Tham khảo tài liệu 'bearing design in machinery episode 1 part 10', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | The surface finish of the two surfaces bearing and journal must be considered. A dimensionless film parameter A relating hn to the average surface finish has been introduced see Hamrock 1994 A Rs2 j R -1 8-4 where Rs j surface finish of the journal surface RMS and Rs b surface finish of the bearing surface RMS . As discussed earlier the range of values assigned to A depends on the operating conditions and varies from 5 to 100. The minimum film thickness is not the only limitation encountered in the design of a journal bearing. Other limitations which depend on the bearing material determine in many cases the maximum allowable bearing load. The most important limitations are as follows. 1. Maximum allowed PV value depending on the bearing material to avoid bearing overheating during the start-up of boundary lubrication. This is particularly important in bearing materials that are not good heat conductors such as plastics materials. 2. Maximum allowed peak pressure to prevent local failure of the bearing material. 3. Maximum allowed peak temperature to prevent melting or softening of the bearing material. In most applications the inner bearing surface is made of a thin layer of a soft white metal babbitt which has a low melting temperature. The design procedure must ensure that the allowed values are not exceeded for otherwise it can result in bearing failure. If the preliminary calculations indicate that these limitations are exceeded it is necessary to introduce design modifications. In most cases the design of hydrodynamic bearing requires trial-and-error calculations to verify that all the requirements are satisfied. RAIMONDI AND BOYD CHARTS AND TABLES Partial Bearings A partial journal bearing has a bearing arc b of less than 360 and only part of the bearing circumference supports the journal. A full bearing is where the bearing arc b 360 in a partial bearing the bearing arc is less than 360 such as b 60 120 and 180 . A partial bearing has two .
