It can be seen from the failure form of the gear teeth that when designing the gear transmission, the tooth surface should have a higher ability to resist wear, pitting, gluing and plastic deformation, while the tooth root should have a higher ability to resist fracture. Therefore, the basic requirements for the material properties of gears are that the tooth surface should be hard and the tooth core should be tough.
Common gear materials
Steel has good toughness and impact resistance. It can also improve its mechanical properties and hardness of tooth surface through heat treatment or chemical heat treatment, so it is most suitable for manufacturing gears.
(1) forged steel
Except for those whose size is too large or whose structural shape is complex and only suitable for casting, forged steel is generally used to make gears. Carbon steel or alloy steel with carbon content of 0.15% ~ 0.6% is commonly used.
Forged steel for gear manufacturing can be divided into:
1) Forged steel used for heat-treated gear teeth. ,
For gears with low strength, speed and accuracy, soft tooth surface (hardness? 350 HBS) shall be used to facilitate tooth cutting and prevent rapid wear and blunt of the cutter. Therefore, gear blanks should be cut after normalizing or tempering. After cutting, it is finished product. Its accuracy is generally grade 8, and it can reach grade 7 when finishing cutting. This kind of gear is easy to manufacture, economical and productive.
2) Forged steel for gears to be finished.
The main gear transmission used in high-speed, heavy-duty and precision machines (such as precision machine tools and aeroengines) requires not only good material performance, high strength of gear teeth and high hardness of gear surface (such as 58-65 HRC), but also fine machining such as gear grinding. At present, most of the gears that need finishing machining are first cut, then surface hardening treatment, and finally finish machining, the accuracy can reach 5 or 4 levels. This kind of gear has high precision and high price. The heat treatment methods used include surface quenching, carburizing, nitriding, soft nitriding and cyaniding. The material used depends on the specific requirements and heat treatment method.
According to the composition and properties of the metal contained in the alloy steel, the toughness, impact resistance, wear resistance and scuffing resistance of the material can be improved respectively, and the mechanical properties of the material and the hardness of the tooth surface can also be improved by heat treatment or chemical heat treatment. Therefore, for the gears with high speed, heavy load, small size and small mass, they are all made of alloy steel with excellent performance (such as 20CrMnTi, 20Cr2Ni4A, etc.).
Due to the advantages of high mechanical properties and small structure size of hard tooth surface gear, some developed countries also generally adopt the middle and hard tooth surface gear transmission in general machinery.
(2) cast steel
The wear resistance and strength of cast steel are good, but it should be annealed, normalized and tempered if necessary. Cast steel is often used for gears of large size.
2 cast iron
The properties of gray cast iron are brittle, the impact resistance and wear resistance are poor, but the resistance to gluing and pitting corrosion is better. The gray cast iron gear is often used in the situation of stable operation, low speed and low power.
3 non metallic materials
For high-speed, light load and low precision gear transmission, in order to reduce noise, non-metallic materials (such as cloth clamping plastic, nylon, etc.) are often used as small gears, while large gears are still made of steel or cast iron. In order to make the big gear have enough anti-wear and anti pitting ability, the hardness of tooth surface should be 250-350 HBS.