1. The failure modes of gears
There are many types of gears with different uses. In the actual production and application process, the failure modes of gears are also various. Gear failure generally occurs on the tooth surface and rarely occurs in other parts. According to the reasons for the failure of the gear during operation, the common failure modes of gears can be analyzed except for gear teeth broken, tooth surface glued, tooth surface fatigue, pitting, and tooth surface. Wear, plastic deformation, etc.
1.1 Gear breaking
Gear tooth breaking is a very dangerous form of final failure. According to different causes, it is divided into overload breaking, fatigue breaking and random breaking.
1.1.1 Overload fracture
When the tooth surface is subjected to an excessive impact load, the stress of the gear tooth exceeds its limit stress, and overload fracture occurs, which is generally a short-term overload. When the gear tooth breaks due to overload, the cross section has a radial or herringbone pattern radiation area, the radiation direction is roughly parallel to the crack propagation direction, the radiation center is the source of fracture, and the fracture surface shows the fatigue line of the shell pattern, and the cast iron gear is prone to overload fracture .
1.1.2 Fatigue fracture
Under the action of cyclic loading, the bending stress at the tooth root is the largest and the stress is concentrated. When the fatigue limit is exceeded, fatigue fracture is prone to occur at the fillet of the tooth root. As the working time and the number of cycles increase, multiple times Repeated action, the cracks gradually expand and deepen, eventually leading to fatigue fracture of the gear teeth. There are many factors that cause gear teeth to break due to fatigue, such as improper gear materials, low machining accuracy, small transition fillets at the tooth root, and insufficient estimation of the actual load during design.
1.1.3 Random fracture
When the gear material is defective and peeled off to form excessive local stress concentration at the fracture, it will cause random fracture. The form of the fracture is similar to that of general fatigue fracture. This failure is actually a secondary failure.
1.2 Tooth surface gluing
In high-speed and heavy-duty transmission, the lubricating oil is destroyed due to the increase in the temperature of the meshing area, so that the two tooth surface metals directly contact and bond to each other. As the tooth surface slides relatively, the softer tooth surface metal It is torn off along the sliding direction to form grooves. This phenomenon is gluing. According to their different characteristics and reasons, gluing can be divided into four types: light gluing, medium gluing, destructive gluing and partial gluing. Gluing the tooth surfaces will cause strong wear and heat, resulting in unstable transmission, resulting in scrap gears.
1.3 Tooth surface pitting
When the gear is working, the contact stress generated at any point on the meshing surface changes according to the pulsation cycle. When the tooth surface contact stress exceeds the contact limit stress of the material, the surface layer of the tooth surface will produce fine fatigue cracks and cracks. The expansion of the surface layer of metal particles peel off and form some small pits, commonly known as pitting pits. Pitting corrosion will reduce the bearing area of the tooth surface, cause shock and noise, and break the gear teeth in severe cases. When the pitting area exceeds the tooth height and the tooth width is 60%, new parts should be replaced.
1.4 Tooth surface wear
There are two types of tooth surface wear: (1), abrasive wear caused by hard particles (such as iron filings, sand, etc.) entering between the tooth surfaces; (2) due to the mutual friction of the tooth surfaces After excessive wear, a large amount of material on the working surface is worn away, and the shape of the tooth profile is damaged, which often leads to severe noise and vibration, and ultimately leads to transmission failure. Therefore, the tooth surface wear of important gears should not exceed 10% of the original tooth thickness. Generally, the wear of the tooth surface of the gear tooth should not exceed 20%-30% of the tooth thickness depending on the purpose of the equipment. If it exceeds the standard, it should be replaced.
1.5 Plastic deformation
The plastic deformation of the tooth surface mainly occurs in the occasions of low speed, heavy load, frequent start and overload. When the working stress of the tooth surface exceeds the yield limit of the material, plastic flow occurs on the tooth surface, which causes the pitch line of the drive gear tooth surface. Grooves are produced and ridges appear on the driven wheel. This failure mostly occurs on non-hard surface gear teeth, and the gear tooth profile is severely deformed, especially when the left and right sides are asymmetrical, new parts should be replaced.
The several main gear tooth failure forms described above can not only be repaired under normal circumstances, but also under the condition that the gear material and processing technology cannot be changed, the occurrence of unfavorable gear failures can be delayed through advance prevention, and the service life of the gear can be increased. .
2. Prevent the form of gear failure
2.1 Improve gear installation accuracy
2.2 Reasonable selection of materials
The selection of gear materials should be based on strength, toughness and process performance requirements, comprehensive consideration, combined with my country’s actual situation, should choose low-carbon alloy carburized steel. For gears that bear heavy loads and impact loads, steel materials mainly made of Ni-Cr and Ni-Cr-Mo alloy carburized steel are used. For gears with relatively stable load or low power and small modulus, it can also be used without Ni-Mn steel of Ni. Compared with gears made of ordinary electric furnace steel, gears made of this kind of steel can increase their contact and bending fatigue life by 3-5 times, and the ultimate load of gears can be increased by 15%-20%.
2.3 Heat treatment
Through the heat treatment process, the gear material can be improved, the hardness can be appropriately increased, the partial overload of the tooth surface can be eliminated or reduced, and the anti-stripping ability of the tooth surface can be improved. For example, deep carburizing and quenching for gears in coal mining machinery can reduce gears Hardening, improve core hardness, small residual tensile stress in transition zone and sufficient hardened layer depth.
2.4 Choose gear oil according to actual conditions
According to data, 34.4% of mechanical failures are due to insufficient lubrication and 19.6% are due to improper lubrication. In other words, 54% of mechanical failures are due to lubrication problems, so choose a good gear Oil is of great significance to increase the service life of gears.
In order to ensure the strength and hardness of the gear, it was decided to use argon arc welding alloy wire surfacing repair, and then use a polishing machine to reshape the treatment plan, so that the gear teeth after welding are few, and the higher hardness and strength can be achieved without heat treatment.
Through the analysis of gear failure forms, the ability to accurately distinguish equipment failures can be improved, the failures can be resolved in time, and economic benefits can be improved.