The surface of a machined part is uneven, it has tiny peaks and valleys. Surface roughness is the measure of the micro-irregularities on the surface.
Surface roughness is closely related to the fitting properties, wear resistance, fatigue strength, contact stiffness, vibration, and noise of mechanical parts.
By far the most commonly used surface roughness parameter is Ra, which is a dimensional unit that can be either micros(um) or microinch(u-in).
The smaller the surface roughness Ra value, the smoother the surface. So, is the smaller the surface roughness value of the part, the better?
Of course not！
Rough part surface may affect the service life and reliability of mechanical products, but if the surface is too smooth, the processing cost will be increased.
The selection of surface roughness values should not only meet the functional requirements of the part’s surface but also consider economic rationality.
Therefore, under the premise of meeting the functional requirements, the general principle is to choose a relatively larger surface roughness value, to lower the production cost.
The general selection principles are listed as follows for reference:
Part 1 :
- On the same part, the roughness value of the contact surface should be smaller than that of the non-contact surface.
- The roughness value of the friction surface should be smaller than that of the non-friction surface.
- We should choose the Ra value of the rolling friction surface to be smaller than that of the sliding friction surface.
- The roughness value of the friction surface with high speed should be smaller than that of the friction surface with low speed.
- We should select a smaller roughness value of the friction surface with high unit pressure than that of the friction surface with low unit pressure.
Part 2 :
- For the surface to withstand cyclic load or the surface is easy to cause stress concentration, such as rounded corners, and grooves, we should select a smaller surface roughness value.
- For the surface that requires wear resistance, we should choose a smaller surface roughness value.
Because the rougher the surface, the smaller the effective contact area between the mating surfaces, resulting in greater pressure, the faster wear.
- For the mating surface with small fit clearance, the interference fit surface requires a reliable connection and withstands heavy load, we should select a relatively lower surface roughness value.
- If the fitting type of a part is the same, The smaller the size, the smaller the surface roughness value should be chosen.
- If the international tolerance grade is the same, the size is smaller, the surface roughness value should be smaller.
The surface roughness value of the shaft should be smaller than that of the hole.
- For the surface with corrosion resistance requirement, we should select a smaller surface roughness value.
Because if the Ra is bigger, it means the surface is rougher, and the corrosive gas or liquid will penetrate to inside of the part, resulting in surface corrosion.
- For the surface which has the sealing requirement, a smaller surface roughness value is to be chosen.
Because if the Ra is too big, the contact surfaces can’t be sealed tightly, and the gas or liquid will leak through the gap of the contacting surfaces.
When you are in doubt about the value of Ra, consulting a team of experienced engineers is a time-saving option.
Feel free to reach out to us for professional support.