for by the difference in the fatigue of the bearing material itself. This disparity is considered statistically when calculating bearing life, and the basic rating life is defined as follows:

 

The basic rating life is based on a 90% statistical model which is expressed as the total number of revolutions 90% of the bearings in an identical group of bearings subjected to identical operating conditions will attain or surpass before flaking due to material fatigue occurs. For bearings operating at fixed constant speeds, the basic rating life (90% reliability) is expressed in the total number of hours of operation.

 

The basic dynamic load rating is an expression of the load capacity of a bearing based on a Constant load which the bearing can sustain for one million revolutions (the basic life rating). For radial bearings this rating applies to pure radial loads, and for thrust bearings it refers to pure axial loads. The basic dynamic load ratings given in the bearing tables of this catalog are for bearings constructed of  BYWB  

standard bearing materials, using standard manufacturing techniques. Please consult BYWB engineering for basic load ratings of bearings on structed of special materials.

 

Machine applications and requisite life

When selecting a bearing, it is essential that the requisite life of the bearing be established in relation to the operating conditions.The requisite life of the bearing is usually determined by the type of machine in which the bearing will be used, and duration of service and operational reliability requirements. When determining bearing size, the fatigue life of the bearing is an important actor; however, besides bearing life, the strength and rigidity of the shaft and housing must also be taken into consideration.

Adjusted life rating factor

The basic bearing life rating ( 90 % reliability factor ) can be calculated through the formulas mentioned earlier in Section 7.2. However, in some applications a bearing fife factor of over 90% reliability may be required. To meet these requirements, bearing life can be lengthened by the use of specially improved bearing materials or special construction techniques. Moreover, according to elastohydrodynamic lubrication theory, it is clear that the bearing operating conditions (lubrication, temperature, speed, etc.) all exert an effect on bearing life. All these adjustment factors are taken into consideration when calculating bearing life, the adjusted bearing life can be determined.

Life adjustment factor for material

The life of a bearing is affected by the material type and quality as well as the manufacturing process. In


this regard, the life is adjusted by the use of an a2 factor.

The basic dynamic load ratings listed in the cataloge are based on BYWB ' s standard material and process, therefore, the adjustment factor a2 =1. When special materials or processes are used the adjustment factor can be larger than 1.

 

BYWB bearings can generally be rotated up to 120 C ~175 C , which depends on series. If bearings are operated at a higher temperature, the bearing must be specially heat treated ( stabilized ) so that inadmissible dimensional change does not occur due to changes in the micro-structure. This special heat treatment might cause the reduction of bearing life because of a hardness change.

 

Life adjustment factor a3 for operating conditions

The operating conditions life adjustment factor a3 is used to adjust for such conditions as lubrication, operating temperature, and other operation factors which have an effect on bearing ' s life.

 

Generally saying, when lubricating conditions are satisfactory, the a3 factor has a value of one; when lubricating conditions are exceptionally favorable, while all other operating conditions are normal, it can have a value greater than one.

 

Basic static load rating

 

When stationary rolling bearings are subjected to static loads, they suffer from partial permanent deformation of the contact surfaces at the contact point between the rolling elements and the raceway. The amount of deformity increases as the load increases, and if this increase in load exceeds certain limits, the subsequent smooth operation of the bearings are impaired.

 

It has been found through experience that a permanent deformity of 0.0001 times the diameter of the rolling element, occurring at the most heavily stressed contact point between the raceway and the rolling elements, can be tolerated without any impairment in running efficiency.

 

The basic rating of the static load refers to a fixed static load limit at which the specified amount of permanent deformation occurs. It applies to pure radial loads for radial bearings and to pure axial loads for thrust bearings.

 

Selecting bearing size using the life equations - BYWB rating life

The nominal or basic rating life can deviate significantly from the actual service life in a given application. Service life in a particular application depends on a variety of influencing factors including lubrication, the degree of contamination, misalignment, proper installation and environmental conditions.

 

Therefore it contains a modified life equation to supplement the basic rating life. This life calculation makes use of a modification factor to account for the lubrication and contamination condition of the bearing and the fatigue limit of the material.


bearings or by exceptionally low rotational speed (nr/min x dpmm less than 10,000). For bearings used under special operating conditions, please consult BYWB engineering.

 

As the operating temperature of the bearing increases, the hardness of the bearing material decreases. Thus, the bearing life correspondingly decreases. 


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