Effect of surface roughness of a part on its service life and workpiece accuracy 5

Surface roughness refers to the small spacing and small peak-to-valley unevenness of the machined surface. The distance (wave distance) between two peaks or troughs is very small (less than 1 mm) and is difficult to distinguish with the naked eye, so it belongs to the microgeometric error. The smaller the surface roughness, the smoother the surface.

The size of the surface roughness has a great impact on the performance of mechanical parts, mainly in the following aspects:

1) Surface roughness affects the wear resistance of parts. The rougher the surface, the smaller the effective contact area between the mating surfaces, the greater the pressure, and the faster the wear.

2) Surface roughness affects the stability of mating properties. For the gap fit, the rougher the surface, the easier it is to wear, so that the gap gradually increases during the working process; For interference fits, the actual effective interference is reduced and the bond strength is reduced due to the flattening of the microscopic convex peaks during assembly.

3) Surface roughness affects the fatigue strength of parts. There are large troughs on the surface of rough parts, and they, like sharp corner notches and cracks, are sensitive to stress concentrations, which affect the fatigue strength of the part.

4) Surface roughness affects the corrosion resistance of parts. The rough surface is easy to cause corrosive gases or liquids to penetrate into the inner layer of the metal through the microscopic valleys on the surface, causing surface corrosion.

5) Surface roughness affects the tightness of parts. Rough surfaces do not fit tightly together, and gases or liquids leak through the gaps between the contact surfaces.

In addition, surface roughness also has an impact on the appearance and measurement accuracy of parts.

The surface roughness of the lathe parts is the surface gloss, strictly speaking, it is the flatness of the surface, because the knife pattern is the line not the surface, and the larger the value, the rougher it is.

The surface roughness of ordinary lathe machined parts can be reached. Generally, the roughness of the surface of the lathe can be controlled from to to, the accuracy of different lathes is different, the higher the speed, the slower the tool, the higher the roughness, and vice versa. The hardness of the material is slightly higher with the appropriate forming tool, and the roughness can even be reached, but the instability is difficult to control, and it is occasionally obtained under special circumstances.

The rough surface of the lathe parts will lead to the reduction of the contact area, the pressure per unit area will increase, the contact deformation will become larger, the friction resistance will become larger, and the wear will become faster; For the fit between the gaps, the surface is rough and easy to wear, which also reduces the strength of the mating parts of the metal products.

The surface of the bed is rough, and the concave place is easy to accumulate corrosive substances, which is difficult to remove, resulting in the surface corrosion of the hardware processing parts. When the lathe parts are overloaded, the grooves of the metal parts are prone to stress concentration, which leads to cracking of the metal parts and the cracking of the metal parts.

1. Affect the mating properties of parts.

For the parts that need to be matched, the roughness of the parts can easily cause wear, which accelerates the speed of gap increase, and in serious cases, it will also destroy the properties of the fit. In some high-precision, small-size cases, the surface roughness will have a greater impact on the properties of the fit. Surface roughness: For interference fits, the actual effective interference will be reduced to reduce the strength of the connection.

2. Affect the strength of parts.

The greater the surface roughness of the part, the more sensitive it is to the corresponding concentration. This effect is even greater under alternating loads. For example, engine crankshafts are often damaged due to surface damage, so there are strict requirements for the surface roughness of grooves or fillets in parts such as crankshafts.

3. Affect the wear resistance of parts.

When two parts come into contact with each other and produce relative motion, the surface of the part increases the loss of frictional energy, mainly due to the need to overcome the resistance between the peaks and valleys of the undulating surface. And the greater the surface roughness, because the friction consumes more energy. The rougher the surface, the smaller the actual effective contact area of the mating surface, resulting in greater pressure per unit and therefore more likely to produce wear.

4. Affect the corrosion resistance of parts.

The rougher the surface of the part, the more corrosive gases and impurities will accumulate on the surface of the part, and these gases and impurities will penetrate through the microscopic depressions on the surface of the part, exacerbating corrosion.

5. Affect the fatigue strength of parts.

The contour valley of the microgeometry error will cause the stress concentration factor, so that the rougher the part, the more sensitive the corresponding concentration. When the part is subjected to alternating load, the stress concentration will reduce the fatigue strength of the part, and the surface of the part will be cracked and damaged.

6. Affect the working progress of machines and instruments.

The surface roughness is large, the coefficient of friction is also large, and the wear also increases. This will not only reduce the sensitivity of the operation of the machine or instrument parts, but also affect the work progress of the machine or instrument. Due to the fact that the actual effective relief area of the rough surface is small.

Therefore, under the same load, the pressure per unit area increases, and the deformation of the surface layer will also increase, which affects the working accuracy of the machine.

Theoretically, the smoother the surface (the smaller the roughness) of the same material, the better the wear resistance.

If it is absolutely smooth, the abrasion resistance is the worst, because the molecules are attracted to each other and tear the material.

In fact, it is impossible to achieve this kind of machining accuracy.

In general, the lower the roughness parameter value, the better the wear resistance.

However, if the roughness value is very small, the wear resistance deteriorates. Explain why.

What is the effect of surface roughness on the performance of part book recall?

Correct Answer: 1) Effect on the nature of the fit; 2) the impact on the loss of friction mill state; 3) Effect on contact stiffness; 4) Effect on fatigue strength; 5) Effect of chain mega on corrosiveness.