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CHAPTER SIXTEENPlain Surface Bearings 565cause rapid heating of the bearing/journal interface, and sei-zure would likely occur very rapidly.16–7 DESIGN OF FULL-FILM HYDRO-DYNAMICALLY LUBRICATED BEARINGSThe following discussion presents several guidelines for bearing design for typical industrial applications. The design procedure is based primarily on information in References 1–5.Surface RoughnessA ground journal with a surface roughness average of 16–32 microinches (μin), or 0.40 to 0.80 μm, is recommended for bearings of good quality. The bearing should be equally smooth or made from one of the softer materials so that “wearing in” can smooth the high spots, creating a good fit between the journal and the bearing. In high-precision equipment, polishing or lapping can be used to produce a surface finish of the order of 8 to 16 μin (0.20 to 0.40 μm).bearing. The clearance between the bearing and the journal is highly exaggerated. Part (a) of the figure shows the rise of pressure as the rotating shaft draws oil into the converging wedge, approaching the point of minimum film thickness. The maximum pressure occurs there and then falls rapidly to nearly zero as the space between the journal and the bear-ing diverges again. The integrated effect of the pressure dis-tribution is a force sufficient to support the shaft on a film of oil without metal-to-metal contact.Figure 16–6(b) shows the pressure distribution axially along the shaft through the line of minimum film thickness or maximum pressure. The highest pressure value occurs in the middle of the bearing length, and it falls rapidly as the ends are approached because the pressure outside the bearing is ambient pressure, typically atmospheric pressure. There is continual leakage flow from both ends of the bear-ing. This illustrates the importance of providing a means of continually supplying oil to the bearing to maintain the full-film operation. Without a steady, adequate supply of oil, the system would not be able to create the pressurized film to carry the shaft, and boundary lubrication would result. The significantly higher frictional forces thus created would (a) Stationary(b) Start-up(c) Full-film operationBearingJournalRadial load, FFFOil fillsthe spacebetweenbearingandjournalehoFIGURE 16–5 Position of the journal relative to the bearing as a function of mode of operationBearingOilJournalehoPmaxOil film pressuredistributionEnd leakage(b) Diametral section through lineof minimum film thicknessBearingOilJournal(a) Section through middleof bearingFIGURE 16–6 Pressure distribution in the oil film for hydrodynamic lubrication