Tham khảo tài liệu 'lubrication fundamentals 2011 part 6', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Figure Effect of viscosity speed and load on film thickness. film cannot be formed some metallic friction and wear commonly occur and very high coefficients of friction may be reached. The portion of the curve between points a and c is a mixed film zone including the minimum value of f corresponding to the ZN P value indicated by b. From the point of view of low friction it would be desirable to operate with ZN P between b and c but in this zone any slight disturbance such as a momentary shock load or reduction in speed might result in film rupture. Consequently good practice is to design with a reasonable factor of safety so that the operating value of ZN P is in the zone to the right of c. The ratio of the operating ZN P to the value of ZN P for the minimum coefficient of friction point b is called the bearing safety factor. Common practice is to use a bearing safety factor on the order of 5. In an operating bearing if it becomes necessary to increase the speed ZN P will increase and it may be necessary to decrease the oil viscosity to keep ZN P and the coefficient of friction in the design range. An increase in load will result in a decrease in ZN P and it may be necessary to increase the oil viscosity to keep ZN P and the coefficient of friction in the design range. Film thickness can be related to ZN P in the manner shown in Figure . The curve is typical of large uniformly loaded medium speed bearings such as are used in steam turbines. In general film thickness increases if ZN P is increased for example if the load is reduced while the oil viscosity and journal speed remain constant. With a proper bearing safety factor the film thickness will be such that normal variation in speed load and oil viscosity will not result in the reduction of film thickness to the point at which metal-to-metal contact will occur. Equations procedures and data for plain bearing design and performance calculations are available in many technical papers and books. Among the
