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Material Selection and Process Analysis of Automobile and Tractor Gear Parts

Automobile and tractor gears are mainly divided into transmissions and differentials. In the transmission, the gear ratio of the engine, crankshaft and main shaft gear is changed; in the differential, the gear is used to increase the torsion torque and adjust the speed of the left and right wheels, and the power of the engine is transmitted to the driving wheel through the gear to drive Cars and tractors are running. The working load of automobile and tractor gears is much larger than that of machine tool gears. Therefore, the requirements for their wear resistance, fatigue strength, core strength and impact toughness are higher than those of machine tool gears. Practice has proved that it is more appropriate to use carburized steel for automobile and tractor gears and use after carburizing treatment.

Take the second and third gears of the second shaft in the transmission of a JN-150 truck (with a deadweight of 8000kg) as an example for analysis.

1. Choose Steel

The production characteristics of automobile and tractor gears are large batches and high output. Therefore, when selecting steel, under the premise of meeting mechanical properties, sufficient attention must be paid to process performance.

20CrMnTi steel has high mechanical properties. After the steel is carburized, quenched, and tempered at low temperature, the surface hardness is 58~62HRC, and the core hardness is 30~45HRC. The process performance of 20CrMdri steel is good. After forging, normalizing is generally used to improve its workability.

20CrMnTi steel has better heat treatment processability and higher hardenability. Due to the influence of the alloy element titanium, it is not sensitive to overheating, so it can be directly cooled and quenched after carburizing. In addition, there is a faster carburizing rate. The advantages of uniform transition layer and small deformation after carburizing and quenching are very advantageous for the manufacture of gear parts with complex shapes and small deformations.

20CrMnTi steel can be used to manufacture important parts with a section thickness of 30mm or less, which can bear high-speed, medium-load, impact and friction, such as various carburized parts such as gears and gear shafts. When the carbon content is at the upper limit, it can be used to manufacture gears with a cross-sectional thickness below 40mm and a modulus greater than 10mm.

According to the specifications and working conditions of the second and third gears of the second shaft in the JN-150 truck transmission, it is more appropriate to use 20CrMnTi steel.

2. The Process Route of the Two-axis Gear

Cutting → forging → normalizing → machining → carburizing, quenching and low temperature tempering → shot peening → grinding inner hole and shift groove → assembling.

3. The Role of the Heat Treatment Process

After carburizing, the surface carbon content is increased to ensure high hardness after quenching, and improve wear resistance and contact fatigue strength. Shot peening can increase the compressive stress of the gear surface, strengthen the surface material, and improve the fatigue resistance of the material.

In addition to high-frequency induction hardened gears and carburized gears, there are also carbonitrided gears. According to different stress conditions and performance requirements, the gears can be quenched and tempered with medium carbon alloy steel and used after nitriding. Cast iron and cast steel can also be used to manufacture gears.