What are the factors that affect gear life?

After the gear is used for a period of time, friction will always occur, which will gradually reduce the gear life. The gear life is the working time before the product fails, or the number of rotations (number of revolutions) of the gear shaft.

Due to quality and use issues,gearFailure phenomena will occur. Common forms of gear tooth failure include: gear tooth fracture, tooth surface pitting, tooth surface gluing, tooth surface wear, tooth surface plastic deformation, and so on.

Gear failure often leads to the failure of the transmission system or the whole machine, which leads to major safety accidents.Therefore, only by grasping the cause of the failure of the product, can the occurrence of failure be avoided.

XNUMX. The influence of gear material selection

For gears, if the material is not selected properly, the parts will be damaged prematurely or even fail.Therefore, the rational selection and use of metal materials is a very important task.

Commonly used materials for gears are forged steel, cast steel, cast iron, and non-metallic materials.Generally, the use performance of the material should be considered first to meet the use requirements of the parts.

Use performance refers to the performance that the material should possess under normal use conditions, including mechanical properties, physical properties and chemical properties.Use performance is a necessary condition to ensure the safety, reliability and durability of parts.

The working conditions of the gear are different, and the damage of the gear teeth is different, which is the basis for determining the strength of the gear and the selection of materials and heat treatment.Sometimes a certain simulation test verification is needed to finally determine the material of the part.

For general mechanical parts, its mechanical properties are mainly used as the basis for material selection.

After comprehensively analyzing the working conditions and failure modes of the parts, and determining the main mechanical performance indexes and their values ​​of the parts through mechanical calculations according to the geometry and size of the parts, the load and the service life of the parts, You can use the manual to select materials.

Secondly, consider that the manufacturability of the material should meet the processing requirements.The manufacturability of a material refers to the ability of the material to adapt to a certain processing.

In the selection of materials, compared with the use performance, the process performance of the material is often in a secondary position.

The requirements of working conditions are the first factors that should be considered when selecting gear materials. Alloy steel is often used to make gears that are high-speed, heavy-duty and work under impact loads.

Finally, economy should be fully considered.When selecting materials, attention should be paid to reducing the total cost of parts.The total cost of the parts includes the price of the material itself, processing fees, management fees and other additional costs such as parts maintenance fees.

Improper selection of materials will not only suffer economic losses, but also make gear failures earlier.

XNUMX. The influence of heat treatment process

1. Normalizing

Normalizing can eliminate internal stress, refine grains, improve mechanical properties and cutting performance.Gears that do not require high mechanical strength can be normalized with medium carbon steel.

Large-diameter gears can be normalized by cast steel.

For carbon steel gears, due to the low hardenability of carbon steel, normalizing and quenching and tempering effects are similar when designing large sizes, but normalizing can reduce the cost and does not require quenching and tempering.If the normalizing treatment is not carried out or the normalizing treatment is not thorough enough, it is easy to have abnormal original austenite grain boundaries and grain orientations, which will increase the tendency of quenching cracking.

2. Tempering

Quenching and tempering is generally used for medium carbon steel and medium carbon alloy steel.For example, 45, 40Cr, 35SiMn, etc.

The hardness of the tooth surface after quenching and tempering is generally 220-260HBS.Because the hardness is not high, the tooth profile can be fine-cut after heat treatment, and it is easy to run-in during use.

The general heat treatment of soft tooth surface gears is quenching and tempering or normalizing, and the hardness is usually between 180-280HBS.

Due to the limitation of the hardenability of steel, large-modulus and high-quality gears should be quenched and tempered after gear opening.

3. Surface hardening

Generally used for medium carbon steel and medium carbon alloy steel, such as 45 steel, 40Cr, etc.After surface hardening, the deformation of the gear teeth is not large, and the accuracy can be reached 7 without grinding the teeth, and the hardness of the tooth surface can reach 52-56HRC.Because the tooth surface has high contact strength and good wear resistance, and the unhardened tooth core still has high toughness, it can withstand a certain impact load.

Surface quenching methods include high frequency quenching and flame quenching.Hardened gear teeth are subjected to final heat treatment after finishing. The heat treatment method is usually carburizing quenching, surface quenching, etc. The hardness is usually 40-60HRC.After the final heat treatment, the gear teeth will inevitably be deformed, so they can be eliminated by grinding or grinding.

In addition, it is required to harden the tooth root and tooth surface of the gear, the purpose of which is to increase the surface hardness of the tooth root at the point with the greatest bending normal stress and the stress concentration point, and to improve the bending fatigue strength.

The purpose of the hardening treatment of the tooth surface is to improve the wear resistance and contact fatigue performance of the tooth surface. According to the technical requirements, the hardened layer of the tooth surface and tooth root is 2-4mm deep.In this way, the low content of some elements will not cause the gear to break.

However, if quenching causes uneven distribution of the depth of the hardened layer, the depth of one side of the hardened layer does not meet the technical requirements, the tooth root has no hardened layer, the depth of the hardened layer is unevenly distributed, and the tooth root is not hardened, which will cause the tooth root to resist bending Fatigue strength is seriously insufficient.

At the same time, when the tooth root transition fillet is too small and there are longitudinal knife marks, it will cause greater stress concentration, and then bending fatigue fracture will be formed at the tooth root.

4. Carburizing and quenching

Carburizing and quenching of gears is one of the common processes for manufacturing gears.Carburizing steel is low-carbon steel and low-carbon alloy steel with a carbon content of 0.15%-0.25%, such as 20, 20Cr, etc.After carburizing and quenching, the tooth surface hardness can reach 56-62HRC. The tooth surface has high contact strength and good wear resistance, while the tooth core still maintains high toughness. It is often used for important gear transmissions subject to impact loads.

Usually, the deformation after carburizing and quenching is large, and the teeth need to be ground.In the process of carburizing the gear and the subsequent quenching process, there are many factors that can cause quenching cracks.For example, whether the raw materials are qualified; whether the steel is operated in a standard manner during the forging process; whether the carbon potential control is normal and stable during the carburizing process; whether the temperature during the carburizing and quenching heating and holding process is too high; and the quality of the quenching medium and whether the tempering Fully in time and so on.

Gear carburizing and quenching When the carburizing potential is too high or the low-temperature tempering after quenching is insufficient or not timely, grinding cracks on the tooth surface may occur.If the control is normal, there will be no grinding cracks.

The gear made of out-of-tolerance material has no quenching cracks during quenching and tempering. After the carburizing treatment and the surface is greatly carburized, even if the carburizing process is normal, because the alloy elements are seriously out of tolerance, cracks will appear during quenching. .

5. Nitriding

Nitriding is a chemical heat treatment.No other heat treatment is required after nitriding, and the tooth surface hardness can reach 60-62HRC.Because the nitriding temperature is low and the deformation of the teeth is small, it is suitable for occasions where it is difficult to grind teeth, such as internal gears.

The nitride layer is generally not thick and brittle, so it is not suitable for impact applications.Commonly used for nitriding steel containing lead, molybdenum, aluminum and other alloying elements, such as 38CrMoAlA.

In the heat treatment process, special attention should be paid to the segregation of chemical components in the steel, which is likely to cause gear fracture.

Third, the impact caused by gear processing methods

Gear transmission is that the teeth of a pair of gears mesh with each other, which requires that the surfaces of the teeth of a pair of gears meshed smoothly and can bear a certain load.

To mesh smoothly, the curve of the tooth surface must be processed well. The curve of the tooth surface generally includes cycloid, involute, and arc. Involute is more commonly used.

Involute processing methods generally have two forming methods and generating methods.

The forming method is to directly use the gear forming milling cutter to mill out the tooth valley. The advantage is that it can be obtained on the milling machine. The disadvantage is that the number of teeth is segmented in order to reduce the number of tools, and a knife is used within a segment of the number of teeth, so that the tooth profile will be changed. Bring system error.

The generative method uses a tool (or abrasive) whose contour is conjugated to the contour of the gear to be cut, and cuts out the tooth through the generative motion between the wheel blank and the tool. The generative method can use a tool (or grinding tool) to cut the tooth. Tool) Machining gears with different numbers of teeth in the same module is the most widely used in gear processing. Commonly used gears include gear shaping, gear hobbing, gear shaving, gear honing, and gear grinding.

Different processing methods will cause different errors, make the teeth of the two wheels unsteady, cause vibration, and aggravate the wear of the gears.

Attention should also be paid to the surface roughness. When the surface roughness of the friction pair is low, due to the small height of the micro-bumps involved in the shearing and plowing action, the size of the resulting wear debris is also small, which may be embedded in the surface grooves of the specimen. In the groove, the wear strength of the surface is greatly reduced, or even does not appear, and the wear performance of the friction pair is improved.

When the surface roughness of the friction pair is high, the height of the micro-protrusions involved in the shearing and plowing action is larger, and the size of the generated wear debris is also large, and it is easy to generate large-size hard wear debris or high ridges.

The ridge breaks off and falls off during the rolling process of the subsequent gear teeth to form wear debris. As the generation of large-size wear debris increases, the wear on the surface of the friction pair increases, the roughness increases, and the wear increases accordingly.

In addition, the traces of tool processing will increase wear and cracks during the gear meshing process, which should be paid attention to.

Fourth, the impact of lubricants

The lubrication method of gear transmission mainly depends on the peripheral speed of the product.

For low-speed or open gear drives, regular manual lubrication or grease is used.

For closed gear transmission, when the product peripheral speed v<12m/s, the large gear is immersed in the oil pool for oil immersion lubrication; when v>12m/s, in order to avoid oil churning loss, oil injection lubrication is often used.

Lubricating oil has an impact on the fatigue pitting corrosion life of gears, and it has a great influence. Improper selection of lubricating oil or low quality lubricating oil can reduce the fatigue pitting corrosion life by half; choosing high-performance lubricating oil can increase the fatigue pitting corrosion life by 2 ―3 times, and the key to high-performance lubricating oil is the reasonable compounding of additives.

If the viscosity of the lubricating oil is too high, it is difficult for the lubricating oil to be transported to the meshing part of the gear. When the lubrication is insufficient, the product is most prone to wear.

Compared with no lubrication, good lubrication can increase the service life of transmission components: the service life of quenched parts can be increased by 20%, and the service life of non-quenched parts can be increased by 200%.

In addition, the installation quality of the gear transmission mechanism and the working temperature also have a great influence on the failure of the gear.

The gear fit is also affected by the shape and position deviation of the gear, as well as the parallelism of the axis.The size of the tooth thickness (actual) of the pinion and the large gear and the size of the center distance of the shaft plus the deviation of the corresponding gear elements, as well as the backlash of the gear teeth, that is, the gap between the non-working tooth surfaces at the working diameter.

Since the working backlash is determined by the assembly backlash and the working state, they include the influence of deflection, installation error, radial runout of the bearing, temperature, and other unknown factors.

These will cause the gear meshing to be inaccurate, and accelerate the failure of the gear. It will also cause the temperature to rise too high, plastic deformation of the gear tooth surface or the phenomenon of tooth surface gluing.

There is also a sudden increase in the applied load, which may cause the gear teeth to break.

Through the above analysis of the influence of gear materials, heat treatment, processing methods, lubricants, installation, temperature applied load, etc., we can find problems in time in future work, and can solve problems based on the knowledge of the materials we have already grasped. Precautions before they occur have a great impact on the life of gears.

Gear is a mechanical transmission part widely used in mechanical equipment. It transmits motion and power between any two shafts in space through a pair of gear teeth meshing with each other, and can change the form and speed of motion.The product has a wide range of applications, constant transmission ratio, high efficiency and long service life, and is an important mechanism of the machine.