Abstract: Five forms of damage to agricultural machinery transmission gears were analyzed, and poor lubrication was identified as the main cause. Users were reminded to choose the correct lubrication method and use lubricating oil that meets the requirements.

Keywords: agricultural machinery; Transmission gear; lubricant

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. The power is transmitted through gears in the gearbox, central drive, and Z-final drive. Gears often work at high speeds and high 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. Types of gear damage

(1) The gear teeth are broken. Due to the effect of normal pressure on the gear teeth, a Z-large bending normal stress will be generated at its root, and there is a significant stress concentration at the root, making it easy for the gear teeth to break from the root. In addition, when the gears suddenly reverse or are overloaded for a short period of time, they are prone to impact and breakage. Preventive measures: When installing gears, they should meet the requirements. When designing gears, the tooth thickness should be appropriately increased, and the impact should be minimized as much as possible during use.

(2) Gear wear. In meshing transmission, except for contact at nodes, when two tooth profiles come into contact at any position, each point on the tooth surface has unequal tangential velocities. 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, which is caused by metal to metal contact, surface adhesion, and then tearing off. The reason is insufficient lubricating oil or incorrect gear meshing. ② Abrasive wear is caused by external particles such as dust and sand. ③ Corrosion wear refers to the chemical erosion of gear surfaces 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 and regular replacement of lubricating oil.

(3) Fatigue pitting corrosion on the tooth surface. Its generation 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, using lubricating oil with high viscosity, and performing 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 fast speed of the gear cause a decrease in the viscosity of the lubricating oil, resulting in the loss of lubrication function, as well as the instantaneous high temperature of the gear. Preventive measures: Use lubricating oil with high viscosity or activated lubricating oil with excellent anti adhesive properties.

(5) Plastic deformation of tooth surface. If the surface hardness of the gear is not high, plastic deformation of the tooth surface will occur, which will damage the smoothness of the transmission. Preventive measures: 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 causes of gear damage. To ensure the correct lubrication of gear transmission, it is necessary to select the lubrication method and lubricating oil correctly according to the usage conditions of the gear.

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 when the gears rotate to lubricate the various lubrication parts of the gearbox. This method is only suitable for small and medium-sized gear transmissions with low speeds.

(2) Oil injection 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 the lubricating oil will rapidly deteriorate. At this point, the oil injection lubrication method can be used, which involves using 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 onto the meshing area. 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 injection point should be from the meshing point, or the oil should be sprayed from the end face of the gear to avoid excessive fluid dynamic pressure between the working tooth profile and affecting the strength and service life of the gear. Simultaneously spraying oil from the end face can also reduce the power consumption of gears.

(3) Oil mist lubrication method: Compressed air is used to spray lubricating oil from the nozzle. After the lubricating oil is atomized, it 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 frictional heat and wear debris, playing a good role in cooling and cleaning. Commonly used in gear transmission 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; The production of animal and vegetable oils is limited and prone to spoilage, so they are only used in situations with special requirements; Mineral oil is widely used due to its abundant production, low cost, and stable performance. Mineral oil can be classified into many types based on their usage 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 according to 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, which is commonly used in domestic tractors, is a spiral bevel gear. This gear can withstand high loads and speeds, and can generally use ordinary vehicle gear oil. When the tractor is used for deep tillage operations, the tooth surface is often under high load and the oil temperature is very high. Therefore, sulfur phosphorus type medium load (GL-4) vehicle gear oil should be selected. Hyperbolic gears widely used both domestically and internationally have advantages such as high reduction ratio, smooth meshing, low noise, and compact structure. Ordinary vehicle lubricating oil used for heavy load operations may cause tooth surface abrasion and adhesion. Therefore, sulfur phosphorus type medium load (GL-4) or sulfur phosphorus type heavy-duty (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 less likely to break and cause scratches on the tooth surface. 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 viscosity of the gear oil is too small, the load-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 ambient temperature and the actual usage of the machinery. Low speed and heavy-duty gears require oil with high viscosity, oiliness, and good extreme pressure due to poor conditions for oil film formation. The formation conditions of oil film in high-speed gear transmission are good, but the stirring loss is large, so low viscosity oil should be selected.

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2025 May 1st Week FK Product Recommendation:

Flange Cartridge Units:

They are units of rigid cast iron housings with piloted flange cartridge and wider inner ring bearing inserts.

Standard painting in green (other colors are available for choices).

Standard duty (FC200) and medium duty (FCX) are available for this series.

End caps by pressed steel are capable of protecting against external polution and dusts.

The self-aligning ability of the ball bearings can compensate for slight static mis-alignemnt of the shaft. The bearing inserts are locked onto the shaft by means of set screws, eccentric locking collars, adapter sleeves and concentric locking collars etc.

Lubricants filled inside the spherical ball bearing inserts can be standard grease, low/hight temperature grease and food grade grease.

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