Tham khảo tài liệu 'bearing design in machinery episode 3 part 2', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Fig. 13-1 Requirement for minimum lubricant viscosity in rolling bearings from SKF 1992 with permission . Solution The pitch diameter according to Eq. 13-17 is dm 4 2 8 65 mm Line I in Fig. 13-1 shows the intersection of dm 65 with the diagonal straight line of 2000 RPM. The horizontal dotted line indicates a minimum viscosity required of 13 cSt mm2 s . Based on the required viscosity the oil grade should be selected. The oil viscosity decreases with temperature and the relation between the oil grade and Copyright 2003 by Marcel Dekker Inc. All Rights Reserved. Fig. 13-2 Viscosity-temperature charts from SKF 1992 with permission . its viscosity depends on the oil temperature. In Fig. 13-2 viscosity-temperature charts for several rolling bearing oil grades are presented. Estimation of the oil temperature inside the operating bearing is required before one can select the oil grade according to Fig. 13-2. It is preferable to estimate the temperature with an error on the high side. This would result in higher viscosity which can ensure a full EHD fluid film at the rolling contact although the friction resistance can be slightly higher. If a lubricant with higher-than-required viscosity is selected an improvement in bearing life can be expected. However since a higher viscosity raises the bearing operating temperature there is a limit to the improvement that can be obtained in this manner. The improvement in the bearing fatigue life due to higher lubricant viscosity above the minimum required viscosity is shown in Fig. 13-3. The life adjustment factor a3 sec. is a function of the viscosity ratio K defined as V _ _ _ K - 13-18 Vmin Copyright 2003 by Marcel Dekker Inc. All Rights Reserved. This results in extra rolling contact stresses which counteract any other benefits obtained from using high-viscosity oil. The fatigue life adjustment factor a3 in Fig. 13-3 is often used as a23 a2a3. This is because experience indicated that there is no significant improvement .
