Tham khảo tài liệu 'handbook of lubrication episode 1 part 3', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 42 CRC Handbook of Lubrication FIGURE 9. Coefficient of friction pattern for a typical lubricated contact. Z is lubricant viscosity N shaft speed and P the unit load transferred radially by the shaft to the bearing. coefficient of friction usually accompanied by a severe rearrangement of surface material with little loss of material. In most other sliding pairs there is no connection between the coefficient of friction and wear rate. Static and Kinetic Friction The force required to begin sliding is usually greater than the force required to sustain sliding. For dry surfaces the reason for the starting or static coefficient of friction being larger than the sliding or kinetic coefficient of friction may most simply be explained in terms of the adhesion of asperities. It is often found that the static coefficient of friction increases with time of standing. This suggests diffusion bonding of the points of contact which progresses with time. Sustained sliding could be viewed as providing a very short standing time of one asperity upon another. This should also produce a decrease in the coefficient of friction as the sliding speed increases which is found in many systems. When a hard sphere slides on some plastics the frictional behavior is such as to require a new definition of static friction. For example for a sphere of steel sliding on Nylon 6-6 the coefficient of friction at 60 C varies with sliding speed as shown in Figure 6. The static coefficient of friction is lower than that at v2. Most observers would however measure the value of g at v2 as the static value of g. The reason is that v1 in the present example is imperceptibly slow. The coefficient of friction at the start of visible sliding at v2 is higher than at v3. In this case it may be useful to define the starting coefficient of friction as that at v2 and the static coefficient of friction as that at or below v1. In lubricated systems the starting friction is often higher than the kinetic friction. When
