Ra, the unit of measurement of surface roughness. It belongs to one-dimensional morphological parameters.
The geometric characteristics of the surface are described by morphological parameters. The most commonly used surface topography parameter is surface roughness, which is expressed by the contour curve of a certain section of the surface. According to different representation methods, it can be divided into one-dimensional, two-dimensional and three-dimensional morphological parameters.
Ra is called the arithmetic mean deviation of the contour or the average value of the center line. It is the arithmetic average of the height of each point on the contour within the measured length range.
The influence of surface roughness on parts is mainly manifested in the following aspects:
1, affecting wear resistance. The rougher the surface, the smaller the effective contact area between mating surfaces, the greater the pressure, the greater the friction resistance and the faster the wear.
2, affect the stability of the game. For clearance fit, the rougher the surface, the easier it is to wear, which makes the clearance gradually increase during working. For interference fit, the actual effective interference decreases and the connection strength decreases, because the microscopic peak is squeezed flat during assembly.
3, affect the fatigue strength. There are large valleys on the surface of rough parts, which are as sensitive to stress concentration as sharp notches and cracks, thus affecting the fatigue strength of parts.
4, affect the corrosion resistance. Rough surface of parts can easily make corrosive gas or liquid penetrate into the internal metal layer through microscopic valleys on the surface, resulting in surface corrosion.
5, affect the sealing performance. Rough surfaces can't fit tightly, and gas or liquid will leak through the gap between the contact surfaces.
6, affect the contact stiffness. Contact stiffness is the ability of the joint surface of parts to resist contact deformation under the action of external force. The stiffness of the machine depends largely on the contact stiffness between parts.