Abstract: This article analyzes five forms of gear damage in agricultural machinery transmission, points out that poor lubrication is the main cause, and reminds users to pay attention to selecting the correct lubrication method and using lubricating oil that meets the requirements.
Keywords: Agricultural machinery; Transmission gear; lubricating oil
Gear transmission has the advantages of high transmission efficiency, compact structure, reliable operation, and long service life. It is the main transmission method in agricultural machinery and is widely used in modern machine manufacturing industry. Power is transmitted through gears in transmissions, central drives, and Z-final drives. Gears often work at high speeds and loads, and gear damage often occurs. In recent years, the power of agricultural machinery has been increasing, and the speed and load capacity have also been increasing, which has put forward higher requirements for the bearing capacity of gears. To fully utilize the load-bearing capacity of gears, reduce the possibility of gear failure, and extend the service life of gears, lubrication quality is an important influencing factor.
1. Gear damage form
(1) The gear teeth are broken. Due to the effect of normal pressure on the gear teeth, a Z-shaped bending normal stress will be generated at its root, and there is a significant stress concentration at the root, making the gear teeth prone to breaking from the root. In addition, when the gear suddenly reverses or is overloaded for a short period of time, it is prone to impact fracture. Preventive measures: When installing gears, they should meet the requirements. When designing gears, the tooth thickness should be appropriately increased, and impact should be minimized as much as possible during use.
(2) Gear wear. In meshing transmission, apart from node contact, when two tooth profiles come into contact at any position, each point on the tooth surface has an unequal tangential velocity. Therefore, there is inevitable relative sliding between the tooth surfaces. Under the action of inter tooth load, it will inevitably cause wear on the tooth surface. There are three main forms of gear wear: ① adhesive wear, caused by metal to metal contact, surface adhesion together, and then tearing off. The reason is insufficient lubricating oil or incorrect gear engagement Abrasive wear is caused by external particles such as dust and sand Corrosion and wear, chemical erosion on the surface of gears caused by contaminated lubricating oil or additives. Preventive measures: When designing gears, the tooth surface hardness and surface roughness should be appropriately increased, and attention should be paid to the cleanliness of lubricating oil and regular replacement.
(3) Fatigue pitting on the tooth surface. Its occurrence is due to excessive contact stress on the tooth surface. Preventive measures: Measures such as increasing the hardness and surface roughness of the tooth surface, selecting lubricating oil with high viscosity, and conducting good running in can be taken to improve the ability of the tooth surface to resist pitting corrosion.
(4) Tooth surface bonding. The reason is that the high temperature and speed of the gears cause a decrease in the viscosity of the lubricating oil, loss of lubrication function, and instantaneous high temperature of the gears. Prevention method: Choose lubricating oil with high viscosity or use activated lubricating oil with excellent anti bonding performance.
(5) Plastic deformation of tooth surface. The surface hardness of gears is not high, which will cause plastic deformation of the tooth surface and damage the smoothness of the transmission. Prevention method: Enhance surface hardness.
2. Gear lubrication
From the above reasons for gear damage, in addition to poor gear quality and low assembly quality, poor quality of lubricating oil, low cleanliness, and unqualified lubrication methods are the main reasons for gear damage. To ensure the correct lubrication of gear transmission, it is necessary to choose the correct lubrication method and lubricating oil according to the usage conditions of the gears.
2.1 Gear lubrication method
(1) Oil bath and splash lubrication. Using the gearbox as the oil tank, immerse the gears into the oil to a certain depth, and rely on the splashing of oil during gear rotation to meet the lubrication of various lubrication parts of the gearbox. This method is only suitable for medium and small gear transmissions with low speeds.
(2) Oil spray lubrication method. When the circumferential speed of the gear exceeds 12-15m/s, if oil bath or splash lubrication is still used, the oil stirring loss will significantly increase and will cause the lubricating oil to deteriorate rapidly. At this point, the oil spray lubrication method can be used, which uses a nozzle to spray pressure oil onto the gear pair. When the circumferential speed of the spur gear is less than 20m/s and the circumferential speed of the helical gear is less than 40m/s, it can be directly sprayed to the meshing point. When the circumferential speed of the gear is higher, lubricating oil should be sprayed onto both gears separately. The higher the circumferential speed of the gear, the farther the distance between the oil injection point and the meshing point should be, or the oil should be sprayed from the end face of the gear to avoid forming excessive fluid dynamic pressure between the working tooth profiles and affecting the strength and service life of the gear. Simultaneously spraying oil from the end face can also reduce the power consumption of the gears.
(3) Oil mist lubrication method: Use compressed air to spray lubricating oil out of the nozzle. After atomization, the lubricating oil diffuses with the compressed air to the friction surface for lubrication. The oil mist lubricating oil film layer is thin but uniform. Oil mist can take away friction heat and wear debris, playing a good role in cooling and cleaning. Commonly used in gear transmissions with a circumferential speed greater than 15m/s.
2.2 Selection of lubricating oil
Lubricating oil is divided into mineral lubricating oil, synthetic lubricating oil, and animal and vegetable oil. The cost of synthetic lubricating oil is very high, and it is only used in places with high or low temperatures or other special requirements; Animal and vegetable oils have limited production and are prone to spoilage, so they are only used in situations with special requirements; Mineral oil has abundant production, low cost, and stable performance, so it is widely used. Mineral oil is divided into many types based on its purpose and viscosity, such as mechanical oil, machine tool hydraulic oil, gear oil, etc. When making specific choices, the following aspects can be considered:
(1) Choose lubricating oil based on the severity of working conditions. The gears in the gearbox are both spur and helical gears, and the load and slip speed are not high, so using ordinary vehicle gear oil can meet the requirements. The main transmission gear of the rear axle gearbox is often used by domestic tractors with spiral bevel gears, which bear large loads and high speeds. Generally, ordinary vehicle gear oil can be used. When the tractor is plowing and loosening deeply, the tooth surface is often under high load and the oil temperature is very high. Therefore, sulfur phosphorus medium load (GL-4) vehicle gear oil should be selected. The hyperbolic gears widely used domestically and internationally have advantages such as high reduction ratio, good meshing smoothness, low noise, and compact structure. When using ordinary vehicle lubricating oil for heavy load operations, the tooth surface may be scratched and glued. Therefore, sulfur phosphorus medium load (GL-4) or sulfur phosphorus heavy load (GL-5) vehicle gear oil should be used.
(2) There are many factors to consider when selecting the viscosity of gear oil. If the viscosity is high, the oil film thickness is not easy to break and it is not easy to cause tooth surface abrasion. But if the viscosity is too high, it is difficult to obtain the required amount of lubricating oil at the meshing part of the gear. If the gear oil viscosity is too small, the bearing capacity is low and it is easy to cause tooth surface abrasion. Therefore, when selecting gear oil, it should be determined based on the local environmental temperature and the actual use of the machinery. Low speed and heavy-duty gears require the selection of oil with high viscosity, good oiliness, and extreme pressure properties due to poor conditions for oil film formation. The oil film formation conditions for high-speed gear transmission are good, but the stirring loss is large, so low viscosity oil should be selected.
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2024-06-14