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Bearing Design in Machinery Episode 3 Part 7

Tham khảo tài liệu 'bearing design in machinery episode 3 part 7', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Fig. 14-1 Friction and wear tests of nonconformal contacts. Although these testing machines are useful for evaluating the performance of solid lubricants and comparing bearing materials for dry friction there are serious reservations concerning the testing accuracy of liquid lubricants for boundary friction or comparing various boundary friction lubricant additives. These reservations concern the basic assumption of boundary lubrication tests that there is only one boundary lubrication friction coefficient independent of sliding speed that can be compared for different lubricants. However measurements indicated that in many cases the friction coefficient is very sensitive to the viscosity or sliding speed. For example certain additives can increase the viscosity which will result in higher hydrodynamic load capacity and in turn reduction of the boundary friction. The friction force has a hydrodynamic component in addition to the contact friction adhesion friction . Therefore it is impossible to completely separate the magnitude of the two friction components. Certain boundary additives to mineral oils may reduce the friction coefficient only because they slightly increase the viscosity. Even for line or point contact there is an EHD effect that increases with velocity and sliding speed. The hydrodynamic effect would reduce the boundary friction because it generates a thin film that separates the surfaces. This argument has practical consequences on the testing of boundary layer lubricants. These Copyright 2003 by Marcel Dekker Inc. All Rights Reserved. tests are intended to measure only the adhesion friction of boundary lubrication however there is an additional viscous component. Currently boundary lubricants are evaluated by measuring the friction at an arbitrary constant sliding speed e.g. four-ball tester operating at constant speed . The current testing methods of liquid boundary lubricants should be reevaluated. Apparently better tests would be obtained by .