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Friction and Lubrication in Mechanical Design Episode 1 Part 2

Tham khảo tài liệu 'friction and lubrication in mechanical design episode 1 part 2', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Introduction 5 1. In any situation where the resultant of tangential forces is smaller than some force parameter specific to that particular situation the friction force will be equal and opposite to the resultant of the applied forces and no tangential motion will occur. 2. When tangential motion occurs the friction force always acts in a direction opposite to that of the relative velocity of the surfaces. 3. The friction force is proportional to the normal load. 4. The coefficient of friction is independent of the apparent contact area. 5. The static coefficient is greater than the kinetic coefficient. 6. The coefficient of friction is independent of sliding speed. Strictly speaking none of these laws is entirely accurate. Moore indicated that laws 3 4 5 and 6 are reasonably valid for dry friction under the following conditions For law 3 the normal load is assumed to be low compared to that causing the real area of contact to approach the apparent area. For law 4 the materials in contact are assumed to have a definite yield point such as metals . It does not apply to elastic and viscoelastic materials. Law 5 does not apply for materials with appreciable viscoelastic properties. Law 6 is not valid for most materials especially for elastomers where the viscoelastic behavior is very significant. A number of workers also found some exceptions to the first friction law. Rabinowicz 18 reported that Stevens 19 Rankin 20 and Courtney-Pratt and Eisner 21 had shown that when the tangential force Fis first applied a very small displacement occurs almost instantaneously in the direction of F with a magnitude in the order of 10-5 or 10 6 cm. Seireg and Weiter 22 conducted experiments to investigate the load displacement and displacement-time characteristics of friction contacts of a ball between two parallel flats under low rates of tangential load application. The tests showed that the frictional joint exhibited creep behavior at room temperatures under loads below the gross