All kinds of household appliances, automobiles and aerospace equipment need to choose sealed bearings with dust cover for the purpose of simplifying the structure of the main engine, convenient maintenance and preventing the pollution of bearings from the surrounding harsh environment and for a long time without lubrication. The sealing effect of sealed bearing directly affects the quality of bearing. For sealed bearing, it is not only required to have high life reliability, but also good sealing performance, in order to reduce vibration and reduce noise.
1. Problems before process improvement
The quality of the sealed bearing with dustproof cover mainly depends on the degree of cooperation between the dustproof cover groove and the dustproof cover of the outer ring. For the dustproof cover groove of the outer ring, the main factors affecting the sealing effect are the diameter size of the dustproof cover notch and the shape of the groove. The shape of the groove is mainly guaranteed by the forming tool under the current technological level conditions, and because the axial local size of the groove is small, the size of the groove before and after heat treatment is basically unchanged. The dustproof cover notch diameter size due to the impact of machining, especially heat treatment deformation. Therefore, whether to control the dustproof cover notch diameter size, become the key process. Take 6314-2Z as an example to illustrate the problems existing before the process improvement of the outer ring dust cover groove. The processing process of the outer ring is as follows: end face, outer diameter surface, safety Angle ※ the other end face, outer diameter surface, safety Angle ※ soft grinding end face ※ Printing ※ Outer channel ※ dustproof cover notch ※ dustproof cover groove ※ Outer corner. The product drawing and working drawing of 6314-2Z outer ring are shown in Figure 1 and Figure 2, respectively. It can be seen from the figure that the diameter of the dustproof cover notch and the bottom of the groove in the product drawing are 128.1 ± 0.08mm and 129.2 ± 0.08mm. The specified size of the working drawing is 128 ±0.06mm and 129.1 ±0.06mm. Obviously, in order to ensure the final requirements of the product in the process, considering the expansion law of heat treatment, a large amount of 0.1mm expansion is subtracted from its basic size; The upper and lower deviations of the size combined with the actual level of the vehicle processing technology are compressed by 0.02mm each
After the outer ring of 6314-2Z is processed by the car, the single plane outer diameter size will change during the heat treatment process. The current heat treatment technical conditions stipulate that the single plane outer diameter variation VDsp is 0.20mm after heat treatment, and the heat treatment swell size is different. So that the diameter of the finished dust cover notch can not be effectively guaranteed. Table 1 shows the diameter changes of dustproof cover notch before and after heat treatment of 20 pieces of a batch of products randomly selected from 6314-2Z outer ring. According to the data in the table, the diameters of dustproof cover slots before heat treatment are all qualified, and only 50% of the diameters of dustproof cover slots after heat treatment meet the product requirements, and the size dispersion of other out-of-tolerance products is very large. For further explanation, the line diagram of diameter size deviation (VDcs) and single plane outside diameter size variation (VDcsp) of dust cover notch before and after heat treatment (FIG. 3) was obtained according to Table 1. It can be seen from FIG. 3a that the average size of notch diameter generally shows a trend of swelling after heat treatment, but for each product, the swelling is large and different. As can be seen from Figure 3b, the variation of notch diameter size after heat treatment is significantly greater than that before heat treatment, and it is very difficult to completely ensure the diameter size of dustproof cover notch only by the existing process.
The main process of bearing assembly is: outer ring final inspection ※ Inner and outer ring channel size sorting ※ Fitting ※ retainer ※ riveting retainer ※ Grease ※ Pressure dust cover ※ packing. Among them, the pressure dust cover needs to be finished with the special dust cover buckle mold in the press. If the slot size of the outer ring dust cover is not qualified, there will be a problem when it matches the dust cover. If the diameter of the dustproof cover notch of the outer ring is small, it will have greater interference with the dustproof cover size, so that the warping deformation will occur after the dustproof cover is pressed. If the diameter of the dustproof cover notch of the outer ring is large, the dustproof cover is not closely matched after pressing, and it is easy to rotate in the dustproof cover groove, or even the phenomenon of falling off the cover. Once the above situation occurs, it will directly affect the use of sealed bearings, and, in addition, bearing with dust cover on both sides are grease bearing, repair is difficult, so effective measures must be taken from the process to solve.
2. Process improvement measures
The fundamental reasons affecting the seal quality were analyzed, and the hard wheel processing technology was adopted to improve the seal quality. In the design process of the turning drawing before heat treatment, the diameter size of the dustproof cover notch should fully take into account the expansion and deformation of the heat treatment, and reserve a certain amount of hard car on the surface of the dustproof cover notch. The amount of hard cars should be as small as possible to improve the durability of hard turning tools and the efficiency of hard cars. Considering that the outer ring size has a tendency to expand during heat treatment, Table 2 shows the reserved (empirical) hard car reservation under different size sections. However, the reserved hard car reservation is not equal to the actual hard car reservation after heat treatment, and the actual hard car reservation is smaller than the reserved hard car reservation. Usually, the cutting depth of hard car is 0.1-0.3mm. After the heat treatment of the outer ring and rough grinding of each surface, the groove surface of the hard dust-proof cover of the CNC lathe can be fully met the technological requirements, so as to ensure the sealing quality.
Still take 6314-2Z as an example to illustrate the improvement measures of the outer ring dustproof cover groove process: increase the surface process of the dustproof cover notch of the hard car, in order to improve the precision of the dustproof cover notch of the hard car by improving the precision of the outer diameter clamping surface, the dustproof cover notch process of the hard car is placed after the coarse grinding process. Before heat treatment, part of the size of the dustproof cover groove in the outer ring working drawing was adjusted (the size in brackets in Figure 2), so that the diameter size of the notch was reduced by 0.40mm compared with that before improvement, from 128 ± 0.06mm to 127.6 ± 0.06mm. That is, 0.5 mm hard car is reserved on the basis of the product drawing size, and the actual hard car is about 0.4 mm. Due to the existence of hard car, the chamfer coordinates between the dustproof cover notch and the end face are also increased accordingly. The size of the dust-proof cover groove forming tool also needs to be adjusted accordingly. The CNC lathe used for the hard car is CL-20A; The tool bar is A25T-SVQBR-16-D, and the tool block is VBGW160408-2U. When cutting, the cutting speed ν=85m/min; Feed rate f= 0.09mm /r; Depth of cut αp is 0.20 to 0.25 mm. Figure 4 is the line diagram of the average outside diameter deviation ΔDcm and the single plane outside diameter variation VDcsp of 20 pieces of dustproof cover notch of a batch of 6314-2Z outer ring after random measurement after taking improvement measures. It can be concluded from the calculation in the figure that the diameter size difference after machining the dustproof cover notch of the hard car is between -0.02 ~ +0.02 mm, and the variation of the single in-plane and out-of-plane diameter VDcsp is 0.01 ~ 0.03mm, which fully meets the product design requirements.
The improvement of the processing technology of the dustproof cover notch of the outer ring effectively solves the quality problem of the bearing seal caused by the diameter size of the dustproof cover notch is too scattered. Because the hard car process is arranged on the outer ring surface after rough grinding, and dustproof cover notch surface is narrow, hard car cutting depth is small, cutting force is small, do not need too much clamping force, therefore, will not cause deformation of the channel and dustproof cover notch, better ensure the quality of the other parts of the outer ring. The increased hard turning process, because of the wear of the hard turning tool block is small, and the efficiency of the hard turning is also very high, the increased cost is much smaller than the loss caused by the unqualified sealing quality.
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