Measurement and analysis of running-in surface

1 Introduction

　　 The cylinder liner-piston ring is the most important frictional pair in the engine and operates in a mixed lubrication state. The wear of the friction pair is such that the soft surface (liner surface) in the mating surface is worn until the surface contour conforms to the hard surface (piston ring surface). In the actual running-in process, the surface morphology of the liner is mainly changed. Based on the actual measurement of the running-in surface, this paper determines the surface topography parameters of the cylinder liner during the stable working phase. Obviously, if the original surface of the cylinder liner can be designed close to the surface appearance after running-in, it will help to shorten or even avoid the run-in period, which will produce direct economic benefits.

2. Measurement and analysis of surface topography

　　 The running-in test was performed on the traction table. Taylorsurf-6 type profiler was used to measure the surface morphology of the cylinder liners before and after the five sets of wear. The cut-off length was 0.8mm and the measurement length was 4mm. The change in surface topography can be described quantitatively using the following parameters:
(1) Skewness Rsk and kurtosis Rku: The skewness Rsk is used to measure the degree to which the distribution curve deviates from the symmetrical position; the kurtosis Rku indicates the sharpness of the distribution curve. If the height of the surface profile is represented by y, then

In the formula, Rq—root mean square deviation of the surface profile n—the number of sampling points (2) root mean square slope Δq: its calculation formula is

In the formula L——measurement length
θ=y'
(3) The average wavelength λ: λ belongs to the hybrid parameter and contains more information than the pure height parameter and spatial parameter. λ is defined as

λ=2πRq/Δq

The measurement results are listed in the table below. The index 1 in the table represents the original surface that has not been run in; the subscript 2 represents the surface after running-in. As can be seen from the table, the running-in surface exhibits the following characteristics: the value of negative skewness and kurtosis increases (Rsk value is approximately -2.5 to -3.0, Rku value is approximately 10 to 20); the root mean square slope decreases (Δq value is about 1.5° to 2°); the average wavelength does not change much before and after running-in (λ value is about 100 μm or so), indicating that the spatial characteristics of the surface did not substantially change before and after running-in. Understanding the average wavelength is important for controlling the actual machining process because in many machining operations using single-edge cutting, the average wavelength is approximately equal to the tool feed rate.

Change of surface topography parameters before and after running-in

Since the micro-convex bodies are subjected to load during the process of mixed lubrication, the micro-convex density D (the number of micro-convex units per unit length) is very important. In the past, when the cylinder-piston ring was lubricated, the D value was basically artificially given, and therefore it has greater limitations. According to the measurement of the surface, the value of D varies greatly along the height of the surface. Figures 1a to e respectively show the variation of the number of microbumps along the surface height over the entire length of the measurement range before and after the five surfaces are worn out. The solid line in the figure represents the surface after running-in, the dotted line represents the original surface before running-in, and 0 is the midline position. It can be seen that the D value of the original surface is generally larger than the D value of the surface after running-in; for a surface itself, the D value is the largest at the midline, that is, when the surface is contacted at different heights, its D value is different. Accurate analysis of the liner-piston ring lubrication characteristics is very important.

Fig.1 Change of the density of microbumps along the surface before and after running-in

3. Wear wear analysis

　　 The wear amount of wear can be analyzed by two parameters of wear depth and wear volume. In the running-in process, the worn-in surface is only the peak of the profile being worn away, and the peaks and valleys are basically unchanged. Therefore, the maximum peak-to-valley height change can be used to measure the wear amount, that is, the wear depth can be regarded as the maximum peak before running-in. The difference between the valley height and the maximum peak-to-valley height after running-in. Since five cut-off lengths were measured for each test piece in this test, five peak-to-valley height averages were used to determine the amount of wear. FIG. 2 is a histogram of wear depth of the above five test pieces.

Figure 2 wear depth histogram

Due to the invariable characteristics of the peaks and valleys during the running-in process, the 100% support on the support rate curve before and after the run-in should be on the same plane, so the wear volume can be obtained from the support rate curve [1]. The wear volume histogram obtained according to the above method is shown in FIG.

Fig. 3 Wear volume histogram

Comparing Fig. 2 and Fig. 3, it can be seen that the wear depth is not completely consistent with the wear volume, which is mainly due to the influence of the surface topographical structural characteristics. Since the method of machining the surface of the cylinder liner analyzed is the same, the difference in wear described above is not large, but the difference may sometimes be very large for the surface obtained by the different processing. Therefore, when the running-in wear is reduced through the surface design, the above two parameters must be comprehensively considered to achieve the desired effect.

4 Conclusion

　　 (1) The cylinder surface has the following features in the stable working phase: the negative skewness value is about 2.5 to 3.0, the kurtosis value is about 10 to 20, and the average slope is about 1.5° to 2°. The average wavelength is about 100 μm.
(2) The density of microbumps varies along the height of the surface, and the density of microbumps is greatest at the midline. At the same time, the density of the surface micro-projections after running-in decreases.
(3) The amount of worn-in wear is influenced by the surface topography. When the wear depth is large, the wear volume is not necessarily large.

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