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Bearing Design in Machinery Episode 2 Part 9

Tham khảo tài liệu 'bearing design in machinery episode 2 part 9', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Fig. 12-20 Pressure wave and film thickness distribution in elastohydrodynamic lubrication. wear and optimize the bearing life. In such cases it is possible to have an optimal design that operates with a full EHD fluid film. For this purpose the designer can calculate the minimum film thickness hmin according to the equations discussed in the following sections. For optimal conditions the minimum film thickness hmin must be greater than the surface roughness. The equivalent surface roughness at the contact Rs RMS is obtained from the roughness of the two individual surfaces in contact Rs1 and Rs2 from the equation see Hamroch 1994 Rs R21 Rs22 1 2 12-38 The surface roughness is measured by a profilometer often referred to as stylus measurement. Microscope devices are used when higher-precision measurements are needed. The ratio A of the film thickness to the size of surface asperities Rs is A hmin 12-39 Rs For a full EHD lubrication of rolling bearings A is usually between 3 and 5. The desired ratio A is determined according to the expected level of vibrations and Copyright 2003 by Marcel Dekker Inc. All Rights Reserved. other disturbances in the machine. Although hmin is very important for successful bearing operation calculation of h0 is often required for determining the viscous shear force referred to as traction force which is the resistance to relative sliding. 12.8 ELASTOHYDRODYNAMIC LUBRICATION OF A LINE CONTACT A theoretical line contact is formed between two cylinders such as in a cylindrical rolling bearing. For this case Pan and Hamrock 1989 introduced the following empirical equation for the minimum film thickness hmin h min Rx 1.714Ur0.694 aEeq 0.568 W 0.128 12-40 Here a is the viscosity-pressure coefficient. Although theoretical equations were derived earlier Eq. 12-40 is more accurate because it is based on actual measurements. The equation is a function of dimensionless terms. The advantage in using dimensionless terms is that any system of units .