From the failure mode of gears, it can be seen that when designing gear transmission, the tooth surface must have high wear resistance, pitting corrosion resistance, bonding resistance and plastic deformation resistance, while the tooth root must have high fracture resistance.
Therefore, the basic requirements for gear materials are: the tooth surface must be hard and the tooth core must be tough.
Common Gear Materials
Hardened gear (hardness 350HBS):
It is used in high-speed, heavy-duty, and precision machines (such as precision machine tools and aircraft engines).
In addition to the excellent material properties, high tooth strength and high tooth surface hardness (such as 58 ~ 65 HRC), it should also be completed by gear grinding.
Currently, most gears that require finishing are first cut, then hardened and finally finished.
Accuracy can reach level 5 or 4.
This type of gear has high precision and high price, so heat treatment methods include surface quenching, carburizing, nitriding, soft nitriding and cyanidation.
Therefore, the material depends on the specific requirements and heat treatment method.
That is, teeth cutting, surface hardening and finishing. The machined gear has a hard tooth surface and the accuracy is grade 5 or 4.
Alloy steel can improve the toughness, impact resistance, wear resistance and cementation resistance of the material according to the composition and properties of the contained metal.
It can also improve the mechanical properties and tooth surface hardness of the material through heat treatment or chemical heat treatment.
Therefore, aviation gears with high speed, heavy load, small size and small weight are made of alloy steel with excellent performance (such as 20CrMnTi, 20Cr2Ni4A, etc.).
2. Cast steel
Cast steel has good wear resistance and strength, but it must be annealed and normalized.
It can also be quenched and tempered when necessary, which is often used for large gears.
2. Cast iron
Gray cast iron is brittle, with low impact and wear resistance, but good adhesion and corrosion resistance.
It is generally used for stable operation, low speed and low power.
3. Non-metallic materials
To reduce noise, commonly used non-metallic materials such as bakelite, nylon, etc. are often used as the pinion, while the large gear is still made of steel or cast iron.
In order for the gear to have sufficient resistance to wear and pitting corrosion, the tooth surface hardness is also required, which should be 250 ~ 350 HBS.
Gear material selection principle
There are many types of gear materials, and many factors must be considered when selecting. The following points are for reference:
1. Gear material must meet the requirements of working conditions
For example, gears used in aircraft must meet the requirements of small weight, high transmission power and high reliability, therefore, alloy steel with high mechanical properties must be selected;
Gear transmission in mining machines generally has high power, low working speed, and high dust content in the surrounding environment, so cast steel or cast iron are often used;
The power of household and office machines is very small, but it requires stable transmission, low noise or no noise, and can operate normally with less lubrication.
Therefore, engineering plastics are often used as gear materials.
In short, working condition requirements are the first factor to be considered when selecting gear materials.
2. Gear size, blank forming method, heat treatment and manufacturing process must be considered.
Billets are generally used for large gears, and the gear material can be cast steel or cast iron.
For gears with high requirements for medium and low sizes, forged material is generally used, and forged steel can be used.
When the size is small and the requirements are not high, round steel can be used as a blank.
Gear surface hardening methods include carburizing, nitriding and surface hardening.
3. Normalized carbon steel, no matter how the blank is made, can only be used to make the gear work under stable load and light impact, and cannot withstand large impact load;
Quenched and tempered carbon steel can be used to manufacture gears that work under moderate impact loads.
4. Alloy steel is often used to manufacture gears with high speed, heavy load and working under impact load.
5. Aircraft gear transmission requires the gear size to be as small as possible, and high-strength alloy steel with surface hardening treatment must be used.
6. For gears with soft tooth surface made of metal, the hardness difference between paired tooth surfaces should be maintained at 30 ~ 50 HBS or higher.
When the hardness of the tooth surface of the pinion and large gear is very different (such as the tooth surface of the pinion is hardened and ground, and the tooth surface of the large gear is normalized or quenched and tempered) and the speed is high, The hard surface of the pinion tooth will have an obvious cold work hardening effect on the soft surface of the large gear tooth, thereby increasing the fatigue limit of the large gear tooth surface.
Therefore, when the hardness difference between the paired tooth surfaces of two gears is large, the allowable contact fatigue stress of large gears can be increased by about 20%, but attention should be paid to the high tooth surfaces. hardness, and the roughness value should be reduced accordingly.
Wrap it up
In short, when selecting gears, it also depends on the occasion of use.
Different places pay attention to different places, and of course the materials used are also different.
When designing, pay attention to the selection of materials to make the gears practical and durable.
