SKF Cooper split roller bearings and bearing units
Table 2 Life adjustment factor Reliability Failure probability SKF rating life Factor n L nm a 1 % % million revolutions – 90 10 L 10m 1 95 5 L 5m 0,64 96 4 L 4m 0,55 97 3 L 3m 0,47 98 2 L 2m 0,37 99 1 L 1m 0,25 Life calculation with multiple load conditions Where varying loads are experienced in operation, using the maximum load condi- tion may lead to an unrealistically low calcu- lated life. For n load conditions constituting the full load cycle (at constant speed), an overall dynamic equivalent load may be cal- culated as follows: ⎡ i = n y 0,3 P = s ∑ P i (10/3) p i s ⎣ i = i ⎦ where P i = dynamic equivalent load under load condition i p i = proportion of time load condition i is applicable Where the load is continuously variable, it may be broken down into a discrete approxi- mation to the actual load cycle. Equivalent dynamic bearing load, P When calculating the bearing rating life, a value for equivalent dynamic bearing load is required for the bearing life equations. The loads acting on a bearing are calcu- lated according to the laws of mechanics using the external forces – such as forces from power transmission, work forces, grav- itational or inertial forces – that are known or can be calculated. In real-world circumstances, the loads acting on a bearing may not be constant, can act both radially and axially, and are subject to other factors that require the load calcula- tions to be modified or, in some cases, simplified. Calculating equivalent dynamic bearing load The load value, P, used in the bearing rating life equations is the equivalent dynamic bearing load and is defined as: a hypothetical load, constant in magnitude and direction, that acts radially on radial bearings and axi- ally and centrically on thrust bearings. This hypothetical load, when applied, would have the same influence on bearing life as the actual loads to which the bearing is subjected. If a bearing is loaded with simultaneously acting radial load F r and axial load F a that are constant in magnitude and direction, the equivalent dynamic bearing load P can be obtained from the general equation P = X F r + Y F a where P = equivalent dynamic bearing load [kN] F r = actual radial bearing load [kN] F a = actual axial bearing load [kN] X = radial load factor for the bearing Y = axial load factor for the bearing An axial load only influences the equivalent dynamic load P for a single row radial bear- ing if the ratio Fa/Fr exceeds a certain limit- ing factor, often expressed as e. With double row bearings, even light axial loads influence the equivalent load and have to be considered. Please see details for each bearing type under Loads ( page 72 for split cylindrical roller bearings and page 103 for split tapered roller bearings). Size selection based on static load When any of the following conditions exist, bearing size should be selected or verified based on the static load that the bearing can accommodate, taking into account the possi- ble effects of permanent deformation: • The bearing is not rotating and is sub- jected to continuous high load or intermit- tent peak loads. • The bearing makes slow oscillating move- ments under load. • The bearing rotates and, in addition to the normal fatigue life dimensioning operating loads, has to sustain temporary high peak loads. • The bearing rotates under load at low speed (n < 10 r/min) and is required to have only a limited life. In such a case, the rating life equations, for a given equivalent load P, would give such a low requisite basic dynamic load rating C, that a bearing selected on a fatigue life basis would be seriously overloaded in service. In such conditions, the resulting deformation can include flattened areas on the rolling elements or indentations in the raceways. The indentations may be irregularly spaced around the raceway, or evenly spaced at positions corresponding to the spacing of the rolling elements. A stationary or slowly oscil- lating bearing supporting a load great enough to cause permanent deformation will generate high levels of vibration and friction when subjected to continuous rota- tion. It is also possible that the internal clear- ance will increase or the character of the housing and shaft fits may be affected. Static load rating The basic static load rating C 0 is defined in ISO 76 as the load that results in a certain value of contact stress at the centre of con- tact of the most heavily loaded rolling ele- ment/raceway. For roller bearings, the con- tact stress value is 4 000 MPa (580 000 psi) . These stress values produce a total per- manent deformation of the rolling element and raceway that is approximately 0,0001 of the rolling element diameter. The loads are purely radial for radial bearings and axial, centrically acting, for thrust bearings. Bearing size 11 1
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