（1） Influence of Surface Quality on Wear Resistance of Parts
The wear resistance of parts is related to the materials, lubrication and surface quality of friction pairs. The wear process can be divided into three stages. The first stage is called the initial wear stage. At this time, the two surfaces only contact at the top of some peaks. The actual contact area is much smaller than the theoretical contact area, and the peak contact part will generate greater pressure. The rougher the surface, the smaller the actual contact area, and the greater the pressure at the crest, so the wear is significant. After initial wear, the contact area of the friction pair surface increases, the pressure decreases, and it enters the normal wear stage (stage II). The wear resistance of parts at this stage is the best. With the passage of time, when the roughness value becomes very small, the ability of the part surface to store lubricating oil drops sharply. The deterioration of the lubrication conditions causes the molecular adhesion between the two surfaces in close contact, and the friction resistance increases, thus entering the third stage of wear, that is, the rapid wear stage. Obviously, there is an optimal parameter for the surface roughness value, and the wear amount is minimum at this time.
The cold work hardening of the surface layer reduces the elastic and plastic deformation at the contact of the friction pair, and the wear resistance can generally be increased by 0.5-1 times. However, excessive cold work hardening will loosen the metal structure, even cause surface cracks and surface peeling, thus affecting the wear resistance of parts.
（2） Influence of surface blood quality on fatigue strength of parts
Under the action of alternating load, the micro uneven concave valley, scratch, crack and other defects on the surface of the part are most likely to cause stress concentration and develop into fatigue cracks, leading to fatigue damage of the part. Reducing the ra value of surface roughness can improve the fatigue strength.
The residual compressive stress of the surface layer can partially offset the tensile stress caused by the working load, so as to improve the fatigue strength of the parts, while the residual tensile stress of the surface layer will aggravate the fatigue cracks and reduce the fatigue strength.
The cold work hardening of the surface layer can prevent the expansion of fatigue cracks and improve the strength of parts. However, if the cooling and hardening process is too large, it is easy to produce cracks, so the degree of cooling and hardening depth should be controlled within a certain range.
（3） Influence of Surface Quality on Parts Fit
The surface geometric characteristics will change the actual effective interference or clearance between mating parts, which will inevitably affect their mating properties and precision. For example, for clearance fit, too large surface roughness will lead to excessive wear and rapid increase of fit clearance. For parts requiring high fitting accuracy such as pneumatic and hydraulic parts, the clearance will increase and the system performance will be affected; For interference fit, the crest of the fitting surface will be squeezed during the assembly process, reducing the actual effective interference and affecting the reliability of the connection.
The surface residual stress will deform the part and cause the shape and size error of the part. It also affects the fit of the part.