About gear transmission types and fault response methods

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When manufacturing gears, several typical errors such as eccentricity, pitch error, base pitch error and tooth profile error are usually generated. There are many reasons for gear manufacturers to generate these errors, including errors from machine tool movement, cutting tool errors, errors from improper installation and debugging of tools, workpieces, and machine tool systems, fixture errors, and gear deformation caused by internal stress during heat treatment. When these gear errors are large, it will cause the gear transmission to rotate slowly and quickly with micro-inertia interference, causing impact and vibration when the gear pair is meshed, causing large noise.
Due to assembly technology and assembly methods, the assembly error of “one end in contact and one end hanging” is usually caused when assembling gears; linear deviation of gear shaft and imbalance of gears, etc. One-end contact or linear deviation of gear shaft will cause uneven load on the gear, causing excessive load on individual gear teeth and local early wear, and even cause gear tooth breakage in severe cases. Gear imbalance will cause impact vibration and noise.

1. Tooth fracture
During gear transmission, the action force of the driving gear and the reaction force of the driven gear both act on the other gear teeth through the contact point. The dangerous situation is that the contact point is located at the top of the gear teeth at a certain moment; at this time, the gear teeth are like a cantilever beam. The bending stress generated at the root of the gear teeth after being loaded is large. If it is suddenly overloaded or impact overloaded, it is easy to cause overload fracture at the root of the gear teeth.

2. Tooth surface wear or scratches
Gear teeth have relative sliding during meshing transmission, coupled with poor lubrication, unclean lubricating oil, lubricating oil deterioration, low speed heavy load or poor heat treatment quality, which can cause adhesive wear, abrasive wear, corrosive wear and scratches on the gear tooth surface.

3. Tooth surface fatigue
The so-called tooth surface fatigue mainly includes pitting and peeling of the tooth surface. The cause of pitting is mainly due to the micro fatigue cracks caused by the pulsating contact stress on the working surface of the gear teeth. When the lubricating oil enters the surface crack area, it first closes the entrance and then squeezes during the meshing process. The lubricating oil in the micro fatigue crack area expands the crack area on the gear tooth surface under high pressure, causing the surface metal particles to fall off from the tooth surface, leaving small pits to form pitting on the tooth surface. When the fatigue crack on the gear tooth surface continues to expand deeper and farther, it will cause a large area or large pieces to fall off, forming tooth surface spalling.

4. Plastic deformation of tooth surface
When the gear material is soft and the load transmitted is large, plastic deformation of the tooth surface is easy to occur. Under the action of excessive friction between the tooth surfaces, the contact stress of the tooth surface will exceed the material’s anti-extrusion yield limit, and the tooth surface material will enter a plastic state, causing plastic flow of the tooth surface metal. This causes the active gear to form grooves on the tooth surface near the pitch line, and the driven gear to form ridges on the tooth surface near the pitch line, thereby destroying the tooth shape.

What is the process flow of gear processing?

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1. Forging and blanking
Hot die forging is still the most widely used blank forging process for automotive gear parts. In recent years, wedge cross rolling technology has been widely promoted in shaft processing. This technology is particularly suitable for blanking for relatively complex step shafts. It not only has high precision and small subsequent processing allowance, but also has high production efficiency.
2. Normalizing
The purpose of this process is to obtain a hardness suitable for subsequent gear cutting and to prepare the organization for the final heat treatment, so as to effectively reduce heat treatment deformation. The material of the gear steel used is usually 20CrMnTi. The general normalizing is greatly affected by personnel, equipment and environment, making it difficult to control the cooling speed and uniformity of the workpiece, resulting in large hardness dispersion and uneven metallographic structure, which directly affects metal cutting and final heat treatment, making thermal deformation large and irregular, and the quality of parts cannot be controlled. For this reason, the isothermal normalizing process is adopted. Practice has proved that the use of isothermal normalizing effectively changes the disadvantages of general normalizing, and the product quality is stable and reliable.


3. Turning
In order to meet the positioning requirements of high-precision gear processing, all gear blanks are processed using CNC lathes, and mechanical clamping is used without re-grinding the turning tool, so that the aperture, end face and outer diameter processing can be completed simultaneously under one clamping, which not only ensures the verticality requirements of the inner hole and the end face, but also ensures that the size discreteness of large-scale gear blank production is small. This improves the accuracy of the gear blank and ensures the processing quality of subsequent gears. In addition, the high efficiency of CNC lathe processing has greatly reduced the number of equipment and has good economic efficiency.
4. Hobbing and gear shaping
The equipment used for processing the gear part still uses a large number of ordinary gear hobbing machines and gear shaping machines. Although they are easy to adjust and maintain, their production efficiency is low. If a large production capacity is to be completed, multiple machines need to be produced simultaneously. With the development of coating technology, it is very convenient to re-coat the hobbing cutter and the slotting cutter after grinding. The coated tool can significantly increase the service life, generally by more than 90%, effectively reducing the number of tool changes and grinding time, and the benefits are significant.


5. Gear shaving Radial gear shaving technology is widely used in mass production of automotive gears due to its high efficiency and easy realization of the requirements for the modification of the designed tooth shape and tooth direction.
6. Heat treatment
Carburizing and quenching are required for automotive gears to ensure their good mechanical properties. For products that are no longer ground after heat treatment, stable and reliable heat treatment equipment is essential.
7. Grinding
It is mainly to finish the inner hole, end face, outer diameter of the shaft and other parts of the heat-treated gear to improve dimensional accuracy and reduce geometric tolerances.