Gears are important parts of machines and instruments, and the accuracy of gears affects the quality of the entire machine or instrument to a certain extent. Due to the complex tooth shape and many parameters, the evaluation of gear accuracy is more complicated.
The requirements of modern industry for gear transmission can be summed up in the following four items:
1. It is required that the change of the transmission ratio within one revolution should be as small as possible to ensure the accuracy of the transmission movement. (Motion Accurate)
2. The change of instantaneous transmission ratio is required to be as small as possible to ensure stable transmission, small shock and vibration, and low noise. (working smoothly)
3. It is required that the working tooth surface under load can be in good contact with gear processing to ensure sufficient bearing capacity and service life. (contact accuracy)
4. The gear pair is required to have an appropriate backlash (the gap between the non-working surfaces of the meshing gear teeth to compensate for thermal deformation and store lubricating oil.)
Gears and gears for different purposes and different working conditions have different emphasis on the above four requirements. For example, the focus of the indexing transmission of the control system or the follow-up system is the movement accuracy, so as to ensure the movement coordination of the main and driven gears. The focus of variable speed gear transmissions in automobiles and tractors is smooth operation to reduce noise. The focus of low-speed and heavy-duty gear transmission (such as the gear transmission of a rolling mill) is the contact accuracy of the tooth surface to ensure good contact of the tooth surface.
The selection of gear processing materials depends on the application requirements. The commonly used gear materials are steel (cast steel and forged steel), cast iron and non-metallic materials.
Cast steel has good wear resistance and strength, but it should be annealed and normalized, and can also be quenched and tempered if necessary.
Steel has good toughness and impact resistance. It can also improve its mechanical properties and tooth surface hardness through heat treatment or chemical heat treatment, so it is most suitable for manufacturing gears. The main gear transmission used in forged steel high-speed, heavy-duty and precision machines (such as precision machine tools, aero-engine gear processing machines) for gears that need to be finished requires excellent material properties, high-strength gear teeth and high-quality tooth surfaces. In addition to hardness (such as 58-65HRC), finishing such as grinding teeth should also be carried out.
At present, the gears that need to be finished are mostly cut first, then surface hardened, and finally finished. The precision can reach level 5 or level 4. This type of gear has high precision and is more expensive, so the heat treatment methods include surface quenching, carburizing and nitriding, soft nitriding and cyaniding. The forged steel used for gears that have been cut after heat treatment should be used for gears that do not require high strength, speed and precision, so that the teeth can be cut easily and the tool will not be worn out quickly.