Journal of Chongqing University (Natural Science Edition) Gear Tooth Surface Electro-erosion Mechanism He Zelong Wei Yunlong Zhang Guanghui Hu Gaoju Cao Xingjin 2, Zhu Xiaolu 2 (1. The national key point of mechanical transmission of Chongqing University, can observe the cloud pattern appearance of tooth surface erosion, electro-erosion The molten liquid metal after discharge is pressed and bonded together before being completely solidified, and is torn apart along with the separation of the tooth flanks, thus forming a microscopic morphology of the cloud pattern. An alternating composite action with friction wear marks, electrical erosion and frictional wear can be referred to as electrical erosion friction wear.
2 tooth surface erosion mechanism 2.1 tooth surface non-contact discharge erosion model The non-contact erosion process is actually the result of corrosion accumulation of multi-channel repeated discharge, so there is not only a problem of spatial distribution (ie, the position where the electrical erosion occurs is moving) And there is a problem of time (that is, some places have electrical erosion first, and some places produce electrical erosion).
Therefore, when studying the physical process of electro-erosion, it is often simplified. First, a physical process model of single-channel non-contact discharge is established. This process can be divided into three independent and closely related phases: ionization preparation phase, discharge thermal erosion phase, and deionization termination phase. Ionization preparation stage: A pair of feeding tooth surfaces, since the shaft voltage is applied to the pair of tooth surfaces, there is an electric field between the tooth surfaces. When the electric field between the tooth surfaces is strong to a certain extent, electrons in the vicinity of the peaks on the cathode tooth surface will move at high speed to the convex peak on the tooth surface as the anode under the action of the electric field, and the atoms in the lubricating medium liquid. Avalanche ionization occurs when the molecule collides with the molecule, and the avalanche ionization rapidly develops from the cathode tooth surface to the anode tooth surface, which is ready for the discharge thermal erosion stage. (2) Stage of discharge thermal erosion: it is the main stage of non-contact discharge erosion. When the discharge amount is constant, if the discharge duration is too short, the heat will not be transferred into the metal inside, so the erosion of the metal material is mainly gasification, and the liquid metal is thrown very little; if the discharge duration is too long, there will be More heat is transferred into the deep part of the metal and lost, which also reduces the amount of corrosion. Only when the discharge duration is appropriate, the thermal effect will be the highest, and the metal erosion will be the most, and the electric erosion is the most serious. (3) End of deionization phase: disappearance of the channel and continued expansion of the bubble, shrinkage of the bubble, rupture of the bubble and end of the discharge.
The meshing of a pair of spur gears is a line contact. Due to the elastic deformation, its contact area is a contact strip of a certain width. The multi-channel repeated discharge process mostly occurs in a limited contact zone working area, and it has not only a quantity with the above single-channel discharge. The touch is still qualitative. Because it is also that multiple channels do not necessarily discharge at the same time, many phenomena after the previous discharge affect the latter discharge process, and also affect the discharge process of multiple channels near it.
The tooth surface of the gear transmission is constantly meshed and separated, that is, the distance between the tooth surfaces is from large to small, to the smallest (ie, the minimum oil film thickness), and then gradually becomes smaller and larger. Assuming that the voltage between the tooth surfaces is constant, as the distance between the tooth surfaces decreases, the electric field strength will gradually increase.
Since there are many peaks on the cathode tooth surface and the anode tooth surface, when the convex peak on the cathode tooth surface is opposite to the peak of the anode tooth surface, the distance between them is the closest, so when the distance between the tooth surfaces has not reached the minimum, The discharge channel may have been broken down to form a discharge channel discharge. A large number of electrolytic products after discharge (such as metal particles, carbon black, small bubbles, residual heat after discharge, etc.) and discharge pit flanges may also affect the normal transfer of the discharge point, that is, the spatial distribution and time of repeated discharge The problem of distribution is such that a multi-channel discharge is formed one after another between the cathode flank and the anode flank. In this process, the peaks on the cathode tooth surface and the anode tooth surface may have come into contact, resulting in contact discharge erosion. Contact discharge plays a major role in replacing non-contact discharge, and frictional wear has also begun. They affect each other in time and space. Finally, with the separation of the meshing flank, the erosion of the pair of teeth ends (of course including both non-contact discharge erosion and contact discharge erosion), while the other pair As the teeth begin to engage, new electrical and frictional wear will begin.
2.2 Tooth surface contact discharge electric erosion model When the inter-electrode voltage between the tooth surfaces is low, the distance between the cathode tooth surface and the anode tooth surface is reduced during the tooth surface meshing process until it is reduced to the minimum, that is, the tooth The minimum oil film thickness between the faces. During this period of time, the electric field strength between the tooth surfaces is constantly increasing. Although the threshold of the lubricant medium has been reached, the polarization and bridging, collision and ionization preparation are insufficient, and it is impossible to have time. Avalanche ionization makes it impossible to form a discharge channel, and it is impossible to cause non-contact discharge erosion.
Thus, during the process of reducing the distance between the cathode tooth surface and the anode tooth surface to the minimum oil film thickness, the peaks on the cathode tooth surface and the peaks on the anode tooth surface have been in contact. Due to the small contact area, the current density is extremely large through the contact area, and the current density reaches a very high value. The Joule heat generated by the contact current instantaneously melts and vaporizes the tooth surface metal material in the contact area, causing an explosive spark discharge. . The discharge channel is composed of molten metal droplets, metal vapor and ionized gas, during which the molten metal material of the cathode tooth surface and the metal material on the anode tooth surface diffuse, resulting in a process of re-alloying, and the ionized gas is also involved. this process. When the distance between the cathode tooth surface and the anode tooth surface is further reduced, there is a mechanical impact force and a pressing force between the tooth surfaces, so that the contact area is increased, the current density is decreased, and the Joule generated by the current is mechanically impacted. Densification by force and squeezing force. Finally, the separation of the cathode tooth surface and the anode tooth surface allows the alloy layer to be firmly bonded to the tooth surface, so the microscopic morphology of the cloud pattern on the tooth surface can be observed under an electron microscope.
2.2.1 Hertz elastic wire contact nominal contact area Any solid surface is microscopically uneven, and there is a certain roughness. Assuming it is a smooth surface, Hertz solved the problem of line contact theory of smooth objects in 1895. When the tooth surfaces are engaged with each other under the normal load, the strip-shaped planar end surface contact line width 2a is increased with the load. Since the deformation of each point on the end contact line is different, the contact compressive stress is not constant and varies according to the elliptical law. The deformation is greatest at the center of the contact line, and the compressive stress is also the largest.
The distance between the lines. Their contact can be converted into a smooth rigid surface in contact with a rough elastic surface having a rough root mean square value of a = rd + d.
Usually the contour height of the actual surface of the machining is in accordance with the Gauss distribution law. The Gauss probability distribution function is: standard deviation, and 2 is the variance. It represents the distribution curve of the standard nominal contact area: width (cm); Fn - bearing normal force (N).
The minimum oil film thickness of the tooth surface is based on the Hooke line contact lubrication state diagram. From the rigid isobaric lubrication to the elastic variable viscosity lubrication various lubrication theories, four different oil film thickness calculation formulas are obtained accordingly.
1) Rigid first-viscosity zone (R/): Gauss probability distribution, which indicates the probability of occurrence at different heights. In theory, the standard Gauss probability distribution curve has a range of (one called +夂), but in fact, 99.9% of all cases are included between (a 3a+3W) and thus +3a is used as the limit of the Gauss distribution. The error is negligible.
When the thickness of the oil film is h, only the part with the contour height z>h is contacted. In the probability density distribution curve, the area of ​​the z>h part is the probability of surface contact, that is, the actual contact area is: 2) rigid-varying viscosity Zone (RV) 2.2.4 Tooth surface contact discharge galvanic heat source is the resistance heat generated by the galvanic current through the contact zone. According to Joule's law, the heat W is 3) the elastic first-class viscosity region (E/): 4) the heat generated in the elastic-varying-viscosity region (EV) per unit time is called the heat source strength, and the b-tooth width can be expressed by the following formula; a lubricating oil viscosity coefficient; U relative speed; Fn contact point pressure; E' comprehensive elastic modulus; R contact point comprehensive radius of curvature; n - lubrication oil viscosity under standard atmospheric pressure.
The actual peak contact area of ​​the actual surface of the tooth surface is distributed according to the probability density function, so the number of points of the contact peak should be calculated according to the probability. The heat generated by the two rough surfaces in a unit volume per unit time is called the volume heat source intensity, and the rough root mean square value can be expressed by the following formula, which is the product and the oil-resistance zone: a P-corrosion contact zone resistivity ( Dcm); actual contact area (cm2) of the galvanic contact zone.
2.2.5 The temperature of the contact point of the tooth surface contact discharge is very short due to the short discharge time of each contact point of the contact discharge erosion. It is too late to carry out the heat transfer process, or the heat transfer process affects the temperature rise of the discharge point. Small, can not be considered. Since the hot melt and the liquid hot melt of the metallic iron in the solid phase are different, and there is latent heat of fusion during melting, the temperature of the electrolytic corrosion discharge point is: T < hour: (g/cm3); heat of the solid metal of the cs melt. 1); hot melt (Ag. IO) with a liquid metal; latent heat of fusion of Qr metal 2.2.6 Electro-oxidation discharge point metal temperature coupler is a product of Austria Voih Company. The relevant parameters are as follows: motor power P = 700W, gear normal modulus m = 2.5, large gear speed 2 = 1490rpm, large gear teeth z2 = 91, small gear teeth z zombie 46, pitch circle helix angle P = 11.3 ° tooth width b = 20 mm, normal pressure angle = 20 °. Lubricating oil viscosity h=0.075Pas. According to the Hertz line contact theory formula, the nominal contact area A can be obtained to obtain the viscous parameter g. and the elastic parameter respectively. According to the Hooke line contact lubrication state diagram, the area at this time is obtained. Calculate the minimum oil film thickness h=2 according to the corresponding minimum oil film thickness calculation formula. 235 is in the elastic variable viscosity region (EV area). The surface roughness of the large and small gears is <72=1.6Mm, and the minimum oil film thickness is h=2.235Mm. The processing surface conforms to the standard Gauss probability distribution curve. According to the probability distribution curve, the probability of the actual contact point can be obtained as p=0.3314%; the electro-erosion discharge current/=1A, and the electric corrosion contact area resistivity P=9.7M>cm The density of the gear metal material is P=7.8g/cm3, and the solid metal of the gear metal material is hot-melt cs=0.(°K) The molten metal of the gear metal material is hot-melt cl=0.813Ag.K) The melting of the gear metal material The latent heat Qr=272.142/g, initial 361.19K. When the electric corrosion current between the tooth surfaces reaches 1A, the current density of the contact point reaches the order of 105A/cm2. The temperature at the point of contact reached more than two thousand degrees, and the metal has long since become a liquid. It then cools and solidifies, and the two tooth faces are bonded together. With the separation of the two tooth faces, the bonding points are torn apart, and a cloud-like pattern is formed on the tooth surface.
3 Conclusion The gear failure is so rapid, not the result of single erosion, nor is it caused entirely by friction and wear, but the result of the combined action of electro-erosion and friction-wear. The result of the alternating combination of electro-erosion and friction wear is significantly accelerated. The failure of the gear. In the case of friction and wear, the lubrication condition and the hardness of the material itself play a major decisive role; when the shaft voltage is large, that is, when the electric field strength between the tooth surfaces is large, the main role of the electric erosion in the gear failure process, In most cases, it is caused by the combination of electrical erosion and frictional wear. Electro-erosion friction wear accelerates the failure of the gear.
The electrolytic erosion process is actually the result of the accumulation of non-contact discharge erosion and contact discharge erosion. Non-contact discharge erosion occurs only when the interelectrode voltage between the tooth faces is high and the lubricant film between the tooth faces is formed well. The erosion of the tooth surface metal material occurs during the period when the lubricating oil medium is broken down to the moment when the cathode tooth surface is in contact with the anode tooth surface, and the tooth surface metal material is in an explosive gasification process of the superheated liquid metal. , throwing molten metal out of the tooth surface. If the interelectrode voltage between the tooth flanks is low and the lubricating oil film is poorly formed, the tooth surface electro-erosion is actually dominated by contact discharge electric erosion. Contact discharge erosion occurs approximately during the period in which the cathode tooth surface is in contact with the anode tooth surface and the cathode tooth surface is shorted to the anode tooth surface. Contact discharge erosion is not only related to the resistivity of the tooth surface material, hot melt, liquefaction heat, thermal conductivity, etc., but also related to the roughness of the contact surface, not only related to the load carrying capacity of the gear, but also the nature and lubrication of the lubricating oil. All have a relationship. Only when the interelectrode voltage between the tooth surfaces is relatively moderate, both non-contact discharge erosion and contact discharge erosion are the result of the combined action of the two.
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