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mechanical design an integrated approach Part 8

Tham khảo tài liệu 'mechanical design an integrated approach part 8', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 400 PART II Applications Table 10.2 Heat transfer coefficient c for self-contained bearings Lubrication system Conditions c Oil ring Still air 7.4 Average air circulation 8.5 Oil bath Still air 9.6 Average air circulation 1L3 projected area i.e. 1 2.5DL . It is to be emphasized that Eq. 10.23 should be used only when ballpark results are sufficient. Heat Developed Under equilibrium conditions the rate at which heat develops within a bearing is equal to the rate at which heat dissipates fW 2ĩĩrn H 10.24 In this expression f coefficient of friction w load r journal radius n journal speed as defined in Section 10.5 and H is given by Eq. 10.22 . A heat-balance computation involving finding average film temperature at the equilibrium is a trial and error procedure 14 15 I 10.10 MATERIALS FOR JOURNALBEARINGS The operating conditions for journal bearing materials are such that rather strict requirements must be placed on the material to be used. For instance in thick-film lubrication any material with sufficient compressive strength and a smooth surface is an adequate bearing material. Small bushings and thrust bearings are often expected to run with thin-film lubrication. Any foreign particles larger than the minimum film thickness present in the oil damage the shaft surface unless they can become imbedded in a relatively soft bearing material. In this section we discuss some of the types of bearing materials in widespread usage. Special uses are for many other materials such as hard wood glass silver ceramics and sapphires 16 I 9 . Alloys Babbitt alloys are the most commonly used materials usually having a tin or lead base. They posses low melting points moduli of elasticity yield strength and good plastic flow. In a bearing the foregoing give good conformability and embeddability characteristics. Conformability measures the capability of the bearing to adapt to shaft misalignment and deflection. Embeddability is the bearing s capability to ingest harder foreign .