CALCULATION FOR RADIAL ROLLING BEARINGS

CALCULATION FOR RADIAL ROLLING BEARINGS The calculations for radial rolling bearings must take account of the following principal factors: .. Actual supported loads and possible shock loads .. Speed of rotation .. Operating temperature .. Hardness of the bearing raceways Other features such as lubrication, sealing and alignment do not enter directly into life calculations but they must be considered in order to avoid introducing unfavorable factors. The life calculation of a radial bearing or a thrust bearing under rotation is established from the dynamic capacity C indicated in the tables of dimensions. The static capacity Co enables one to determine the maximum load under certain conditions. 1.6.1 DYNAMIC CAPACITY C The dynamic capacity of a bearing is the constant radial load which it can support during 1 000 000 revolutions before the first signs of fatigue appear on a bearing race or rolling element. 1.6.2 NOMINAL LIFE The life of a radial bearing is the number of revolutions (or the number of hours at constant speed) that it will maintain before showing the first signs of material fatigue. The relationship between the life is millions of revolutions L10, the dynamic capacity C and the supported load P, is given by the formula: L10 = ( C/P ) p In which : L10 - Basic rating life (106 Revolutions) C - Basic dynamic load rating (Newton) P - Equivalent dynamic load (Newton) P - is equal to 10/3 for needle or roller bearings and 3 for ball bearings. The formula above is independent of speed of rotation, which must not exceed the recommended limit in respect of the radial bearing or the thrust bearing used and the method of lubrication. If the speed of rotation n (rpm) is constant, the life is given in hours by the function: L10 h = (L10 x 106)/60 n hours The life in hours is then inversely proportional to the speed. The above formulae will ensure that 90% of the bearings operating under the same conditions will attain at least the calculated L10 life, known as the nominal life (the figure being the percentage of bearings which may not attain this life). The formulae are based on the use of standard quality bearing steel and assume a satisfactory method of lubrication. 1.6.3 MODIFIEDL LIFE Lna In various conditions modified life can be determined (in millions of revolutions) following the general formula: Lna = a1 a2 a3 L10 Lna = adjusted rating life, millions of revolutions In which a1, a2 and a3 are correction factors linked to reliability, material and lubrication respectively. 1.6.4 Reliability correction factor a1 A reliability factor in excess of 90% may be required in certain industries, such as aviation, for reasons of security and to reduce the risk of very costly immobilization. The table below indicates the values of the correction factor a1 as a function of reliability: