Ball and Roller Bearings

Load Rating and Life3. Load Rating and Life3.1 Bearing lifeEven in bearings operating under normal conditions, thesurfaces of the raceway and rolling elements areconstantly being subjected to repeated compressivestresses which causes flaking of these surfaces to occur.This flaking is due to material fatigue and will eventuallycause the bearings to fail. The effective life of a bearingis usually defined in terms of the total number ofrevolutions a bearing can undergo before flaking of eitherthe raceway surface or the rolling element surfacesoccurs.Other causes of bearing failure are often attributed toproblems such as seizing, abrasions, cracking, chipping,scuffing, rust, etc. However, these so called "causes" ofbearing failure are usually themselves caused byimproper installation, insufficient or improper lubrication,faulty sealing or inaccurate bearing selection. Since theabove mentioned "causes" of bearing failure can beavoided by taking the proper precautions, and are notsimply caused by material fatigue, they are consideredseparately from the flaking aspect.3.2 Basic rating life and basic dynamic load ratingA group of seemingly identical bearings when subjectedto identical load and operating conditions will exhibit awide diversity in their durability.This "life" disparity can be accounted for by thedifference in the fatigue of the bearing material itself.This disparity is considered statistically when calculatingbearing life, and the basic rating life is defined as follows.The basic rating life is based on a 90% statistical modelwhich is expressed as the total number of revolutions90% of the bearings in an identical group of bearingssubjected to identical operating conditions will attain orsurpass before flaking due to material fatigue occurs. Forbearings operating at fixed constant speeds, the basicrating life (90% reliability) is expressed in the total numberof hours of operation.Basic dynamic load rating expresses a rolling bearing'scapacity to support a dynamic load. The basic dynamicload rating is the load under which the basic rating life ofthe bearing is 1 million revolutions. This is expressed aspure radial load for radial bearings and pure axial load forthrust bearings. These are referred to as "basic dynamicload rating (Cr)" and "basic dynamic axial load rating (Ca)."The basic dynamic load ratings given in the bearingtables of this catalog are for bearings constructed of NTNstandard bearing materials, using standard manufacturingtechniques.The relationship between the basic rating life, the basicdynamic load rating and the bearing load is given informula.For ball bearings: L 10 C 3 3.1PFor roller bearings: L 10 C 10/3 3.2Pwhere,L10 : Basic rating life 10 6 revolutionsC : Basic dynamic load rating, N {kgf}(Cr: radial bearings, Ca: thrust bearings)P : Equivalent dynamic load, N {kgf}(Pr: radial bearings, Pa: thrust bearings)n : Rotational speed, min -1The relationship between Rotational speed n and speedfactor fn as well as the relation between the basic ratinglife L10h and the life factor fn is shown in Table 3.1 andFig. 3.1.Table 3.1 Correlation of bearing basic rating life, life factor,and speed factorClassification Ball bearing Roller bearingBasic rating lifeL10h hLife factorfhSpeed factorfnnmin -160,00040,00030,00020,00015,00010,0008,0006,0004,0003,0002,0001,5001,0001001080060040030020015080604030201510 6 C( )60nfn0.0820.090.120.140.160.18h80,00060,00040,00030,00020,00015,0000.2010,0000.220.240.260.280.300.350.40.50.60.70.80.91.11.21.31.40.101.01.49fnPL10h8,0006,0004,0003,0002,0001,5001,0009008007006005004003002003CP33.3( n )Ball bearings= 500 fh 3 60n1/34.55.4543.532.521.91.81.71.61.51.41.31.21.10.950.900.850.800.75fh n fn L10h1.00.74min -160,00040,00030,00020,00015,00010,0008,0006,0004,0003,0002,0001,5001,0008006004003002001501008060403020151010 6 C( )0.1060.120.140.160.180.200.220.240.260.280.300.350.40.50.60.70.80.91.01.11.21.31.41.44hPRoller bearings80,00060,00040,00030,00020,00015,00010,0008,0006,0004,0003,0002,0001,5001,0009008007006005004004.64.543.532.521.91.81.71.61.51.41.31.21.11.00.950.90300 0.8520010/3fn0.80CPfh0.76= 500 fh33.3( n )3/1010/3Fig. 3.1 Bearing life rating scaleA-17