SKF Cooper split roller bearings and bearing units

1 Overview, selection and application recommendations Bearing size Size selection based on rating life Bearing life is defined as the number of rev- olutions (or the number of operating hours) at a given speed that the bearing is capable of enduring before the first sign of metal fatigue (spalling) occurs on a rolling element or the raceway of the inner or outer ring. Tests on seemingly identical bearings, under identical operating conditions, result in a large variation in the number of cycles, or time, needed to cause metal fatigue. Therefore, bearing life estimates based on rolling contact fatigue (RCF) are insufficiently accurate and so a statistical approach is needed to determine bearing size. The basic rating life, L 10 , is the fatigue life that 90% of a sufficiently large group of apparently identical bearings, operating under identical operating conditions, can be expected to attain or exceed. To determine a relevant bearing size using the definition given here, compare the calcu- lated rating life against the service life expectations of the bearing application, using experience from previous dimension- ing where available. Otherwise, use the guidelines regarding specification life of vari- ous bearing applications provided in table 1 . Basic rating life If you consider only the load and speed, you can use the basic rating life, L 10 . The basic rating life of a bearing according to ISO 281 is ⎛ C ⎞ p L 10 = J⎝ P ⎠ If the speed is constant, it is often preferable to calculate the life expressed in operating hours using 10 6 L 10h = LLLK L 10 60 n where L 10 = basic rating life (at 90% reliability) [millions of revolutions] L 10h = basic rating life (at 90% reliability) [millions of hours] C = basic dynamic load rating [kN] P = equivalent dynamic bearing load [kN] n = rotational speed [r/min] p = exponent of the life equation = 10/3 for roller bearings Life adjustment factor For modern high-quality bearings, the cal- culated basic rating life can deviate signifi- cantly from the actual service life in a given application. Service life in a particular appli- cation depends not only on load and bearing size, but also on a variety of influencing fac- tors including lubrication, degree of contam- ination, proper mounting and other environ- mental conditions. ISO 281 uses a modified life factor to sup- plement the basic rating life ⎛ C ⎞ p L nm =a 1 L 10 = a 1 J⎝ P ⎠ If the speed is constant, the life can be expressed in operating hours, using the equation ⎛ 10 6 ⎞ L nmh = JKL L nm ⎝ 60 n ⎠ where L nm =Rating life (at 100 – n 1) % reliability) [millions of revolutions] L nmh =Rating life (at 100 – n 1) % reliability) [operating hours] L 10 = basic rating life (at 90% reliability) [mil- lions of revolutions] a 1 = life adjustment factor for reliability ( table 2 , values in accordance with ISO 281) C = basic dynamic load rating [kN] P = equivalent dynamic bearing load [kN] n = rotational speed [r/min] p = exponent of the life equation = 10/3 for roller bearings 1) The factor n represents the failure probability, which is the difference between the requisite reliability and 100%. Since the life adjustment factor a 1 is related to fatigue, it is less relevant for load levels, P, below the fatigue load limit P u . Dimension- ing with life adjustment factors reflecting very high reliability (such as 99%) will result in large bearings for given loads. In such cases, the bearing load must be checked against the minimum load requirement for the bearing. See also Requisite minimum load ( page 13 ). Table 1 Guideline values of specification life Type of operation Specification life Operating hours Operation for use 8 hours a day 20 000 ... 50 000 Continuous operation main drives, large electrical machinery, flywheels, mining 70 000 ... 100 000 Continuous operation and an exceptionally high degree of reliability 100 000 ... 200 000 10