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The car transmission is responsible for shifting and twisting, reversing the vehicle and temporarily cutting off the power using the neutral gear, so that the car can adapt to driving under various conditions. Therefore, the relative movement of the transmission gear pair is very frequent, and as the mileage increases, the wear of the gear pair is inevitable. Analysis of the causes of the wear of the gear pair and the improvement measures are found to be very positive for improving the service life of the gear pair.
1Working characteristics During the running of the car, the gear pair of the car transmission is constantly subjected to impact and withstand alternating loads due to frequent shifting of gears, speed and load. From the kinematics analysis of the gear transmission, it is known that there is rolling and sliding on the surface of the meshing gear. Therefore, the friction between the tooth surfaces is a form of mixed friction, that is, rolling friction and sliding friction exist simultaneously. The direction and magnitude of rolling friction and sliding friction and the corresponding frictional resistance change from the beginning of the gear engagement to the final disengagement.
In order to reduce the wear of the gear pair to the gear pair, lubrication of the gear pair is essential. When the gear pair is in operation, its lubrication is intermittent, and the lubricating oil is thinned due to the poor heat dissipation. The tooth surface is more compact than the pressure, so the oil film is easily destroyed. Establish an oil film. Lubrication of the gear pair is both boundary lubrication and liquid lubrication. Its relative proportion in operation is unstable and varies with sliding speed, pressure and other factors. When the sliding speed is high, the proportion of liquid lubrication is large, and the friction factor is small; on the contrary, when the sliding speed is low, the boundary lubrication is basically, and the friction factor is large.
2 Gear Pair Wear Analysis Normal wear of the transmission gear pair can see a very uniform smooth surface on the tooth surface. In general, the tooth surface wear of the gear pair is greatest in the pitch circle area of ​​the gear. The tooth surface (including the tooth top) above the pitch circle wears less, and the wear between the root and its transition to the pitch circle area is relatively more severe than the tooth top portion. There are many factors that cause the gear tooth surface to wear, and the most important forms of wear are fatigue wear and adhesive wear.
2.1 Fatigue wear The main form of tooth surface wear is fatigue spalling due to the fact that the tooth surface is substantially rolling friction. During the meshing process of the gear pair, the contact stress on the tooth surface is pulsating, that is, the contact stress gradually increases to a maximum value when approaching the meshing, and the contact stress gradually decreases to zero as it gradually moves out of the meshing region. This process is repeated once for each engagement. Therefore, there is a certain microscopic unevenness of the gear surface, and micro-plastic deformation occurs when meshing, so fatigue occurs under the repeated action of such alternating load.
The process of fatigue wear includes the following three consecutive stages: the generation of micro cracks. Microscopic cracks are mainly caused by maximum contact shear stress, as shown. Under this force, micro-plastic deformation occurs on the gear surface close to the surface layer or surface layer, and the surface is hardened to form an initial crack.
The development of cracks under the surface layer and the distribution of the distribution of the lubricating oil are schematic diagrams of the crack propagation. Under the constant action of alternating contact stress, the micro-cracks expand along an angle of about 45° with the surface. At the same time, the gear oil is squeezed into the crack to form an oil wedge at the tip end of the crack, as shown in (a). When another gear surface of the pair of gear pairs is in contact with the crack and the crack is sealed, the oil sealed in the crack generates a high pressure during the subsequent rolling process, thereby promoting the growth of the crack, such as (b) ) shown.
The tooth surface metal peels off. As the crack continues to develop in depth, the small piece of metal between the crack and the surface layer is broken under the continuation of the load, and finally the metal of the surface is peeled off to form a pit, as shown in (c).
We know that the curvature of the points on the involute profile is not the same, and the loads on the meshing points of the profile are not exactly the same. The maximum contact stress occurs on the root surface near the node. Therefore, pitting damage occurs near the root surface of the pitch line, starting with needle-sized pockmarks or large-diameter ecchymoses, which then gradually expand, even into a single piece, forming obvious exfoliation marks.
2.2 Adhesive wear root and tooth tip due to the relative sliding speed is very large, in the case of gears in high-speed heavy-duty and poor lubrication, poor heat dissipation, etc., local high temperature occurs at the contact, plastic deformation of the tooth surface under high temperature and high pressure The contacts are glued together. When the two teeth slide, the softer gear surface is torn off causing adhesive wear.
Poor gear lubrication, poor assembly accuracy, and heavy loads can cause scratches and gluing of the gears. As the gear transmission develops toward high speed and heavy load, the form of abrasion and gluing of the gear will become more prominent. Because of the relative sliding of the root and the top of the tooth, these parts are prone to gluing and extend to the vicinity of the pitch line. It is characterized in that a flaw is formed along the sliding direction and is approximately perpendicular to the pitch line. The larger the gear, the greater the tendency of the glue to damage the ring, as the relative sliding speed increases as the profile size increases.
The wear and tear of the gear tooth surface is no less harmful than fatigue wear.
When the transfer load is large and the lubricating oil or oil is deficient, it will cause strong local heating and surface sticking, resulting in sticking.
2.3 Tooth surface plastic deformation If the material of the gear is soft, the load and friction are very large. During the meshing process, the material of the surface of the tooth surface will be plastically deformed along the direction of friction.
Since the frictional force on the driving gear is directed away from the pitch line toward the tooth tip and the root, respectively, after the plastic deformation occurs, the gear tooth surface forms a groove along the pitch line, and the friction on the driven gear The force acts on the root and the top of the tooth toward the pitch line, respectively, and the tooth surface forms a rib shape along the pitch line.
3 Main measures to improve gear pair wear Tooth surface wear is the inevitable result of long-term work of gears, but improper use, assembly, adjustment, etc., or poor meshing and gaps do not meet technical requirements, can accelerate gear wear. Therefore, it is very important to improve gear pair wear. The main measures are as follows.
3.1 Reasonable selection of gear oil The choice of gear oil should be based on the type of gear, the size of the load, the speed of the sliding speed, select different grades of oil, and then determine the viscosity grade of the oil according to the lowest and highest operating temperatures used, see table 1. At present, all countries in the world adopt API (American Petroleum Institute) quality classification standards for automotive gear oils, which are divided into five grades of GL-5; the most representative viscosity classification is SAE (American Society of Automotive Engineers) J306 It is divided into 7 grades such as 70W, 75W, 80W, 85W, 90140 and 250. The W letter indicates that there is a low temperature requirement. For example, GL-4 medium-duty vehicle gear oil for manual transmissions for general purpose and imported production lines; 3 manual gears for domestic manual transmissions using spiral bevel gears; GL-4 gear oil for domestic manual transmissions using hyperbolic gears; A lubricant with a higher viscosity should be used after the fatigue crack on the tooth surface.
Table 1 Viscosity of Vehicle Gear Oil Selection Ambient Temperature / C Vehicle Gear Oil Viscosity Level 3.2 Strictly Replace Gear Oil According to the Term of Use The transmission gear oil is generally fixed (predetermined mileage) and periodically (changed season). However, if the gear oil is found to be degraded during use, even if the oil change time is not available, it should be replaced in time according to the actual situation. The quality of the transmission gear oil is usually checked by empirical methods, that is, the gear oil is viewed or hand-turned, such as oil discoloration (yellow earth color), the viscosity is lowered, the oil contains more impurities or white foam, etc., indicating that the oil has deteriorated.
3.3 Guarantee the proper oil level and the amount of oil in the oil transmission, generally based on the fuel filler. For example, the oil level of the CA141 and BJ2021 transmissions is not less than 10 mm below the lower edge of the fuel filler; Dongfeng EQ14 (X Yuejin NJ130 transmission oil level is flush with the lower edge of the fuel filler.
3.4 Improve gear machining accuracy and repair quality Improve the accuracy of tooth surface machining, ensure that the gap of each component meets the technical standards, and effectively reduce the phenomenon of gear pitting; strictly carry out reasonable gear maintenance work according to the maintenance cycle, and timely clean and replace the gear oil after the initial mileage is completed. , so that it is always in a good state of technology; when repairing gear transmission parts, it must be adjusted according to technical requirements.
In short, the wear of the transmission gear pair is unavoidable in the long-term use process, but as long as the gear pair is created with good lubrication conditions and the maintenance and repair quality is continuously improved, the wear of the gear pair can be slowed down and the service life of the gear pair can be improved.