>> P.20
Basic Dynamic Load Ratingand LifeLifeBasic rating lifeThe basic rating life is dened as the total number of revolutions that 90% of a group of identical bearings can be operated individually under the same condi-tions free from any material damage caused by rolling Rolling bearings will suffer damage due to various fatigue.causes during service. Damage such as abnormal For rotation at a constant rotational speed, the basic wear, seizure, and cracks is caused by improper use, rating life can be represented by the total service including incorrect mounting, lack of oil, dust intrusion hours.and so on, and can be avoided by remedying these causes. However, bearings will eventually be dam-aged due to fatigue-flaking even if used properly. Basic dynamic load ratingWhen a bearing rotates under load, the raceways and The basic dynamic load rating is dened as the con-the rolling elements are subjected to repeated stresses stant radial load (in the case of radial bearings) or the concentrated on the part close to the surface. Fatigue, constant axial load acting along the bearing central therefore, occurs in the surface layer, producing axis (in the case of thrust bearings) that allows a damage in the form of scaling. This is called aking basic rating life of 1,000,000 revolutions.(spalling). When this occurs, the bearing can no lon-ger be used.Bearing LifeCalculation of rating lifeThe relationship among the basic rating life, basic dynamic load rating and dynamic equivalent load Bearing life is dened as the total number of revolu-(bearing load) of rolling bearings is as follows: ……………………………………(1)pCP=( )tions (or total service hours at a constant rotational speed) before a sign of the first flaking appears on the L10rolling surface of raceway or rolling elements. where, L10 :Basic rating life,106 rev.C :Basic dynamic load rating,NP p :Dynamic equivalent load,N: xponent, Roller bearing: 10/3 Ball bearing: 3Accordingly, when the rotational speed per minute is given, the basic rating life is represented as the total service hours according to the following equations:Lh=106L1060n= 500 fph……………………(2) …………………………………(3)CPf h=f nf n=( )n1/p33.3 …………………………………(4)where, Lh :Basic rating life represented by service hours,h:Rotational speed,min-1n fh :Life factorfn :Velocity factorIn addition, the rating life can be calculated by obtain-ing fh and fn from the life calculation scales of Fig. 2.However, even when bearings of the same size, structure, material and heat treatment are subjected to the same conditions, the bearing lives will show variation (See Fig. 1.). This results from the statistical nature of the fatigue phenomenon.In selecting a bearing, it is incorrect to take an aver-age life for all bearings as the design standard. It is more practical to consider a bearing life that is reliable for the greater proportion of bearings used. Therefore, the basic rating life dened in the following is used.Rolling fatigue lifeFig. 1 Variation of rolling fatigue lifeA17Average lifeBasic rating lifeFailure probability density (Frequency of failure)
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