Understand surface roughness, the effect of roughness on single-acting/double-acting and multi-stage cylinders
In technical communication, many people are used to using "surface finish". In fact, "surface finish" is according to human visual point of view, and "surface roughness" is put forward according to the actual surface micro-geometry. Because it is to be in line with international standards (ISO), the expression term "surface finish" is no longer used in the national standard, and the term "surface roughness" should be used for formal and rigorous expressions.
Surface roughness refers to the small spacing and small peaks and valleys of the processed surface. The distance (wave distance) between its two crests or two troughs is very small (below 1mm), which is a microscopic geometric shape error.
Specifically, it refers to the level of the tiny peaks and valleys Z and the distance S. Generally divided by S:
S＜1mm is the surface roughness,
1≤S≤10mm is the waviness,
S＞10mm is the f shape
Surface roughness formation factors
Surface roughness is generally formed by the processing method used and other factors, such as the friction between the tool and the surface of the part during the processing, the plastic deformation of the surface layer metal when the chips are separated, and the high-frequency vibration in the process system, electrical processing Discharge pits and so on. Due to the difference in processing methods and workpiece materials, the depth, density, shape and texture of the traces left on the processed surface are different.
Evaluation Basis of Surface Roughness
1) Sampling length
The unit length of each parameter, the sampling length is the length of a reference line for evaluating the surface roughness. Generally use 0.08mm, 0.25mm, 0.8mm, 2.5mm, 8mm as the reference length under the ISO1997 standard.
Selection of sampling length L and evaluation length Ln of Ra, Rz, Ry
2) Evaluation length
Consists of N reference lengths. The surface roughness of each part of the component surface cannot truly reflect the true roughness parameters on a reference length, but N sampling lengths are needed to evaluate the surface roughness. Under the ISO1997 standard, the assessment length is generally N equal to 5.
3) Baseline
The baseline is the contour centerline used to evaluate the surface roughness parameters.
Surface roughness evaluation parameters
1) Height characteristic parameters
Ra profile arithmetic mean deviation: the arithmetic mean of the absolute value of the profile deviation within the sampling length (lr). In actual measurement, the more the number of measurement points, the more accurate Ra is.
Rz Maximum profile height: the distance between the top line of the profile peak and the bottom line of the valley.
In the common range of amplitude parameters, Ra is preferred. Before 2006, there was another evaluation parameter in the national standard, which was "the ten-point height of micro-roughness" represented by Rz, and the maximum height of the contour was represented by Ry. After 2006, the national standard canceled the ten-point height of micro-roughness and adopted Rz. Indicates the maximum height of the contour.
2) Spacing feature parameters
Rsm The average width of the contour element. Within the sampling length, the average value of the profile microscopic unevenness spacing. The microscopic unevenness distance refers to the length of the profile peak and the adjacent profile valley on the midline. In the case of the same Ra value, the Rsm value is not necessarily the same, so the reflected texture will also be different. Surfaces that value texture usually pay attention to the two indicators of Ra and Rsm.
The Rmr shape characteristic parameter is expressed by the contour support length ratio, which is the ratio of the contour support length to the sampling length. The profile support length is the sum of the length of each section of the profile obtained by intercepting a straight line parallel to the center line and a distance of c from the top line of the profile within the sampling length.
VDI3400, Ra, Rmax comparison table
The Ra indicator is often used in actual domestic production; the Rmax indicator is commonly used in Japan, which is equivalent to the Rz indicator; the European and American countries often use the VDI3400 standard to indicate surface roughness. When making European mold orders, the VDI indicator is often used. For example, I often hear customers say "The surface of this product is made according to VDI30."
The surface of VDI3400 has a corresponding relationship with the commonly used standard Ra. Many people often need to check the data to find the corresponding value. The following table is very complete and it is recommended to collect.
Comparison table between VDI3400 standard and Ra
Ra and Rmax comparison table
The piston rod of the single-acting/double-acting and multi-stage cylinders is an important part. It is usually made of 45# steel to make a solid rod or a hollow tube. The piston rod is subject to abrasive erosion during use and is very easy to wear. At present, the traditional domestic process is hard chromium plating (coating thickness 0.03~0.05mm) and polishing, and the surface roughness Ra is 0.1~0.2μm. The plating solution is based on chromic acid and sulfuric acid as a catalyst. The process advantages are: stable plating solution, easy to operate, relatively high quality chromium coating on the surface, giving the oil rod bright, high hardness, excellent wear resistance, etc.
The processing of the piston rod can affect the quality of the product, so pay attention to the selection, processing the piston rod can improve the surface corrosion resistance, and the piston rod can delay the generation or expansion of fatigue cracks, thereby improving the fatigue strength of the cylinder rod.
The hydraulic cylinder tube is formed by rolling to form a cold hardened layer on the rolling surface, which reduces the elastic and plastic deformation of the contact surface of the grinding pair, thereby improving the wear resistance of the cylinder rod surface and avoiding grinding Burns. After rolling, the surface roughness value is reduced, which can improve the matching properties