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Affected by surface topography, due to the presence of surface microprotrusions, the pressure P distribution of the contact zone changes from an elliptical distribution to a higher-order pressure Pa distribution, as shown. The force on each microprotrusion increases with its height, and the radius of curvature of the peak increases and decreases with the increase of the thickness of the oil film. When the two surfaces have ideal longitudinal roughness contact, the microprotrusions can be tightly fitted, and the microprotrusions are not easily unstable, and plastic folding is not easy to form, so the surface of the longitudinal roughness has more surface than the surface of the lateral roughness. High resistance to pitting and flaking.
For the boundary lubrication condition, the microprotrusion is an idealized isosceles triangle rigid body, and the top end is subjected to a concentrated positive pressure Pw and a tangential force (friction force) Pf, and the base body is an elastic body. The micro-convex body first tilts under the action of Pf force, then collapses under the action of Pw force, and the elastic support stiffness is k, then the micro-convex rotation angle is:=2Pfhx2k (1) The above formula is proportional to h, indicating that h is large The more easily the microprotrusions fall, resulting in folding, which is prone to pitting and flaking.
The effect of slip rate is greater when the slip ratio of the two surfaces is contacted, and the friction is also larger. The micro-convex body is easy to fall, forming a fold, causing cracks, and the surface is prone to pitting and peeling. The greater the load affected by the load, the more prone to pitting and spalling. With a pair of 35 and 40 steel HBS>350 gears, the circumferential speed is 17.9m / s, the tangential force is 146N / mm test, pitting and spalling increase sharply with the increase of the number of cycles, the tangential force is 138N / mm In the following cases, there are few pitting corrosions, and the development speed is very slow. When the running is not completed, the pitting will have a tendency to stop. Therefore, it is important to understand the ultimate load of non-progressive pitting when designing gears. When designing, attention should be paid to the matching of the hardness of the tooth surface. A pair of meshing gears must have a hardness difference. For the helical gear transmission, due to the meshing characteristics, the hardness difference has a more outstanding effect on improving the anti-pitting ability of the tooth surface. The hard tooth surface improves the hardness of the soft tooth surface during operation. Generally, the hardness of the pinion gear is higher than the hardness of the large gear by 70. The hardened layer thickness of the hard tooth surface is: = (24) Z0 (2) where: Z0 is the point at which the max acting point is away from the surface layer. Near the pitch line Z0=0.786b.
Lubrication affects the lubricant to expand the crack and accelerate the pitting and spalling of the tooth surface. In general, the thicker the oil film, the smaller the surface micro-convex interaction and the higher the surface resistance to pitting and peeling. According to experiments, the higher the oil temperature, the smaller the viscosity of the oil, the more likely it is pitting. The range of non-progressive ultimate load is 130146 N/mm at an oil temperature of 50, 250302 N/mm at 20 o'clock, and 102131 N/mm at 80 o'clock. It can be seen that the viscosity of the oil is different and the ultimate load varies greatly.
Pitting is caused by fatigue cracks on the tooth surface or shallow surface below the tooth surface. It is a necessary condition for pitting or flaking; after the oil enters the crack, it is sealed, causing a large oil pressure to cause crack propagation, which is pitting. Or sufficient conditions for flaking. The formation process of pitting or spalling is three stages: plastic folding, crack propagation and brittle fracture. Pitting occurred on the surface of the toothed high tooth that was chased, 23 mm below the pitch line. Non-progressive pitting is within the ultimate contact stress HP, and destructive pitting occurs when it exceeds HP. Pitting is to form an etch pit on the surface layer. The depth of the pit is generally about 100400m and the diameter is 12mm. For soft flank, the typical shape of the crater is mostly composed of two straight lines and one curve, and the apex angle is about 90120. The apex of the fan shape is nipple-shaped, pointing to the root of the driving gear. The crater on the driven gear has more irregular shapes, and the individual also has a fan shape, but points to the top of the tooth. In addition to pitting on the hard tooth surface, there is also a large area of ​​flakes that are irregular and irregular. Cracks occur on the entire tooth surface, and the cracks occur most in the place where pitting occurs. The direction of the crack is opposite in the direction of the main and driven tooth surfaces. On one tooth surface, the tooth top high tooth surface is opposite to the root high tooth surface. The plastic flow direction is in the same direction as the crack. Peeling is the formation of pitting, and then occurs under too high stress and stress cycles. The cracks are connected to each other and eventually fracture to spall. Factors affecting pitting and spalling are surface morphology, load, slip and lubrication conditions.
Research on Pitting and Shedding of Tooth Surface Fatigue